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  • GFZ Data Services
  • John Wiley & Sons
  • 2015-2019  (1,265)
  • 1950-1954
  • 1935-1939
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  • 1
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    On the onset of ionospheric precursors 40 min before strong earthquakes (2015)
    John Wiley & Sons
    Details
    Publication Date: 2021-02-10
    Description: Heki (2011) and Heki and Enomoto (2013) claimed that anomalous, yet similar, increases of ionospheric total electron content (TEC) started ~40 min prior to the 2011 Tohoku-Oki, as well as before other Mw〉8 earthquakes. The authors concluded that the reported TEC anomalies were likely related to the pending earthquakes, suggesting also that TEC monitoring may be useful for future earthquake prediction. Here we carefully examine the findings of Heki (2011) and Heki and Enomoto (2013) by performing new analyses of the same TEC data. Our interpretation is that the 40 min onset of the ionospheric precursors is an artifact induced by the definition of the reference line adopted in analyzing TEC variations. We also discuss this repeatability in the tectonic and geodynamic context of the earthquakes. By performing a Superimposed Epoch Analysis of TEC data, we show that, however, the TEC increase reported by Heki (2011) was not particularly anomalous. We conclude that the TEC precursors reported by Heki (2011) and Heki and Enomoto (2013) are not useful for developing short-term earthquake prediction capabilities.
    Description: Published
    Description: 1383–1393
    Description: 1A. Geomagnetismo e Paleomagnetismo
    Description: JCR Journal
    Description: restricted
    Keywords: Ionosphere ; Total Electron Content ; Earthquake precursors ; Short-term earthquake prediction ; 01. Atmosphere::01.02. Ionosphere::01.02.99. General or miscellaneous ; 01. Atmosphere::01.03. Magnetosphere::01.03.99. General or miscellaneous ; 04. Solid Earth::04.06. Seismology::04.06.99. General or miscellaneous
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 2
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    Does geothermal exploitation trigger earthquakes in Tuscany? (2016)
    John Wiley & Sons
    Details
    Publication Date: 2017-04-04
    Description: For the past 25 years, power production has been accompanied by a small but steady increase in seismicity near geothermal wells. A new project seeks to explore why.
    Description: Published
    Description: 6T. Sismicità indotta e caratterizzazione sismica dei sistemi naturali
    Description: N/A or not JCR
    Description: open
    Keywords: seismic monitoring ; array ; geothermal energy ; 04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 3
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    Segmentation and eruptive activity along the East Pacific Rise at 16°N, in relation with the nearby Mathematician hotspot (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4380–4399, doi:10.1002/2014GC005560.
    Description: The 16°N segment of the East Pacific Rise is the most overinflated and shallowest of this fast-spreading ridge, in relation with an important magma flux due to the proximity of the Mathematician hotspot. Here, we analyze the detailed morphology of the axial dome and of the Axial Summit Trough (AST), the lava morphology, and the geometry of fissures and faults, in regard to the attributes of the magma chamber beneath and of the nearby hotspot. The data used are 1 m resolution bathymetry combined with seafloor photos and videos. At the dome summit, the AST is highly segmented by 10 third-order and fourth-order discontinuities over a distance of 30 km. Often, two contiguous and synchronous ASTs coexist. Such a configuration implies a wide (1100 m minimum) zone of diking. The existence of contiguous ASTs, their mobility, their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot, plus the existence of a second magma lens under the western half of the summit plateau, clearly reflect the influence of the hotspot on the organization of the spreading system. The different ASTs exhibit contrasted widths and depths. We suggest that narrow ASTs reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the ASTs. AST widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits.
    Description: This work was supported by a PhD Scholarship of Brittany Region and Université de Bretagne Occidentale (France) and has benefited from funding by the Europôle Mer and Labex-Mer of the European Institute for Marine Studies (IUEM).
    Description: 2015-05-26
    Keywords: East Pacific Rise ; Axial summit trough ; Hotspot ; Segmentation ; Lava ; Spreading processes
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Lucky Strike seamount : implications for the emplacement and rifting of segment-centered volcanoes at slow spreading mid-ocean ridges (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4157–4179, doi:10.1002/2014GC005477.
    Description: The history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow spreading Mid-Atlantic Ridge at the Lucky Strike Segment is deduced using near-bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (〈1 Myr), associated with focused eruptions, and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction. This volcanic phase likely requires efficient melt pooling and a long-lived crustal magma chamber as a melt source. A reduction in melt supply triggers formation of an axial graben rifting the central volcano, and the onset of seafloor spreading may eventually split it. At Lucky Strike, this results in two modes of crustal construction. Eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows, accumulating a thick layer of extrusives. Away from the volcano summit, deformation and volcanic emplacement is distributed throughout the rift valley floor, lacking a clear locus of accretion and deformation. Volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber. The mode of rapid volcano construction and subsequent rifting observed at the Lucky Strike seamount is common at other central volcanoes along the global mid-ocean ridge system.
    Description: he TowCam delployment for seafloor imaging during the Graviluck'06 cruise was supported by NSF grant OCE-0623744 to A.S. and D.J.F., and by WHOI Deep Ocean Exploration Institute funding (AS & DF). D.J.F. also benefitted from a visiting position at IPGP to carry out this work. The field data acquisition for the Lustre'96 cruise was supported by NSF grant OCE-9505579. The Flores, Sudaçores, and SISMOMAR cruises where funded by CNRS/IFREMER (France).
    Description: 2015-05-07
    Keywords: Mid-ocean ridges ; Volcanism ; Tectonics ; Rifting ; Sonar ; Faulting
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4230–4253, doi:10.1002/2014GC005509.
    Description: Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid- and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low- to high-temperature mantle wedge transition lies slightly trenchward of the arc.
    Description: Phil Wannamaker and Virginie Maris gratefully acknowledge funding by the U.S. National Science Foundation under grants EAR08–43725 and EAR08–38043 through the Earthscope and Geophysics programs. The 2D inversion capability received development support under U.S. Department of Energy contract DE-PS36-04GO94001. Rob Evans was supported through Earthscope grant EAR08–44041 and Shane McGary through a National Defense Science and Engineering Graduate (NDSEG) fellowship. Fieldwork in Canada was made possible by an NSERC Discovery Grant and a Canadian Foundation for Innovation award to Martyn Unsworth.
    Description: 2015-05-11
    Keywords: Cascadia ; Electrical resistivity ; Magnetotellurics ; Plate coupling ; Episodic tremor and slip ; Arc magmatism
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Characterizing the eddy field in the Arctic Ocean halocline (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 8800–8817, doi:10.1002/2014JC010488.
    Description: Ice-Tethered Profilers (ITP), deployed in the Arctic Ocean between 2004 and 2013, have provided detailed temperature and salinity measurements of an assortment of halocline eddies. A total of 127 mesoscale eddies have been detected, 95% of which were anticyclones, the majority of which had anomalously cold cores. These cold-core anticyclonic eddies were observed in the Beaufort Gyre region (Canadian water eddies) and the vicinity of the Transpolar Drift Stream (Eurasian water eddies). An Arctic-wide calculation of the first baroclinic Rossby deformation radius Rd has been made using ITP data coupled with climatology; Rd ∼ 13 km in the Canadian water and ∼8 km in the Eurasian water. The observed eddies are found to have scales comparable to Rd. Halocline eddies are in cyclogeostrophic balance and can be described by a Rankine vortex with maximum azimuthal speeds between 0.05 and 0.4 m/s. The relationship between radius and thickness for the eddies is consistent with adjustment to the ambient stratification. Eddies may be divided into four groups, each characterized by distinct core depths and core temperature and salinity properties, suggesting multiple source regions and enabling speculation of varying formation mechanisms.
    Description: Funding was provided by the National Science Foundation Polar Programs award ARC-1107623.
    Description: 2015-06-22
    Keywords: Arctic halocline ; Rossby deformation radius ; Mesoscale eddies
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Influence of late Pleistocene glaciations on the hydrogeology of the continental shelf offshore Massachusetts, USA (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4651–4670, doi:10.1002/2014GC005569.
    Description: Multiple late Pleistocene glaciations that extended onto the continental shelf offshore Massachusetts, USA, may have emplaced as much as 100 km3 of freshwater (salinity 〈5 ppt) in continental shelf sediments. To estimate the volume and extent of offshore freshwater, we developed a three-dimensional, variable-density model that couples fluid flow and heat and solute transport for the continental shelf offshore Massachusetts. The stratigraphy for our model is based on high-resolution, multichannel seismic data. The model incorporates the last 3 Ma of climate history by prescribing boundary conditions of sea level change and ice sheet extent and thickness. We incorporate new estimates of the maximum extent of a late Pleistocene ice sheet to near the shelf-slope break. Model results indicate that this late Pleistocene ice sheet was responsible for much of the emplaced freshwater. We predict that the current freshwater distribution may reach depths up to 500 meters below sea level and up to 30 km beyond Martha's Vineyard. The freshwater distribution is strongly dependent on the three-dimensional stratigraphy and ice sheet history. Our predictions improve our understanding of the distribution of offshore freshwater, a potential nonrenewable resource for coastal communities along recently glaciated margins.
    Description: This work was funded by NSF-OCE-0824368.
    Description: 2015-06-05
    Keywords: Pleistocene ; Hydrogeology ; Continental shelf ; Glaciation ; Freshwater resources
    Repository Name: Woods Hole Open Access Server
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  • 8
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    Moored observations of synoptic and seasonal variability in the East Greenland Coastal Current (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 8838–8857, doi:10.1002/2014JC010134.
    Description: We present a year-round assessment of the hydrographic variability within the East Greenland Coastal Current on the Greenland shelf from five synoptic crossings and 4 years of moored hydrographic data. From the five synoptic sections the current is observed as a robust, surface intensified flow with a total volume transport of 0.66 ± 0.18 Sv and a freshwater transport of 42 ± 12 mSv. The moorings showed heretofore unobserved variability in the abundance of Polar and Atlantic water masses in the current on synoptic scales. This is exhibited as large vertical displacement of isotherms (often greater than 100 m). Seasonally, the current is hemmed into the coast during the fall by a full depth Atlantic Water layer that has penetrated onto the inner shelf. The Polar Water layer in the current then thickens through the winter and spring seasons increasing the freshwater content in the current; the timing implies that this is probably driven by the seasonally varying export of freshwater from the Arctic and not the local runoff from Greenland. The measured synoptic variability is enhanced during the winter and spring period due to a lower halocline and a concurrent enhancement in the along-coast wind speed. The local winds force much of the high-frequency variability in a manner consistent with downwelling, but variability distinct from downwelling is also visible.
    Description: This work was funded by the National Science Foundation grant OCE-1130008, NASA grant NNX13AK88G, and the Ocean and Climate Change Institute at the Woods Hole Oceanographic Institution.
    Description: 2015-06-23
    Keywords: Greenland ; Freshwater ; Coastal current ; Fjord
    Repository Name: Woods Hole Open Access Server
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  • 9
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    Global estimate of submarine groundwater discharge based on an observationally constrained radium isotope model (2015)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: © The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geophysical Research Letters 41 (2014): 8438–8444, doi:10.1002/2014GL061574.
    Description: Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) × 1013 m3 yr−1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.
    Description: This work was supported by the Ministry of Oceans and Fisheries, Korea, through the Korea Institute of Marine Science and Technology (KIMST) (20120176) and National Research Foundation (NRF) of Korea (2013R1A2A1A05004343 and 2013R1A1A1058203). Charette and Moore's contributions were supported by the US National Science Foundation through the GEOTRACES project.
    Keywords: Submarine groundwater discharge ; Radium ; Inverse modeling ; Land-ocean interaction ; Brackish groundwater ; Coastal flux
    Repository Name: Woods Hole Open Access Server
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  • 10
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    Quantifying flow regimes in a Greenland glacial fjord using iceberg drifters (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 8411–8420, doi:10.1002/2014GL062256.
    Description: Large, deep-keeled icebergs are ubiquitous in Greenland's outlet glacial fjords. Here we use the movement of these icebergs to quantify flow variability in Sermilik Fjord, southeast Greenland, from the ice mélange through the fjord to the shelf. In the ice mélange, a proglacial mixture of sea ice and icebergs, we find that icebergs consistently track the glacier speed, with slightly faster speeds near terminus and episodic increases due to calving events. In the fjord, icebergs accurately capture synoptic circulation driven by both along-fjord and along-shelf winds. Recirculation and in-/out-fjord variations occur throughout the fjord more frequently than previously reported, suggesting that across-fjord velocity gradients cannot be ignored. Once on the shelf, icebergs move southeastward in the East Greenland Coastal Current, providing wintertime observations of this freshwater pathway.
    Description: Funding for this study was provided by National Science Foundation grants OCE-1130008 and ARC-0909274, and by the University of Oregon.
    Description: 2015-06-11
    Keywords: Icebergs ; Fjord circulation ; Ice mélange ; East Greenland Current ; Greenland ice sheet ; Iceberg melt
    Repository Name: Woods Hole Open Access Server
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  • 11
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    Development and evolution of detachment faulting along 50 km of the Mid-Atlantic Ridge near 16.5°N (2015)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4692–4711, doi:10.1002/2014GC005563.
    Description: A multifaceted study of the slow spreading Mid-Atlantic Ridge (MAR) at 16.5°N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along ∼50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, noncorrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal a new detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multiscale corrugations are observed 2–3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low-angle (6°) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust 〉2 Ma, indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two-sided detachment faulting.
    Description: This work was supported by the National Science Foundation grant OCE-1155650.
    Description: 2015-06-05
    Keywords: Oceanic detachment faults ; AUV Sentry ; Mid-Atlantic Ridge
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  • 12
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    Boundary mixing in Orkney Passage outflow (2015)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 8627–8645, doi:10.1002/2014JC010099.
    Description: One of the most remarkable features of contemporary oceanic climate change is the warming and contraction of Antarctic Bottom Water over much of global ocean abyss. These signatures represent changes in ventilation mediated by mixing and entrainment processes that may be location-specific. Here we use available data to document, as best possible, those mixing processes as Weddell Sea Deep and Bottom Waters flow along the South Orkney Plateau, exit the Weddell Sea via Orkney Passage and fill the abyssal Scotia Sea. First, we find that an abrupt transition in topography upstream of Orkney Passage delimits the extent of the coldest waters along the Plateau's flanks and may indicate a region of especially intense mixing. Second, we revisit a control volume budget by Heywood et al. (2002) for waters trapped within the Scotia Sea after entering through Orkney Passage. This budget requires extremely vigorous water mass transformations with a diapycnal transfer coefficient of inline image m2 s−1. Evidence for such intense diapycnal mixing is not found in the abyssal Scotia Sea interior and, while we do find large rates of diapycnal mixing in conjunction with a downwelling Ekman layer on the western side of Orkney Passage, it is insufficient to close the budget. This leads us to hypothesize that the Heywood budget is closed by a boundary mixing process in which the Ekman layer associated with the Weddell Sea Deep Water boundary current experiences relatively large vertical scale overturning associated with tidal forcing along the southern boundary of the Scotia Sea.
    Description: KLP gratefully acknowledges salary support from Woods Hole Oceanographic Institution bridge support funds. ACNG acknowledges the support of a Philip Leverhulme Prize. LJ and MPM were supported by the ANDREX project, funded by the U.K. National Environment Research Council (NE/E01366X/1).
    Description: 2015-06-16
    Keywords: Mixing ; Transport ; Control volume
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  • 13
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    Trend and interannual variability in southeast Greenland Sea Ice : impacts on coastal Greenland climate variability (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 8619–8626, doi:10.1002/2014GL062107.
    Description: We describe the recent occurrence of a region of diminished sea ice cover or “notch” offshore of the Kangerdlugssuaq Fiord, the site of the largest tidewater glacier along Greenland's southeast coast. The notch's location is consistent with a topographically forced flux of warm water toward the fiord, and the decrease of the sea ice cover is shown to be associated with a regional warming of the upper ocean that began in the mid-1990s. Sea ice in the vicinity of the notch also exhibits interannual variability that is shown to be associated with a seesaw in surface temperature and sea ice between southeast and northeast Greenland that is not describable solely in terms of the North Atlantic Oscillation. We therefore argue that other modes of atmospheric variability, including the Lofoten Low, are required to fully document the changes to the climate that are occurring along Greenland's east coast.
    Description: G.W.K.M. was supported by the Natural Sciences and Engineering Research Council of Canada. F.S. and M.O. were supported by NSF OCE 1130008 and NASA NNX13AK88G.
    Description: 2015-06-02
    Keywords: Greenland ; Sea ice ; Interannual variability ; Lofoten Low ; Icelandic Low
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  • 14
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    The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment (2015)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 1115–1132, doi:10.1002/2014GC005709.
    Description: The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (〉103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5°C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3°C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.
    Description: Funding for the NoMELT experiment was provided by the National Science Foundation through the following grant numbers: OCE-0927172, OCE-0928270, OCE-1459649, and OCE-0928663.
    Description: 2015-10-18
    Keywords: Water ; Lithosphere-asthenosphere boundary ; Olivine ; Magnetotellurics
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  • 15
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    Meteorological buoy observations from the central Iceland Sea (2015)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Atmospheres 120 (2015): 3199–3208, doi:10.1002/2014JD022584.
    Description: We present the first continuous in situ atmospheric observations from the central Iceland Sea collected from a meteorological buoy deployed for a 2 year period between 23 November 2007 and 21 August 2009. We use these observations to evaluate the ERA-Interim reanalysis product and demonstrate that it represented low-level meteorological fields and surface turbulent fluxes in this region very well. The buoy observations showed that moderate to strong winds were common from any direction, while wind speeds below 5 ms−1 were relatively rare. The observed low-level air temperature and surface heat fluxes were related to the wind direction with cold-air outbreaks most common from the northwest. Mean wintertime turbulent heat fluxes were modest (〈60 Wm−2), but the range was substantial. High heat flux events, greater than 200 Wm−2, typically occurred every 1–2 weeks in the winter, with each event lasting on average 2.5 days with an average total turbulent heat flux of ∼200 Wm−2 out of the ocean. The most pronounced high heat flux events over the central Iceland Sea were associated with cold-air outbreaks from the north and west forced by a deep Lofoten Low over the Norwegian Sea.
    Description: This work was funded in part by the Ocean and Climate Change Institute at the Woods Hole Oceanographic Institution and NSF grant OCE-1433958.
    Description: 2015-10-24
    Keywords: Iceland Sea ; Met buoy ; Heat flux ; Nordic Seas ; Cold-air outbreak
    Repository Name: Woods Hole Open Access Server
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  • 16
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    Modeled alongshore circulation and force balances onshore of a submarine canyon (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 1887–1903, doi:10.1002/2014JC010555.
    Description: Alongshore force balances, including the role of nonlinear advection, in the shoaling and surf zones onshore of a submarine canyon are investigated using a numerical modeling system (Delft3D/SWAN). The model is calibrated with waves and alongshore flows recorded over a period of 1.5 months at 26 sites along the 1.0, 2.5, and 5.0 m depth contours spanning about 2 km of coast. Field observation-based estimates of the alongshore pressure and radiation-stress gradients are reproduced well by the model. Model simulations suggest that the alongshore momentum balance is between the sum of the pressure and radiation-stress gradients and the sum of the nonlinear advective terms and bottom stress, with the remaining terms (e.g., wind stress and turbulent mixing) being negligible. The simulations also indicate that unexplained residuals in previous field-based estimates of the momentum balance may be owing to the neglect of the nonlinear advective terms, which are similar in magnitude to the sum of the forcing (pressure and radiations stress gradients) and to the bottom stress.
    Description: Funding was provided by a joint WHOI-USGS postdoctoral scholarship, NSF, ONR, and ASD(R&E).
    Description: 2015-09-23
    Keywords: Alongshore momentum balance ; Nonlinear advection ; Surfzone currents
    Repository Name: Woods Hole Open Access Server
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  • 17
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    Mechanisms of surface wave energy dissipation over a high-concentration sediment suspension (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 1638–1681, doi:10.1002/2014JC010245.
    Description: Field observations from the spring of 2008 on the Louisiana shelf were used to elucidate the mechanisms of wave energy dissipation over a muddy seafloor. After a period of high discharge from the Atchafalaya River, acoustic measurements showed the presence of 20 cm thick mobile fluid-mud layers during and after wave events. While total wave energy dissipation (D) was greatest during the high energy periods, these periods had relatively low normalized attenuation rates (κ = Dissipation/Energy Flux). During declining wave-energy conditions, as the fluid-mud layer settled, the attenuation process became more efficient with high κ and low D. The transition from high D and low κ to high κ and low D was caused by a transition from turbulent to laminar flow in the fluid-mud layer as measured by a Pulse-coherent Doppler profiler. Measurements of the oscillatory boundary layer velocity profile in the fluid-mud layer during laminar flow reveal a very thick wave boundary layer with curvature filling the entire fluid-mud layer, suggesting a kinematic viscosity 2–3 orders of magnitude greater than that of clear water. This high viscosity is also consistent with a high wave-attenuation rates measured by across-shelf energy flux differences. The transition to turbulence was forced by instabilities on the lutocline, with wavelengths consistent with the dispersion relation for this two-layer system. The measurements also provide new insight into the dynamics of wave-supported turbidity flows during the transition from a laminar to turbulent fluid-mud layer.
    Description: This work was supported by Office of Naval Research Award N00014-06-1–0718, which was part of the ONR Multidisciplinary University Research Initiative (MUD-MURI): entitled ‘‘Mechanisms of Fluid-Mud Interactions Under Waves.’’ Additional support was provided by National Science Foundation grant 1059914.
    Description: 2015-09-19
    Keywords: Fluid mud ; Wave dissipation ; Laminar and turbulent wave boundary layers ; Lutocline instabilities ; Wave-supported turbidity flows
    Repository Name: Woods Hole Open Access Server
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    Relationships among seismic velocity, metamorphism, and seismic and aseismic fault slip in the Salton Sea Geothermal Field region (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 120 (2015): 2600–2615, doi:10.1002/2014JB011579.
    Description: The Salton Sea Geothermal Field is one of the most geothermally and seismically active areas in California and presents an opportunity to study the effect of high-temperature metamorphism on the properties of seismogenic faults. The area includes numerous active tectonic faults that have recently been imaged with active source seismic reflection and refraction. We utilize the active source surveys, along with the abundant microseismicity data from a dense borehole seismic network, to image the 3-D variations in seismic velocity in the upper 5 km of the crust. There are strong velocity variations, up to ~30%, that correlate spatially with the distribution of shallow heat flow patterns. The combination of hydrothermal circulation and high-temperature contact metamorphism has significantly altered the shallow sandstone sedimentary layers within the geothermal field to denser, more feldspathic, rock with higher P wave velocity, as is seen in the numerous exploration wells within the field. This alteration appears to have a first-order effect on the frictional stability of shallow faults. In 2005, a large earthquake swarm and deformation event occurred. Analysis of interferometric synthetic aperture radar data and earthquake relocations indicates that the shallow aseismic fault creep that occurred in 2005 was localized on the Kalin fault system that lies just outside the region of high-temperature metamorphism. In contrast, the earthquake swarm, which includes all of the M 〉 4 earthquakes to have occurred within the Salton Sea Geothermal Field in the last 15 years, ruptured the Main Central Fault (MCF) system that is localized in the heart of the geothermal anomaly. The background microseismicity induced by the geothermal operations is also concentrated in the high-temperature regions in the vicinity of operational wells. However, while this microseismicity occurs over a few kilometer scale region, much of it is clustered in earthquake swarms that last from hours to a few days and are localized near the MCF system.
    Description: This work was funded by USGS NEHRP proposal G10AP00101 and NSF proposal 0943906.
    Description: 2015-10-28
    Keywords: Geothermal field ; Earthquakes ; Fault creep
    Repository Name: Woods Hole Open Access Server
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  • 19
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    Eddy stirring and horizontal diffusivity from Argo float observations : geographic and depth variability (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 3989–3997, doi:10.1002/2015GL063827.
    Description: Stirring along isopycnals is a significant factor in determining the distribution of tracers within the ocean. Salinity anomalies on density surfaces from Argo float profiles are used to investigate horizontal stirring and estimate eddy mixing lengths. Eddy mixing length and velocity fluctuations from the ECCO2 global state estimate are used to estimate horizontal diffusivity at a 300 km scale in the upper 2000 m with near-global coverage. Diffusivity varies by over two orders of magnitude with latitude, longitude, and depth. In all basins, diffusivity is elevated in zonal bands corresponding to strong current regions, including western boundary current extension regions, the Antarctic Circumpolar Current, and equatorial current systems. The estimated mixing lengths and diffusivities provide an observationally based data set that can be used to test and constrain predictions and parameterizations of eddy stirring.
    Description: This work was supported by the National Science Foundation under grants OCE-13-55668 and OCE-95-21468 and the Office of Naval Research under grants N00014-12-1-0336 and N00014-13-1-0484.
    Description: 2015-11-21
    Keywords: Eddy stirring ; Diffusivity ; Mixing length ; Spice ; Argo
    Repository Name: Woods Hole Open Access Server
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    Inorganic carbon system dynamics in landfast Arctic sea ice during the early-melt period (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 3542-3566, doi:10.1002/2014JC010620.
    Description: We present the results of a 6 week time series of carbonate system and stable isotope measurements investigating the effects of sea ice on air-sea CO2 exchange during the early melt period in the Canadian Arctic Archipelago. Our observations revealed significant changes in sea ice and sackhole brine carbonate system parameters that were associated with increasing temperatures and the buildup of chlorophyll a in bottom ice. The warming sea-ice column could be separated into distinct geochemical zones where biotic and abiotic processes exerted different influences on inorganic carbon and pCO2 distributions. In the bottom ice, biological carbon uptake maintained undersaturated pCO2 conditions throughout the time series, while pCO2 was supersaturated in the upper ice. Low CO2 permeability of the sea ice matrix and snow cover effectively impeded CO2 efflux to the atmosphere, despite a strong pCO2 gradient. Throughout the middle of the ice column, brine pCO2 decreased significantly with time and was tightly controlled by solubility, as sea ice temperature and in situ melt dilution increased. Once the influence of melt dilution was accounted for, both CaCO3 dissolution and seawater mixing were found to contribute alkalinity and dissolved inorganic carbon to brines, with the CaCO3 contribution driving brine pCO2 to values lower than predicted from melt-water dilution alone. This field study reveals a dynamic carbon system within the rapidly warming sea ice, prior to snow melt. We suggest that the early spring period drives the ice column toward pCO2 undersaturation, contributing to a weak atmospheric CO2 sink as the melt period advances.
    Description: We acknowledge support from the Polar Continental Shelf Program (PCSP) of Natural Resources Canada, the Natural Sciences and Engineering Research Council of Canada, the Northern Scientific Training Program, Canada Economic Development, and Fisheries and Oceans Canada.
    Description: 2015-11-19
    Keywords: Sea ice ; Carbon cycling ; CO2 ; Brines ; Stable isotopes ; Arctic Ocean
    Repository Name: Woods Hole Open Access Server
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    Multicentury changes in ocean and land contributions to the climate-carbon feedback (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 744–759, doi:10.1002/2014GB005079.
    Description: Improved constraints on carbon cycle responses to climate change are needed to inform mitigation policy, yet our understanding of how these responses may evolve after 2100 remains highly uncertain. Using the Community Earth System Model (v1.0), we quantified climate-carbon feedbacks from 1850 to 2300 for the Representative Concentration Pathway 8.5 and its extension. In three simulations, land and ocean biogeochemical processes experienced the same trajectory of increasing atmospheric CO2. Each simulation had a different degree of radiative coupling for CO2 and other greenhouse gases and aerosols, enabling diagnosis of feedbacks. In a fully coupled simulation, global mean surface air temperature increased by 9.3 K from 1850 to 2300, with 4.4 K of this warming occurring after 2100. Excluding CO2, warming from other greenhouse gases and aerosols was 1.6 K by 2300, near a 2 K target needed to avoid dangerous anthropogenic interference with the climate system. Ocean contributions to the climate-carbon feedback increased considerably over time and exceeded contributions from land after 2100. The sensitivity of ocean carbon to climate change was found to be proportional to changes in ocean heat content, as a consequence of this heat modifying transport pathways for anthropogenic CO2 inflow and solubility of dissolved inorganic carbon. By 2300, climate change reduced cumulative ocean uptake by 330 Pg C, from 1410 Pg C to 1080 Pg C. Land fluxes similarly diverged over time, with climate change reducing stocks by 232 Pg C. Regional influence of climate change on carbon stocks was largest in the North Atlantic Ocean and tropical forests of South America. Our analysis suggests that after 2100, oceans may become as important as terrestrial ecosystems in regulating the magnitude of the climate-carbon feedback.
    Description: We are grateful for support from the U.S. Department of Energy Office of Science and the National Science Foundation (NSF). J.T.R. and F.H. received support from the Regional and Global Climate Modeling Program in the Climate and Environmental Sciences Division of the Biological and Environmental Research (BER) Program in the U.S. Department of Energy Office of Science. J.T.R., K.L., E.M., W.F., J.K.M., S.C.D., and N.N.M. received funding from the NSF project “Collaborative Research: Improved Regional and Decadal Predictions of the Carbon Cycle“ (AGS-1048827, AGS-1021776, and AGS-1048890). The Community Earth System Modeling project receives support from both NSF and BER.
    Description: 2015-12-02
    Keywords: Atlantic meridional overturning circulation ; Net primary production ; Stratification ; Ecosystems ; Carbon-concentration feedback
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    Hydrodynamic and sediment transport modeling of New River Inlet (NC) under the interaction of tides and waves (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 4028–4047, doi:10.1002/2014JC010425.
    Description: The interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi-3-D nearshore community model, NearCoM-TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semidiurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave-current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward-directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel-shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primarily by waves.
    Description: Funding was provided by the Office of Naval Research (N00014-13-1–0120 and N00014-14-1-0586) and the Office of the Assistant Secretary of Defense for Research and Engineering.
    Description: 2015-12-07
    Keywords: Wave-current interaction ; Sediment transport ; New River ; Morphological evolution ; Tidal inlet
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  • 23
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    Strong sea surface cooling in the eastern equatorial Pacific and implications for Galápagos Penguin conservation (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 6432–6437, doi:10.1002/2015GL064456.
    Description: The Galápagos is a flourishing yet fragile ecosystem whose health is particularly sensitive to regional and global climate variations. The distribution of several species, including the Galápagos Penguin, is intimately tied to upwelling of cold, nutrient-rich water along the western shores of the archipelago. Here we show, using reliable, high-resolution sea surface temperature observations, that the Galápagos cold pool has been intensifying and expanding northward since 1982. The linear cooling trend of 0.8°C/33 yr is likely the result of long-term changes in equatorial ocean circulation previously identified. Moreover, the northward expansion of the cold pool is dynamically consistent with a slackening of the cross-equatorial component of the regional trade winds—leading to an equatorward shift of the mean position of the Equatorial Undercurrent. The implied change in strength and distribution of upwelling has important implications for ongoing and future conservation measures in the Galápagos.
    Description: K.B.K. acknowledges support from the Alfred P. Sloan Foundation, the James E. and Barbara V. Moltz Fellowship administered by the Woods Hole Oceanographic Institution (WHOI) Ocean and Climate Change Institute (OCCI), and the National Science Foundation (NSF) Physical Oceanography program (grant OCE–1233282). S.J. acknowledges support from WHOI. C.W.B. was supported by the NOAA Center for Satellite Applications and Research.
    Description: 2016-02-06
    Keywords: Upwelling ; Conservation ; Penguins ; Galapagos ; Ocean circulation ; Climate
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  • 24
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    Water exchange between the continental shelf and the cavity beneath Nioghalvfjerdsbræ (79 North Glacier) (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 7648–7654, doi:10.1002/2015GL064944.
    Description: The mass loss at Nioghalvfjerdsbræ is primarily due to rapid submarine melting. Ocean data obtained from beneath the Nioghalvfjerdsbræ ice tongue show that melting is driven by the presence of warm (1°C) Atlantic Intermediate Water (AIW). A sill prevents AIW from entering the cavity from Dijmphna Sund, requiring that it flow into the cavity via bathymetric channels to the south at a pinned ice front. Comparison of water properties from the cavity, Dijmphna Sund, and the continental shelf support this conclusion. Overturning circulation rates inferred from observed melt rates and cavity stratification suggest an exchange flow between the cavity and the continental shelf of 38mSv, sufficient to flush cavity waters in under 1 year. These results place upper bounds on the timescales of external variability that can be transmitted to the glacier via the ice tongue cavity.
    Description: NASA Grant Number: NNX13AK88G, NSF Grant Number: OCE-1434041
    Description: 2016-03-22
    Keywords: 79North ; Ice tongue ; NEGIS ; Nioghalvfjerdsfjorden ; Circulation ; Ice-ocean
    Repository Name: Woods Hole Open Access Server
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  • 25
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    Extended evolution : a conceptual framework for integrating regulatory networks and niche construction (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 324 (2015): 565–577, doi:10.1002/jez.b.22631.
    Description: This paper introduces a conceptual framework for the evolution of complex systems based on the integration of regulatory network and niche construction theories. It is designed to apply equally to cases of biological, social and cultural evolution. Within the conceptual framework we focus especially on the transformation of complex networks through the linked processes of externalization and internalization of causal factors between regulatory networks and their corresponding niches and argue that these are an important part of evolutionary explanations. This conceptual framework extends previous evolutionary models and focuses on several challenges, such as the path-dependent nature of evolutionary change, the dynamics of evolutionary innovation and the expansion of inheritance systems.
    Description: NSF Grant Numbers: 1243575, 1127611
    Repository Name: Woods Hole Open Access Server
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  • 26
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    Near-inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the Arabian Sea during the monsoons (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 6492–6507, doi:10.1002/2014JC010198.
    Description: We present the horizontal kinetic energy (KE) balance of near-inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15.26° N in the Arabian Sea during the southwest monsoon. The highly sheared and stratified transition layer at the mixed-layer base varies between 5 m and 35 m and correlates negatively with the wind stress. Results from the mixed layer near-inertial KE (NIKE) balance suggest that wind energy at times can energize the transition layer and at other times is fully utilized within the mixed layer. A simple two layer model is utilized to study the shear evolution in the transition layer and shown to match well with observations. The shear production in this model arises from alignment of wind stress and shear. Although the winds are unidirectional during the monsoon, the shear in the transition layer is predominantly near-inertial. The near-inertial shear bursts in the observations show the same phasing and magnitude at near-inertial frequencies as the wind-shear alignment term.
    Description: NASA Grant Number: NNX12AD47G, NSF Grant Number: 0928138, ONR Grant Numbers: N00014-11-1-0429 and N00014-10-1-0273, NSF Grant Number: OCE-0745508
    Description: 2016-03-26
    Keywords: Near inertial energy ; Transition layer ; Near inertial shear
    Repository Name: Woods Hole Open Access Server
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    Surfzone to inner-shelf exchange estimated from dye tracer balances (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 6289–6308, doi:10.1002/2015JC010844.
    Description: Surfzone and inner-shelf tracer dispersion are observed at an approximately alongshore-uniform beach. Fluorescent Rhodamine WT dye, released near the shoreline continuously for 6.5 h, is advected alongshore by breaking-wave- and wind-driven currents, and ejected offshore from the surfzone to the inner-shelf by transient rip currents. Novel aerial-based multispectral dye concentration images and in situ measurements of dye, waves, and currents provide tracer transport and dilution observations spanning about 350 m cross-shore and 3 km alongshore. Downstream dilution of near-shoreline dye follows power law decay with exponent −0.33, implying that a tenfold increase in alongshore distance reduces the concentration about 50%. Coupled surfzone and inner-shelf dye mass balances close, and in 5 h, roughly half of the surfzone-released dye is transported offshore to the inner-shelf. Observed cross-shore transports are parameterized well ( inline image, best fit slope inline image) using a bulk exchange velocity and mean surfzone to inner-shelf dye concentration difference. The best fit cross-shore exchange velocity inline image is similar to a temperature-derived exchange velocity on another day with similar wave conditions. The inline image magnitude and observed inner-shelf dye length scales, time scales, and vertical structure indicate the dominance of transient rip currents in surfzone to inner-shelf cross-shore exchange during moderate waves at this alongshore-uniform beach.
    Description: National Science Foundation Graduate Research Fellowship Grant Number: DGE1144086, California Sea Grant Number: R/CONT-207TR
    Description: 2016-03-19
    Keywords: Surfzone ; Inner-shelf ; Tracer ; Cross-shore transport ; Mixing ; Pollution
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  • 28
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    Influence of ice thickness and surface properties on light transmission through Arctic sea ice (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 5932–5944, doi:10.1002/2015JC010914.
    Description: The observed changes in physical properties of sea ice such as decreased thickness and increased melt pond cover severely impact the energy budget of Arctic sea ice. Increased light transmission leads to increased deposition of solar energy in the upper ocean and thus plays a crucial role for amount and timing of sea-ice-melt and under-ice primary production. Recent developments in underwater technology provide new opportunities to study light transmission below the largely inaccessible underside of sea ice. We measured spectral under-ice radiance and irradiance using the new Nereid Under-Ice (NUI) underwater robotic vehicle, during a cruise of the R/V Polarstern to 83°N 6°W in the Arctic Ocean in July 2014. NUI is a next generation hybrid remotely operated vehicle (H-ROV) designed for both remotely piloted and autonomous surveys underneath land-fast and moving sea ice. Here we present results from one of the first comprehensive scientific dives of NUI employing its interdisciplinary sensor suite. We combine under-ice optical measurements with three dimensional under-ice topography (multibeam sonar) and aerial images of the surface conditions. We investigate the influence of spatially varying ice-thickness and surface properties on the spatial variability of light transmittance during summer. Our results show that surface properties such as melt ponds dominate the spatial distribution of the under-ice light field on small scales (〈1000 m2), while sea ice-thickness is the most important predictor for light transmission on larger scales. In addition, we propose the use of an algorithm to obtain histograms of light transmission from distributions of sea ice thickness and surface albedo.
    Description: U.S. National Science Foundation Office of Polar Programs NSF OPP ANT-1126311, National Oceanic and Atmospheric Administration Office of Exploration and Research NOAA OER NA14OAR4320158, European Research Council Advanced Investigator Grant Number: 294757
    Keywords: Melt ponds ; Light transmittance ; Albedo ; ROV ; Spatial variability ; Shortwave radiation
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  • 29
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    Short-term variability in euphotic zone biogeochemistry and primary productivity at Station ALOHA : a case study of summer 2012 (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 1145–1164, doi:10.1002/2015GB005141.
    Description: Time-series observations are critical to understand the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time-series program has maintained near-monthly sampling at Station ALOHA (22°45′N, 158°00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and has identified ecosystem variability over seasonal to interannual timescales. To further extend the temporal resolution of these near-monthly time-series observations, an extensive field campaign was conducted during July–September 2012 at Station ALOHA with near-daily sampling of upper water-column biogeochemistry, phytoplankton abundance, and activity. The resulting data set provided biogeochemical measurements at high temporal resolution and documents two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea-surface salinity minimum feature which was prominent in the upper water column (0–50 m) for a period of approximately 30 days. The shipboard observations during July–September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea-surface salinity minimum feature in the NPSG.
    Description: NOAA Climate Observation Division; National Science Foundation (NSF) Center for Microbial Oceanography: Research and Education (C-MORE) Grant Numbers: EF0424599, OCE-1153656, OCE-1260164; Gordon and Betty Moore Foundation Marine Microbiology Investigator
    Description: 2016-02-13
    Keywords: Primary productivity ; Microbial ecology ; Station ALOHA ; Temporal variability ; Biogeochemistry
    Repository Name: Woods Hole Open Access Server
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    Nature of the Jurassic Magnetic Quiet Zone (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 8367–8372, doi:10.1002/2015GL065394.
    Description: The nature of the Jurassic Quiet Zone (JQZ), a region of low-amplitude oceanic magnetic anomalies, has been a long-standing debate with implications for the history and behavior of the Earth's geomagnetic field and plate tectonics. To understand the origin of the JQZ, we studied high-resolution sea surface magnetic anomalies from the Hawaiian magnetic lineations and correlated them with the Japanese magnetic lineations. The comparison shows the following: (i) excellent correlation of anomaly shapes from M29 to M42; (ii) remarkable similarity of anomaly amplitude envelope, which decreases back in time from M19 to M38, with a minimum at M41, then increases back in time from M42; and (iii) refined locations of pre-M25 lineations in the Hawaiian lineation set. Based on these correlations, our study presents evidence of regionally and possibly globally coherent pre-M29 magnetic anomalies in the JQZ and a robust extension of Hawaiian isochrons back to M42 in the Pacific crust.
    Description: National Science Foundation Grant Numbers: OCE-1029965, OCE-1233000, OCE-1029573
    Description: 2016-04-24
    Keywords: Jurassic Quiet Zone ; Magnetic anomaly ; Plate tectonics
    Repository Name: Woods Hole Open Access Server
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    Compositional dependence of lower crustal viscosity (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 8333–8340, doi:10.1002/2015GL065459.
    Description: We calculate the viscosity structure of the lower continental crust as a function of its bulk composition using multiphase mixing theory. We use the Gibbs free-energy minimization routine Perple_X to calculate mineral assemblages for different crustal compositions under pressure and temperature conditions appropriate for the lower continental crust. The effective aggregate viscosities are then calculated using a rheologic mixing model and flow laws for the major crust-forming minerals. We investigate the viscosity of two lower crustal compositions: (i) basaltic (53 wt % SiO2) and (ii) andesitic (64 wt % SiO2). The andesitic model predicts aggregate viscosities similar to feldspar and approximately 1 order of magnitude greater than that of wet quartz. The viscosity range calculated for the andesitic crustal composition (particularly when hydrous phases are stable) is most similar to independent estimates of lower crust viscosity in actively deforming regions based on postglacial isostatic rebound, postseismic relaxation, and paleolake shoreline deflection.
    Description: Woods Hole Oceanographic Institution Summer Student Fellowship Program; NSF. Grant Numbers EAR-13-16333, EAR-1220075
    Description: 2016-04-23
    Keywords: Crustal viscosity ; Crustal composition ; Flow laws ; Lower crust ; Andesite
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    S-wave splitting in the offshore South Island, New Zealand : insights into plate-boundary deformation (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 2829–2847, doi:10.1002/2015GC005882.
    Description: Local and regional S-wave splitting in the offshore South Island of the New Zealand plate-boundary zone provides constraints on the spatial and depth extent of the anisotropic structure with an enhanced resolution relative to land-based and SKS studies. The combined analysis of offshore and land measurements using splitting tomography suggests plate-boundary shear dominates in the central and northern South Island. The width of this shear zone in the central South Island is about 200 km, but is complicated by stress-controlled anisotropy at shallow levels. In northern South Island, a broader (〉200 km) zone of plate-boundary parallel anisotropy is associated with the transitional faulting between the Alpine fault and Hikurangi subduction and the Hikurangi subduction zone itself. These results suggest S-phases of deep events (∼90 km) in the central South Island are sensitive to plate-boundary derived NE-SW aligned anisotropic media in the upper-lithosphere, supporting a “thin viscous sheet” deformation model.
    Description: United States National Ocean Bottom Seismograph Instrumentation Pool
    Description: 2016-02-29
    Keywords: Anisotropy ; Deformation ; Tomography ; S-wave splitting ; Plate-boundary ; Stress
    Repository Name: Woods Hole Open Access Server
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    Sediment transport-based metrics of wetland stability (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geophysical Research Letters 42 (2015): 7992–8000, doi:10.1002/2015GL065980.
    Description: Despite the importance of sediment availability on wetland stability, vulnerability assessments seldom consider spatiotemporal variability of sediment transport. Models predict that the maximum rate of sea level rise a marsh can survive is proportional to suspended sediment concentration (SSC) and accretion. In contrast, we find that SSC and accretion are higher in an unstable marsh than in an adjacent stable marsh, suggesting that these metrics cannot describe wetland vulnerability. Therefore, we propose the flood/ebb SSC differential and organic-inorganic suspended sediment ratio as better vulnerability metrics. The unstable marsh favors sediment export (18 mg L−1 higher on ebb tides), while the stable marsh imports sediment (12 mg L−1 higher on flood tides). The organic-inorganic SSC ratio is 84% higher in the unstable marsh, and stable isotopes indicate a source consistent with marsh-derived material. These simple metrics scale with sediment fluxes, integrate spatiotemporal variability, and indicate sediment sources.
    Description: U.S. Geological Survey Coastal and Marine Geology Program; Global Change and Land Use Program
    Keywords: Sediment transport ; Tidal wetlands ; Wetland stability ; Wetland vulnerability
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    The multiple fates of sinking particles in the North Atlantic Ocean (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 1471–1494, doi:10.1002/2014GB005037.
    Description: The direct respiration of sinking organic matter by attached bacteria is often invoked as the dominant sink for settling particles in the mesopelagic ocean. However, other processes, such as enzymatic solubilization and mechanical disaggregation, also contribute to particle flux attenuation by transferring organic matter to the water column. Here we use observations from the North Atlantic Ocean, coupled to sensitivity analyses of a simple model, to assess the relative importance of particle-attached microbial respiration compared to the other processes that can degrade sinking particles. The observed carbon fluxes, bacterial production rates, and respiration by water column and particle-attached microbial communities each spanned more than an order of magnitude. Rates of substrate-specific respiration on sinking particle material ranged from 0.007 ± 0.003 to 0.173 ± 0.105 day−1. A comparison of these substrate-specific respiration rates with model results suggested sinking particle material was transferred to the water column by various biological and mechanical processes nearly 3.5 times as fast as it was directly respired. This finding, coupled with strong metabolic demand imposed by measurements of water column respiration (729.3 ± 266.0 mg C m−2 d−1, on average, over the 50 to 150 m depth interval), suggested a large fraction of the organic matter evolved from sinking particles ultimately met its fate through subsequent remineralization in the water column. At three sites, we also measured very low bacterial growth efficiencies and large discrepancies between depth-integrated mesopelagic respiration and carbon inputs.
    Description: U.S. Environmental Protection Agency (EPA) STAR Grant Number: FP-91744301-0; National Science Foundation Grant Numbers OCE-1061883, EF-0424599, OCE-1155438, OCE-1059884, OCE-1031143; Gordon and Betty Moore Foundation Grant Numbers: 3301, 3789; Gordon and Betty Moore Foundation; Woods Hole Oceanographic Institution
    Description: 2016-03-25
    Keywords: Carbon cycle ; Particle flux ; Bacterial growth efficiency ; Bacterial respiration ; Microbial respiration
    Repository Name: Woods Hole Open Access Server
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    Decoupling of net community and export production on submesoscales in the Sargasso Sea (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 1266–1282, doi:10.1002/2014GB004913.
    Description: Determinations of the net community production (NCP) in the upper ocean and the particle export production (EP) should balance over long time and large spatial scales. However, recent modeling studies suggest that a horizontal decoupling of flux-regulating processes on submesoscales (≤10 km) could lead to imbalances between individual determinations of NCP and EP. Here we sampled mixed-layer biogeochemical parameters and proxies for NCP and EP during 10, high-spatial resolution (~2 km) surface transects across strong physical gradients in the Sargasso Sea. We observed strong biogeochemical and carbon flux variability in nearly all transects. Spatial coherence among measured biogeochemical parameters within transects was common but rarely did the same parameters covary consistently across transects. Spatial variability was greater in parameters associated with higher trophic levels, such as chlorophyll in 〉5.0 µm particles, and variability in EP exceeded that of NCP in nearly all cases. Within sampling transects, coincident EP and NCP determinations were uncorrelated. However, when averaged over each transect (30 to 40 km in length), we found NCP and EP to be significantly and positively correlated (R = 0.72, p = 0.04). Transect-averaged EP determinations were slightly smaller than similar NCP values (Type-II regression slope of 0.93, standard deviation = 0.32) but not significantly different from a 1:1 relationship. The results show the importance of appropriate sampling scales when deriving carbon flux budgets from upper ocean observations.
    Description: NASA Ocean Carbon and Biogeochemistry program Grant Number: NNX11AL94G; WHOI Postdoctoral Scholar fellowship; NASA ACE Grant Number: NNX12AJ25G; NSF Grant Number: OCE-0752366
    Description: 2016-02-29
    Keywords: Submesoscale biogeochemical variability ; Export production ; Net community production ; Sargasso Sea carbon flux balance
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    Rapid growth and concerted sexual transitions by a bloom of the harmful dinoflagellate Alexandrium fundyense (Dinophyceae) (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography 60 (2015): 2059–2078, doi:10.1002/lno.10155.
    Description: Transitions between life cycle stages by the harmful dinoflagellate Alexandrium fundyense are critical for the initiation and termination of its blooms. To quantify these transitions in a single population, an Imaging FlowCytobot (IFCB), was deployed in Salt Pond (Eastham, Massachusetts), a small, tidally flushed kettle pond that hosts near annual, localized A. fundyense blooms. Machine-based image classifiers differentiating A. fundyense life cycle stages were developed and results were compared to manually corrected IFCB samples, manual microscopy-based estimates of A. fundyense abundance, previously published data describing prevalence of the parasite Amoebophrya, and a continuous culture of A. fundyense infected with Amoebophrya. In Salt Pond, a development phase of sustained vegetative division lasted approximately 3 weeks and was followed by a rapid and near complete conversion to small, gamete cells. The gametic period (∼3 d) coincided with a spike in the frequency of fusing gametes (up to 5% of A. fundyense images) and was followed by a zygotic phase (∼4 d) during which cell sizes returned to their normal range but cell division and diel vertical migration ceased. Cell division during bloom development was strongly phased, enabling estimation of daily rates of division, which were more than twice those predicted from batch cultures grown at similar temperatures in replete medium. Data from the Salt Pond deployment provide the first continuous record of an A. fundyense population through its complete bloom cycle and demonstrate growth and sexual induction rates much higher than are typically observed in culture.
    Description: National Science Foundation Grant Number: OCE-0430724, OCE-0911031, and OCE-1314642; National Institutes of Health Grant Number: NIEHS-1P50-ES021923-01; National Park Service (NPS) Cooperative Agreement Grant Number: H238015504; Gordon and Betty Moore Foundation Grant Number: #2649 to HMS; IOF Grant Number: MOHAB PIOF-GA-252260
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    Heterogeneous and asymmetric crustal accretion : new constraints from multibeam bathymetry and potential field data from the Rainbow area of the Mid-Atlantic Ridge (36°15'N) (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 2994–3014, doi:10.1002/2015GC005743.
    Description: At slow-spreading mid-ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma-poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multibeam bathymetry and potential field data from the Rainbow area of the Mid-Atlantic Ridge (35°40'N–36°40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T, and the fraction of magmatic accretion, M. Estimated T values range from 0.2–0.4 on ridge segments to 0.6-0.8 at the Rainbow nontransform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high-temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep-penetrating second-generation faults.
    Description: NSF Grant Numbers: OCE-0961151 , OCE-0961680
    Description: 2016-03-13
    Keywords: Oceanic detachment faults ; Geomorphology ; Crustal accretion ; Slow-spreading ridges ; Nontransform discontinuities ; Gravity anomalies
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    How did ocean warming affect Australian rainfall extremes during the 2010/2011 La Niña event? (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 9942–9951, doi:10.1002/2015GL065948.
    Description: Extreme rainfall conditions in Australia during the 2010/2011 La Niña resulted in devastating floods claiming 35 lives, causing billions of dollars in damages, and far-reaching impacts on global climate, including a significant drop in global sea level and record terrestrial carbon uptake. Northeast Australian 2010/2011 rainfall was 84% above average, unusual even for a strong La Niña, and soil moisture conditions were unprecedented since 1950. Here we demonstrate that the warmer background state increased the likelihood of the extreme rainfall response. Using atmospheric general circulation model experiments with 2010/2011 ocean conditions with and without long-term warming, we identify the mechanisms that increase the likelihood of extreme rainfall: additional ocean warming enhanced onshore moisture transport onto Australia and ascent and precipitation over the northeast. Our results highlight the role of long-term ocean warming for modifying rain-producing atmospheric circulation conditions, increasing the likelihood of extreme precipitation for Australia during future La Niña events.
    Description: Australian Research Council (ARC); ARC Centre of Excellence for Climate System Science; ARC Laureate Fellowship program; Penzance and John P. Chase Memorial Endowed Funds; Ocean Climate Change Institute at WHOI
    Description: 2016-05-19
    Keywords: Ocean warming ; Precipitation;extremes ; Australia ; Attribution ; La Nina
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    Noble gas signatures in Greenland : tracing glacial meltwater sources (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 9311–9318, doi:10.1002/2015GL065778.
    Description: This study represents the first comprehensive noble gas study in glacial meltwater from the Greenland Ice Sheet. It shows that most samples are in disequilibrium with surface collection conditions. A preliminary Ne and Xe analysis suggests that about half of the samples equilibrated at a temperature of ~0°C and altitudes between 1000 m and 2000 m, with a few samples pointing to lower equilibration altitudes and temperatures between 2°C and 5°C. Two samples suggest an origin as melted ice and complete lack of equilibration with surface conditions. A helium component analysis suggests that this glacial meltwater was isolated from the atmosphere prior to the 1950s, with most samples yielding residence times ≤ 420 years. Most samples represent a mixture between a dominant atmospheric component originating as precipitation and basal meltwater or groundwater, which has accumulated crustal 4He over time.
    Description: University of Michigan; Packard Foundation; Department of Earth and Environmental Sciences Turner fellowship
    Description: 2016-05-06
    Keywords: Noble gases ; Greenland ; Glacial meltwater ; Water residence times
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    Structure of the mantle beneath the Alboran Basin from magnetotelluric soundings (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 4261–4274, doi:10.1002/2015GC006100.
    Description: We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3-D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20–30 km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics.
    Description: NSF Grant Number: EAR080-9074; Spanish National Projects Grant Number: CTM2009-07039-E/MAR, CTM2011-30400-C02-02
    Description: 2016-06-19
    Keywords: Magnetotellurics ; Western Mediterranean ; Subduction ; Roll-back
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    Nitrogen cycling in the secondary nitrite maximum of the eastern tropical North Pacific off Costa Rica (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 2061–2081, doi:10.1002/2015GB005187.
    Description: Nitrite is a central intermediate in the marine nitrogen cycle and represents a critical juncture where nitrogen can be reduced to the less bioavailable N2 gas or oxidized to nitrate and retained in a more bioavailable form. We present an analysis of rates of microbial nitrogen transformations in the oxygen deficient zone (ODZ) within the eastern tropical North Pacific Ocean (ETNP). We determined rates using a novel one-dimensional model using the distribution of nitrite and nitrate concentrations, along with their natural abundance nitrogen (N) and oxygen (O) isotope profiles. We predict rate profiles for nitrate reduction, nitrite reduction, and nitrite oxidation throughout the ODZ, as well as the contributions of anammox to nitrite reduction and nitrite oxidation. Nitrate reduction occurs at a maximum rate of 25 nM d−1 at the top of the ODZ, at the same depth as the maximum rate of nitrite reduction, 15 nM d−1. Nitrite oxidation occurs at maximum rates of 10 nM d−1 above the secondary nitrite maximum, but also in the secondary nitrite maximum, within the ODZ. Anammox contributes to nitrite oxidation within the ODZ but cannot account for all of it. Nitrite oxidation within the ODZ that is not through anammox is also supported by microbial gene abundance profiles. Our results suggest the presence of nitrite oxidation within the ETNP ODZ, with implications for the distribution and physiology of marine nitrite-oxidizing bacteria, and for total nitrogen loss in the largest marine ODZ.
    Description: National Science Foundation. Grant Numbers OCE 05-26277, OCE 09-610998; WHOI Coastal Ocean Institute
    Description: 2016-06-15
    Keywords: Nitrite ; Nitrate ; Stable isotopes
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    The annual cycle of gross primary production, net community production, and export efficiency across the North Pacific Ocean (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 30 (2016): 361–380, doi:10.1002/2015GB005318.
    Description: We measured triple oxygen isotopes and oxygen/argon dissolved gas ratios as nonincubation-based geochemical tracers of gross oxygen production (GOP) and net community production (NCP) on 16 container ship transects across the North Pacific from 2008 to 2012. We estimate rates and efficiency of biological carbon export throughout the full annual cycle across the North Pacific basin (35°N–50°N, 142°E–125°W) by constructing mixed layer budgets that account for physical and biological influences on these tracers. During the productive season from spring to fall, GOP and NCP are highest in the Kuroshio region west of 170°E and decrease eastward across the basin. However, deep winter mixed layers (〉200 m) west of 160°W ventilate ~40–90% of this seasonally exported carbon, while only ~10% of seasonally exported carbon east of 160°W is ventilated in winter where mixed layers are 〈120 m. As a result, despite higher annual GOP in the west than the east, the annual carbon export (sequestration) rate and efficiency decrease westward across the basin from export of 2.3 ± 0.3 mol C m−2 yr−1 east of 160°W to 0.5 ± 0.7 mol C m−2 yr−1 west of 170°E. Existing productivity rate estimates from time series stations are consistent with our regional productivity rate estimates in the eastern but not western North Pacific. These results highlight the need to estimate productivity rates over broad spatial areas and throughout the full annual cycle including during winter ventilation in order to accurately estimate the rate and efficiency of carbon sequestration via the ocean's biological pump.
    Description: This work was funded by a NDSEG Fellowship from the Office of Naval Research, a NSF Graduate Research Fellowship, and an ARCS Foundation Fellowship to H.I.P. and by NSF Ocean Sciences (0628663 and 1259055 to P.D.Q.).
    Description: 2016-08-27
    Keywords: North Pacific ; Carbon cycle ; Productivity ; Biological pump ; Gross oxygen production ; Net community production
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    Segment-scale variations in seafloor volcanic and tectonic processes from multibeam sonar imaging, Mid-Atlantic Ridge Rainbow region (35°45′–36°35′N) (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 17 (2016): 3560–3579, doi:10.1002/2016GC006433.
    Description: Along-axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid-ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow spreading Mid-Atlantic Ridge (35°45′–36°35′N) centered on the Rainbow nontransform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multidirectional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style, and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone-dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment, respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif.
    Description: National Science Foundation Grant Numbers: OCE-0961151, OCE-0961680
    Description: 2017-03-07
    Keywords: Mid-ocean ridges ; Oceanic core complex ; Rainbow massif ; Backscatter
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    Predicting Atlantic seasonal hurricane activity using outgoing longwave radiation over Africa (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 43 (2016): 7152–7159, doi:10.1002/2016GL069792.
    Description: Seasonal hurricane activity is a function of the amount of initial disturbances (e.g., easterly waves) and the background environment in which they develop into tropical storms (i.e., the main development region). Focusing on the former, a set of indices based solely upon the meridional structure of satellite-derived outgoing longwave radiation (OLR) over the African continent are shown to be capable of predicting Atlantic seasonal hurricane activity with very high rates of success. Predictions of named storms based on the July OLR field and trained only on the time period prior to the year being predicted yield a success rate of 87%, compared to the success rate of NOAA's August outlooks of 53% over the same period and with the same average uncertainty range (±2). The resulting OLR indices are statistically robust, highly detectable, physically linked to the predictand, and may account for longer-term observed trends.
    Description: Alfred P. Sloan Foundation; Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution; Ocean and Climate Change Institute
    Description: 2017-01-08
    Keywords: Hurricanes ; Tropical cyclones ; Seasonal predictability ; Africa ; Outgoing longwave radiation
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    Evolution of a Canada Basin ice-ocean boundary layer and mixed layer across a developing thermodynamically forced marginal ice zone (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 6223–6250, doi:10.1002/2016JC011778.
    Description: A comprehensive set of autonomous, ice-ocean measurements were collected across the Canada Basin to study the summer evolution of the ice-ocean boundary layer (IOBL) and ocean mixed layer (OML). Evaluation of local heat and freshwater balances and associated turbulent forcing reveals that melt ponds (MPs) strongly influence the summer IOBL-OML evolution. Areal expansion of MPs in mid-June start the upper ocean evolution resulting in significant increases to ocean absorbed radiative flux (19 W m−2 in this study). Buoyancy provided by MP drainage shoals and freshens the IOBL resulting in a 39 MJ m−2 increase in heat storage in just 19 days (52% of the summer total). Following MP drainage, a near-surface fresh layer deepens through shear-forced mixing to form the summer mixed layer (sML). In late summer, basal melt increases due to stronger turbulent mixing in the thin sML and the expansion of open water areas due in part to wind-forced divergence of the sea ice. Thermal heterogeneities in the marginal ice zone (MIZ) upper ocean led to large ocean-to-ice heat fluxes (100–200 W m−2) and enhanced basal ice melt (3–6 cm d−1), well away from the ice edge. Calculation of the upper ocean heat budget shows that local radiative heat input accounted for at least 89% of the observed latent heat losses and heat storage (partitioned 0.77/0.23). These results suggest that the extensive area of deteriorating sea ice observed away from the ice edge during the 2014 season, termed the “thermodynamically forced MIZ,” was driven primarily by local shortwave radiative forcing.
    Description: This material is based upon research supported by, or in part by, the U.S. Office of Naval Research under award numbers N0001414WX20089, N0001415WX01195, and N00014-12-1- 0140.
    Keywords: IOBL-OML evolution ; Ephemeral pycnocline ; Summer mixed layer ; Ocean heat storage ; Thermodynamic MIZ ; Melt pond drainage
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    Sinking phytoplankton associated with carbon flux in the Atlantic Ocean (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography 61 (2016): 1172–1187, doi:10.1002/lno.10253.
    Description: The composition of sinking particles and the mechanisms leading to their transport ultimately control how much carbon is naturally sequestered in the deep ocean by the “biological pump.” While detrital particles often contain much of the sinking carbon, sinking of intact phytoplankton cells can also contribute to carbon export, which represents a direct flux of carbon from the atmosphere to the deep ocean by circumventing the surface ocean food web. Phytoplankton that contributed to carbon flux were identified in sinking material collected by short-term sediment trap deployments conducted along a transect off the eastern shore of South America. Particulate organic carbon flux at 125 m depth did not change significantly along the transect. Instead, changes occurred in the composition and association of phytoplankton with detrital particles. The fluxes of diatoms, coccolithophores, dinoflagellates, and nano-sized cells at 125 m were unrelated to the overlying surface population abundances, indicating that functional-group specific transport mechanisms were variable across locations. The dominant export mechanism of phytoplankton at each station was putatively identified by principal component analysis and fell into one of three categories; (1) transport and sinking of individual, viable diatom cells, (2) transport by aggregates and fecal pellets, or (3) enhanced export of coccolithophores through direct settling and/or aggregation
    Description: Funding for the DeepDOM cruise was provided by the National Science Foundation (NSF) grant OCE-1154320 to E. B. Kujawinski and K. Longnecker, WHOI. Partial research support was provided by NSF through grants OCE-0925284, and OCE-1316036 to S.T. Dyhrman. C.A. Durkin was supported by a Woods Hole Oceanographic Institution Devonshire Postdoctoral Scholarship.
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    Mixing of dissolved oxygen in Chesapeake Bay driven by the interaction between wind-driven circulation and estuarine bathymetry (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 5639–5654, doi:10.1002/2016JC011924.
    Description: Field observations collected in Chesapeake Bay demonstrate how wind-driven circulation interacts with estuarine bathymetry to control when and where the vertical mixing of dissolved oxygen occurs. In the across-Bay direction, the lateral Ekman response to along-Bay wind forcing contributes to the vertical mixing of dissolved oxygen in two ways. First, the lateral tilting of the pycnocline/oxycline, consistent with the thermal wind relationship, advects the region of high vertical gradient into the surface and bottom boundary layers where mixing can occur. Second, upwelling of low-oxygen water to the surface enhances the atmospheric influx. In the along-Bay direction, the abrupt change in bottom depth associated with Rappahannock Shoal results in surface convergence and downwelling, leading to localized vertical mixing. Water that is mixed on the shoal is entrained into the up-Bay residual bottom flow resulting in increases in bottom dissolved oxygen that propagate up the system. The increases in dissolved oxygen are often associated with increases in temperature and decreases in salinity, consistent with vertical mixing. However, the lagged arrival moving northward suggests that the propagation of this signal up the Bay is due to advection.
    Description: National Science Foundation Grant Number: OCE-1338518
    Description: 2017-02-08
    Keywords: Mixing ; Hypoxia ; Chesapeake Bay
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    Comment: On phytoplankton perception by calanoid copepods (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Association for the Sciences of Limnology and Oceanography, 2016. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 61 (2016): 1163–1168, doi:10.1002/lno.10294.
    Description: Three publications recently reported that calanoid copepods, feeding on phytoplankton cells by using a feeding current, perceived such cells by mechanoperception. There was no evidence of remote chemically-mediated perception of those cells. These observations differ from earlier findings that feeding-current producing calanoids are able to detect phytoplankton cells by chemoperception at a distance from their particle-collecting setae of their cephalic appendages. The results on mechanoperception and the earlier published data on chemoperception will be presented and discussed. In addition, the concentration of chemicals within the phycosphere of food cells will be re-examined. We conclude that chemoperception of phytoplankton cells by calanoid copepods in a feeding current is feasible.
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    Atmospheric forcing during active convection in the Labrador Sea and its impact on mixed-layer depth (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 6978–6992, doi:10.1002/2015JC011607.
    Description: Hydrographic data from the Labrador Sea collected in February–March 1997, together with atmospheric reanalysis fields, are used to explore relationships between the air-sea fluxes and the observed mixed-layer depths. The strongest winds and highest heat fluxes occurred in February, due to the nature and tracks of the storms. While greater numbers of storms occurred earlier and later in the winter, the storms in February followed a more organized track extending from the Gulf Stream region to the Irminger Sea where they slowed and deepened. The canonical low-pressure system that drives convection is located east of the southern tip of Greenland, with strong westerly winds advecting cold air off the ice edge over the warm ocean. The deepest mixed layers were observed in the western interior basin, although the variability in mixed-layer depth was greater in the eastern interior basin. The overall trend in mixed-layer depth through the winter in both regions of the basin was consistent with that predicted by a 1-D mixed-layer model. We argue that the deeper mixed layers in the west were due to the enhanced heat fluxes on that side of the basin as opposed to oceanic preconditioning.
    Description: National Science Foundation (RP); Natural Science and Engineering Research Council of Canada Grant Number: OCE-1259618
    Description: 2017-03-22
    Keywords: Labrador Sea ; Convection ; Impact of storms ; Storm tracks ; Mixed layers
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    Polyphosphate dynamics at Station ALOHA, North Pacific subtropical gyre (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Association for the Sciences of Limnology and Oceanography, 2015. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 61 (2016): 227–239, doi:10.1002/lno.10206.
    Description: Polyphosphate (polyP) was examined within the upper water column (≤ 150 m) of Station ALOHA (22° 45′N, 158° 00′W) during two cruises conducted in May–June 2013 and September 2013. Phosphorus molar ratios of particulate polyP to total particulate phosphorus (TPP) were relatively low, similar to previously reported values from the temperate western North Atlantic, and did not exhibit strong vertical gradients, reflecting a lack of polyP recycling relative to other forms of TPP with depth. Furthermore, relationships among polyP:TPP, soluble reactive phosphorus (SRP), and alkaline phosphatase activity (APA) were also consistent with previous observations from the Atlantic Ocean. To ascertain potential mechanisms of biological polyP production and utilization, surface seawater was incubated following nutrient additions. Results were consistent with polyP:TPP enrichment under opposite extremes of APA, suggesting diverse polyP accumulation/retention mechanisms. Addition of exogenous polyP (45 ± 5 P atoms) to field incubations did not increase chlorophyll content relative to controls, suggesting that polyP was not bioavailable to phytoplankton at Station ALOHA. To clarify this result, phytoplankton cultures were screened for the ability to utilize exogenous polyP. PolyP bioavailability was variable among model diatoms of the genus Thalassiosira, yet chain length did not influence polyP bioavailability. Thus, microbial community composition may influence polyP dynamics in the ocean, and vice versa.
    Description: This work was supported by a Postdoctoral Fellowship from the Ford Foundation (JMD), the National Science Foundation under grants OCE 1225801 (JMD), OCE 1316036 (STD), EF 04-24599 (DMK), the Woods Hole Oceanographic Coastal Ocean Institute, the Center for Microbial Oceanography: Research and Education, and the Gordon and Betty Moore Foundation (DMK). Additional support was provided by grants from the Simons Foundation to DMK and STD.
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    Emerging opportunities and challenges in phenology : a review (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 7 (2016): e01436, doi:10.1002/ecs2.1436.
    Description: Plant phenology research has gained increasing attention because of the sensitivity of phenology to climate change and its consequences for ecosystem function. Recent technological development has made it possible to gather invaluable data at a variety of spatial and ecological scales. Despite our ability to observe phenological change at multiple scales, the mechanistic basis of phenology is still not well understood. Integration of multiple disciplines, including ecology, evolutionary biology, climate science, and remote sensing, with long-term monitoring data across multiple spatial scales is needed to advance understanding of phenology. We review the mechanisms and major drivers of plant phenology, including temperature, photoperiod, and winter chilling, as well as other factors such as competition, resource limitation, and genetics. Shifts in plant phenology have significant consequences on ecosystem productivity, carbon cycling, competition, food webs, and other ecosystem functions and services. We summarize recent advances in observation techniques across multiple spatial scales, including digital repeat photography, other complementary optical measurements, and solar-induced fluorescence, to assess our capability to address the importance of these scale-dependent drivers. Then, we review phenology models as an important component of earth system modeling. We find that the lack of species-level knowledge and observation data leads to difficulties in the development of vegetation phenology models at ecosystem or community scales. Finally, we recommend further research to advance understanding of the mechanisms governing phenology and the standardization of phenology observation methods across networks. With the opportunity for “big data” collection for plant phenology, we envision a breakthrough in process-based phenology modeling.
    Description: U.S. National Science Foundation Grant Numbers: PLR-1417763, DBI-959333, AGS-1005663; University of Chicago and the MBL Lillie Research Innovation Award; NEXT Program; KAKENHI (MEXT, Japan); National Science Foundation of China Grant Number: 41571103; NERC Grant Number: NE/J02080X/1
    Keywords: Cameras ; Greenness ; ILTER ; Modeling ; Phenology ; Scale ; International LTER
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    Prey capture by the cosmopolitan hydromedusae, Obelia spp., in the viscous regime (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Association for the Sciences of Limnology and Oceanography, 2016. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 61 (2016): 2309–2317, doi:10.1002/lno.10390.
    Description: Obelia spp. are cnidarian hydromedusae with a cosmopolitan distribution but very little is known about their feeding. The small size of Obelia (bell diameter ∼ 1 mm, tentacle width ∼ 0.05 mm) suggests that feeding occurs in a viscous regime characterized by thick boundary layers. During feeding observations with a natural prey assemblage the majority of prey were captured at the tentacle tips during the contraction phase. Swimming kinematics from high speed videography confirmed that swimming was a low Re number process (Re 〈 50) and showed that maximum tentacle velocities occurred at the tentacle tips midway through a bell contraction. Flow visualizations from particle image velocimetry demonstrated that fluid motion between the tentacles was limited and that velocities were highest at the tentacle tips, leading to a thinning of boundary layer in this region. The highest nematocyst densities were observed in this same region of the tentacle tips. Taken together, the body kinematics, flow visualizations and nematocyst distributions of Obelia explain how these predators are able to shed viscous boundary layers to effectively capture microplanktonic prey. Our findings help explain how other small feeding-current medusae whose feeding interactions are governed by viscosity are able to successfully forage.
    Description: National Science Foundation Grant Numbers: OCE- 1155084, DBI- 1455471, OCE- 1536672, OCE- 1536688
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    Tectonic structure of the Mid-Atlantic Ridge near 16°30′N (2017)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 17 (2016): 3993–4010, doi:10.1002/2016GC006514.
    Description: The 16°30'N area of the Mid-Atlantic Ridge represents an area of present-day detachment faulting. Here we present shipboard bathymetric, magnetic and gravity data acquired up to 65 km from the ridge axis that reveal a varied tectonic history of this region. Magnetic data are used to calculate spreading rates and examine spreading rate variability along and across the axis. Bathymetric and gravity data are used to infer the crustal structure. A central magnetic anomaly 40% narrower than expected is observed along much of the study area. Misalignment between modern-day spreading center and magnetic anomalies indicates tectonic reorganization of the axis within the past 780 ka. Observed magnetic anomalies show a pattern of anomalous skewness consistent with rotation of magnetic vectors probably associated with detachment faulting. Relatively thin crust north of a small (∼7 km) nontransform offset coincides with a weakly magmatic spreading axis. In contrast, to the south a robust axial volcanic ridge is underlain by thicker crust. Variations in crustal structure perpendicular to the axis occur over tens of kilometers, indicating processes which occur over timescales of 1–2 Ma.
    Description: National Science Foundation Grant Number: OCE-1155650
    Description: 2017-04-22
    Keywords: Mid-Ocean Ridge ; Oceanic detachment fault ; Crustal accretion ; Gravity anomalies ; Magnetic anomalies
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    Liquid freshwater transport estimates from the East Greenland Current based on continuous measurements north of Denmark Strait (2017)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 93-109, doi:10.1002/2016JC012106.
    Description: Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kögur section north of Denmark Strait between August 2011 and July 2012. The 11 month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012 to 2014. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of −131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s.
    Description: European Union Seventh Framework Programme Grant Numbers: (FP7 2007–2013), 308299; US National Science Foundation Grant Number: OCE-0959381
    Description: 2017-07-10
    Keywords: Freshwater ; East Greenland Current ; Mooring observations ; Time series
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    Seasonal controls of aragonite saturation states in the Gulf of Maine (2017)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 372–389, doi:10.1002/2016JC012373.
    Description: The Gulf of Maine (GoME) is a shelf region especially vulnerable to ocean acidification (OA) due to natural conditions of low pH and aragonite saturation states (Ω-Ar). This study is the first to assess the major oceanic processes controlling seasonal variability of the carbonate system and its linkages with pteropod abundance in Wilkinson Basin in the GoME. Two years of seasonal sampling cruises suggest that water-column carbonate chemistry in the region undergoes a seasonal cycle, wherein the annual cycle of stratification-overturn, primary production, respiration-remineralization and mixing all play important roles, at distinct spatiotemporal scales. Surface production was tightly coupled with remineralization in the benthic nepheloid layer during high production seasons, which results in occasional aragonite undersaturation. From spring to summer, carbonate chemistry in the surface across Wilkinson Basin reflects a transition from a production-respiration balanced system to a net autotropic system. Mean water-column Ω-Ar and abundance of large thecosomatous pteropods show some correlation, although patchiness and discrete cohort reproductive success likely also influence their abundance. Overall, photosynthesis-respiration is the primary driving force controlling Ω-Ar variability during the spring-to-summer transition as well as over the seasonal cycle. However, calcium carbonate (CaCO3) dissolution appears to occur near bottom in fall and winter when bottom water Ω-Ar is generally low but slightly above 1. This is accompanied by a decrease in pteropod abundance that is consistent with previous CaCO3 flux trap measurements. The region might experience persistent subsurface aragonite undersaturation in 30–40 years under continued ocean acidification.
    Description: Coastal Ocean Institute at Woods Hole Oceanographic Institution; National Science Foundation Grant Number: (OCE-1316040)
    Description: 2017-07-22
    Keywords: Ocean acidification ; Carbonate chemistry ; Aragonite saturation ; Pteropod ; Gulf of Maine ; Wilkinson Basin
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    Effects of climate change and anthropogenic modification on a disturbance-dependent species in a large riverine system (2017)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 8 (2017): e01653, doi:10.1002/ecs2.1653.
    Description: Humans have altered nearly every natural disturbance regime on the planet through climate and land-use change, and in many instances, these processes may have interacting effects. For example, projected shifts in temperature and precipitation will likely influence disturbance regimes already affected by anthropogenic fire suppression or river impoundments. Understanding how disturbance-dependent species respond to complex and interacting environmental changes is important for conservation efforts. Using field-based demographic and movement rates, we conducted a metapopulation viability analysis for piping plovers (Charadrius melodus), a threatened disturbance-dependent species, along the Missouri and Platte rivers in the Great Plains of North America. Our aim was to better understand current and projected future metapopulation dynamics given that natural disturbances (flooding or high-flow events) have been greatly reduced by river impoundments and that climate change could further alter the disturbance regime. Although metapopulation abundance has been substantially reduced under the current suppressed disturbance regime (high-flow return interval ~ 20 yr), it could grow if the frequency of high-flow events increases as predicted under likely climate change scenarios. We found that a four-year return interval would maximize metapopulation abundance, and all subpopulations in the metapopulation would act as sources at a return interval of 15 yr or less. Regardless of disturbance frequency, the presence of even a small, stable source subpopulation buffered the metapopulation and sustained a low metapopulation extinction risk. Therefore, climate change could have positive effects in ecosystems where disturbances have been anthropogenically suppressed when climatic shifts move disturbance regimes toward more historical patterns. Furthermore, stable source populations, even if unintentionally maintained through anthropogenic activities, may be critical for the persistence of metapopulations of early-successional species under both suppressed disturbance regimes and disturbance regimes where climate change has further altered disturbance frequency or scope.
    Description: Nebraska Environmental Trust Nebraska State Wildlife Grant Program; Nebraska Wildlife Conservation Fund; U.S. Army Corps of Engineers; U.S. Fish and Wildlife Service (USFWS); USFWS North Atlantic Landscape Conservation Cooperative; Virginia Tech
    Keywords: Charadrius melodus ; Early-successional ; Endangered species conservation ; Floods ; Global change ; Land-use change ; Piping plovers ; Population viability analysis ; Succession ; Vortex
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    Dissipation processes in the Tongue of the Ocean (2016)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 3159–3170, doi:10.1002/2015JC011165.
    Description: The Tongue of the Ocean (TOTO) region located within the Bahamas archipelago is a relatively understudied region in terms of both its biological and physical oceanographic characteristics. A prey-field mapping cruise took place in the fall between 15 September 2008 and 1 October 2008, consisting of a series of transects and “clovers” to study the spatial and temporal variability. The region is characterized by a deep scattering layer (DSL), which is preyed on by nekton that serves as the food for beaked whale and other whale species. This study marks the first of its kind where concurrent measurements of acoustic backscatter and turbulence have been conducted for a nekton scattering layer well below the euphotic zone. Turbulence data collected from a Deep Microstructure Profiler are compared to biological and shear data collected by a 38 kHz Simrad EK 60 echo sounder and a hydrographic Doppler sonar system, respectively. From these measurements, the primary processes responsible for the turbulent production in the TOTO region are assessed. The DSL around 500 m and a surface scattering layer (SSL) are investigated for raised ε values. Strong correlation between turbulence levels and scattering intensity of prey is generally found in the SSL with dissipation levels as large as ∼10−7 W kg−1, 3 orders of magnitude above background levels. In the DSL and during the diel vertical migration, dissipation levels ∼10−8 W kg−1 were observed.
    Description: U.S. Office of Naval Research Grant Number: N00014-08-1-1162-01
    Keywords: Biological mixing ; Turbulence ; Deep scattering layer ; Tongue of the Ocean
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    Interannual variability of winter-spring temperature in the Middle Atlantic Bight : relative contributions of atmospheric and oceanic processes (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 4209–4227, doi:10.1002/2016JC011646.
    Description: Relative contributions between the local atmospheric and oceanic processes on the interannual variability of winter-spring shelf temperature in the Middle Atlantic Bight (MAB) are investigated based on a regional ocean model. The model demonstrates sufficient capability to realistically simulate the interannual temperature changes during 2003–2014. On interannual time scales, the mean winter/spring temperature in the MAB is determined by the combination of the initial temperature at the beginning of the season and the mean cumulative air-sea flux, while the mean cumulative ocean advective flux plays a secondary role. In spite of the overall importance of air-sea flux in determining the winter and spring temperature, the relative contributions between air-sea flux and ocean advective flux on the evolution of the temperature anomaly in each individual year varies. The predictability of spring (April–June) temperature based on winter (January–March) temperature is weak because the temporal decorrelation time scale changes significantly from year to year. Both the highly variable shelf temperature and its decorrelation time scale are affected by the changes in the relative contributions between the air-sea flux and ocean advective flux.
    Description: National Science Foundation Grant Number: OCE-1435602
    Description: 2016-12-18
    Keywords: Interannual variability ; Winter-spring temperature ; Air-sea flux ; Ocean advective flux ; Decorrelation time scale ; Predictability
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    Finite-frequency wave propagation through outer rise fault zones and seismic measurements of upper mantle hydration (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 43 (2016): 7982–7990, doi:10.1002/2016GL070083.
    Description: Effects of serpentine-filled fault zones on seismic wave propagation in the upper mantle at the outer rise of subduction zones are evaluated using acoustic wave propagation models. Modeled wave speeds depend on azimuth, with slowest speeds in the fault-normal direction. Propagation is fastest along faults, but, for fault widths on the order of the seismic wavelength, apparent wave speeds in this direction depend on frequency. For the 5–12 Hz Pn arrivals used in tomographic studies, joint-parallel wavefronts are slowed by joints. This delay can account for the slowing seen in tomographic images of the outer rise upper mantle. At the Middle America Trench, confining serpentine to fault zones, as opposed to a uniform distribution, reduces estimates of bulk upper mantle hydration from ~3.5 wt % to as low as 0.33 wt % H2O.
    Description: NSF Grant Number: OCE-0841063
    Description: 2017-02-14
    Keywords: Outer rise faulting ; Serpentinization ; Seismic anisotropy
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    Regulation of South China Sea throughflow by pressure difference (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 4077–4096, doi:10.1002/2015JC011177.
    Description: Sea Surface Height (SSH) data from the European Centre for Medium-Range Weather Forecasts-Ocean Reanalysis System 4 (ECMWF-ORAS4) are used to determine the pressure difference in connection with variability of the South China Sea ThroughFlow (SCSTF) from 1958 to 2007. Two branches of SCSTF, the Karimata-Sunda Strait ThroughFlow (KSSTF) and the Mindoro Strait ThroughFlow (MSTF), are examined. Using the ensemble empirical mode decomposition method (EEMD), time series of pressure difference and volume transport are decomposed into intrinsic mode functions and trend functions, with the corresponding variability on different time scales. Pressure difference agrees with the KSSTF volume transport on decadal time scale; while for the MSTF, pressure difference varies similarly with volume transport on interannual time scale. Separating the dynamic height difference into the thermal and haline terms, for the KSSTF more than half of the dynamic height difference (32 cm) is due to the thermal contributions; while the remaining dynamic height difference (23 cm) is due to the haline contributions. For the MSTF, the dynamic height difference (29 cm) is primarily due to the thermal contribution (26 cm).
    Description: This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant XDA11010304), the National Natural Science Foundation of China (grant number 41306015 and 41476013) and the Independent Research Project Program of State Key Laboratory of Tropical Oceanography (grant LTOZZ1603).
    Description: 2016-12-12
    Keywords: South China Sea throughflow ; Pressure difference ; Dynamic height ; Thermal term ; Haline term
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    Forum for Arctic Modeling and Observational Synthesis (FAMOS) : past, current, and future activities (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 3803–3819, doi:10.1002/2016JC011898.
    Description: The overall goal of the Forum for Arctic Modeling and Observational Synthesis (FAMOS) community activities reported in this special issue is to enhance understanding of processes and mechanisms driving Arctic Ocean marine and sea ice changes, and the consequences of those changes especially in biogeochemical and ecosystem studies. Major 2013–2015 FAMOS accomplishments to date are: identification of consistent errors across Arctic regional models; approaches to reduce these errors, and recommendations for the most effective coupled sea ice-ocean models for use in fully coupled regional and global climate models. 2013–2015 FAMOS coordinated analyses include many process studies, using models together with observations to investigate: dynamics and mechanisms responsible for drift, deformation and thermodynamics of sea ice; pathways and mechanisms driving variability of the Atlantic, Pacific and river waters in the Arctic Ocean; processes of freshwater accumulation and release in the Beaufort Gyre; the fate of melt water from Greenland; characteristics of ocean eddies; biogeochemistry and ecosystem processes and change, climate variability, and predictability. Future FAMOS collaborations will focus on employing models and conducting observations at high and very high spatial and temporal resolution to investigate the role of subgrid-scale processes in regional Arctic Ocean and coupled ice-ocean and atmosphere-ice-ocean models.
    Description: National Science Foundation Office of Polar Programs. Grant Number: PLR-1313614 and PLR- 1313647
    Keywords: Introduction ; Arctic ; Modeling
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    Studies of the Canadian Arctic Archipelago water transport and its relationship to basin-local forcings : results from AO-FVCOM (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 4392–4415, doi:10.1002/2016JC011634.
    Description: A high-resolution (up to 2 km), unstructured-grid, fully coupled Arctic sea ice-ocean Finite-Volume Community Ocean Model (AO-FVCOM) was employed to simulate the flow and transport through the Canadian Arctic Archipelago (CAA) over the period 1978–2013. The model-simulated CAA outflow flux was in reasonable agreement with the flux estimated based on measurements across Davis Strait, Nares Strait, Lancaster Sound, and Jones Sounds. The model was capable of reproducing the observed interannual variability in Davis Strait and Lancaster Sound. The simulated CAA outflow transport was highly correlated with the along-strait and cross-strait sea surface height (SSH) difference. Compared with the wind forcing, the sea level pressure (SLP) played a dominant role in establishing the SSH difference and the correlation of the CAA outflow with the cross-strait SSH difference can be explained by a simple geostrophic balance. The change in the simulated CAA outflow transport through Davis Strait showed a negative correlation with the net flux through Fram Strait. This correlation was related to the variation of the spatial distribution and intensity of the slope current over the Beaufort Sea and Greenland shelves. The different basin-scale surface forcings can increase the model uncertainty in the CAA outflow flux up to 15%. The daily adjustment of the model elevation to the satellite-derived SSH in the North Atlantic region outside Fram Strait could produce a larger North Atlantic inflow through west Svalbard and weaken the outflow from the Arctic Ocean through east Greenland.
    Description: NSF Grant Numbers: OCE-1203393, PLR-1203643; National Natural Science Foundation of China Grant Number: 41276197; Shanghai Pujiang Program Grant Number: 12PJ1404100; Shanghai Shuguang Program
    Description: 2016-12-25
    Keywords: Water transport ; Canadian Arctic Archipelago ; Atmospheric forcing ; Sea surface height
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    Observations of bubbles in natural seep flares at MC 118 and GC 600 using in situ quantitative imaging (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 2203–2230, doi:10.1002/2015JC011452.
    Description: This paper reports the results of quantitative imaging using a stereoscopic, high-speed camera system at two natural gas seep sites in the northern Gulf of Mexico during the Gulf Integrated Spill Research G07 cruise in July 2014. The cruise was conducted on the E/V Nautilus using the ROV Hercules for in situ observation of the seeps as surrogates for the behavior of hydrocarbon bubbles in subsea blowouts. The seeps originated between 890 and 1190 m depth in Mississippi Canyon block 118 and Green Canyon block 600. The imaging system provided qualitative assessment of bubble behavior (e.g., breakup and coalescence) and verified the formation of clathrate hydrate skins on all bubbles above 1.3 m altitude. Quantitative image analysis yielded the bubble size distributions, rise velocity, total gas flux, and void fraction, with most measurements conducted from the seafloor to an altitude of 200 m. Bubble size distributions fit well to lognormal distributions, with median bubble sizes between 3 and 4.5 mm. Measurements of rise velocity fluctuated between two ranges: fast-rising bubbles following helical-type trajectories and bubbles rising about 40% slower following a zig-zag pattern. Rise speed was uncorrelated with hydrate formation, and bubbles following both speeds were observed at both sites. Ship-mounted multibeam sonar provided the flare rise heights, which corresponded closely with the boundary of the hydrate stability zone for the measured gas compositions. The evolution of bubble size with height agreed well with mass transfer rates predicted by equations for dirty bubbles.
    Description: Gulf of Mexico Research Initiative
    Keywords: Natural seeps ; Natural gas bubble ; Clathrate hydrate ; Plume ; Quantitative imaging
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    A gradient of nutrient enrichment reveals nonlinear impacts of fertilization on Arctic plant diversity and ecosystem function (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecology and Evolution 7 (2017): 2449–2460, doi:10.1002/ece3.2863.
    Description: Rapid environmental change at high latitudes is predicted to greatly alter the diversity, structure, and function of plant communities, resulting in changes in the pools and fluxes of nutrients. In Arctic tundra, increased nitrogen (N) and phosphorus (P) availability accompanying warming is known to impact plant diversity and ecosystem function; however, to date, most studies examining Arctic nutrient enrichment focus on the impact of relatively large (〉25x estimated naturally occurring N enrichment) doses of nutrients on plant community composition and net primary productivity. To understand the impacts of Arctic nutrient enrichment, we examined plant community composition and the capacity for ecosystem function (net ecosystem exchange, ecosystem respiration, and gross primary production) across a gradient of experimental N and P addition expected to more closely approximate warming-induced fertilization. In addition, we compared our measured ecosystem CO2 flux data to a widely used Arctic ecosystem exchange model to investigate the ability to predict the capacity for CO2 exchange with nutrient addition. We observed declines in abundance-weighted plant diversity at low levels of nutrient enrichment, but species richness and the capacity for ecosystem carbon uptake did not change until the highest level of fertilization. When we compared our measured data to the model, we found that the model explained roughly 30%–50% of the variance in the observed data, depending on the flux variable, and the relationship weakened at high levels of enrichment. Our results suggest that while a relatively small amount of nutrient enrichment impacts plant diversity, only relatively large levels of fertilization—over an order of magnitude or more than warming-induced rates—significantly alter the capacity for tundra CO2 exchange. Overall, our findings highlight the value of measuring and modeling the impacts of a nutrient enrichment gradient, as warming-related nutrient availability may impact ecosystems differently than single-level fertilization experiments.
    Description: NASA Terrestrial Ecology Grant Number: NNX12AK83G; National Science Foundation Division of Graduate Education Grant Number: DGE-11-44155
    Keywords: Arctic ; Climate change ; Ecosystem function ; Ecosystem respiration ; Gross primary productivity ; Net ecosystem ; CO2 exchange ; Plant diversity
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    Extraction efficiency and quantification of dissolved metabolites in targeted marine metabolomics (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography: Methods 15 (2017): 417–428, doi:10.1002/lom3.10181.
    Description: The field of metabolomics seeks to characterize the suite of small molecules that comprise the end-products of cellular regulation. Metabolomics has been used in biomedical applications as well as environmental studies that explore ecological and biogeochemical questions. We have developed a targeted metabolomics method using electrospray ionization–liquid chromatography tandem mass spectrometry to analyze metabolites dissolved in seawater. Preparation of samples from the marine environment presents challenges because dilute metabolites must be concentrated and desalted. We present the extraction efficiencies of 89 metabolites in our targeted method using solid phase extraction (SPE). In addition, we calculate the limits of detection and quantification for the metabolites in the method and compare the instrument response factors in five different matrices ranging from deionized water to spent medium from cultured marine microbes. High background organic matter content reduces the instrument response factor for only a small group of metabolites, yet enhances the extraction efficiency for other metabolites on the SPE cartridge used here, a modified styrene-divinylbenzene polymer called PPL. Aromatic or larger uncharged compounds, in particular, are reproducibly well retained on the PPL polymer. This method is suitable for the detection of dissolved metabolites in marine samples, with limits of detection ranging from 〈 1 pM to ∼ 2 nM dependent on the dual impacts of seawater matrix on extraction efficiency and on instrument response factors.
    Description: Gordon and Betty Moore Foundation Grant Number: 3304; National Science Foundation Grant Number: OCE-1154320; Simons Foundation International
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    Diploastrea heliopora Sr/Ca and δ18O records from northeast Luzon, Philippines : an assessment of interspecies coral proxy calibrations and climate controls of sea surface temperature and salinity (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography 32 (2017): 424–438, doi:10.1002/2017PA003098.
    Description: The Indo-Pacific coral Diploastrea heliopora reveals regional multidecadal- to centennial- scale climate variability using coral carbonate δ18O (δ18Oc) as a combined proxy for sea surface temperature (SST) and sea surface salinity (SSS). However, to assess the coral's full potential in resolving climatic events, an independent SST proxy would be more advantageous. We examined both Sr/Ca and δ18O of Diploastrea against an adjacent Porites lobata core collected from northeast Luzon, Philippines. Winter Sr/Ca data from Diploastrea show a significant correlation to SST (r = −0.41, p 〈 0.05, (root-mean-square of the residual) RMSR = 0.81°C) and provide a proxy with similar sensitivity as Porites (r = −0.57, p 〈 0.05, RMSR = 0.62°C). An interspecies SST record is shown to be robust and used for a reconstruction of the Pacific Decadal Oscillation during boreal winter (r = −0.70, p = 0.02). While we were unable to generate a robust Diploastrea δ18O-SSS calibration at interannual timescale, the freshening trend toward the present, commonly observed in the region, is qualitatively captured in Diploastrea δ18O. Comparison with Porites δ18O and instrumental SSS records shows that the magnitude of freshening is consistent between coral species. Wet and dry season Porites δ18O provide support for the relative influence of El Niño–Southern Oscillation events and local precipitation to SSS variability at our site. The multiproxy, multispecies approach of this study further strengthens the evidence for Diploastrea as an alternate climate archive in the Indo-Pacific region and seals its potential in helping resolve less understood global-scale climate phenomena.
    Description: National Research Foundation Singapore (NRF) Grant Number: NRF-RF2012-03
    Keywords: Diploastrea heliopora ; Sr/Ca ; Oxygen isotopes ; Paleoclimate reconstructions
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    Deformation and faulting of subduction overriding plate caused by a subducted seamount (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 43 (2016): 8936–8944, doi:10.1002/2016GL069785.
    Description: We conducted numerical experiments to simulate elastoplastic deformation of the overriding plate caused by a subducted seamount. Calculations revealed development of a distinct pair of fault-like shear zones, including a landward dipping forethrust fault initiated from the seamount top and a seaward dipping backthrust fault from the landward base of the seamount. Significant dome-shaped surface uplift was predicted above the thrust faults. Lesser-developed seaward dipping backthrust faults were calculated to develop under certain conditions. The overriding plate was calculated to deform in two stages: In Stage I, elastic deformation leads to the formation of fault-like shear zones. After major faults have cut through the entire plate, plastic deformation on faults dominates Stage II. On the subduction interface, compressional normal stress was calculated to increase on the landward leading flank of the seamount and decrease on the seaward trailing flank. These changes, together with associated stress singularities at seamount edges, could affect earthquake processes.
    Description: NSF Grant Number: OCE-1141785; Mariana Trench Initiative of SCSIO; WHOI DOEI Graduate Student Fellowship
    Description: 2017-03-03
    Keywords: Seamount ; Overriding plate ; Elastoplasticity ; Fault formation ; Earthquakes
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    On the recent destabilization of the Gulf Stream path downstream of Cape Hatteras (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 43 (2016): 9836–9842, doi:10.1002/2016GL069966.
    Description: Mapped satellite altimetry reveals interannual variability in the position of initiation of Gulf Stream meanders downstream of Cape Hatteras. The longitude where the Gulf Stream begins meandering varies by 1500 km. There has been a general trend for the destabilization point to shift west, and 5 of the last 6 years had a Gulf Stream destabilization point upstream of the New England Seamounts. Independent in situ data suggest that this shift has increased both upper-ocean/deep-ocean interaction events at Line W and open-ocean/shelf interactions across the Middle Atlantic Bight (MAB) shelf break. Mooring data and along-track altimetry indicate a recent increase in the number of deep cyclones that stir Deep Western Boundary Current waters from the MAB slope into the deep interior. Temperature profiles from the Oleander Program suggest that recent enhanced warming of the MAB shelf may be related to shifts in the Gulf Stream's destabilization point.
    Description: NSF Grant Numbers: OCE-1332834, OCE-1558521
    Description: 2017-03-28
    Keywords: Gulf Stream ; Altimetry ; Cape Hatteras ; Middle Atlantic Bight ; Deep Western Boundary Current ; Line W
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    Concentrations and cycling of DMS, DMSP, and DMSO in coastal and offshore waters of the Subarctic Pacific during summer, 2010-2011 (2017)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 3269–3286, doi:10.1002/2016JC012465.
    Description: Concentrations of dimethylsulfide (DMS), measured in the Subarctic Pacific during summer 2010 and 2011, ranged from ∼1 to 40 nM, while dissolved dimethylsulfoxide (DMSO) concentrations (range 13-23 nM) exceeded those of dissolved dimethyl sulfoniopropionate (DMSP) (range 1.3–8.8 nM). Particulate DMSP dominated the reduced sulfur pool, reaching maximum concentrations of 100 nM. Coastal and off shore waters exhibited similar overall DMS concentration ranges, but sea-air DMS fluxes were lower in the oceanic waters due to lower wind speeds. Surface DMS concentrations showed statistically significant correlations with various hydrographic variables including the upwelling intensity (r2 = 0.52, p 〈 0.001) and the Chlorophyll a/mixed layer depth ratio (r2 = 0.52, p 〈 0.001), but these relationships provided little predictive power at small scales. Stable isotope tracer experiments indicated that the DMSP cleavage pathway always exceeded the DMSO reduction pathway as a DMS source, leading to at least 85% more DMS production in each experiment. Gross DMS production rates were positively correlated with the upwelling intensity, while net rates of DMS production were significantly correlated to surface water DMS concentrations. This latter result suggests that our measurements captured dominant processes driving surface DMS accumulation across a coastal-oceanic gradient.
    Description: Natural Sciences and Engineering Research Council of Canada, from the Peter Wall Institute for Advanced Studies
    Description: 2017-10-24
    Keywords: Dimethylsulfide ; DMSP ; DMSO ; DMS turnover ; Rate measurements ; Isotopic tracers ; Sea-air flux ; Upwelling
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    Observations and a linear model of water level in an interconnected inlet-bay system (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 2760–2780, doi:10.1002/2016JC012318.
    Description: A system of barrier islands and back-barrier bays occurs along southern Long Island, New York, and in many coastal areas worldwide. Characterizing the bay physical response to water level fluctuations is needed to understand flooding during extreme events and evaluate their relation to geomorphological changes. Offshore sea level is one of the main drivers of water level fluctuations in semienclosed back-barrier bays. We analyzed observed water levels (October 2007 to November 2015) and developed analytical models to better understand bay water level along southern Long Island. An increase (∼0.02 m change in 0.17 m amplitude) in the dominant M2 tidal amplitude (containing the largest fraction of the variability) was observed in Great South Bay during mid-2014. The observed changes in both tidal amplitude and bay water level transfer from offshore were related to the dredging of nearby inlets and possibly the changing size of a breach across Fire Island caused by Hurricane Sandy (after December 2012). The bay response was independent of the magnitude of the fluctuations (e.g., storms) at a specific frequency. An analytical model that incorporates bay and inlet dimensions reproduced the observed transfer function in Great South Bay and surrounding areas. The model predicts the transfer function in Moriches and Shinnecock bays where long-term observations were not available. The model is a simplified tool to investigate changes in bay water level and enables the evaluation of future conditions and alternative geomorphological settings.
    Description: New York State Department of Environmental Conservation Grant Number: (NYS-DEC); U.S. Geological Survey (USGS)
    Keywords: Water level ; Back-barrier bays ; Tidal variations ; Storm effects ; Dredging ; Long Island
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    Physical linkages between an offshore canyon and surf zone morphologic change (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 3451–3460, doi:10.1002/2016JC012319.
    Description: The causes of surf zone morphologic changes observed along a sandy beach onshore of a submarine canyon were investigated using field observations and a numerical model (Delft3D/SWAN). Numerically simulated morphologic changes using four different sediment transport formulae reproduce the temporal and spatial patterns of net cross-shore integrated (between 0 and 6.5 m water depths) accretion and erosion observed in a ∼300 m alongshore region, a few hundred meters from the canyon head. The observations and simulations indicate that the accretion or erosion results from converging or diverging alongshore currents driven primarily by breaking waves and alongshore pressure gradients. The location of convergence or divergence depends on the direction of the offshore waves that refract over the canyon, suggesting that bathymetric features on the inner shelf can have first-order effects on short-term nearshore morphologic change.
    Description: WHOI-USGS postdoctoral scholarship, NSF, ONR
    Description: 2017-10-29
    Keywords: Surf zone morphologic change ; Surf zone currents
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    Distributions, sources, and transformations of dissolved and particulate iron on the Ross Sea continental shelf during summer (2017)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 6371–6393, doi:10.1002/2017JC013068.
    Description: We report water column dissolved iron (dFe) and particulate iron (pFe) concentrations from 50 stations sampled across the Ross Sea during austral summer (January–February) of 2012. Concentrations of dFe and pFe were measured in each of the major Ross Sea water masses, including the Ice Shelf Water and off-shelf Circumpolar Deep Water. Despite significant lateral variations in hydrography, macronutrient depletion, and primary productivity across several different regions on the continental shelf, dFe concentrations were consistently low (〈0.1 nM) in surface waters, with only a handful of stations showing elevated concentrations (0.20–0.45 nM) in areas of melting sea ice and near the Franklin Island platform. Across the study region, pFe associated with suspended biogenic material approximately doubled the inventory of bioavailable iron in surface waters. Our data reveal that the majority of the summertime iron inventory in the Ross Sea resides in dense shelf waters, with highest concentrations within 50 m of the seafloor. Higher dFe concentrations near the seafloor are accompanied by an increased contribution to pFe from authigenic and/or scavenged iron. Particulate manganese is also influenced by sediment resuspension near the seafloor but, unlike pFe, is increasingly associated with authigenic material higher in the water column. Together, these results suggest that following depletion of the dFe derived from wintertime convective mixing and sea ice melt, recycling of pFe in the upper water column plays an important role in sustaining the summertime phytoplankton bloom in the Ross Sea polynya.
    Description: National Science Foundation's United States Antarctic Program Grant Numbers: ANT-0944174 , ANT-0944165; National Science Foundation Grant Number: OCE-0649505
    Description: 2018-02-17
    Keywords: Ross Sea ; Iron ; Manganese ; Particles ; Continental shelf
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    Late spring nitrate distributions beneath the ice-covered northeastern Chukchi Shelf (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 122 (2017): 2409–2417, doi:10.1002/2017JG003881.
    Description: Measurements of late springtime nutrient concentrations in Arctic waters are relatively rare due to the extensive sea ice cover that makes sampling difficult. During the SUBICE (Study of Under-ice Blooms In the Chukchi Ecosystem) cruise in May–June 2014, an extensive survey of hydrography and prebloom concentrations of inorganic macronutrients, oxygen, particulate organic carbon and nitrogen, and chlorophyll a was conducted in the northeastern Chukchi Sea. Cold (〈−1.5°C) winter water was prevalent throughout the study area, and the water column was weakly stratified. Nitrate (NO3−) concentration averaged 12.6 ± 1.92 μM in surface waters and 14.0 ± 1.91 μM near the bottom and was significantly correlated with salinity. The highest NO3− concentrations were associated with winter water within the Central Channel flow path. NO3− concentrations were much reduced near the northern shelf break within the upper halocline waters of the Canada Basin and along the eastern side of the shelf near the Alaskan coast. Net community production (NCP), estimated as the difference in depth-integrated NO3− content between spring (this study) and summer (historical), varied from 28 to 38 g C m−2 a−1. This is much lower than previous NCP estimates that used NO3− concentrations from the southeastern Bering Sea as a baseline. These results demonstrate the importance of using profiles of NO3− measured as close to the beginning of the spring bloom as possible when estimating local NCP. They also show that once the snow melts in spring, increased light transmission through the sea ice to the waters below the ice could fuel large phytoplankton blooms over a much wider area than previously known.
    Description: NSF Office of Polar Programs Grant Numbers: PLR-1304563, PLR-1303617
    Description: 2018-03-18
    Keywords: Chukchi Sea ; Nitrate ; Phytoplankton
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    Thermal segmentation of mid-ocean ridge-transform faults (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 18 (2017): 3405–3418, doi:10.1002/2017GC006967.
    Description: 3-D finite element simulations are used to calculate thermal structures and mantle flow fields underlying mid-ocean ridge-transform faults (RTFs) composed of two fault segments separated by an orthogonal step over. Using fault lengths and slip rates, we derive an empirical scaling relation for the critical step over length ( inline image), which marks the transition from predominantly horizontal to predominantly vertical mantle flow at the base of the lithosphere under a step over. Using the ratio of step over length (LS) to inline image, we define three degrees of segmentation: first-degree, corresponding to type I step overs ( inline image ≥ 3); second-degree, corresponding to type II step overs (1 ≤  inline image 〈 3); and third-degree, corresponding to type III step overs ( inline image 〈1). In first-degree segmentation, thermal structures and mantle upwelling patterns under a step over are similar to those of mature ridges, where normal mid-ocean ridge basalts (MORBs) form. The seismogenic area under first-degree segmentation is characteristic of two, isolated faults. Second-degree segmentation creates pull-apart basins with subdued melt generation, and intratransform spreading centers with enriched MORBs. The seismogenic area of RTFs under second-degree segmentation is greater than that of two isolated faults, but less than that of an unsegmented RTF. Under third-degree segmentation, mantle flow is predominantly horizontal, resulting in little lithospheric thinning and little to no melt generation. The total seismogenic area under third-degree segmentation approaches that of an unsegmented RTF. Our scaling relations characterize the degree of segmentation due to step overs along transform faults and provide insight into RTF frictional processes, seismogenic behavior, and melt transport.
    Description: NSF Grant Numbers: OCE-1352565, OCE-14-58201; NOAA. Grant Number: NA10NOS4000073; 2011 ExxonMobil Geosciences
    Description: 2018-03-13
    Keywords: Oceanic transform fault ; Intratransform spreading center ; Fault segmentation ; Fault thermal structure ; Melt transport
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    Evidence for the maintenance of slowly varying equatorial currents by intraseasonal variability (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geophysical Research Letters 45 (2018): 1923–1929, doi:10.1002/2017GL076662.
    Description: Recent evidence from mooring data in the equatorial Atlantic reveals that semiannual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we show that intraseasonal variability, with time scales of tens of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealized model setup that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability.
    Description: This study was supported by the Deutsche Forschungsgemeinschaft as part of the Sonderforschungsbereich 754 “Climate‐Biogeochemistry Interactions in the Tropical Ocean” and through several research cruises with RV Meteor, RV Maria S. Merian, and RV L'Atalante, by the German Federal Ministry of Education and Research as part of the cooperative projects RACE (03F0605B), SACUS (03G0837A), and MiKlip2 (ATMOS‐MODINI; 01LP1517D) and by the European Union 7th Framework Programme (FP7 2007–2013) under grant agreement 603521 PREFACE project. Support for the moored observations was additionally provided by the U.S. National Science Foundation.
    Keywords: Equatorial deep jets ; Seasonal cycle ; Momentum flux convergence ; Tropical instability waves ; Equatorial basin mode resonance
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    Export of strongly diluted Greenland meltwater from a major glacial fjord (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 4163-4170, doi:10.1029/2018GL077000.
    Description: The Greenland Ice Sheet has been, and will continue, losing mass at an accelerating rate. The influence of this anomalous meltwater discharge on the regional and large‐scale ocean could be considerable but remains poorly understood. This uncertainty is in part a consequence of challenges in observing water mass transformation and meltwater spreading in coastal Greenland. Here we use tracer observations that enable unprecedented quantification of the export, mixing, and vertical distribution of meltwaters leaving one of Greenland's major glacial fjords. We find that the primarily subsurface meltwater input results in the upwelling of the deep fjord waters and an export of a meltwater/deepwater mixture that is 30 times larger than the initial meltwater release. Using these tracer data, the vertical structure of Greenland's summer meltwater export is defined for the first time showing that half the meltwater export occurs below 65 m.
    Description: National Science Foundation Grant Number: OCE-1536856
    Description: 2018-11-05
    Keywords: Greenland ; Ocean-glacier interactions ; Fjord circulation ; Meltwater ; Noble gas ; Overturning
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    Coherent seismic arrivals in the P wave coda of the 2012 Mw 7.2 Sumatra earthquake : water reverberations or an early aftershock? (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 3147-3159, doi:10.1002/2018JB015573.
    Description: Teleseismic records of the 2012 Mw 7.2 Sumatra earthquake contain prominent phases in the P wave train, arriving about 50 to 100 s after the direct P arrival. Azimuthal variations in these arrivals, together with back‐projection analysis, led Fan and Shearer (2016a, https://doi.org/10.1002/2016GL067785) to conclude that they originated from early aftershock(s), located ∼150 km northeast of the mainshock and landward of the trench. However, recently, Yue et al. (2017, https://doi.org/10.1002/2017GL073254) argued that the anomalous arrivals are more likely water reverberations from the mainshock, based mostly on empirical Green's function analysis of a M6 earthquake near the mainshock and a water phase synthetic test. Here we present detailed back‐projection and waveform analyses of three M6 earthquakes within 100 km of the Mw 7.2 earthquake, including the empirical Green's function event analyzed in Yue et al. (2017, https://doi.org/10.1002/2017GL073254). In addition, we examine the waveforms of three M5.5 reverse‐faulting earthquakes close to the inferred early aftershock location in Fan and Shearer (2016a, https://doi.org/10.1002/2016GL067785). These results suggest that the reverberatory character of the anomalous arrivals in the mainshock coda is consistent with water reverberations, but the origin of this energy is more likely an early aftershock rather than delayed and displaced water reverberations from the mainshock.
    Description: National Science Foundation Grant Numbers: EAR-1261681, EAR-1620251; Weston Howland Jr. Postdoctoral Scholarship
    Description: 2018-10-25
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    Long‐term nutrient addition increases respiration and nitrous oxide emissions in a New England salt marsh (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecology and Evolution 8 (2018): 4958-4966, doi:10.1002/ece3.3955.
    Description: Salt marshes may act either as greenhouse gas (GHG) sources or sinks depending on hydrological conditions, vegetation communities, and nutrient availability. In recent decades, eutrophication has emerged as a major driver of change in salt marsh ecosystems. An ongoing fertilization experiment at the Great Sippewissett Marsh (Cape Cod, USA) allows for observation of the results of over four decades of nutrient addition. Here, nutrient enrichment stimulated changes to vegetation communities that, over time, have resulted in increased elevation of the marsh platform. In this study, we measured fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in dominant vegetation zones along elevation gradients of chronically fertilized (1,572 kg N ha−1 year−1) and unfertilized (12 kg N ha−1 year−1) experimental plots at Great Sippewissett Marsh. Flux measurements were performed using darkened chambers to focus on community respiration and excluded photosynthetic CO2 uptake. We hypothesized that N‐replete conditions in fertilized plots would result in larger N2O emissions relative to control plots and that higher elevations caused by nutrient enrichment would support increased CO2 and N2O and decreased CH4 emissions due to the potential for more oxygen diffusion into sediment. Patterns of GHG emission supported our hypotheses. Fertilized plots were substantially larger sources of N2O and had higher community respiration rates relative to control plots, due to large emissions of these GHGs at higher elevations. While CH4 emissions displayed a negative relationship with elevation, they were generally small across elevation gradients and nutrient enrichment treatments. Our results demonstrate that at decadal scales, vegetation community shifts and associated elevation changes driven by chronic eutrophication affect GHG emission from salt marshes. Results demonstrate the necessity of long‐term fertilization experiments to understand impacts of eutrophication on ecosystem function and have implications for how chronic eutrophication may impact the role that salt marshes play in sequestering C and N.
    Keywords: Carbon dioxide ; Cavity ringdown spectroscopy ; Great Sippewissett Marsh ; Methane ; Nitrous oxide ; Nutrient enrichment
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    Opening of Hess Deep rift at the Galapagos triple junction (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 3942-3950, doi:10.1029/2018GL077555.
    Description: At the Galapagos triple junction, the westward propagating Cocos‐Nazca (C‐N) Rift breaks into ~0.5 Ma crust accreted at the East Pacific Rise. Rifting transitions to full magmatic seafloor spreading in the wake of the propagating tip. The 25‐km‐long Hess Deep rift is the transitional segment from rifting to spreading. Intrarift ridge (IRR), located within Hess Deep rift, is interpreted as a detachment fault, which exhumes deep‐seated rocks to the seafloor. Although transitional segments must have occurred throughout the westward propagation of C‐N Rift, IRR is the only obvious detachment fault along the base of the Rift scarps in the last ~5 Ma of its propagation. IRR formation may be in response to a decrease in spreading rate (~40 to 〈20 mm/yr) and presumed lower melt supply, resulting from the formation of the Galapagos microplate ~1.4 Ma, which now controls the opening at the C‐N Rift tip.
    Description: D.K.S. and H.S were supported in part by WHOI.
    Description: 2018-10-24
    Keywords: Seafloor spreading ; Mid‐ocean ridges ; Oceanic triple junctions ; Oceanic detachment faults ; Galapagos Triple Junction ; Galapagos microplate
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    Climate change and functional traits affect population dynamics of a long‐lived seabird (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Animal Ecology 87 (2018): 906-920, doi:10.1111/1365-2656.12827.
    Description: Recent studies unravelled the effect of climate changes on populations through their impact on functional traits and demographic rates in terrestrial and freshwater ecosystems, but such understanding in marine ecosystems remains incomplete. Here, we evaluate the impact of the combined effects of climate and functional traits on population dynamics of a long‐lived migratory seabird breeding in the southern ocean: the black‐browed albatross (Thalassarche melanophris, BBA). We address the following prospective question: “Of all the changes in the climate and functional traits, which would produce the biggest impact on the BBA population growth rate?” We develop a structured matrix population model that includes the effect of climate and functional traits on the complete BBA life cycle. A detailed sensitivity analysis is conducted to understand the main pathway by which climate and functional trait changes affect the population growth rate. The population growth rate of BBA is driven by the combined effects of climate over various seasons and multiple functional traits with carry‐over effects across seasons on demographic processes. Changes in sea surface temperature (SST) during late winter cause the biggest changes in the population growth rate, through their effect on juvenile survival. Adults appeared to respond to changes in winter climate conditions by adapting their migratory schedule rather than by modifying their at‐sea foraging activity. However, the sensitivity of the population growth rate to SST affecting BBA migratory schedule is small. BBA foraging activity during the pre‐breeding period has the biggest impact on population growth rate among functional traits. Finally, changes in SST during the breeding season have little effect on the population growth rate. These results highlight the importance of early life histories and carry‐over effects of climate and functional traits on demographic rates across multiple seasons in population response to climate change. Robust conclusions about the roles of various phases of the life cycle and functional traits in population response to climate change rely on an understanding of the relationships of traits to demographic rates across the complete life cycle.
    Description: NSF Grant Number: OPP‐1246407; European Research Council Advanced Grant Grant Numbers: ERC‐2012‐ADG_20120314, 322989
    Keywords: Birds ; Climate change ; Foraging behaviours ; Non‐breeding season ; Phenotypic traits ; Pre‐breeding season ; Timing of breeding ; Wing length
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    Deformation of the Pacific/North America plate boundary at Queen Charlotte Fault : the possible role of rheology (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Published 2018. This article is a U.S. Government work and is in the public domain in the USA. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 4223-4242, doi:10.1002/2017JB014770.
    Description: The Pacific/North America (PA/NA) plate boundary between Vancouver Island and Alaska is similar to the PA/NA boundary in California in its kinematic history and the rate and azimuth of current relative motion, yet their deformation styles are distinct. The California plate boundary shows a broad zone of parallel strike slip and thrust faults and folds, whereas the 49‐mm/yr PA/NA relative plate motion in Canada and Alaska is centered on a single, narrow, continuous ~900‐km‐long fault, the Queen Charlotte Fault (QCF). Using gravity analysis, we propose that this plate boundary is centered on the continent/ocean boundary (COB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the COB is a boundary between materials of contrasting elastic properties, once a fault is established there, it will probably remain stable. We propose that deformation progressively shifted to the COB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by fault reactivation on Pacific crust and by an eastward dipping QCF. Underthrusting of Pacific slab under Haida Gwaii occurs at convergence angles 〉14°–15° and may have been responsible for the emergence of the archipelago. The calculated slab entry dip (5°–8°) suggests that the slab probably does not extend into the asthenosphere. The PA/NA plate boundary at the QCF can serve as a structurally simple site to investigate the impact of rheology and composition on crustal deformation and the initiation of slab underthrusting.
    Keywords: Ocean‐continent boundary ; Strike‐slip fault ; Subduction initiation ; Bimaterial interface ; Yakutat terrane ; Haida Gwaii
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    Intense storms increase the stability of tidal bays (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of [publisher] for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 5491-5500, doi:10.1029/2018GL078208.
    Description: Coastal bays and, specifically, back‐barrier tidal basins host productive ecosystems, coastal communities, and critical infrastructure. As sea level continues to rise and tropical cyclones increase in intensity, these coastal systems are increasingly at risk. Developing a sediment budget is imperative to understanding how storm events affect the system's resilience, where net import of sediment indicates growth and resilience against sea level rise, and net export of sediment indicates deterioration. Using high‐resolution numerical simulations, we show that intense storms import sediment into a system of bays in Virginia, USA. Duration and magnitude of storm surge are among the most important factors in sediment import, suggesting that intense storms increase the stability of tidal bays by providing the sediment necessary to counteract sea level rise. Since climate models project that tropical cyclones will increase in intensity in coming decades, our results have significant implications for the resilience of tidal bays and the future of coastal communities worldwide.
    Description: National Science Foundation Grant Numbers: NSF 1237733, NSF 1637630, NSF 163630
    Description: 2018-11-21
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    Constraining marsh carbon budgets using long‐term C burial and contemporary atmospheric CO2 fluxes (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 123 (2018): 867-878, doi:10.1002/2017JG004336.
    Description: Salt marshes are sinks for atmospheric carbon dioxide that respond to environmental changes related to sea level rise and climate. Here we assess how climatic variations affect marsh‐atmosphere exchange of carbon dioxide in the short term and compare it to long‐term burial rates based on radiometric dating. The 5 years of atmospheric measurements show a strong interannual variation in atmospheric carbon exchange, varying from −104 to −233 g C m−2 a−1 with a mean of −179 ± 32 g C m−2 a−1. Variation in these annual sums was best explained by differences in rainfall early in the growing season. In the two years with below average rainfall in June, both net uptake and Normalized Difference Vegetation Index were less than in the other three years. Measurements in 2016 and 2017 suggest that the mechanism behind this variability may be rainfall decreasing soil salinity which has been shown to strongly control productivity. The net ecosystem carbon balance was determined as burial rate from four sediment cores using radiometric dating and was lower than the net uptake measured by eddy covariance (mean: 110 ± 13 g C m−2 a−1). The difference between these estimates was significant and may be because the atmospheric measurements do not capture lateral carbon fluxes due to tidal exchange. Overall, it was smaller than values reported in the literature for lateral fluxes and highlights the importance of investigating lateral C fluxes in future studies.
    Description: National Science Foundation Grant Numbers: OCE-1637630, OCE-1238212, 1426308
    Description: 2018-08-06
    Keywords: Salt marsh ; Eddy covariance ; Carbon budget ; Interannual variation ; Burial rate ; Salinity
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    Effect of growth temperature on photosynthetic capacity and respiration in three ecotypes of Eriophorum vaginatum (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecology and Evolution 8 (2018): 3711-3725, doi:10.1002/ece3.3939.
    Description: Ecotypic differentiation in the tussock‐forming sedge Eriophorum vaginatum has led to the development of populations that are locally adapted to climate in Alaska's moist tussock tundra. As a foundation species, E. vaginatum plays a central role in providing topographic and microclimatic variation essential to these ecosystems, but a changing climate could diminish the importance of this species. As Arctic temperatures have increased, there is evidence of adaptational lag in E. vaginatum, as locally adapted ecotypes now exhibit reduced population growth rates. Whether there is a physiological underpinning to adaptational lag is unknown. Accordingly, this possibility was investigated in reciprocal transplant gardens. Tussocks of E. vaginatum from sites separated by ~1° latitude (Coldfoot: 67°15′N, Toolik Lake: 68°37′, Sagwon: 69°25′) were transplanted into the Toolik Lake and Sagwon sites and exposed to either an ambient or an experimental warming treatment. Five tussocks pertreatment combination were measured at each garden to determine photosynthetic capacity (i.e., Vcmax and Jmax) and dark respiration rate (Rd) at measurement temperatures of 15, 20, and 25°C. Photosynthetic enhancements or homeostasis were observed for all ecotypes at both gardens under increased growth temperature, indicating no negative effect of elevated temperature on photosynthetic capacity. Further, no evidence of thermal acclimation in Rd was observed for any ecotype, and there was little evidence of ecotypic variation in Rd. As such, no physiological contribution to adaptational lag was observed given the increase in growth temperature (up to ~2°C) provided by this study. Despite neutral to positive effects of increased growth temperature on photosynthesis in E. vaginatum, it appears to confer no lasting advantage to the species.
    Description: Division of Polar Programs Grant Numbers: 1417645, 1417763, 1418010; West Chester University, Department of Biology
    Keywords: Adaptational lag ; Eriophorum vaginatum ; Moist tussock tundra ; Photosynthetic capacity ; Respiration ; Temperature acclimation
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    Salinification in the South China Sea since late 2012 : a reversal of the freshening since the 1990s (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 2744-2751, doi:10.1002/2017GL076574.
    Description: Salinification has occurred in the South China Sea from late 2012 to the present, as shown by satellite Aquarius/Soil Moisture Active Passive data and Argo float data. This salinification follows a 20 year freshening trend that started in 1993. The salinification signal is strongest near the surface and extends downward under the seasonal thermocline to a depth of 150 m. The salinification occurs when the phase of the Pacific Decadal Oscillation switches from negative to positive. Diagnosis of the salinity budget suggests that an increasing net surface freshwater loss and the horizontal salt advection through the Luzon Strait driven by the South China Sea throughflow contributed to this ongoing salinification. In particular, a decrease in precipitation and enhanced Luzon Strait transport dominated the current intense salinification. Of particular interest is whether this salinification will continue until it reaches the previous maximum recorded in 1992.
    Description: Major State Research Development Program of China Grant Number: 2016YFC1402603; National Natural Science Foundation of China Grant Numbers: 41776025, 41476014, 41776026, 41676018; NOAA Climate Program Office MAPP Program Grant Number: NA15OAR4310088; NSF Physical Oceanography Program Grant Number: 1537136; National Science Foundation Grant Number: ICER‐1663704; Pearl River S&T Nova Program of Guangzhou; Open Project Program of State Key Laboratory of Tropical Oceanography Grant Number: LTOZZ1601
    Description: 2018-09-05
    Keywords: South China Sea ; Salinification ; Argo floats ; Aquarius/SMPA ; PDO
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    Archie's saturation exponent for natural gas hydrate in coarse‐grained reservoirs (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 2069-2089, doi:10.1002/2017JB015138.
    Description: Accurately quantifying the amount of naturally occurring gas hydrate in marine and permafrost environments is important for assessing its resource potential and understanding the role of gas hydrate in the global carbon cycle. Electrical resistivity well logs are often used to calculate gas hydrate saturations, Sh, using Archie's equation. Archie's equation, in turn, relies on an empirical saturation parameter, n. Though n = 1.9 has been measured for ice‐bearing sands and is widely used within the hydrate community, it is highly questionable if this n value is appropriate for hydrate‐bearing sands. In this work, we calibrate n for hydrate‐bearing sands from the Canadian permafrost gas hydrate research well, Mallik 5L‐38, by establishing an independent downhole Sh profile based on compressional‐wave velocity log data. Using the independently determined Sh profile and colocated electrical resistivity and bulk density logs, Archie's saturation equation is solved for n, and uncertainty is tracked throughout the iterative process. In addition to the Mallik 5L‐38 well, we also apply this method to two marine, coarse‐grained reservoirs from the northern Gulf of Mexico Gas Hydrate Joint Industry Project: Walker Ridge 313‐H and Green Canyon 955‐H. All locations yield similar results, each suggesting n ≈ 2.5 ± 0.5. Thus, for the coarse‐grained hydrate bearing (Sh 〉 0.4) of greatest interest as potential energy resources, we suggest that n = 2.5 ± 0.5 should be applied in Archie's equation for either marine or permafrost gas hydrate settings if independent estimates of n are not available.
    Description: DOE Grant Number: DE‐FE0023919; Gas Hydrate Project of the U.S. Geological Survey's Coastal and Marine Geology Program
    Description: 2018-08-17
    Keywords: Gas hydrate ; Resistivity ; Velocity ; Hydrate saturation ; Mallik ; Gulf of Mexico
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    Impact of pore fluid chemistry on fine‐grained sediment fabric and compressibility (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 5495-5514, doi:10.1029/2018JB015872.
    Description: Fines, defined here as grains or particles, less than 75 μm in diameter, exist nearly ubiquitously in natural sediment, even those classified as coarse. Macroscopic sediment properties, such as compressibility, which relates applied effective stress to the resulting sediment deformation, depend on the fabric of fines. Unlike coarse grains, fines have sizes and masses small enough to be more strongly influenced by electrical interparticle forces than by gravity. These electrical forces acting through pore fluids are influenced by pore fluid chemistry changes. Macroscopic property dependence on pore fluid chemistry must be accounted for in sediment studies involving subsurface flow and sediment stability analyses, as well as in engineered flow situations such as groundwater pollutant remediation, hydrocarbon migration, or other energy resource extraction applications. This study demonstrates how the liquid limit‐based electrical sensitivity index can be used to predict sediment compressibility changes due to pore fluid chemistry changes. Laboratory tests of electrical sensitivity, sedimentation, and compressibility illustrate mechanisms linking microscale and macroscale processes for selected pure, end‐member fines. A specific application considered here is methane extraction via depressurization of gas hydrate‐bearing sediment, which causes a dramatic pore water salinity drop concurrent with sediment being compressed by the imposed effective stress increase.
    Description: DOI U.S. Geological Survey (USGS); U.S. Department of Energy (DOE) Grant Numbers: DE‐FE00‐28966, DE‐FE00‐26166
    Description: 2019-01-17
    Keywords: Fine‐grained sediment fabric ; Electrical sensitivity ; Pore‐fluid chemistry ; Sedimentation ; Compressibility ; Methane hydrate
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    Variability of the Iceland‐Scotland overflow water transport through the Charlie‐Gibbs fracture zone : results from an eddying simulation and observations (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 5808-5823, doi:10.1029/2018JC013895.
    Description: Observations show that the westward transport of the Iceland‐Scotland overflow water (ISOW) through the Charlie‐Gibbs Fracture Zone (CGFZ) is highly variable. This study examines (a) where this variability comes from and (b) how it is related to the variability of ISOW transport at upstream locations in the Iceland Basin and other ISOW flow pathways. The analyses are based on a 35‐year 1/12° eddying Atlantic simulation that represents well the main features of the observed ISOW in the area of interest, in particular, the transport variability through the CGFZ. The results show that (a) the variability of the ISOW transport is closely correlated with that of the barotropic transports in the CGFZ associated with the meridional displacement of the North Atlantic Current front and is possibly induced by fluctuations of large‐scale zonal wind stress in the Western European Basin east of the CGFZ; (b) the variability of the ISOW transport is increased by a factor of 3 from the northern part of the Iceland Basin to the CGFZ region and transport time series at these two locations are not correlated, further suggesting that the variability at the CGFZ does not come from the upstream source; and (c) the variability of the ISOW transport at the CGFZ is strongly anticorrelated to that of the southward ISOW transport along the eastern flank of the Mid‐Atlantic Ridge, suggesting an out‐of‐phase covarying transport between these two ISOW pathways.
    Description: Woods Hole Oceanographic Institution; National Oceanic and Atmospheric Administration Grant Number: NA15OAR4310088; U.S. National Science Foundation Grant Numbers: 1537136, OCE‐0926656
    Description: 2019-02-20
    Keywords: Iceland‐Scotland overflow water ; Charlie‐Gibbs fracture zone ; Variability ; Volume transport ; Eddying simulation
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    Relationship between Greenland Ice Sheet surface speed and modeled effective pressure (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 123 (2018): 2258-2278, doi:10.1029/2017JF004581.
    Description: We use a numerical subglacial hydrology model and remotely sensed observations of Greenland Ice Sheet surface motion to test whether the inverse relationship between effective pressure and regional melt season surface speeds observed at individual sites holds on a regional scale. The model is forced with daily surface runoff estimates for 2009 and 2010 across an ~8,000‐km2 region on the western margin. The overall subglacial drainage system morphology develops similarly in both years, with subglacial channel networks growing inland from the ice sheet margin and robust subglacial pathways forming over bedrock ridges. Modeled effective pressures are compared to contemporaneous regional surface speeds derived from TerraSAR‐X imagery to investigate spatial relationships. Our results show an inverse spatial relationship between effective pressure and ice speed in the mid‐melt season, when surface speeds are elevated, indicating that effective pressure is the dominant control on surface velocities in the mid‐melt season. By contrast, in the early and late melt seasons, when surface speeds are slower, effective pressure and surface speed have a positive relationship. Our results suggest that outside of the mid‐melt season, the influence of effective pressures on sliding speeds may be secondary to the influence of driving stress and spatially variable bed roughness.
    Description: National Aeronautics and Space Administration (NASA). Grant Number: NXX10AI30G National Science Foundation (NSF) American Geophysical Union Horton Research Grant; National Science Foundation Graduate Research Fellowship; National Science Foundation's Office of Polar Programs (NSF‐OPP) Grant Numbers: PLR‐1418256, ARC‐1023364, ARC‐0520077; Woods Hole Oceanographic Institution's Ocean and Climate Change Institute (OCCI)
    Description: 2019-03-27
    Keywords: Glaciology ; Greenland ; Subglacial hydrology ; Numerical modeling ; Ice dynamics
    Repository Name: Woods Hole Open Access Server
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    The interannual variability of the breakdown of fall stratification on the New Jersey Shelf (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 6503-6520, doi:10.1029/2018JC014049.
    Description: During the seasonal evolution of stratification on the New Jersey shelf in the fall, strong thermal stratification that was established in the preceding summer is broken down through wind‐driven processes and surface cooling. Ten years of output from a Regional Ocean Modeling Systems run and a one‐dimensional mixed layer model is used here to examine the interannual variability in the strength of the stratification and in the processes that reduce stratification in fall. Our analysis shows that the strength of the stratification at the end of the summer is not correlated with the timing of shelf destratification. This indicates that processes that occur within the fall are more important for the timing of stratification breakdown than are the initial fall conditions. Furthermore, wind‐driven processes reduce a greater fraction of the stratification in each year than does the surface cooling during the fall. Winds affect the density gradients on the shelf through both changes to the temperature and salinity fields. Processes associated with the downwelling‐favorable winds are more effective than those during upwelling‐favorable winds in breaking down the vertical density gradients. In the first process, cross‐shelf advective fluxes during storms act to decrease stratification during downwelling‐favorable winds and increase stratification during upwelling‐favorable winds. Second, there is also enhanced velocity shear during downwelling‐favorable winds, which allows for more shear instabilities that break down stratification via mixing. Observational data and model output from Tropical Storm Ernesto compare favorably and suggest that downwelling‐favorable winds act through the mechanisms identified from the Regional Ocean Modeling Systems results.
    Description: DOC | National Oceanic and Atmospheric Administration (NOAA) Grant Number: NA13OAR4830233; NSF | GEO | Division of Ocean Sciences (OCE) Grant Number: 1558960
    Description: 2019-03-12
    Keywords: Middle Atlantic Bight ; Fall stratification ; Ekman buoyancy flux ; ROMS ; Interannual variability ; Storms
    Repository Name: Woods Hole Open Access Server
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    Coupled model biases breed spurious low‐frequency variability in the tropical Pacific Ocean (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geophysical Research Letters 45 (2018): 10,609-10,618, doi:10.1029/2018GL079455.
    Description: Coupled general circulation model (GCM) biases in the tropical Pacific are substantial, including a westward extended cold sea surface temperature (SST) bias linked to El Niño–Southern Oscillation (ENSO). Investigation of internal climate variability at centennial timescales using multicentury control integrations of 27 GCMs suggests that a Pacific Centennial Oscillation emerges in GCMs with too strong ENSO variability in the equatorial Pacific, including westward extended SST variability. Using a stochastic model of climate variability (Hasselmann type), we diagnose such centennial SST variance in the western equatorial Pacific. The consistency of a simple stochastic model with complex GCMs suggests that a previously defined Pacific Centennial Oscillation may be driven by biases in high‐frequency ENSO forcing in the western equatorial Pacific. A cautious evaluation of long‐term trends in the tropical Pacific from GCMs is necessary because significant trends in historical and future simulations are possible consequences of biases in simulated internal variability alone.
    Description: Singapore Ministry of Educaton; National Science Foundation Grant Numbers: OISE‐1743738, AGS‐1602581, AGS‐1401400, AGS‐1243204; Singapore Ministry of Education Academic Research Fund Tier 2 Grant Number: MOE2016‐T2‐1‐016; LDEO. Grant Number: 8258
    Keywords: Model bias ; ENSO ; Centennial variability ; Climate model ; Tropical Pacific
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    Seasonal asymmetry in the evolution of surface ocean pCO2 and pH thermodynamic drivers and the influence on sea‐air CO2 flux (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Global Biogeochemical Cycles 32 (2018): 1476-1497, doi:10.1029/2017GB005855.
    Description: It has become clear that anthropogenic carbon invasion into the surface ocean drives changes in the seasonal cycles of carbon dioxide partial pressure (pCO2) and pH. However, it is not yet known whether the resulting sea‐air CO2 fluxes are symmetric in their seasonal expression. Here we consider a novel application of observational constraints and modeling inferences to test the hypothesis that changes in the ocean's Revelle factor facilitate a seasonally asymmetric response in pCO2 and the sea‐air CO2 flux. We use an analytical framework that builds on observed sea surface pCO2 variability for the modern era and incorporates transient dissolved inorganic carbon concentrations from an Earth system model. Our findings reveal asymmetric amplification of pCO2 and pH seasonal cycles by a factor of two (or more) above preindustrial levels under Representative Concentration Pathway 8.5. These changes are significantly larger than observed modes of interannual variability and are relevant to climate feedbacks associated with Revelle factor perturbations. Notably, this response occurs in the absence of changes to the seasonal cycle amplitudes of dissolved inorganic carbon, total alkalinity, salinity, and temperature, indicating that significant alteration of surface pCO2 can occur without modifying the physical or biological ocean state. This result challenges the historical paradigm that if the same amount of carbon and nutrients is entrained and subsequently exported, there is no impact on anthropogenic carbon uptake. Anticipation of seasonal asymmetries in the sea surface pCO2 and CO2 flux response to ocean carbon uptake over the 21st century may have important implications for carbon cycle feedbacks.
    Description: Cooperative Institute for Climate Science Grant Number: NA17RJ2612; David and Lucile Packard Foundation/MBARI Grant Number: 4696; NOAA Office of Climate Observations Grant Number: NA11OAR4310066; NOAA. Grant Number NA11OAR4310066; KBR Grant Numbers: A08OAR4320752, NA17RJ2612
    Keywords: Revelle Factor ; Carbon cycle ; Seasonal cycle ; CO2 fluxes ; Ocean acidification
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    A phytoplankton model for the allocation of gross photosynthetic energy including the trade‐offs of diazotrophy (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 123 (2018): 1796-1816, doi:10.1029/2017JG004263.
    Description: Gross photosynthetic activity by phytoplankton is directed to linear and alternative electron pathways that generate ATP, reductant, and fix carbon. Ultimately less than half is directed to net growth. Here we present a phytoplankton cell allocation model that explicitly represents a number of cell metabolic processes and functional pools with the goal of evaluating ATP and reductant demands as a function of light, nitrate, iron, oxygen, and temperature for diazotrophic versus nondiazotrophic growth. We employ model analogues of Synechoccocus and Crocosphaera watsonii, to explore the trade‐offs of diazotrophy over a range of environmental conditions. Model analogues are identical in construction, except for an iron quota associated with nitrogenase, an additional respiratory demand to remove oxygen in order to protect nitrogenase and an additional ATP demand to split dinitrogen. We find that these changes explain observed differences in growth rate and iron limitation between diazotrophs and nondiazotrophs. Oxygen removal imparted a significantly larger metabolic cost to diazotrophs than ATP demand for fixing nitrogen. Results suggest that diazotrophs devote a much smaller fraction of gross photosynthetic energy to growth than nondiazotrophs. The phytoplankton cell allocation model model provides a predictive framework for how photosynthate allocation varies with environmental conditions in order to balance cellular demands for ATP and reductant across phytoplankton functional groups.
    Description: DOC | NOAA | Climate Program Office (CPO) Grant Number: NA100AR4310093; National Science Foundation (NSF) Grant Number: EF‐0424599; Center for Microbial Oceanography Research and Education (CMORE) Grant Number: NSF EF‐0424599; NOAA Global Carbon Program Grant Number: NA100AR4310093
    Description: 2018-11-01
    Keywords: Phytoplankton ; Diazotroph ; Photosynthesis ; Resource allocation ; Biogeochemistry
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    Submarine deep‐water lava flows at the base of the western Galápagos Platform (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 19 (2018): 3945–3961, doi:10.1029/2018GC007632.
    Description: To investigate the initial phases of magmatism at the leading edge of the upwelling mantle plume, we mapped, photographed, and collected samples from two long, deep‐water lava flows located at the western base of the Galápagos Platform using the remotely operated vehicle Hercules. Lavas were recovered from four areas on the seafloor west of Fernandina volcano, including the western flow fronts of two deep‐water flows, heavily sedimented terrain between the two flows, and the eastern, shallower end of one flow. The sediment cover and morphologies are distinct between the western flow fronts and the eastern region based on seafloor imagery, suggesting that the long lava flows are not a single eruptive unit. Major and trace element concentrations reveal both tholeiitic and alkalic compositions and support the interpretation that multiple eruptive units comprise the deep‐water flows. Alkalic lavas have higher [La/Sm]N ratios (2.05–2.12) and total alkali contents (5.18–5.40) compared to tholeiitic lavas, which have [La/Sm]N ratios ranging from 1.64 to 1.68 and total alkali contents ranging from 3.07 to 4.08 wt%. Radiogenic isotope ratios are relatively homogeneous, suggesting a similar mantle source. We use petrologic models to assess three alternative mechanisms for the formation of the alkalic magmas: (1) high‐pressure crystallization of clinopyroxene, (2) mixing of high silica and mafic magmas, and (3) variable extents of melting of the same mantle source. Our modeling indicates that the alkalic samples form from lower extents of melting compared to the tholeiitic lavas and suggests that the deep‐water alkalic lavas are analogous to the initial, preshield building phase observed south of Hawaii and at the base of Loihi Seamount.
    Description: Dalio Explorer Fund; National Science Foundation (NSF) Grant Number: OCE‐1634952
    Description: 2019-04-25
    Keywords: Submarine volcanism ; Galápagos ; Alkalic magmatism ; Mantle plume ; Mantle melting ; Radiogenic isotopes
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    Examining the origins of ocean heat content variability in the eastern North Atlantic subpolar gyre (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 11,275-11,283, doi:10.1029/2018GL079122.
    Description: We analyze sources of ocean heat content (OHC) variability in the eastern North Atlantic subpolar gyre from both Eulerian and Lagrangian perspectives within two ocean simulations from 1990 to 2015. Heat budgets reveal that while the OHC seasonal cycle is driven by air‐sea fluxes, interannual OHC variability is driven by both air‐sea fluxes and the divergence of ocean heat transport, the latter of which is dominated by the oceanic flux through the southern face of the study area. Lagrangian trajectories initialized along the southern face and run backward in time indicate that interannual variability in the subtropical‐origin volume flux (i.e., the upper limb of the overturning circulation) drives variability in the temperature flux through the southern face. As such, the heat carried by the imported subtropical waters is an important component of the eastern subpolar gyre heat budget on interannual time scales.
    Description: NSF. Grant Number NSF‐OCE‐12‐59102; NASA Grant Number: NNX13AO21H
    Description: 2019-04-27
    Keywords: Subpolar gyre ; Heat budget ; Ocean heat content ; Subtropical gyre ; Overturning circulation ; Lagrangian trajectories
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    Turbulent mixing in a far‐field plume during the transition to upwelling conditions : microstructure observations from an AUV (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 9765-9773, doi:10.1029/2018GL078543.
    Description: A REMUS 600 autonomous underwater vehicle was used to measure turbulent mixing within the far‐field Chesapeake Bay plume during the transition to upwelling. Prior to the onset of upwelling, the plume was mixed by a combination of energetic downwelling winds and bottom‐generated shear resulting in a two‐layer plume structure. Estimates of turbulent dissipation and buoyancy flux from a nose‐mounted microstructure system indicate that scalar exchange within the plume was patchy and transient, with direct wind mixing constrained to the near surface by stratification within the plume. Changing wind and tide conditions contributed to temporal variability. Following the separation of the upper plume from the coast, alongshore shear became a significant driver of mixing on the shoreward edge of the plume.
    Description: NSF Grant Numbers: OCE‐1334231, OCE‐1745258, OCE‐1334398
    Description: 2019-03-23
    Keywords: River plume ; Upwelling ; Turbulence ; Autonomous underwater vehicle ; Mixing
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    Near-surface environmentally forced changes in the Ross Ice Shelf observed with ambient seismic noise (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 11,187-11,196, doi:10.1029/2018GL079665.
    Description: Continuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies 〉5 Hz. These firn‐trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long‐term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event. Modeling demonstrates high spectral sensitivity to near‐surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0°C for the melt event. Percolation‐related refrozen layers and layer thinning may also contribute to long‐term spectral changes after the melt event. Single‐station observations are inverted for elastic structure for multiple stations across the ice shelf. High‐frequency ambient noise seismology presents opportunities for continuous assessment of near‐surface ice shelf or other firn environments.
    Description: NSF Office of Polar Programs Grant Number: PLR-1142518
    Description: 2019-04-16
    Keywords: Ross Ice Shelf ; Antarctica ; Firn ; Ambient noise ; Temporal monitoring ; Resonances
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    North Atlantic natural variability modulates emergence of widespread Greenland melt in a warming climate (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 9171-9178, doi:10.1029/2018GL079682.
    Description: Record‐breaking melt over Greenland in recent decades is linked not only to climate change but also to natural variability, including persistent atmospheric high‐pressure conditions in the negative phase of the North Atlantic Oscillation and warm North Atlantic Ocean temperatures during the positive phase of the Atlantic Multidecadal Oscillation. However, the relative importance of natural variability for Greenland melt under varying degrees of greenhouse forcing is still unclear. Using reanalysis data and a large ensemble of climate model simulations, we find that a negative North Atlantic Oscillation and positive Atlantic Multidecadal Oscillation consistently promote heightened summer melt under various forcing conditions. Moreover, timing of widespread 21st century Greenland melt varies considerably between ensemble members due to different phasing of these modes of natural variability. These results indicate the importance of natural modes of variability across a range of external forcing conditions for interannual melt variability and the emergence of widespread Greenland melt.
    Description: U.S. National Science Foundation Grant Number: ANS‐1736738; Woods Hole Oceanographic Institution Summer Student Fellow program Grant Number: AGS‐1355339
    Description: 2019-03-10
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    Global search for autumn‐lead sea surface salinity predictors of winter precipitation in southwestern United States (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 8445-8454, doi:10.1029/2018GL079293.
    Description: Sea surface salinity (SSS) is sensitive to changes in ocean evaporation and precipitation, that is, to changes in the oceanic water cycle. Through the close connection between the oceanic and terrestrial water cycle, SSS can be used as an indicator of rainfall on land. Here we search globally for teleconnections between autumn‐lead September‐October‐November SSS signals and winter December‐January‐February precipitation over southwestern United States. The SSS‐based model (R2 = 0.61) outperforms the sea surface temperature‐based model (R2 = 0.54). Further, a fresh tropical Pacific in autumn, indicated by low SSS, corresponds with wet winters. Recent studies suggest that anomalously high rainfall in the tropics may excite Rossby waves that can export water to the extratropics. Thus, incorporating SSS, a sensitive indicator of regional oceanic rainfall, can enhance the accuracy of existing precipitation prediction frameworks that rely on sea surface temperature‐based climate indices and, by extension, improve watershed management.
    Description: NSF Grant Numbers: ICER‐1663704, ICER‐1663138, DGE1144152, DGE1745303; Woods Hole Oceanographic Institution
    Description: 2019-02-22
    Keywords: Sea surface salinity ; Sea surface temperature ; Prediction ; Model ; Water cycle
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    Controls on mid‐ocean ridge normal fault seismicity across spreading rates from rate‐and‐state friction models (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union., 2018. This article is posted here by permission of American Geophysical Union.for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 6719-6733, doi:10.1029/2018JB015545.
    Description: Recent seismic and geodetic observations have led to a growing realization that a significant amount of fault slip at plate boundaries occurs aseismically and that the amount of aseismic slip varies across tectonic settings. Seismic moment release rates measured along the fast‐spreading East Pacific Rise suggest that the majority of fault slip occurs aseismically. By contrast, at the slow‐spreading Mid‐Atlantic Ridge seismic slip may represent up to 60% of total fault displacement. In this study, we use rate‐and‐state friction models to quantify the seismic coupling coefficient, defined as the fraction of total fault slip that occurs seismically, on mid‐ocean ridge normal faults and investigate controls on fault behavior that might produce variations in coupling observed at oceanic spreading centers. We find that the seismic coupling coefficient scales with the ratio of the downdip width of the seismogenic area (W) to the critical earthquake nucleation size (h*). At mid‐ocean ridges, W is expected to increase with decreasing spreading rate. Thus, the relationship between seismic coupling and W/h* predicted from our models explains the first‐order variations in seismic coupling coefficient as a function of spreading rate.
    Description: National Science Foundation (NSF) Grant Numbers: EAR‐10‐10432, OCE‐10‐61203; NSF | GEO | Division of Earth Sciences (EAR); NSF | GEO | Division of Ocean Sciences (OCE)
    Description: 2019-02-16
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