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  • 1
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    Mature diffuse tectonic block boundary revealed by the 2020 southwestern Puerto Rico seismic sequence (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-10-27
    Description: This paper is not subject to U.S. copyright. The definitive version was published in ten Brink, U. S., Vanacore, E. A., Fielding, E. J., Chaytor, J. D., Lopez-Venegas, A. M., Baldwin, W. E., Foster, D. S., & Andrews, B. D. Mature diffuse tectonic block boundary revealed by the 2020 southwestern Puerto Rico seismic sequence. Tectonics, 41(3), (2022): e2021TC006896, https://doi.org/10.1029/2021TC006896.
    Description: Distributed faulting typically tends to coalesce into one or a few faults with repeated deformation. The progression of clustered medium-sized (≥Mw4.5) earthquakes during the 2020 seismic sequence in southwestern Puerto Rico (SWPR), modeling shoreline subsidence from InSAR, and sub-seafloor mapping by high-resolution seismic reflection profiles, suggest that the 2020 SWPR seismic sequence was distributed across several short intersecting strike-slip and normal faults beneath the insular shelf and upper slope of Guayanilla submarine canyon. Multibeam bathymetry map of the seafloor shows significant erosion and retreat of the shelf edge in the area of seismic activity as well as slope-parallel lineaments and submarine canyon meanders that typically develop over geological time. The T-axis of the moderate earthquakes further matches the extension direction previously measured on post early Pliocene (∼〉3 Ma) faults. We conclude that although similar deformation has likely taken place in this area during recent geologic time, it does not appear to have coalesced during this time. The deformation may represent the southernmost part of a diffuse boundary, the Western Puerto Rico Deformation Boundary, which accommodates differential movement between the Puerto Rico and Hispaniola arc blocks. This differential movement is possibly driven by the differential seismic coupling along the Puerto Rico—Hispaniola subduction zone. We propose that the compositional heterogeneity across the island arc retards the process of focusing the deformation into a single fault. Given the evidence presented here, we should not expect a single large event in this area but similar diffuse sequences in the future.
    Description: 2022-08-08
    Keywords: Rupture of multiple faults ; Intra-arc deformation ; Earthquake-generated submarine canyon ; Anisotropic arc composition ; Caribbean seismic hazard
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  • 2
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    Shallow calcium carbonate cycling in the North Pacific Ocean (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-11-06
    Description: Author Posting. © American Geophysical Union, 2022. 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 36(5), (2022): e2022GB007388, https://doi.org/10.1029/2022gb007388.
    Description: The cycling of biologically produced calcium carbonate (CaCO3) in the ocean is a fundamental component of the global carbon cycle. Here, we present experimental determinations of in situ coccolith and foraminiferal calcite dissolution rates. We combine these rates with solid phase fluxes, dissolved tracers, and historical data to constrain the alkalinity cycle in the shallow North Pacific Ocean. The in situ dissolution rates of coccolithophores demonstrate a nonlinear dependence on saturation state. Dissolution rates of all three major calcifying groups (coccoliths, foraminifera, and aragonitic pteropods) are too slow to explain the patterns of both CaCO3 sinking flux and alkalinity regeneration in the North Pacific. Using a combination of dissolved and solid-phase tracers, we document a significant dissolution signal in seawater supersaturated for calcite. Driving CaCO3 dissolution with a combination of ambient saturation state and oxygen consumption simultaneously explains solid-phase CaCO3 flux profiles and patterns of alkalinity regeneration across the entire N. Pacific basin. We do not need to invoke the presence of carbonate phases with higher solubilities. Instead, biomineralization and metabolic processes intimately associate the acid (CO2) and the base (CaCO3) in the same particles, driving the coupled shallow remineralization of organic carbon and CaCO3. The linkage of these processes likely occurs through a combination of dissolution due to zooplankton grazing and microbial aerobic respiration within degrading particle aggregates. The coupling of these cycles acts as a major filter on the export of both organic and inorganic carbon to the deep ocean.
    Description: This work was funded by NSF OCE-1220301 to W.B., NSF OCE-1220600 to J.F.A., and startup funding for A.V.S.
    Description: 2022-11-06
    Keywords: Calcium carbonate ; Dissolution ; Carbon cycle
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  • 3
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    Evaluating models for lithospheric loss and intraplate volcanism beneath the Central Appalachian Mountains (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Long, M. D., Wagner, L. S., King, S. D., Evans, R. L., Mazza, S. E., Byrnes, J. S., Johnson, E. A., Kirby, E., Bezada, M. J., Gazel, E., Miller, S. R., Aragon, J. C., & Liu, S. Evaluating models for lithospheric loss and intraplate volcanism beneath the Central Appalachian Mountains. Journal of Geophysical Research: Solid Earth, 126(10), (2021): e2021JB022571, https://doi.org/10.1029/2021JB022571.
    Description: The eastern margin of North America has been shaped by a series of tectonic events including the Paleozoic Appalachian Orogeny and the breakup of Pangea during the Mesozoic. For the past ∼200 Ma, eastern North America has been a passive continental margin; however, there is evidence in the Central Appalachian Mountains for post-rifting modification of lithospheric structure. This evidence includes two co-located pulses of magmatism that post-date the rifting event (at 152 and 47 Ma) along with low seismic velocities, high seismic attenuation, and high electrical conductivity in the upper mantle. Here, we synthesize and evaluate constraints on the lithospheric evolution of the Central Appalachian Mountains. These include tomographic imaging of seismic velocities, seismic and electrical conductivity imaging along the Mid-Atlantic Geophysical Integrative Collaboration array, gravity and heat flow measurements, geochemical and petrological examination of Jurassic and Eocene magmatic rocks, and estimates of erosion rates from geomorphological data. We discuss and evaluate a set of possible mechanisms for lithospheric loss and intraplate volcanism beneath the region. Taken together, recent observations provide compelling evidence for lithospheric loss beneath the Central Appalachians; while they cannot uniquely identify the processes associated with this loss, they narrow the range of plausible models, with important implications for our understanding of intraplate volcanism and the evolution of continental lithosphere. Our preferred models invoke a combination of (perhaps episodic) lithospheric loss via Rayleigh-Taylor instabilities and subsequent small-scale mantle flow in combination with shear-driven upwelling that maintains the region of thin lithosphere and causes partial melting in the asthenosphere.
    Description: The authors acknowledge support from the U.S. National Science Foundation EarthScope and GeoPRISMS programs via grants EAR-1460257 (R. L. Evans), EAR-1249412 (E. Gazel), EAR-1249438 (E. A. Johnson), EAR-1250988 (S. D. King), EAR-1251538 (E. Kirby), and EAR-1251515 (M. D. Long). The collection and dissemination of most of the geophysical data and models discussed in this study were facilitated by the Incorporated Research Institutions for Seismology (IRIS). The facilities of the IRIS Consortium are supported by the United States National Science Foundation under Cooperative Agreement EAR-1261681.
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  • 4
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    Hydrothermal exploration of the southern Chile Rise: sediment‐hosted venting at the Chile Triple Junction (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in German, C., Baumberger, T., Lilley, M., Lupton, J., Noble, A., Saito, M., Thurber, A., & Blackman, D. Hydrothermal exploration of the southern Chile Rise: sediment‐hosted venting at the Chile Triple Junction. Geochemistry Geophysics Geosystems, 23(3), (2022): e2021GC010317, https://doi.org/10.1029/2021gc010317.
    Description: We report results from a hydrothermal plume survey along the southernmost Chile Rise from the Guamblin Fracture Zone to the Chile Triple Junction (CTJ) encompassing two segments (93 km cumulative length) of intermediate spreading-rate mid-ocean ridge axis. Our approach used in situ water column sensing (CTD, optical clarity, redox disequilibrium) coupled with sampling for shipboard and shore based geochemical analyses (δ3He, CH4, total dissolvable iron (TDFe) and manganese, (TDMn)) to explore for evidence of seafloor hydrothermal venting. Across the entire survey, the only location at which evidence for submarine venting was detected was at the southernmost limit to the survey. There, the source of a dispersing hydrothermal plume was located at 46°16.5’S, 75°47.9’W, coincident with the CTJ itself. The plume exhibits anomalies in both δ3He and dissolved CH4 but no enrichments in TDFe or TDMn beyond what can be attributed to resuspension of sediments covering the seafloor where the ridge intersects the Chile margin. These results are indicative of sediment-hosted venting at the CTJ.
    Description: We acknowledge University of California Ship Funds for their support of that shiptime and the NOAA Ocean Exploration and Research Grant NA08OAR4600757 which supported the research presented here. Finally, we thank two anonymous reviewers whose important contributions helped to improve the final version of this paper. This is PMEL contribution number 5341.
    Keywords: Hydrothermal ; Geochemistry ; Chile Rise ; Chile Triple Junction ; Sediment hosted
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  • 5
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    Zircon U-Pb age constraints on NW Himalayan exhumation from the Laxmi Basin, Arabian Sea (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-10-19
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Zhou, P., Stockli, D. F., Ireland, T., Murray, R. W., & Clift, P. D. Zircon U-Pb age constraints on NW Himalayan exhumation from the Laxmi Basin, Arabian Sea. Geochemistry Geophysics Geosystems, 23(1), (2022): e2021GC010158, https://doi.org/10.1029/2021GC010158.
    Description: The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program recovered a discontinuous erosional record for the Indus River drainage dating back to at least 9.8 Ma, and with a single sample from 15.6 Ma. We dated detrital zircon grains by U-Pb geochronology to reconstruct how erosion patterns changed through time. Long-term increases in detrital zircon U-Pb components of 750–1,200 and 1,500–2,300 Ma record increasing preferential erosion of the Himalaya relative to the Karakoram between 8.3–7.0 and 5.9–5.7 Ma. The average contribution of Karakoram-derived sediment to the Indus Fan fell from 70% of the total at 8.3–7.0 Ma to 35% between 5.9 and 5.7 Ma. An increase in the contribution of 1,500–2,300 Ma zircons starting between 2.5 and 1.6 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon age spectra is consistent with bulk sediment Nd isotope data. The initial change in spatial erosion patterns at 7.0–5.9 Ma occurred during a time of drying climate in the foreland. The increase in ILH erosion postdated the onset of dry-wet glacial-interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the ILH thrust duplex, especially during the Pliocene, also played an important role, while the influence of the Nanga Parbat Massif to the total sediment flux was modest.
    Description: This work was partially funded by a grant from the USSSP, as well as additional funding from the Charles T. McCord Chair in petroleum geology at LSU, and the Chevron (Gulf) Centennial professorship and the UTChron Laboratory at the University of Texas.
    Keywords: Erosion ; Zircon ; Monsoon ; Himalaya ; Provenance
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  • 6
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    Sources, mechanisms, and timescales of sediment delivery to a New England salt marsh (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-10-20
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Baranes, H., Woodruff, J., Geyer, W., Yellen, B., Richardson, J. & Griswold, F. Sources, mechanisms, and timescales of sediment delivery to a New England salt marsh. Journal of Geophysical Research: Earth Surface, 127, (2022): e2021JF006478, https://doi.org/10.1029/2021jf006478.
    Description: he availability and delivery of an external clastic sediment source is a key factor in determining salt marsh resilience to future sea level rise. However, information on sources, mechanisms, and timescales of sediment delivery are lacking, particularly for wave-protected mesotidal estuaries. Here we show that marine sediment mobilized and delivered during coastal storms is a primary source to the North and South Rivers, a mesotidal bar-built estuary in a small river system impacted by frequent, moderate-intensity storms that is typical to New England (United States). On the marsh platform, deposition rates, clastic content, and dilution of fluvially-sourced contaminated sediment by marine material all increase down-estuary toward the inlet, consistent with a predominantly marine-derived sediment source. Marsh clastic deposition rates are also highest in the storm season. We observe that periods of elevated turbidity in channels and over the marsh are concurrent with storm surge and high wave activity offshore, rather than with high river discharge. Flood tide turbidity also exceeds ebb tide turbidity during storm events. Timescales of storm-driven marine sediment delivery range from 2.5 days to 2 weeks, depending on location within the estuary; therefore the phasing of storm surge and waves with the spring-neap cycle determines how effectively post-event suspended sediment is delivered to the marsh platform. This study reveals that sediment supply and the associated resilience of New England mesotidal salt marshes involves the interplay of coastal and estuarine processes, underscoring the importance of looking both up- and downstream to identify key drivers of environmental change.
    Description: The project described in this publication was in part supported by Grant or Cooperative Agreement No. G20AC00071 from the U.S. Geological Survey and a Department of Interior Northeast Climate Adaptation Science Center graduate fellowship awarded to H.E.B (G12AC00001).
    Keywords: Salt marsh ; Sediment ; Estuary ; Tides ; Massachusetts
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  • 7
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    Turbidity currents can dictate organic carbon fluxes across river‐fed fjords: an example from Bute Inlet (BC, Canada) (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hage, S., Galy, V., Cartigny, M., Heerema, C., Heijnen, M., Acikalin, S., Clare, M., Giesbrecht, I., Gröcke, D., Hendry, A., Hilton, R., Hubbard, S., Hunt, J., Lintern, D., McGhee, C., Parsons, D., Pope, E., Stacey, C., Sumner, E., Tank, S., & Talling, P. Turbidity currents can dictate organic carbon fluxes across river‐fed fjords: an example from Bute Inlet (BC, Canada). Journal of Geophysical Research: Biogeosciences, 127(6), (2022): e2022JG006824, https://doi.org/10.1029/2022jg006824.
    Description: The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may lower the concentrations of atmospheric CO2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river-fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within Bute Inlet, a fjord in western Canada. We show that 62% ± 10% of the OC supplied by the two river sources is buried across the fjord surficial (30–200 cm) sediment. The sandy subenvironments (channel and lobe) contain 63% ± 14% of the annual terrestrial OC burial in the fjord. In contrast, the muddy subenvironments (overbank and distal basin) contain the remaining 37% ± 14%. OC in the channel, lobe, and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord’s surface area, at least 3 times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long-term (〉100 years) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient at storing OC supplied by rivers in their near-surface deposits.
    Description: S.H. acknowledges funding by the IAS postgraduate grant scheme, a Research Development funds offered by Durham University, and the NOCS/WHOI exchange program. S.H. has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 899546. The field campaign and geochemical analyses were supported by Natural Environment Research Council grants NE/M007138/1, NE/W30601/1, NE/N012798/1, NE/K011480/1 and NE/M017540/1. M.J.B.C. was funded by a Royal Society Research Fellowship (DHF\R1\180166). M.A.C. was supported by the U.K. National Capability NERC CLASS program (NE/R015953/1) and NERC grants (NE/P009190/1 and NE/P005780/1). C.J.H. and M.S.H. were funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 721403 - ITN SLATE. E.L.P. was supported by a Leverhulme Early Career Fellowship (ECF-2018-267).
    Keywords: Fjords ; Organic carbon ; Sediment ; Submarine channel ; Carbon burial ; Rivers
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  • 8
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    Little change in ice age water mass structure from Cape Basin benthic neodymium and carbon isotopes (2022)
    American Geophysical Union
    Details
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hines, S. K. V., Bolge, L., Goldstein, S. L., Charles, C. D., Hall, I. R., & Hemming, S. R. Little change in ice age water mass structure from Cape Basin benthic neodymium and carbon isotopes. Paleoceanography and Paleoclimatology, 36(11), (2021): e2021PA004281, https://doi.org/10.1029/2021PA004281.
    Description: A common conception of the deep ocean during ice age episodes is that the upper circulation cell in the Atlantic was shoaled at the Last Glacial Maximum compared to today, and that this configuration facilitated enhanced carbon storage in the deep ocean, contributing to glacial CO2 draw-down. Here, we test this notion in the far South Atlantic, investigating changes in glacial circulation structure using paired neodymium and benthic carbon isotope measurements from International Ocean Discovery Program Site U1479, at 2,615 m water depth in the Cape Basin. We infer changes in circulation structure across the last glacial cycle by aligning our site with other existing carbon and neodymium isotope records from the Cape Basin, examining vertical isotope gradients, while determining the relative timing of inferred circulation changes at different depths. We find that Site U1479 had the most negative neodymium isotopic composition across the last glacial cycle among the analyzed sites, indicating that this depth was most strongly influenced by North Atlantic Deep Water (NADW) in both interglacial and glacial intervals. This observation precludes a hypothesized dramatic shoaling of NADW above ∼2,000 m. Our evidence, however, indicates greater stratification between mid-depth and abyssal sites throughout the last glacial cycle, conditions that developed in Marine Isotope Stage 5. These conditions still may have contributed to glacial carbon storage in the deep ocean, despite little change in the mid-depth ocean structure.
    Description: This work was supported by NSF grant OCE-1831415 (S. K. V. Hines, S. L. Goldstein., S. R. Hemming.).
    Description: 2022-04-25
    Keywords: Ocean circulation ; Neodymium isotopes ; Carbon isotopes
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  • 9
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    Increasing coral reef resilience through successive marine heatwaves (2022)
    American Geophysical Union
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    Publication Date: 2022-12-06
    Description: Author Posting. © American Geophysical Union, 2021. 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 48(17), (2021): e2021GL094128, https://doi.org/10.1029/2021GL094128.
    Description: Ocean warming is causing declines of coral reefs globally, raising critical questions about the potential for corals to adapt. In the central equatorial Pacific, reefs persisting through recurrent El Niño heatwaves hold important clues. Using an 18-year record of coral cover spanning three major bleaching events, we show that the impact of thermal stress on coral mortality within the Phoenix Islands Protected Area (PIPA) has lessened over time. Disproportionate survival of extreme thermal stress during the 2009–2010 and 2015–2016 heatwaves, relative to that in 2002–2003, suggests that selective mortality through successive heatwaves may help shape coral community responses to future warming. Identifying and facilitating the conditions under which coral survival and recovery can keep pace with rates of warming are essential first steps toward successful stewardship of coral reefs under 21st century climate change.
    Description: Support was provided by the US National Science Foundation (NSF) 1737311 to A. L. Cohen; The Atlantic Donor Advised Fund to A. L. Cohen; a Woods Hole Oceanographic Institution post-doctoral scholarship to M. D. Fox; the Robertson Foundation, The Prince Albert Foundation, the New England Aquarium, and the Akiko Shiraki Dynner Fund.
    Keywords: Coral reefs ; Thermal stress ; ENSO ; Adaptation ; Oceanography ; Central Pacific
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