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  • American Geophysical Union  (225,276)
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  • 1
    Publication Date: 2020-02-06
    Description: Author Posting. © American Geophysical Union, 2019. 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 124(16), (2019): 8953-8971, doi: 10.1029/2019JD030424.
    Description: Multiyear droughts are a common occurrence in southwestern North America (SWNA), but it is unclear what causes these persistent dry periods. The ocean‐atmosphere conditions coinciding with droughts have traditionally been studied using correlation and composite methods, which suggest that cool conditions in the tropical Pacific are associated with SWNA droughts and warm conditions are associated with wet periods in SWNA. Nevertheless, the extent to which multiyear droughts are truly consistent with this paradigm remains unknown. This is, in part, because the temporal trajectory of ocean‐atmosphere conditions during these dry periods have not been sufficiently characterized. Here we examine the continuum of ocean‐atmosphere trajectories before, during, and after multiyear droughts in SWNA using observation‐based data and an ensemble of climate model simulations from the Community Earth System Model. An examination of sea surface temperature patterns at the beginning, middle, and end of SWNA droughts shows that an El Niño event tends to precede SWNA droughts, a cool tropical Pacific occurs during droughts, and central Pacific El Niño events end droughts. However, moderate El Niño events can occur in the middle of persistent droughts, so a warm tropical Pacific does not always end these dry periods. These findings are important for drought predictability and emphasize the need to improve simulations of the magnitude, life cycle, and frequency of occurrence of El Niño events.
    Description: L. Parsons thanks the Washington Research Foundation for funding support and thanks R. Jnglin Wills and D. Battisti for suggestions related to tropical Pacific‐SWNA comparisons. We thank B. Otto‐Bliesner and acknowledge the CESM1(CAM5) Last Millennium Ensemble Community Project and supercomputing resources provided by NSF/CISL/Yellowstone. Support for the Twentieth Century Reanalysis Project version 2c data set is provided by the U.S. Department of Energy, Office of Science Biological and Environmental Research (BER), and by the National Oceanic and Atmospheric Administration Climate Program Office. GPCC Precipitation data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site (https://www.esrl.noaa.gov/psd/).
    Description: 2020-02-06
    Keywords: drought ; climate dynamics ; teleconnections ; southwest ; climate model ; tropical Pacific
    Repository Name: Woods Hole Open Access Server
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  • 2
    Publication Date: 2020-02-14
    Description: Author Posting. © American Geophysical Union, 2019. 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 124(8), (2019): 6388-6413, doi: 10.1029/2018JC014881.
    Description: For ice concentrations less than 85%, internal ice stresses in the sea ice pack are small and sea ice is said to be in free drift. The sea ice drift is then the result of a balance between Coriolis acceleration and stresses from the ocean and atmosphere. We investigate sea ice drift using data from individual drifting buoys as well as Arctic‐wide gridded fields of wind, sea ice, and ocean velocity. We perform probabilistic inverse modeling of the momentum balance of free‐drifting sea ice, implemented to retrieve the Nansen number, scaled Rossby number, and stress turning angles. Since this problem involves a nonlinear, underconstrained system, we used a Monte Carlo guided search scheme—the Neighborhood Algorithm—to seek optimal parameter values for multiple observation points. We retrieve optimal drag coefficients of CA=1.2×10−3 and CO=2.4×10−3 from 10‐day averaged Arctic‐wide data from July 2014 that agree with the AIDJEX standard, with clear temporal and spatial variations. Inverting daily averaged buoy data give parameters that, while more accurately resolved, suggest that the forward model oversimplifies the physical system at these spatial and temporal scales. Our results show the importance of the correct representation of geostrophic currents. Both atmospheric and oceanic drag coefficients are found to decrease with shorter temporal averaging period, informing the selection of drag coefficient for short timescale climate models.
    Description: The scripts developed for this publication are available at the GitHub (https://github.com/hheorton/Freedrift_inverse_submit). The Neighborhood Algorithm was developed and kindly supplied by M. Sambridge (http://www.iearth.org.au/codes/NA/). Ice‐Tethered Profiler data are available via the Ice‐Tethered Profiler program website (http://whoi.edu/itp). Buoy data were collected as part of the Marginal Ice Zone program (www.apl.washington.edu/miz) funded by the U.S. Office of Naval Research. The ice drift data were kindly supplied by N. Kimura. H. H. was funded by the Natural Environment Research Council (Grants NE/I029439/1 and NE/R000263/1). M. T. was partially funded by the SKIM Mission Science Study (SKIM‐SciSoc) Project ESA RFP 3‐15456/18/NL/CT/gp. T. A. was supported at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. M. T. and H. H. thank Dr. Nicolas Brantut for early discussions on the implementation of inverse modeling techniques.
    Description: 2020-02-14
    Keywords: sea ice drift ; observations ; inverse modeling ; drag coefficients
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  • 3
    Publication Date: 2020-02-12
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 4
    Publication Date: 2020-02-22
    Description: Author Posting. © American Geophysical Union, 2019. 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 46(16), (2019): 9851-9860, doi:10.1029/2019GL083726.
    Description: Coral reef calcification is expected to decline due to climate change stressors such as ocean acidification and warming. Projections of future coral reef health are based on our understanding of the environmental drivers that affect calcification and dissolution. One such driver that may impact coral reef health is heterotrophy of oceanic‐sourced particulate organic matter, but its link to calcification has not been directly investigated in the field. In this study, we estimated net ecosystem calcification and oceanic particulate organic carbon (POCoc) uptake across the Kāne'ohe Bay barrier reef in Hawai'i. We show that higher rates of POCoc uptake correspond to greater net ecosystem calcification rates, even under low aragonite saturation states (Ωar). Hence, reductions in offshore productivity may negatively impact coral reefs by decreasing the food supply required to sustain calcification. Alternatively, coral reefs that receive ample inputs of POCoc may maintain higher calcification rates, despite a global decline in Ωar.
    Description: Data needed for calculations are available in the supporting information. Additional data can be provided upon request directly from the corresponding author or accessed by links provided in the supporting information. The authors declare no competing financial interests. We thank Texas Sea Grant for providing partial funding for this project to A. Kealoha through the Grants‐In‐Aid of Graduate Research Program. We also thank the NOAA Nancy Foster Scholarship for PhD program funding to A. Kealoha and Texas A&M University for funds awarded to Shamberger that supported this work. This research was also supported by funding from National Science Foundation Grant OCE‐1538628 to Rappé. The Hawaii Institute of Marine Biology (particularly the Rappé Lab and Jason Jones), NOAA's Coral Reef Ecosystem Program, Connie Previti, Serena Smith, and Chris Maupin were instrumental in sample collection and data analysis.
    Description: 2020-02-22
    Keywords: coral reefs ; ocean acidification ; climate change ; heterotrophy
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  • 5
    Publication Date: 2020-03-11
    Description: Author Posting. © American Geophysical Union, 2019. 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 124 (2019): 10023–10055, doi: 10.1029/2019JB017648.
    Description: We studied long‐term evolution of nontransform discontinuities (NTDs) on the Mid‐Atlantic Ridge from 0‐ to ~20‐ to 25‐Ma crust using plate reconstructions of multibeam bathymetry, long‐range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly (RMBA), and gravity‐derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi‐stable. Fast, short‐term (〈2 Myr) propagation is driven by reduced magma supply (increased tectonic extension) in the propagating ridge tip when NTD ridge‐axis offsets are small (≲5 km). Propagation at larger offsets generally is slower and longer term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge‐axis offset, the evolution of NTDs appears to be controlled by three factors: (1) gross volume and distribution of magma supplied to ridge segments as controlled by 3‐D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma‐poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long‐wavelength (5‐10 Myr) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle.
    Description: We thank Tingting Wang for providing plate‐reconstruction codes, Ross Parnell‐Turner for technical support, and Anouk Beniest and an anonymous reviewer for comments that helped to improve the manuscript. We benefited greatly from discussion with the Deep Sea Geodynamics Group of the South China Sea Institute of Oceanology. Figures were drawn using the GMT software of Wessel and Smith (1998). This study was supported by National Natural Science Foundation of China (91628301, 41890813, and U1606401), Chinese Academy of Sciences (Y4SL021001, QYZDY‐SSW‐DQC005, and 133244KYSB20180029), Chinese National 985 Project (1350141509), International Exchange Program for Graduate Students of Tongji University (2016020006), China Scholarship Council (201706260034), and Woods Hole Oceanographic Institution. We thank the crews and science parties of the ARSRP, MAREAST, MODE94, and MODE98 expeditions for their contributions to data acquisition. ARSRP and MAREAST data acquisition was funded by Office of Naval Research grant N00014‐90‐J‐6121 and by U.S. National Science Foundation grant OCE‐9503561, respectively. Access to the original data used in this study is available at https://doi.org/10.26025/z2z7‐kd89.
    Description: 2020-03-11
    Keywords: Mid‐Atlantic Ridge ; nontransform discontinuity ; plate reconstruction ; propagating rift
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  • 6
    Publication Date: 2020-03-16
    Description: Author Posting. © American Geophysical Union, 2019. 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 46(20), (2019): 11206-11218, doi: 10.1029/2019GL084347.
    Description: The emperor penguin, an iconic species threatened by projected sea ice loss in Antarctica, has long been considered to forage at the fast ice edge, presumably relying on large/yearly persistent polynyas as their main foraging habitat during the breeding season. Using newly developed fine‐scale sea icescape data and historical penguin tracking data, this study for the first time suggests the importance of less recognized small openings, including cracks, flaw leads and ephemeral short‐term polynyas, as foraging habitats for emperor penguins. The tracking data retrieved from 47 emperor penguins in two different colonies in East Antarctica suggest that those penguins spent 23% of their time in ephemeral polynyas and did not use the large/yearly persistent, well‐studied polynyas, even if they occur much more regularly with predictable locations. These findings challenge our previous understanding of emperor penguin breeding habitats, highlighting the need for incorporating fine‐scale seascape features when assessing the population persistence in a rapidly changing polar environment.
    Description: This study was supported financially and logistically by the Australian Antarctic Division, the Australian Government's Cooperative Research Centre program through the Antarctic Climate & Ecosystems Cooperative Research Centre, and by the Australian Research Council's Special Research Initiative for Antarctic Gateway Partnership (Project ID SR140300001), the French Polar Institute (Institut Paul Emile Victor, IPEV) research projects, and the postdoctoral scholar award from Woods Hole Oceanographic Institution. S. J. acknowledges support from NSF award 1744794 and 1643901. C. B. and Y. R.‐C. acknowledge support from the BNP Paribas Foundation as part of program SENSEI (SENtinels of the SEa Ice). Y. R.‐C. and R. R. R. acknowledge support from the WWF‐UK through R. Downie. Special thanks go to Y. le Maho in charge of the research program in Terre Adelie in 1996/1997, M. LaRue for the field opportunity in the Ross Sea, illuminating sea icescapes and movements of emperor penguins during the breeding season, D. Ainley for interesting discussions, D. Iles for the proofreading and all colleagues and volunteers involved in the research on emperor penguins in Terre Adélie and at the Mawson Coast, especially D. Rodary and W. Bonneau. All animals in this study were treated in accordance with the IPEV and Polar Environment Committees guidelines, and Australian Antarctic Program Animal Ethics Committee permits. Data and data products related to the paper are available on the following repository http://www.usap‐dc.org/view/dataset/601209 with the doi: 10.15784/601209.
    Description: 2020-03-16
    Keywords: emperor penguin ; sea ice ; iceberg ; fast ice ; polynya ; foraging ecology
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  • 7
    Publication Date: 2020-04-16
    Description: Author Posting. © American Geophysical Union, 2019. 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 124 (2019): 7201-7225, doi: 10.1029/2019JC015520.
    Description: The oceanographic response and atmospheric forcing associated with downwelling along the Alaskan Beaufort Sea shelf/slope is described using mooring data collected from August 2002 to September 2004, along with meteorological time series, satellite data, and reanalysis fields. In total, 55 downwelling events are identified with peak occurrence in July and August. Downwelling is initiated by cyclonic low‐pressure systems displacing the Beaufort High and driving westerly winds over the region. The shelfbreak jet responds by accelerating to the east, followed by a depression of isopycnals along the outer shelf and slope. The storms last 3.25 ± 1.80 days, at which point conditions relax toward their mean state. To determine the effect of sea ice on the oceanographic response, the storms are classified into four ice seasons: open water, partial ice, full ice, and fast ice (immobile). For a given wind strength, the largest response occurs during partial ice cover, while the most subdued response occurs in the fast ice season. Over the two‐year study period, the winds were strongest during the open water season; thus, the shelfbreak jet intensified the most during this period and the cross‐stream Ekman flow was largest. During downwelling, the cold water fluxed off the shelf ventilates the upper halocline of the Canada Basin. The storms approach the Beaufort Sea along three distinct pathways: a northerly route from the high Arctic, a westerly route from northern Siberia, and a southerly route from south of Bering Strait. Differences in the vertical structure of the storms are presented as well.
    Description: The authors thank Paula Fratantoni and Dan Torres for processing the moored profiler and ADCP data, respectively. Data from the SBI mooring array can be found at https://archive.eol.ucar.edu/projects/sbi/all_data.shtml. Funding for the analysis was provided by the following grants: National Science Foundation Grants OCE‐1259618 (N. F. and R. P.), OCE‐1756361 (N. F.), and PLR‐1504333 (N. F. and R. P.); National Oceanic and Atmospheric Administration Grant NA14‐OAR4320158 (R. P. and P. L.); and the Natural Sciences and Engineering Research Council of Canada (K. M.).
    Description: 2020-04-16
    Keywords: downwelling ; Beaufort Sea ; shelfbreak ; North Slope ; Arctic cyclone
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  • 8
    Publication Date: 2020-04-14
    Description: Author Posting. © American Geophysical Union, 2019. 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 124 (2019): 7153– 7177, doi: 10.1029/2019JC015261.
    Description: Data from a late spring survey of the northeast Chukchi Sea are used to investigate various aspects of newly ventilated winter water (NVWW). More than 96% of the water sampled on the shelf was NVWW, the saltiest (densest) of which tended to be in the main flow pathways on the shelf. Nearly all of the hydrographic profiles on the shelf displayed a two‐layer structure, with a surface mixed layer and bottom boundary layer separated by a weak density interface (on the order of 0.02 kg/m3). Using a polynya model to drive a one‐dimensional mixing model, it was demonstrated that, on average, the profiles would become completely homogenized within 14–25 hr when subjected to the March and April heat fluxes. A subset of the profiles would become homogenized when subjected to the May heat fluxes. Since the study domain contained numerous leads within the pack ice—many of them refreezing—and since some of the measured profiles were vertically uniform in density, this suggests that NVWW is formed throughout the Chukchi shelf via convection within small openings in the ice. This is consistent with the result that the salinity signals of the NVWW along the central shelf pathway cannot be explained solely by advection from Bering Strait or via modification within large polynyas. The local convection would be expected to stir nutrients into the water column from the sediments, which explains the high nitrate concentrations observed throughout the shelf. This provides a favorable initial condition for phytoplankton growth on the Chukchi shelf.
    Description: The authors are indebted to Commanding Officer John Reeves, Executive Officer Gregory Stanclik, Operations Officer Jacob Cass, and the entire crew of the USCGC Healy for their hard work and dedication in making the SUBICE cruise a success. We also acknowledge Scott Hiller for his assistance with Healy's meteorological data. We thank an anonymous reviewer for helpful input that improved the paper. Funding for A. P., R. P., C. N., and F. B. was provided by the National Science Foundation (NSF) under grant PLR‐1303617. K. M. was funded by the Natural Sciences and Engineering Research Council of Canada. K. V. acknowledges the Bergen Research Foundation under Grant BFS2016REK01. K. A. was supported by the NSF grant PLR‐1304563. The CTD and shipboard ADCP data are available from https://www.rvdata.us/search/cruise/HLY1401, and the nutrient data can be accessed from https://arcticdata.io/catalog/view/doi:10.18739/A2RG3Z and http://ocean.stanford.edu/subice/. The shipboard meteorological data reside at http://ocean.stanford.edu/subice/.
    Description: 2020-04-14
    Keywords: brine rejection ; Chukchi Sea ; convection ; winter water
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  • 9
    Publication Date: 2020-04-14
    Description: Author Posting. © American Geophysical Union, 2019. 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 46 (2019): 12909-12918, doi: 10.1029/2019GL084217.
    Description: Seismic signals from ocean‐solid Earth interactions are ubiquitously recorded on our planet. However, these wavefields are typically incoherent in the time domain limiting their utilization for understanding ocean dynamics or solid Earth properties. In contrast, we find that during large storms such as hurricanes and Nor'easters the interaction of long‐period ocean waves with shallow seafloor features located near the edge of continental shelves, known as ocean banks, excites coherent transcontinental Rayleigh wave packets in the 20‐ to 50‐s period band. These “stormquakes” migrate coincident with the storms but are effectively spatiotemporally focused seismic point sources with equivalent earthquake magnitudes that can be greater than 3.5. Stormquakes thus provide new coherent sources to investigate Earth structure in locations that typically lack both seismic instrumentation and earthquakes. Moreover, they provide a new geophysical observable with high spatial and temporal resolution with which to investigate ocean wave dynamics during large storms.
    Description: We would like to thank the Editor Dr. Hayes, Dr. Ekström, Dr. McNamara, Dr. Pollitz, and the other two reviewers for their constructive suggestions, which have led to improvements in our paper. We would also like to thank Dr. Ardhuin and Dr. Gualtieri for helpful discussions, and specifically Dr. Ardhuin for sharing codes to model ocean wave and seafloor topography interference (Ardhuin et al., 2015). The seismic data were provided by Data Management Center (DMC) of the Incorporated Research Institutions for Seismology (IRIS). The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms, related metadata, and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR‐1261681. The earthquake catalogs were downloaded from the Global Centroid Moment Tensor GCMT project (Ekström et al., 2012), and the International Seismological Centre (ISC) (International Seismological Centre, 2013). The ocean wave models are obtained from the Environmental Modeling Center at the National Weather Service (NWS) of the National Oceanic and Atmospheric Administration (NOAA; Tolman, 2014). The hurricane tracks are obtained from the National Hurricane Center (NHC) of NOAA (Landsea & Franklin, 2013). The topography is obtained from the ETOPO1 Arc‐Minute Global Relief Model provided by the National Geophysical Data Center (NGDC) of NOAA. Toponymic information, including undersea features, are obtained from the GEONet Names Server (GNS), which is based on the Geographic Names Database, containing official standard names approved by the U.S. Board on Geographic Names and maintained by the National Geospatial‐Intelligence Agency (www.nga.mil, last accessed 21 March 2019). The Bahamas Banks geographic polygons are obtained from the U.S. Geological Survey (USGS) Geographic Names Information System (GNIS) database of names. The AELUMA code can be obtained on request through the IRIS data service product website at https://ds.iris.edu/ds/products/infrasound-aeluma/request(last accessed 21 March 2019). W. F. acknowledges support from the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Weston Howland Jr. Postdoctoral Scholarship. C. D. G and M. A. H. H acknowledge support from NSF Grant EAR‐1358520. The processed data are available from the authors upon request.
    Description: 2020-04-14
    Keywords: stormquake ; surface wave ; USArray ; hurriance ; Nor'Easter ; ambient noise
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  • 10
    Publication Date: 2020-04-21
    Description: Author Posting. © American Geophysical Union, 2019. 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 46, (2019): 12108-12116, doi: 10.1029/2019GL084183.
    Description: The accelerated calving of ice shelves buttressing the Antarctic Ice Sheet may form unstable ice cliffs. The marine ice cliff instability hypothesis posits that cliffs taller than a critical height (~90 m) will undergo structural collapse, initiating runaway retreat in ice‐sheet models. This critical height is based on inferences from preexisting, static ice cliffs. Here we show how the critical height increases with the timescale of ice‐shelf collapse. We model failure mechanisms within an ice cliff deforming after removal of ice‐shelf buttressing stresses. If removal occurs rapidly, the cliff deforms primarily elastically and fails through tensile‐brittle fracture, even at relatively small cliff heights. As the ice‐shelf removal timescale increases, viscous relaxation dominates, and the critical height increases to ~540 m for timescales greater than days. A 90‐m critical height implies ice‐shelf removal in under an hour. Incorporation of ice‐shelf collapse timescales in prognostic ice‐sheet models will mitigate the marine ice cliff instability, implying less ice mass loss.
    Description: We thank Greg Hirth, Brad Hager, and Bill Durham for their useful comments. The manuscript benefited from constructive reviews by Dan Martin and an anonymous reviewer and editorial handling by Mathieu Morlighem. This work was supported by an NSF‐GRFP (Fiona Clerc), and NSF Awards OPP‐1739031 (Brent Minchew) and EAR‐19‐03897 (Mark Behn). Code reproducing our results is found at this address (https://doi.org/10.5281/zenodo.3379074).
    Description: 2020-04-21
    Keywords: marine ice cliff ; buttressing ice shelf ; Antarctic Ice Sheet ; ice‐shelf collapse ; brittle‐ductile transition ; marine ice cliff instability
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  • 11
    Publication Date: 2020-04-30
    Description: Author Posting. © American Geophysical Union, 2019. 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 124 (2019): 7575-7590, doi: 10.1029/2019JC015339.
    Description: Satellite altimetry reveals substantial decadal variability in sea level 𝜁 across the tropical Pacific during 1993–2015. An ocean state estimate that faithfully reproduces the observations is used to elucidate the origin of these low-frequency tropical Pacific 𝜁 variations. Analysis of the hydrostatic equation reveals that recent decadal 𝜁 changes in the tropical Pacific are mainly hermosteric in nature, related to changes in upper-ocean heat content. A forcing experiment performed with the numerical model suggests that anomalous wind stress was an important driver of the relevant heat storage and thermosteric variation. Closed budget diagnostics further clarify that the wind-stress-related thermosteric 𝜁 variation resulted from the joint actions of large-scale ocean advection and local surface heat flux, such that advection controlled the budget over shorter, intraseasonal to interannual time scales, and local surface heat flux became increasingly influential at longer decadal periods. In particular, local surface heat flux was important in contributing to a recent reversal of decadal 𝜁 trends in the tropical Pacific. Contributions from local surface heat flux partly reflect damping latent heat flux tied to wind-stress-driven sea-surface-temperature variations.
    Description: This work was supported by NSF Awards OCE‐1558966 and OCE‐1834739. Support of the ECCO project by the NASA Physical Oceanography, Cryospheric Science, and Modeling, Analysis and Prediction programs is also acknowledged. We thank Ou Wang (NASA JPL) for performing the forcing perturbation experiment. Comments from two anonymous reviewers were helpful. Altimetry observations used in Figures 1 and 2 were downloaded from CSIRO (http://www.cmar.csiro.au/sealevel/sl_data_cmar.html). ECCOv4 output is available on the group website (https://ecco.jpl.nasa.gov/).
    Description: 2020-04-30
    Keywords: sea‐level change ; sea‐level variability ; decadal variability ; tropical Pacific ; state estimation
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  • 12
    Publication Date: 2020-05-06
    Description: Author Posting. © American Geophysical Union, 2019. 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 124, (2019): 9141-9170, doi: 10.1029/2019JC015210.
    Description: The observational network around the North Atlantic has improved significantly over the last few decades with subsurface profiling floats and satellite observations and the recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC). These have shown decadal time scale changes across the North Atlantic including in heat content, heat transport, and the circulation. However, there are still significant gaps in the observational coverage. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and can be used to understand the observed changes. However, the ability of the reanalyses to represent the dynamics must also be assessed. We use an ensemble of global ocean reanalyses to examine the time mean state and interannual‐decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture processes and whether any understanding can be gained. In particular, we examine aspects of the circulation including convection, AMOC and gyre strengths, and transports. We find that reanalyses show some consistency, in particular showing a weakening of the subpolar gyre and AMOC at 50°N from the mid‐1990s until at least 2009 (related to decadal variability in previous studies), a strengthening and then weakening of the AMOC at 26.5°N since 2000, and impacts of circulation changes on transports. These results agree with model studies and the AMOC observations at 26.5°N since 2005. We also see less spread across the ensemble in AMOC strength and mixed layer depth, suggesting improvements as the observational coverage has improved.
    Description: This work was initiated through the EU COST‐EOS‐1402 project which supported the development of this paper by funding project meetings, both in person and virtual. We would like to thank Aida Azcarate for organizing the funding for the meetings and would like to thank Martha Buckley, Gokhan Danabasoglu, and Simon Josey for useful discussions. Jackson, Storto and Zuo were partially funded, by the Copernicus Marine Environment Monitoring Service (CMEMS: 23‐GLO‐RAN) and Zuo was partially funded by the Copernicus Climate Change Service. Jackson was also partially funded by the joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). Haines and Robson acknowledge funding under the NERC RAPID projects RAMOC and DYNAMOC (NE/M005127/1) respectively, and Robson also acknowledges funding from the ACSIS project. Mignac was supported for PhD scholarship by the CAPES Foundation, Ministry of Education of Brazil (Proc. BEX 1386/15‐8). Forget acknowledges support from the Simons Foundation (549931) and the NASA IDS program (6937342). Work by Piecuch was carried out under the ECCO project, funded by the NASA Physical Oceanography, Cryospheric Science, and Modeling, Analysis and Prediction programs, and supported by the Independent Research and Development Program at Woods Hole Oceanographic Institution. Wilson was funded by the NERC UK‐OSNAP project (NE/K010875.1) as part of the international OSNAP program. NorCPM‐v1 reanalysis was cofunded by the Center for Climate Dynamics at the Bjerknes Center, the Norwegian Research Council under the EPOCASA (229774/E10) and SFE (270733) research projects, the NordForsk under the Nordic Centre of Excellence (ARCPATH, 76654), and the Trond Mohn Foundation under the project BFS2018TMT01. NorCPM‐v1 reanalysis received a grant for computer time from the Norwegian Program for supercomputer (NOTUR2, project NN9039K) and a storage grant (NORSTORE, NS9039K). Data for the figures are available to download (from https://doi.org/10.5281/zenodo.2598509). Data from some reanalysis products are available to download (from http://marine.copernicus.eu/services-portfolio/access-to-products/) under product names GLOBAL_REANALYSIS_PHY_001_025 (GLORYS2v4), GLOBAL_REANALYSIS_PHY_001_026 (C‐GLORSv7, GLORYS2v4, GloSea5 and ORAS5) and GLOBAL_REANALYSIS_PHY_001_030 (GLORYS12V1).
    Description: 2020-05-06
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  • 13
    Publication Date: 2020-05-07
    Description: Author Posting. © American Geophysical Union, 2019. 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 20, (2019): 5939-5967, doi: 10.1029/2019GC008654.
    Description: The Icelandic hotspot has erupted basaltic magma with the highest mantle‐derived 3He/4He over a period spanning much of the Cenozoic, from the early‐Cenozoic Baffin Island‐West Greenland flood basalt province (49.8 RA), to mid‐Miocene lavas in northwest Iceland (40.2 to 47.5 RA), to Pleistocene lavas in Iceland's neovolcanic zone (34.3 RA). The Baffin Island lavas transited through and potentially assimilated variable amounts of Precambrian continental basement. We use geochemical indicators sensitive to continental crust assimilation (Nb/Th, Ce/Pb, MgO) to identify the least crustally contaminated lavas. Four lavas, identified as “least crustally contaminated,” have high MgO (〉15 wt.%), and Nb/Th and Ce/Pb that fall within the mantle range (Nb/Th = 15.6 ± 2.6, Ce/Pb = 24.3 ± 4.3). These lavas have 87Sr/86Sr = 0.703008–0.703021, 143Nd/144Nd = 0.513094–0.513128, 176Hf/177Hf = 0.283265–0.283284, 206Pb/204Pb = 17.7560–17.9375, 3He/4He up to 39.9 RA, and mantle‐like δ18O of 5.03–5.21‰. The radiogenic isotopic compositions of the least crustally contaminated lavas are more geochemically depleted than Iceland high‐3He/4He lavas, a shift that cannot be explained by continental crust assimilation in the Baffin suite. Thus, we argue for the presence of two geochemically distinct high‐3He/4He components within the Iceland plume. Additionally, the least crustally contaminated primary melts from Baffin Island‐West Greenland have higher mantle potential temperatures (1510 to 1630 °C) than Siqueiros mid‐ocean ridge basalts (1300 to 1410 °C), which attests to a hot, buoyant plume origin for early Iceland plume lavas. These observations support the contention that the geochemically heterogeneous high‐3He/4He domain is dense, located in the deep mantle, and sampled by only the hottest plumes.
    Description: We acknowledge support from NSF EAR‐1624840 (to M.G.J.), NSF EAR‐1900652 (to M.G.J.), and NSF OCE‐1259218 (to M.D.K). We thank Don Francis for generously providing us access to his collection of Baffin Island lavas. We appreciate helpful discussion and feedback from Roberta Rudnick, Matthew Rioux, Douglas Wilson, and Keith Putirka. Jonathan Pinko is thanked for his help with sample preparation. Rick Carlson's continued generosity is gratefully acknowledged, especially discussions regarding 142Nd/144Nd evolution in the Earth. We acknowledge Al Hofmann for suggesting the use of Nb/Th, instead of Nb/U, in older rocks. We are grateful for helpful discussion with Maud Boyet while in Paris celebrating one of the author's birthdays. We thank Lotte Larsen and Asger Pedersen for advice and discussion regarding West Greenland samples. We thank C. Herzberg and G. Fitton for thorough and helpful reviews, which greatly improved this manuscript. All data published in this manuscript are available in the EarthChem data repository (https://doi.org/10.1594/IEDA/111373).
    Description: 2020-05-07
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  • 14
    Publication Date: 2020-05-07
    Description: Author Posting. © American Geophysical Union, 2019. 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 124(12), (2019): 12631-12659, doi:10.1029/2018JB016858.
    Description: 809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508‐m Hole 735B and 1.4 from 158‐m Hole 1105A. With mapping, it provides the first 3‐D view of the upper levels of a 660‐km2 lower crustal batholith. It is laterally and vertically zoned, representing a complex interplay of cyclic intrusion, and ongoing deformation, with kilometer‐scale upward and lateral migration of interstial melt. Transform wall dives over the gabbro‐peridotite contact found only evolved gabbro intruded directly into the mantle near the transform. There was no high‐level melt lens, rather the gabbros crystallized at depth, and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal‐plastic deformation and faulting. The residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif—likely near the crust‐mantle boundary. Thus, basalts erupted to the seafloor from 〉1,550 mbsf. By contrast, the Mid‐Atlantic Ridge lower crust drilled at 23°N and at Atlantis Massif experienced little high‐temperature deformation and limited late‐stage melt transport. They contain primitive cumulates and represent direct intrusion, storage, and crystallization of parental MORB in thinner crust below the dike‐gabbro transition. The strong asymmetric spreading of the SWIR to the south was due to fault capture, with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains.
    Description: The first author wishes to also recognize grants OCE1434452 and OCE1637130 from The National Science Foundation (NSF) for synthesis of the Atlantis Bank site survey data and post‐cruise rock analysis and for analysis of Expedition 360 and 362T cores and data. Additional support was also gratefully received from The Investment in Science Fund at WHOI.
    Description: 2020-05-07
    Keywords: lower ocean crust ; crustal accretion ; SW Indian Ridge ; crust‐mantle boundary ; Ocean core complex ; ocean drilling
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  • 15
    Publication Date: 2020-05-11
    Description: Author Posting. © American Geophysical Union, 2019. 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 124, (2019): 8439-8454, doi: 10.1029/2019JC015637.
    Description: An Iranian tanker with 136,000 tons of natural gas condensates collided with a freighter in the East China Sea in January 2018 and, after drifting ablaze for 8 days and over 200 km, capsized on the edge of the shelf near the Kuroshio Current. Different from the crude oil, the condensates consist of hydrocarbons that have relatively high solubility in seawater. We postulate that the leakage from the remaining condensate cargo at 110 m depth may result in a bottom layer of condensate‐enriched water in the vicinity of the resting tanker. A model is constructed in this study to simulate the dispersion of contaminated water through the processes of oceanic advection, diffusion, biodegradation, and volatilization. It is found that the scope and magnitude of the dispersion are most sensitive to the biodegradation. Even though the biodegradation time scale depends on several factors that are not well quantified in this region, using any value within the estimated range from a previous study results in significant contamination in the broad area. The dispersion is particularly effective in this incident because the tanker capsized near the Kuroshio Current—a fast‐moving western boundary current. The Kuroshio acts as a fast conduit to spread the contaminant to the east coast of Japan and the interior Pacific Ocean. In addition, we identify that the Tsushima Warm Current, a perennial flow into the Japan Sea, is the second major conduit for spreading the polluted water. This study indicates that dissolved hydrocarbons are the main environmental risk for maritime spills of natural gas condensates.
    Description: Chris Reddy at WHOI provided invaluable guidance at the beginning of this study. Jian Zhao at UMD participated in some early discussions and helped the model development. Lei Chen has been financially supported by China Scholarship Council to study at WHOI for 2 years as a WHOI guest student. Jiayan Yang's participation in this study has been supported by the Woods Hole Oceanographic Institution‐Ocean University of China (WHOI‐OUC) Collaborative Initiative and the W. Van Alan Clark Chair for Excellence in Oceanography from WHOI. This work is supported by National Natural Science Foundation of China major project (41490640, 41490643). The daily oceanic velocity field used in the model is Global Ocean Sea Physical Analysis and Forecasting Products distributed by CMEMS, which can be available online (http://marine.copernicus.eu/services‐portfolio/access‐to‐products/?option=com_csw&view=details&product_id=GLOBAL_ANALYSIS_FORECAST_PHY_001_024). The model output data are available freely from the database of ZENODO (https://zenodo.org/record/3405388#.XXk‐5yhKhPY).
    Description: 2020-05-11
    Keywords: maritime spill ; natural gas condensates ; bio‐degradation ; East China and Japan Seas ; Kuroshio Current ; Tsushima Warm Current
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  • 16
    Publication Date: 2020-05-13
    Description: Mineral dust plays an important role in the atmospheric radiation budget as well as in the ocean carbon cycle through fertilization and by ballasting of settling organic matter. However, observational records of open‐ocean dust deposition are sparse. Here, we present the spatial and temporal evolution of Saharan dust deposition over 2 years from marine sediment traps across the North Atlantic, directly below the core of the Saharan dust plume, with highest dust fluxes observed in summer. We combined the observed deposition fluxes with model simulations and satellite observations and argue that dust deposition in the Atlantic is predominantly controlled by summer rains. The dominant depositional pathway changes from wet deposition in summer to dry deposition in winter. Wet deposition has previously been suggested to increase the release of dust‐derived nutrients and their bioavailability, which may be a key contributor to surface‐ocean productivity in remote and oligotrophic parts of the oceans.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed , info:eu-repo/semantics/article
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  • 17
    Publication Date: 2020-05-15
    Description: Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology, 34, (2019): 2141-2157, doi: 10.1029/2019PA003731.
    Description: Dissolution of calcite in deep ocean sediments, which is required to balance global marine CaCO3 production and burial fluxes, is still a poorly understood process. In order to assess the mechanisms of dissolution in sediments, we analyzed four multicore tops taken along a depth transect on the Ontong‐Java Plateau. These cores were taken directly on the equator, and span water column calcite saturation states from ∼0.93 to ∼0.74, allowing us to assess the effect of dissolution on carbonate sediment composition. The top 2 cm of each multicore was sectioned and sieved to separate coccolith from foraminiferal calcite, and the %CaCO3, δ13C, Δ14C, and Mg/Ca were evaluated. The mass ratio of coccoliths to foraminifera increases by a factor of 3 as a function of water depth, reflecting the preferential dissolution of foraminifera. Carbon isotope (δ13C and Δ14C) data suggest that most dissolution takes place at the sediment‐water interface and primarily affects foraminifera. Mg/Ca analyses indicate that the Mg content of the entire foraminiferal assemblage decreases as a function of dissolution. In contrast, coccolith dissolution takes place within the sediments, rather than at the interface. Together these two processes cause coccoliths to be up to 1,000 radiocarbon years younger than foraminifera from the same depth horizon. Despite this within‐sediment coccolith dissolution flux, sediments below the calcite saturation horizon remain enriched in coccolith calcite. Combined with global seafloor hypsometry and calcium carbonate content, this enrichment suggests that globally, coccoliths may outweigh foraminifera in deep ocean sediments by a factor of 1.8.
    Description: A. V. S. thanks the NOSAMS facility and the WHOI/NOSAMS postdoc scholar program, James Funds, and the Bessette family for funding and support. A. Q. acknowledges Williams College research and travel funds. We thank the Stanley W. Watson Director's Discretionary Fund for the Picarro‐Automate analyzer. We thank Ellen Roosen at the WHOI core repository for help with sample identification and sectioning. Thanks to Gretchen Swarr and the WHOI plasma mass spectrometry facility. Finally, we thank Bill Martin and Wally Broecker for enlightening discussions on dissolution and radiocarbon dating of deep ocean sediments. All data are included as supporting information files and are archived with NOAA's World Data Service for Paleoceanography (WDS Paleo; https://www.ncdc.noaa.gov/paleo/study/28150).
    Description: 2020-05-15
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  • 18
    Publication Date: 2020-05-20
    Description: Author Posting. © American Geophysical Union, 2019. 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-Planets 124, (2019): 3095-3118, doi: 10.1029/2019JE005937.
    Description: We applied localized gravity/topography admittance and correlation analysis, as well as the Markov chain Monte Carlo method, to invert for loading and flexural parameters of 21 subregions on Mars with five distinct tectonic types. The loading styles of the five tectonic types are distinct: The surface and subsurface loading in the polar and plain regions can be assumed to be largely uncorrelated, in contrast to the correlated loading associated with the volcanic montes and Valles Marineris. For the impact basins, we consider the initial topographic depression and mantle plug before postimpact surface loading. Our analyses yield four main results: (1) The inverted effective lithospheric thickness (Te) is highly dependent on assumptions of loading type. (2) There is a trend of increasing Te from the Noachian southern highlands (20–60 km) to the Hesperian northern lowlands (〉90 km) and from the Hesperian Elysium Mons (〈55 km) to the Hesperian/Amazonian Olympus Mons (〉105 km). These Te estimates are consistent with the thermal states at the time of loading, corresponding to a global secular cooling history with decreasing heat flux. (3) Our analyses suggest high‐density basaltic surface loading at the volcanic montes and Isidis basin, in contrast to the low‐density sedimentary surface loading at the Utopia and Argyre basins. (4) We find some degree of correlation between the surface and subsurface loading for the northern polar cap and the northern plains, likely due to earlier, larger polar deposits and ancient buried features, respectively.
    Description: The gravity model JGMRO120d and topography model MarsTopo719 used in this paper were retrieved from the Geosciences Node of NASA's Planetary Data System (http://pds‐geosciences.wustl.edu/mro/mro‐m‐rss‐5‐sdp‐v1/mrors_1xxx/data/shadr/) and from the SHTOOLS package (http://sourceforge.net/projects/shtools/), respectively. The MATLAB codes to reproduce the data analysis, parameter estimation, and key figures are available in a github repository (https://github.com/MinaDing/marslithosphere/tree/v1.0.0, DOI: 10.5281/zenodo.3530057). We are grateful to Mark Wieczorek and Frederik Simons for sharing relevant software online. We thank Ken Tanaka for providing a digital map of Mars chronographic ages. We thank Brandon Johnson for consultation on the loading processes of impact basins. We also thank Editor Laurent Montesi and Steven A. Hauck, as well as Patrick McGovern and anonymous reviewers for their invaluable feedbacks. This work was supported by National Natural Science Foundation of China (41806067, 41890813, 91628301 and U1606401), Key Laboratory of Ocean and Marginal Sea Geology, Chinese Academy of Sciences (OMG18‐02), Chinese Academy of Sciences (Y4SL021001, QYZDY‐SSW‐DQC005 and 133244KYSB20180029), Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0205), Radio Science Gravity investigation of the NASA Mars Reconnaissance Orbiter mission (M.T.Z.), and National Science Foundation (EAR 1220280) and Henry Bigelow Chair for Excellence in Oceanography (J.L.).
    Description: 2020-05-20
    Keywords: Mars ; lithospheric flexure ; tectonic loading styles ; lithospheric strength ; Markov chain Monte Carlo method ; inverse spectral method
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  • 19
    Publication Date: 2020-06-04
    Description: Author Posting. © American Geophysical Union, 2020. 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 47(1), (2020): e2019GL085378, doi:10.1029/2019GL085378.
    Description: Retrospectively comparing future model projections to observations provides a robust and independent test of model skill. Here we analyze the performance of climate models published between 1970 and 2007 in projecting future global mean surface temperature (GMST) changes. Models are compared to observations based on both the change in GMST over time and the change in GMST over the change in external forcing. The latter approach accounts for mismatches in model forcings, a potential source of error in model projections independent of the accuracy of model physics. We find that climate models published over the past five decades were skillful in predicting subsequent GMST changes, with most models examined showing warming consistent with observations, particularly when mismatches between model‐projected and observationally estimated forcings were taken into account.
    Description: Z. H. conceived the project, Z. H. and H. F. D. created the figures, and Z. H., H. F. D., T. A., and G. S. helped gather data and wrote the article text. A public GitHub repository with code used to analyze the data and generate figures and csv files containing the data shown in the figures is available online (https://github.com/hausfath/OldModels). Additional information on the code and data used in the analysis can be found in the supporting information. We would like to thank Piers Forster for providing the ensemble of observationally‐informed radiative forcing estimates. No dedicated funding from any of the authors supported this project.
    Description: 2020-06-04
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  • 20
    Publication Date: 2020-06-17
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in E. J., Donnelly, J. P., van Hengstum, P. J., Wiman, C., Sullivan, R. M., Winkler, T. S., d'Entremont, N. E., Toomey, M., & Albury, N. Intense hurricane activity over the past 1500 years at South Andros Island, the Bahamas. Paleoceanography and Paleoclimatology, 34(11), (2019): 1761-1783, doi:10.1029/2019PA003665.
    Description: Hurricanes cause substantial loss of life and resources in coastal areas. Unfortunately, historical hurricane records are too short and incomplete to capture hurricane‐climate interactions on multi‐decadal and longer timescales. Coarse‐grained, hurricane‐induced deposits preserved in blue holes in the Caribbean can provide records of past hurricane activity extending back thousands of years. Here we present a high resolution record of intense hurricane events over the past 1500 years from a blue hole on South Andros Island on the Great Bahama Bank. This record is corroborated by shorter reconstructions from cores collected at two nearby blue holes. The record contains coarse‐grained event deposits attributable to known historical hurricane strikes within age uncertainties. Over the past 1500 years, South Andros shows evidence of four active periods of hurricane activity. None of these active intervals occurred in the past 163 years. We suggest that Intertropical Convergence Zone position modulates hurricane activity on the island based on a correlation with Cariaco Basin titanium concentrations. An anomalous gap in activity on South Andros Island in the early 13th century corresponds to a period of increased volcanism. The patterns of hurricane activity reconstructed from South Andros Island closely match those from the northeastern Gulf of Mexico but are anti‐phased with records from New England. We suggest that either changes in local environmental conditions (e.g., SSTs) or a northeastward shift in storm tracks can account for the increased activity in the western North Atlantic when the Gulf of Mexico and southeastern Caribbean are less active.
    Description: This work was funded by the National Science Foundation Graduate Research Fellowship (to E.J.W.), National Science Foundation grant OCE‐1356708 (to J.P.D. and P.J.vH.), the Dalio Explore Foundation and the USGS Land Change Science Program (M.R.T.). We are grateful to members of WHOI Coastal Systems Group, in particular Stephanie Madsen, for their help in the processing core samples. We thank two anonymous reviewers, Matthew Lachniet, Marci Robinson (USGS) and Miriam Jones (USGS) for their helpful feedback on earlier versions of this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The data are available on the National Climatic Data Center (http://www.ncdc.noaa.gov/dataaccess/paleoclimatology‐data) and WHOI Coastal Systems Group (https://web.whoi.edu/coastal‐group/) websites.
    Keywords: Paleohurricanes ; carbonate tidal flats ; blue holes ; Andros ; Bahamas
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  • 21
    Publication Date: 2020-06-13
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(1),(2020): e2019JD031619, doi:10.1029/2019JD031619.
    Description: High‐resolution paleoclimate records are essential for improving our understanding of internal variability and the detection and attribution of forced climate system responses. The densely populated northeastern United States is at risk from increasing temperatures, severe droughts, and extreme precipitation, but the region has limited annual and seasonal‐resolution paleoclimate records beyond the instrumental record. Chamaecyparis thyoides, L. (B.S.P.), Atlantic white cedar, a wetland conifer found within 200 km of the Atlantic coastline of the United States, is a promising tree‐ring proxy that can fill in these data gaps. Here, we develop and analyze a new network of Atlantic white cedar tree‐ring chronologies across the northeastern United States and demonstrate that site selection is important for regional paleoclimate reconstructions. Ring width variability reflects winter through summer temperatures at inland and hydrologically stable sites in the northernmost section of the species' range. Ombrotrophic sites along the coast record hydrological signals and correlate with growing season precipitation. We demonstrate skillful regional climate field reconstructions for the last several centuries and show the increased skill from incorporating our moisture sensitive sites into broad‐scale products like the North American Drought Atlas. This comprehensive understanding of the species' climate responses leads to a tree‐ring network that provides the long‐term multivariate climate context at multidecadal and centennial time scales for the large‐scale ocean‐atmospheric processes that influence the climate of the region. We use this network to examine the covariance of temperature and drought across the New England area over the past two centuries.
    Description: This research is funded by the U.S. National Science Foundation Paleo Perspectives on Climate Change program (P2C2; AGS‐1304262 and AGS‐1501856). The authors of this paper thank the many field assistants who helped develop the northeastern AWC network. We thank the 300 Committee Land Trust, Dartmouth National Resources Trust, Orleans Conservation Trust, Marine Biological Laboratory, Trustees of Reservations, National Park Service, U.S. Forest Service, The Nature Conservancy, and private land owners who allowed access to field sites.
    Description: 2020-06-13
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  • 22
    Publication Date: 2020-06-20
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(1),(2020): e2019JC015167, doi:10.1029/2019JC015167.
    Description: Manganese (Mn) is distributed widely throughout the global ocean, where it cycles between three oxidation states that each play important biogeochemical roles. The speciation of Mn in seawater was previously operationally defined on filtration, with soluble Mn presumed to be Mn(II) and solid‐phase Mn as Mn(III/IV) oxides. Recent findings of abundant soluble Mn(III) complexes (Mn(III)‐L) highlights the need to reexamine the redox cycling of Mn, as these complexes can donate or accept electrons. To better understand the complex cycling of Mn in coastal waters, the distribution of Mn species at four Northwest Atlantic sites with different characteristics was examined. Diurnal influences on Mn speciation were investigated within a productive site. At all sites, Mn(III)‐L complexes dominated, particularly in surface waters, and Mn oxides were low in abundance in surface waters but high in bottom waters. Despite intrasite similarities, Mn speciation was highly variable between our stations, emphasizing the diverse processes that impact Mn redox. Diel Mn measurements revealed that the cycling of Mn is also highly variable over time, even on time scales as short as hours. We observed a change of over 100 nM total Mn over 17 hrs and find that speciation changed drastically. These changes could include contributions from biological, light‐mediated, and/or abiotic mechanisms but more likely point to the importance of lateral mixing at coastal sites. This exploration demonstrates the spatial and temporal variability of the Mn redox cycle and indicates that single timepoint vertical profiling is not sufficient when describing the geochemistry of dynamic coastal systems.
    Description: This work was funded by grants from the Chemical Oceanography program of the National Science Foundation (OCE‐1355720 to CMH and CHL). Véronique Oldham thanks Woods Hole Oceanographic Institution for the receipt of the WHOI Postdoctoral Scholarship. Thanks also to Kevin Sutherland, Jen Karolweski, Gabriella Farfan, Kalina Grabb, Kaitlin Bowman, Alison Agather, and Lindsey Starr for the shipboard sampling assistance, as well as the captain and crew of the R/V Endeavor who made the sampling for this research possible. All data presented in the manuscript are available through the Biological and Chemical Oceanography Data Management Office (BCO‐DMO) under Project 756930 at the following link (https://www.bco‐dmo.org/project/756930).
    Description: 2020-06-20
    Keywords: Manganese ; Redox ; Trace Metal ; Diel Cycle ; Coastal Ocean ; Organic Complexation
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  • 23
    Publication Date: 2020-07-06
    Description: Author Posting. © American Geophysical Union, 2020. 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 47(1), (2020): e2019GL085455, doi:10.1029/2019GL085455.
    Description: The meridional coherence, connectivity, and regional inhomogeneity in long‐term sea surface temperature (SST) variability over the Northwest Atlantic continental shelf and slope from 1982–2018 are investigated using observational data sets. A meridionally concurrent large SST warming trend is identified as the dominant signal over the length of the continental shelf and slope between Cape Hatteras in North Carolina and Cape Chidley, Newfoundland and Labrador, Canada. The linear trends are 0.37 ± 0.06 and 0.39 ± 0.06 °C/decade for the shelf and slope regions, respectively. These meridionally averaged SST time series over the shelf and slope are consistent with each other and across multiple longer observational data sets with records dating back to 1900. The coherence between the long‐term meridionally averaged time series over the shelf and slope and basin‐wide averaged SST in the North Atlantic implies approximately two thirds of the warming trend during 1982–2018 may be attributed to natural climate variability and the rest to externally forced change including anthropogenic warming.
    Description: We are grateful to the Editor Dr. Kathleen Donohue and two anonymous reviewers. This work was supported by NOAA's Climate Program Office's Modeling, Analysis, Predictions, and Projections (MAPP) program (NA19OAR4320074). We acknowledge our participation in MAPP's Marine Prediction Task Force. The data of NOAA OISST used in this study are available at NOAA Earth System Research Laboratory (https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.highres.html). The HadISST data set is available at Met Office, Hadley Centre (https://www.metoffice.gov.uk/hadobs/hadisst/). The COBE SST and NOAA ERSST data sets are available at NOAA Earth System Research Laboratory's Physical Sciences Division (https://www.esrl.noaa.gov/psd/data/gridded/data.cobe.html; https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v5.html). The near‐surface air temperature is available at Global Historical Climatology Network‐Monthly Database (https://www.ncdc.noaa.gov/data‐access/land‐based‐station‐data/land‐based‐datasets/global‐historical‐climatology‐network‐monthly‐version‐4). The data of SSH are available at Copernicus Marine Environment Monitoring Service (http://marine.copernicus.eu/services‐portfolio/access‐to‐products/?option=com_csw&view=details&product_id=SEALEVEL_GLO_PHY_ L4_REP_OBSERVATIONS_008_047).
    Description: 2020-07-06
    Keywords: sea surface temperature ; continental shelf ; continental slope ; long‐term change ; Northwest Atlantic
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  • 24
    Publication Date: 2020-07-10
    Description: Author Posting. © American Geophysical Union, 2020. 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 21(2), (2020): e2019GC008414, doi:10.1029/2019GC008414.
    Description: X‐ray fluorescence (XRF) core scanning of marine and lake sediments has been extensively used to study changes in past environmental and climatic processes over a range of timescales. The interpretation of XRF‐derived element ratios in paleoclimatic and paleoceanographic studies primarily considers differences in the relative abundances of particular elements. Here we present new XRF core scanning data from two long sediment cores in the Andaman Sea in the northern Indian Ocean and show that sea level related processes influence terrigenous inputs based proxies such as Ti/Ca, Fe/Ca, and elemental concentrations of the transition metals (e.g., Mn). Zr/Rb ratios are mainly a function of changes in median grain size of lithogenic particles and often covary with changes in Ca concentrations that reflect changes in biogenic calcium carbonate production. This suggests that a common process (i.e., sea level) influences both records. The interpretation of lighter element data (e.g., Si and Al) based on low XRF counts is complicated as variations in mean grain size and water content result in systematic artifacts and signal intensities not related to the Al or Si content of the sediments. This highlights the need for calibration of XRF core scanning data based on discrete sample analyses and careful examination of sediment properties such as porosity/water content for reliably disentangling environmental signals from other physical properties. In the case of the Andaman Sea, reliable extraction of a monsoon signal requires accounting for the sea level influence on the XRF data.
    Description: The staff at the Bremen Core Repository is thanked for their help with core handling and Sam Müller at the University of Kiel provided technical assistance with the XRF scanner. We thank two anonymous reviewers for their insightful comments that improved the manuscript significantly. This work was partially funded through DFG Grant HA 5751/3. P. A. and K. N.‐K. acknowledge support from UK‐IODP and Natural and Environment Research Council, UK. The authors express their thanks to all those who contributed to the success of the National Gas Hydrate Program Expedition 01 (NGHP01) and Expedition 353. The data set supporting the conclusions of this article is available in the PANGEA repository (doi: https://doi.pangaea.de/10.1594/PANGAEA.910533).
    Description: 2020-07-10
    Repository Name: Woods Hole Open Access Server
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  • 25
    Publication Date: 2020-07-10
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(1), (2020): e2019JG005222, doi:10.1029/2019JG005222.
    Description: Wetlands play an important role in reducing global warming potential in response to global climate change. Unfortunately, due to the effects of human disturbance and natural erosion, wetlands are facing global extinction. It is essential to implement engineering measures to restore damaged wetlands. However, the carbon sink capacity of restored wetlands is unclear. We examined the seasonal change of greenhouse gas emissions in both restored wetland and natural wetland and then evaluated the carbon sequestration capacity of the restored wetland. We found that (1) the carbon sink capacity of the restored wetland showed clear daily and seasonal change, which was affected by light intensity, air temperature, and vegetation growth, and (2) the annual daytime (8–18 hr) sustained‐flux global warming potential was −11.23 ± 4.34 kg CO2 m−2 y−1, representing a much larger carbon sink than natural wetland (−5.04 ± 3.73 kg CO2 m−2 y−1) from April to December. In addition, the results showed that appropriate tidal flow management may help to reduce CH4 emission in wetland restoration. Thus, we proposed that the restored coastal wetland, via effective engineering measures, reliably acted as a large net carbon sink and has the potential to help mitigate climate change.
    Description: We would like to thank Yangtze Delta Estuarine Wetland Ecosystem Ministry of Education & Shanghai Observation and Research Station for providing sites during our research. This research was supported by the National Key Research and Development Program of China (Grant 2017YFC0506002), the National Natural Science Foundation of China Overseas and Hong Kong‐Macao Scholars Collaborative Research Fund (Grant 31728003), the China Postdoctoral Science Foundation (Grant 2018M640362), the Shanghai University Distinguished Professor (Oriental Scholars) Program (Grant JZ2016006), the Open Fund of Shanghai Key Lab for Urban Ecological Processes and Eco‐Restoration (Grant SHUES2018B06), and the Scientific Projects of Shanghai Municipal Oceanic Bureau (Grant 2018‐03). The complete data set is available at https://data.4tu.nl/repository/uuid:536b2614‐c4ca‐43d2‐84dd‐6180fd859544.
    Keywords: blue carbon ; restored wetland ; sustained‐flux global warming potential (SGWP) ; greenhouse gas (GHG) ; carbon sequestration capacity
    Repository Name: Woods Hole Open Access Server
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  • 26
    Publication Date: 2020-07-15
    Description: The Antarctic Circumpolar Current (ACC) is the world's largest current system connecting all three major basins of the global ocean. Our knowledge of glacial‐interglacial changes in ACC dynamics in the southeast Pacific is not well constrained and presently only based on reconstructions covering the last glacial cycle. Here we use a combination of mean sortable silt grain size of the terrigenous sediment fraction (10–63 μm, "Sortable Silt") and X‐ray fluorescence scanner‐derived Zr/Rb ratios as flow strength proxies to examine ACC variations at the Pacific entrance to the Drake Passage (DP) in the vicinity of the Subantarctic Front. Our results indicate that at the DP entrance, ACC strength varied by ~6–16% on glacial‐interglacial time scales, yielding higher current speeds during interglacial times and reduced current speeds during glacials. We provide evidence that previous observations of a reduction in DP throughflow during the last glacial period are part of a consistent pattern extending for at least the last 1.3 Ma. The orbital‐scale cyclicity follows well‐known global climate changes from prevailing ca. 41‐kyr cycles in the early part of the record (1.3 Ma to 850 ka; marine isotope stage 21) across the mid‐Pleistocene transition into the middle and late Pleistocene 100‐kyr world. A comparison to a bottom water flow record from the deep western boundary current off New Zealand (Ocean Drilling Program Site 1123) reveals anti‐phased changes between the two sites. The enhanced supply of deep water along the DP and into the Atlantic Ocean during interglacials corresponds to a weakened flow of the SW Pacific deep western boundary current.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 27
    Publication Date: 2020-07-17
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(2), (2020): e2019JC015254, doi:10.1029/2019JC015254.
    Description: The Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) modeling system was used to examine axial wind effects on vertical stratification and sediment transport in a convergent estuary. The model demonstrated that stratification dynamics in the upper estuary (Kelvin number 〈1; Ke= fB/√ g'hs) are dominated by longitudinal wind straining, whereas the dominant mechanism governing estuarine stratification in the lower estuary (Kelvin number ~1) is lateral wind straining. Barotropic advection contributes to seaward sediment transport and peaks during spring tides, whereas estuarine circulation causes landward sediment transport with a maximum during neap tides. Down‐estuary winds impose no obvious effects on longitudinal sediment flux, whereas up‐estuary winds contribute to enhanced seaward sediment flux by increasing the tidal oscillatory flux. The model also demonstrates that bottom friction is significantly influenced by vertical stratification over channel regions, which is indirectly affected by axial winds.
    Description: This research was funded by the National Natural Science Foundation of China (Grants 41576089, 51761135021, and 41890851), the National Key Research and Development Program of China (2016YFC0402603) and the Guangdong Provincial Water Conservancy Science and Technology Innovation Project (Grant 201719). We thank Professor Liangwen Jia at the Sun Yat‐sen University for his kindly providing the surficial sediment samples data in 2011. We also thank graduate students Guang Zhang and Yuren Chen from the Sun Yat‐sen University for their help in data analysis. We are grateful to two anonymous reviewers for their insightful comments to help improve this manuscript. The data related to this article is available online at the Zenodo website (https://zenodo.org/record/3606471).
    Description: 2020-07-17
    Keywords: sediment transport ; vertical stratification ; wind effects ; China, Pearl River Delta ; COAWST model system
    Repository Name: Woods Hole Open Access Server
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  • 28
    Publication Date: 2020-07-24
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(2), (2020): e2019JC015856, doi:10.1029/2019JC015856.
    Description: Summer temperature and velocity measurements from 14 years in 15 m of water over the inner shelf off Oregon were used to investigate interannual temperature variability and the capacity of the across‐shelf heat flux to buffer net surface warming. There was no observable trend in summer mean temperatures, and the standard deviation of interannual variability (0.5°C) was less than the standard deviation in daily temperatures each summer (1.6°C, on average). Yet net surface heat flux provided a nearly constant source of heat each year, with a standard deviation less than 15 urn:x-wiley:jgrc:media:jgrc23812:jgrc23812-math-0001 of the interannual mean. The summer mean across‐shelf upwelling circulation advected warmer water offshore near the surface, cooling the inner shelf and buffering the surface warming. In most years (11 out of 14), this two‐dimensional heat budget roughly closed with a residual less than 20 urn:x-wiley:jgrc:media:jgrc23812:jgrc23812-math-0002 of the leading term. Even in years when the heat budget did not balance, the observed temperature change was negligible, indicating that an additional source of cooling was needed to close the budget. A comparison of the residual to the interannual variability in fields such as along‐shelf wind stress, stratification, and along‐shelf currents found no significant correlation, and further investigation into the intraseasonal dynamics is recommended to explain the results. An improved understanding of the processes that contribute to warming or cooling of the coastal ocean has the potential to improve predictions of the impact of year‐to‐year changes in local winds and circulation, such as from marine heat waves or climate change, on coastal temperatures.
    Description: The authors would like to acknowledge the David and Lucile Packard Foundation and The Gordon and Betty Moore Foundation for their support of the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) mooring program. This paper is PISCO contribution 504. The contributions of A. Kirincich and S. Lentz were supported by National Science Foundation (NSF) Grant OCE‐1558874). E. Lemagie was partially supported by NSF Grant OCE‐1558874 as well as the Woods Hole Oceanographic Institution Postdoctoral Scholars program. Temperature and velocity data were collected and made available by PISCO (www.piscoweb.org). The NDBC and NWPO3 buoy data are freely available from NOAA (www.ndbc.noaa.gov). Surface heat flux reanalyses were download online: ERA5 was accessed through www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5, and NCEP and OAFlux data were downloaded from www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html and http://oaflux.whoi.edu/, respectively.
    Description: 2020-07-24
    Keywords: inner shelf ; heat budget ; temperature ; PISCO ; Oregon coast ; upwelling
    Repository Name: Woods Hole Open Access Server
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  • 29
    Publication Date: 2020-07-31
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(2), (2020): e2019JB018203, doi:10.1029/2019JB018203.
    Description: Cold, low‐density diapirs arising from hydrated mantle and/or subducted sediments on the top of subducting slabs have been invoked to transport key chemical signatures to the source region of arc magmas. However, to date there have been few quantitative models to constrain melting in such diapirs. Here we use a two‐phase Darcy‐Stokes‐energy model to investigate thermal evolution, melting, and depletion in a buoyant sediment diapir ascending through the mantle wedge. Using a simplified 2‐D circular geometry, we investigate diapir evolution in three scenarios with increasing complexity. In the first two scenarios we consider instantaneous heating of a diapir by thermal diffusion with and without the effect of the latent heat of melting. Then, these simplified calculations are compared to numerical simulations that include melting, melt segregation, and the influence of depletion on the sediment solidus along pressure‐temperature‐time (P ‐T ‐t ) paths appropriate for ascent through the mantle wedge. The high boundary temperature induces a rim of high porosity, into which new melts are focused and then migrate upward. The rim thus acts like an annulus melt channel, while the effect of depletion buffers additional melt production. Solid matrix flow combined with recrystallization of melt pooled near the top of the diapir can result in large gradients in depletion across the diapir. These large depletion gradients can either be preserved if the diapir leaks melt during ascent, or rehomogenized in a sealed diapir. Overall our numerical simulations predict less melt production than the simplified thermal diffusion calculations. Specifically, we show that diapirs whose ascent paths favor melting beneath the volcanic arc will undergo no more than ~40–50% total melting.
    Description: We thank careful reviews by Juliane Dannberg, Harro Schmeling, and Bernhard steinberger. This work is supported by NSF‐1316333 (MB & NZ), NSF‐1551023 (MB), NSF‐1316310 (CK), and by China's Thousand Talents Plan (2015) and NSFC‐41674098 funding to NZ. The public data repository of Deal.ii (www.dealii.org) is thanked for distributing the software and examples that are used in this study. Computational work was conducted in High‐performance Computing Platform of Peking University, Kenny cluster of WHOI, and Pawsey Supercomputing Centre of Western Australia. We thank Timo Heister and Juliane Dannberg for deal.II technical assistance. The data of mantle wedge thermal structure and diapir trajectories, and the source code to compute the model results are available in the Mendeley data (http://dx.doi.org/10.17632/73n8zkc68s.1).
    Description: 2020-07-31
    Keywords: sedimentary diapirs ; subduction wedge ; melt migration
    Repository Name: Woods Hole Open Access Server
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  • 30
    Publication Date: 2020-02-10
    Description: An edited version of this paper was published by AGU. Copyright (2016) American Geophysical Union.
    Description: The accuracy of earthquake locations and their correspondence with subsurface geology depends strongly on the accuracy of the available seismic velocity model. Most modern methods to construct a velocity model for earthquake location are based on the inversion of passive source seismological data. Another approach is the integration of high-resolution geological and geophysical data to construct deterministic velocity models in which earthquake locations can be directly correlated to the geological structures. Such models have to be kinematically consistent with independent seismological data in order to provide precise hypocenter solutions. We present the Altotiberina (AT) seismic model, a three-dimensional velocity model for the Upper Tiber Valley region (Northern Apennines, Italy), constructed by combining 300 km of seismic reflection profiles, 6 deep boreholes (down to 5 km depth), detailed data from geological surveys and direct measurements of P- and S-wave velocities performed in situ and in laboratory. We assess the robustness of the AT seismic model by locating 11,713 earthquakes with a non-linear, global-search inversion method and comparing the probabilistic hypocenter solutions to those calculated in three previously published velocity models, constructed by inverting passive seismological data only. Our results demonstrate that the AT seismic model is able to provide higher-quality hypocenter locations than the previous velocity models. Earthquake locations are consistent with the subsurface geological structures and show a high degree of spatial correlation with specific lithostratigraphic units, suggesting a lithological control on the seismic activity evolution.
    Description: Published
    Description: 8113-8135
    Description: 4T. Sismicità dell'Italia
    Description: JCR Journal
    Keywords: deterministic velocity model ; earthquakes ; nonlinear hypocenter location ; lithological control on seismicity ; 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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  • 31
    Publication Date: 2020-08-04
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(2), (2020): e2019JC015220, doi:10.1029/2019JC015220.
    Description: Wave‐supported gravity flows (WSGFs) have been identified as a key process driving the offshore delivery of fine sediment across continental shelves. However, our understanding on the various factors controlling the maximum sediment load and the resulting gravity current speed remains incomplete. We adopt a new turbulence‐resolving numerical model for fine sediment transport to investigate the formation, evolution, and termination of WSGFs. We consider the simplest scenario in which fine sediments are supported by the wave‐induced fluid turbulence at a low critical shear stress of erosion over a flat sloping bed. Under the energetic wave condition reported on the Northern California Coast with a shelf slope of 0.005, simulation results show that WSGFs are transitionally turbulent and that the sediment concentration cannot exceed 30 kg/m urn:x-wiley:jgrc:media:jgrc23843:jgrc23843-math-0001 (g/L) due to the attenuation of turbulence by the sediment‐induced stable density stratification. Wave direction is found to be important in the resulting gravity current intensity. When waves are in cross‐shelf direction, the downslope current has a maximum velocity of 1.2 cm/s, which increases to 2.1 cm/s when waves propagate in the along‐shelf direction. Further analysis on the wave‐averaged momentum balance confirms that when waves are parallel to the slope (cross‐shelf) direction, the more intense wave‐current interaction results in larger wave‐averaged Reynolds shear stress and thus in a smaller current speed. Findings from this study suggest that the more intense cross‐shelf gravity current observed in the field may be caused by additional processes, which may enhance the sediment‐carrying capacity of flow, such as the ambient current or bedforms.
    Description: This study is supported by NSF (OCE‐1537231 and OCE‐1924532) and Office of Naval Research (N00014‐17‐1‐2796). Numerical simulations presented in this study were carried out using the Mills and Canviness clusters at University of Delaware, and the SuperMIC cluster at Louisiana State University via XSEDE (TG‐OCE100015). Z. Cheng would like to express thanks for the support of a postdoctoral scholarship from Woods Hole Oceanographic Institution. The source code and the case setup to reproduce the same results are publicly available via the repository maintained by GitHub: https://github.com/yueliangyi/TURBID (source code) and https://github.com/yueliangyi/TURBID/tree/master/spike/wave_supported_gravity_flow (case setup), respectively.
    Description: 2020-08-04
    Keywords: wave‐supported gravity flows ; turbulence‐resolving numerical simulation ; wave direction ; intermittently turbulent flow
    Repository Name: Woods Hole Open Access Server
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  • 32
    Publication Date: 2020-08-19
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in DeGrandpre, M., Evans, W., Timmermans, M., Krishfield, R., Williams, B., & Steele, M. Changes in the arctic ocean carbon cycle with diminishing ice cover. Geophysical Research Letters, 47(12), (2020): e2020GL088051, doi:10.1029/2020GL088051.
    Description: Less than three decades ago only a small fraction of the Arctic Ocean (AO) was ice free and then only for short periods. The ice cover kept sea surface pCO2 at levels lower relative to other ocean basins that have been exposed year round to ever increasing atmospheric levels. In this study, we evaluate sea surface pCO2 measurements collected over a 6‐year period along a fixed cruise track in the Canada Basin. The measurements show that mean pCO2 levels are significantly higher during low ice years. The pCO2 increase is likely driven by ocean surface heating and uptake of atmospheric CO2 with large interannual variability in the contributions of these processes. These findings suggest that increased ice‐free periods will further increase sea surface pCO2, reducing the Canada Basin's current role as a net sink of atmospheric CO2.
    Description: This research was made possible by grants from the NSF Arctic Observing Network program (ARC‐1107346, PLR‐1302884, PLR‐1504410, and OPP‐1723308). In addition, M. S. was supported by ONR (Grant 00014‐17‐1‐2545), NASA (Grant NNX16AK43G), and NSF (Grants PLR‐1503298 and OPP‐1751363).
    Keywords: Arctic Ocean ; ice concentration ; seawater CO2 ; interannual variability ; Canada Basin ; shipboard CO2 measurements
    Repository Name: Woods Hole Open Access Server
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  • 33
    Publication Date: 2020-08-28
    Description: Author Posting. © American Geophysical Union, 2020. 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 21(3), (2020): e2019GC008847, doi:10.1029/2019GC008847.
    Description: To learn more about magnetic properties of the lower ocean crust and its contributions to marine magnetic anomalies, gabbro samples were collected from International Ocean Discovery Program Hole U1473A at Atlantis Bank on the Southwest Indian Ridge. Detailed magnetic property work links certain magnetic behaviors and domain states to specific magnetic mineral populations. Measurements on whole rocks and mineral separates included magnetic hysteresis, first‐order reversal curves, low‐temperature remanence measurements, thermomagnetic analysis, and magnetic force microscopy. Characteristics of the thermomagnetic data indicate that the upper ~500 m of the hole has undergone hydrothermal alteration. The thermomagnetic and natural remanent magnetization data are consistent with earlier observations from Hole 735B that show remanence arises from low‐Ti magnetite and that natural remanent magnetizations are up to 25 A m−1 in evolved Fe‐Ti oxide gabbros, but are mostly 〈1 A m−1. Magnetite is present in at least three forms. Primary magnetite is associated with coarse‐grained oxides that are more frequent in the upper part of the hole. This magnetic population is linked to dominantly “pseudo‐single‐domain” behavior that arises from fine‐scale lamellar intergrowths within the large oxides. Deeper in the hole the magnetic signal is more commonly dominated by an interacting single‐domain assemblage most likely found along crystal discontinuities in olivine and/or pyroxene. A third contribution is from noninteracting single‐domain inclusions within plagioclase. Because the concentration of the highly magnetic, oxide‐rich gabbros is greatest toward the surface, the signal from coarse oxides will likely dominate the near‐bottom magnetic anomaly signal at Atlantis Bank.
    Description: This work used samples and data provided by the International Ocean Discovery Program. Funding was provided by the U.S. Science Support Program (J.B.). I.L. has benefited from a Smithsonian Edward and Helen Hintz Secretarial Scholarship. We thank the members of the IODP Expedition 360 Science Party, and the captain and crew of the JOIDES Resolution. Part of this work was done as a Visiting Fellow at the Institute for Rock Magnetism (IRM) at the University of Minnesota. The IRM is made possible through the Instrumentation and Facilities program of the National Science Foundation, Earth Sciences Division, and by funding from the University of Minnesota. We would like to thank IRM staff M. Jackson, P. Solheid, and D. Bilardello for their generous assistance. Many thanks to A. Butula, K. Vernon, and J. Marquardt for their assistance with rock magnetic measurements at UWM and to L. McHenry for assistance with XRD. We also thank two anonymous reviewers for their thoughtful comments that improved the manuscript. Magnetic data associated with this manuscript are available in the Magnetics Information Consortium (MagIC) database at https://www.earthref.org/MagIC/doi/10.1029/2019GC008847. XRD data are available at https://zenodo.org/record/3611642.
    Description: 2020-08-28
    Keywords: marine magnetic anomalies ; ocean crust magnetization ; magnetic mineralogy ; IODP ; Expedition 360 ; JOIDES Resolution
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  • 34
    Publication Date: 2020-09-01
    Description: Seismic hazard modeling is a multidisciplinary science that aims to forecast earthquake occurrence and its resultant ground shaking. Such models consist of a probabilistic framework that quantifies uncertainty across a complex system; typically, this includes at least two model components developed from Earth science: seismic source and ground motion models. Although there is no scientific prescription for the forecast length, the most common probabilistic seismic hazard analyses consider forecasting windows of 30 to 50 years, which are typically an engineering demand for building code purposes. These types of analyses are the topic of this review paper. Although the core methods and assumptions of seismic hazard modeling have largely remained unchanged for more than 50 years, we review the most recent initiatives, which face the difficult task of meeting both the increasingly sophisticated demands of society and keeping pace with advances in scientific understanding. A need for more accurate and spatially precise hazard forecasting must be balanced with increased quantification of uncertainty and new challenges such as moving from time‐independent hazard to forecasts that are time dependent and specific to the time period of interest. Meeting these challenges requires the development of science‐driven models, which integrate all information available, the adoption of proper mathematical frameworks to quantify the different types of uncertainties in the hazard model, and the development of a proper testing phase of the model to quantify its consistency and skill. We review the state of the art of the National Seismic Hazard Modeling and how the most innovative approaches try to address future challenges.
    Description: Published
    Description: e2019RG000653
    Description: 6T. Studi di pericolosità sismica e da maremoto
    Description: JCR Journal
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 35
    Publication Date: 2020-09-02
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 36
    Publication Date: 2020-09-07
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(2), (2020): e2019JC015700, doi:10.1029/2019JC015700.
    Description: The formation mechanism as well as its temporal change of the North Pacific subtropical mode water (NPSTMW) is investigated using a 50‐year (1960–2009) ocean general circulation model hindcast. The volume budget analysis suggests that the formation of the NPSTMW is mainly controlled by the air‐sea interaction and ocean dynamics, but there is a regime shift of the relative importance between the two around late‐1980s. While the local air‐sea interaction process is a main driver of the NPSTMW formation prior to late‐1980s, ocean dynamics including the vertical entrainment become dominant since then. The NPSTMW formation is affected by the North Pacific Oscillation simultaneously in the early period, but with a few years lag in the later period. The interdecadal change of the driving mechanism of the interannual variability of the NPSTMW is probably due to the stronger (weaker) influence of local atmospheric forcing in the western North Pacific and unfavorable (favorable) wind stress curl condition for the remote oceanic forcing from the central North Pacific during the former (later) period. This regime shift may be related to the change of centers of the actions of the wind stress curl since the late‐1980s.
    Description: The CORE2 data set was obtained from https://data1.gfdl.noaa.gov/nomads/forms/core/COREv2.html. The World Ocean Atlas 2009 and the Polar Hydrographic Climatology data set were obtained from https://www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html and http://psc.apl.washington.edu/nonwp_projects/PHC/Climatology.html, respectively. The OSCAR data were taken from https://podaac.jpl.nasa.gov/dataset/OSCAR_L4_OC_third‐deg. The database of mixed layer depth is downloaded from http://mixedlayer.ucsd.edu. The data set of the Argo floats was taken from http://uskess.whoi.edu/. The sea surface height data observed by the satellite are available from AVISO (http://www.aviso.altimetry.fr/duacs/). The EN4 data set was downloaded from https://www.metoffice.gov.uk/hadobs/en4/. This study was supported by the National Research Foundation of Korea (NRF) Grant NRF‐2009‐C1AAA001‐0093, funded by the Korea government (MEST). The numerical simulation in this paper was supported by the Supercomputing Center of Korea Institute of Science and Technology Information (KISTI), with its supercomputing resources and technical support (KSC‐2018‐CRE‐0117). Y.‐O. Kwon was funded by National Science Foundation (NSF) EaSM2 OCE‐1242989. Y. H. Kim was partly supported by research projects entitled “Investigation and prediction system development of marine heatwave around the Korean Peninsula originated from the subarctic and western Pacific” (20190344) funded by the Ministry of Oceans and Fisheries (MOF). G. Pak was supported by in‐house projects of the Korea Institute of Ocean Science & Technology (PE99711, PE99811).
    Description: 2020-09-07
    Keywords: ocean general circulation model ; North Pacific subtropical mode water ; Kuroshio Extension ; volume budget ; regime shift ; North Pacific Oscillation
    Repository Name: Woods Hole Open Access Server
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  • 37
    Publication Date: 2020-09-17
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(4), (2020): e2019JC016006, doi:10.1029/2019JC016006.
    Description: Equatorward flow of Middle Atlantic Bight (MAB) shelf waters meets poleward flowing South Atlantic Bight shelf waters over the continental shelf near Cape Hatteras, NC, leading to net export of shelf waters into the deep ocean. This export occurs in close proximity to the Gulf Stream, which separates from the continental margin near Cape Hatteras. Observations from sustained underwater glider surveys of the outer continental shelf and slope north of Cape Hatteras from spring 2017 to spring 2019 are used to examine the mean and variability of MAB shelf water export in the region. The 0.3 Sv (1 Sv = 106 m3 s−1) time‐mean export of MAB shelf water south of 37°N was dominated by discrete export events; 50% of export occurred during the 17% of the time during which transport was more than 1 standard deviation above the mean. These events typically occurred in late spring and summer of both years when equatorward flow into the region peaked. Export of MAB shelf water was correlated with equatorward flow into the region, which was itself correlated with the density gradient across the continental shelf break. Observations during specific time periods that capture extrema in MAB shelf water export are examined to highlight the variability in shelf‐deep ocean exchange scenarios in the Hatteras region. These include near‐surface export driven by hurricanes, subsurface export below the northern edge of the Gulf Stream, and a multi‐month near‐cessation of export.
    Description: Patrick Deane at WHOI and the Instrument Development Group at the Scripps Institution of Oceanography were key to the success of the Spray glider operations. Mike Muglia, Trip Taylor, and Nick DeSimone at the East Carolina University Coastal Studies Institute (CSI) provided support for glider deployments and recoveries. WHOI Summer Student Fellow Devon Gaynes assisted with analysis related to 2017 hurricanes. Spray glider observations used here are available from http://spraydata.ucsd.edu and should be cited using the following DOIs: 10.21238/S8SPRAY2675 (Todd & Owens, 2016) and 10.21238/S8SPRAY0880 (Todd, 2020). Buoy winds are available from the National Data Buoy Center (https://www.ndbc.noaa.gov). SST imagery was obtained from the Mid‐Atlantic Regional Association Coastal Ocean Observing System (MARACOOS) THREDDS server (http://tds.maracoos.org/thredds/ARCHIVE-SST.html). Automated Tropical Cyclone Forecast System data are available online (https://ftp.nhc.noaa.gov/atcf/). PEACH was funded by the National Science Foundation (OCE‐1558521). Colormaps are from Thyng et al. (2016).
    Description: 2020-09-17
    Keywords: Cape Hatteras ; shelf‐deep ocean exchange ; underwater glider
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  • 38
    Publication Date: 2020-09-17
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(4), (2020): e2020JC016046, doi:10.1029/2020JC016046.
    Description: Momentum input from westerly winds blowing over the Southern Ocean can be modulated by mesoscale surface currents and result in changes in large‐scale ocean circulation. Here, using an eddy‐resolving 1/20 degree ocean model configured near Drake Passage, we evaluate the impact of current‐wind interaction on vertical processes. We find a reduction in momentum input from the wind, reduced eddy kinetic energy, and a modification of Ekman pumping rates. Wind stress curl resulting from current‐wind interaction leads to net upward motion, while the nonlinear Ekman pumping term associated with horizontal gradients of relative vorticity induces net downward motion. The spatially averaged mixed layer depth estimated using a density criteria is shoaled slightly by current‐wind interaction. Current‐wind interaction, on the other hand, enhances the stratification in the thermocline below the mixed layer. Such changes have the potential to alter biogeochemical processes including nutrient supply, biological productivity, and air‐sea carbon dioxide exchange.
    Description: The MITgcm can be obtained online (http://mitgcm.org). The geostrophic current product derived from the sea level anomaly can be downloaded in the Copernicus Marine and Environment Monitoring Service of Ssalto/Duacs gridded “allsat” series and along‐track Sea Level Anomalies, Absolute Dynamic Topographies and Geostrophic velocities over the Global Ocean, Mediterranean Sea, Black Sea, European Seas and Acrtic Ocean areas, in Delayed‐Time and in Near‐Real‐Time. Resources supporting this work were provided by the NASA High‐End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center with the award number SMD‐15‐5752. H. S., J. M., and D. J. M. were supported by the NSF MOBY project (OCE‐1048926 and OCE‐1048897). H. S. acknowledges the support by National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF‐2019R1C1C1003663) and Yonsei University Research Fund of 2018‐22‐0053. D. J. M. also gratefully acknowledges NSF and NASA support, along with the Holger W. Jannasch and Columbus O'Donnell Iselin shared chairs for Excellence in Oceanography. H. Seo acknowledges the support from the ONR (N00014‐17‐1‐2398), NOAA (NA10OAR4310376), and the Andrew W. Mellon Foundation Endowed Fund for Innovative Research at WHOI. We also thank two anonymous referees whose comments significantly improved the presentation of results.
    Description: 2020-09-17
    Keywords: Southern Ocean ; eddy‐wind interaction ; Ekman pumping ; stratification ; eddy kinetic energy ; mixed layer depth
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  • 39
    Publication Date: 2020-09-16
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(4), (2020): e2019JC015470, doi:10.1029/2019JC015470.
    Description: This study is to quantify the effects of mesoscale eddies on air‐sea heat fluxes and related air‐sea variables in the South China Sea. Using satellite observations of sea surface temperature (SST) and sea surface height anomaly and a high‐resolution air‐sea heat flux product for the 16‐year period from 2000 to 2015, we conducted the composite patterns of air‐sea fluxes and variables associated with anticyclonic eddies (AEs) and cyclonic eddies (CEs). It is found that the SST‐sea surface height correlations over eddies are not always positive. Only 56% of AEs are corresponded with positive SST anomalies (SSTA), that is, SST+ AEs, and 58% of CEs with negative SSTA, that is, SST− CEs. The percentage of these eddies increases with eddy amplitude and shows slight seasonal variations, higher in winter and lower in summer. Composites of SSTA, air‐sea variables, and fluxes are constructed over all eddies, including both SST+ eddies and SST− eddies. All composites show asymmetric patterns, showing that the centers (where the extrema are located) of the fluxes and variables shift westward and poleward (equatorward) relative to the AEs (CEs) cores. Besides, composites of latent heat flux (LHF), sensible heat flux (SHF), and air temperature show monopole patterns, while composites of wind speed and specific humidity show dipole patterns. For SST+ AEs, the coupling strength is 39.6 ± 6.5 W/m2 (7.2 ± 1.7 W/m2) per degree increase of SSTA for LHF (SHF). For SST− CEs, the coupling strength is 39.0 ± 2.0 W/m2 (9.0 ± 0.96 W/m2) per degree decrease of SSTA for LHF (SHF).
    Description: This research was conducted while Y. Liu was a visiting graduate student at Woods Hole Oceanographic Institution (WHOI). She sincerely thanks the WHOI Academic Programs Office for hosting her visit and is grateful to the support from China Scholarship Council (CSC). This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA19060101), the Key R & D project of Shandong Province (Grant 2019JZZY010102), the Key deployment project of Center for Ocean Mega‐Science, CAS (Grant COMS2019R02), the CAS Program (Grant Y9KY04101L), and the National Natural Science Foundation of China (Grant 41776183 and 41906157). Dr. Xiangze Jin is acknowledged for providing the OAFluxHR analysis and for his programming support and guidance to this study. Heat flux data used in this paper can be downloaded (from https://figshare.com/articles/Eddy‐induced_heat_flux_in_the_South_China_Sea/11949735). AVISO SSH data are downloaded from the website (http://www.aviso.altimetry.fr), OISST from the ftp://eclipse.ncdc.noaa.gov/ site, and OAFluxHR analysis will be available from the project website (http://oaflux.whoi.edu).
    Description: 2020-09-16
    Keywords: mesoscale eddies ; air‐sea coupling ; South China Sea
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  • 40
    Publication Date: 2020-09-21
    Description: We compare differences and similarities in the annual stratospheric HNO3 cycle derived from ground‐based measurements at the South Pole during 1993 and 1995, after correcting an error in earlier published profile retrievals for 1993 which led to under estimation of mixing ratios. The data series presented here provide profiling over the range ∼16–48 km, and cover the fall‐winter‐spring cycle in the behavior of HNO3 in the extreme Antarctic with a large degree of temporal overlap. With the exception of one gap of 20 days, the combined data sets cover a full annual cycle. The record shows an increase in HNO3 above 30 km occurring about 20 days before sunset, which appears to be the result of higher altitude heterogeneous conversion of NOx as photolysis diminishes. Both years show a strong increase in HNO3 beginning about polar sunset, in a layer peaking at about 25 km, as additional NOx is heterogeneously converted to nitric acid. When temperatures drop to the polar stratospheric cloud (PSC) formation range near the end of May, gas phase HNO3 is rapidly reduced in the lower stratosphere, although at least 2–3 weeks of temperatures ≤192 K appear to be required to complete most of the gas‐phase removal at the upper end of the depletion range (22–25 km). Despite a significant difference in residual sulfate loading from the explosion of Mount Pinatubo, there appears to be little gross difference in the timing and effects of PSC formation in removing gas phase HNO3 in these 2 years, though removal may be more rapid in 1995. Incorporation of gas phase HNO3 into PSCs appears to be nearly complete up to ∼25 km by midwinter. We also see a repeat of the formation of gas phase HNO3 in the middle stratosphere in early midwinter of 1995 with about the same timing as in 1993, suggesting that this phenomenon is driven by a repetition of dynamical transport and appropriate temperatures and pressures in the polar night, and not (as has been suggested) by ion‐based heterogeneous chemistry that requires triggering by large relativistic electron fluxes. High‐altitude HNO3 production peaks during a period of ∼20 days, but appears to persist for up to ∼40 days in the 40–45 km range, ceasing well before sunrise. This HNO3 descends rapidly throughout the production period, at a rate in good agreement with theoretically determined midwinter subsidence rates. As noted in earlier studies, later warming of this region above PSC evaporation temperatures does not cause reappearance of large amounts of HNO3, indicating that most PSCs gravitationally sink out of the stratosphere before early spring. We present evidence that smaller PSCs do evaporate to ∼1 to 3.5 ppbv of HNO3 in the lower stratosphere, however, working downward from ∼25 km as temperatures rise during the late winter. There is a delay of ∼15 days after sunrise before photolysis causes significant depletion in the altitude range below ∼30 km, where subsidence has carried virtually all higher‐altitude HNO3 by polar sunrise. Some continued subsidence and photolysis combine to keep mixing ratios less than ∼5 ppbv below 30 km until the final breakdown of the vortex in November brings larger amounts of HNO3 with air from lower latitudes.
    Description: Published
    Description: 17739-17750
    Description: 5A. Ricerche polari e paleoclima
    Description: JCR Journal
    Keywords: ozone depletion ; HNO3 ; Antarctic stratosphere ; 01.01. Atmosphere
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 41
    Publication Date: 2020-09-21
    Description: [1] We present the first intercomparison between the two most comprehensive records of gas‐phase HNO3 profiles in the Antarctic stratosphere, covering the greater part of 1993 and 1995. We compare measurements by the Stony Brook Ground‐Based Millimeter‐wave Spectrometer (GBMS) at the South Pole with Version 5 HNO3 data from the Microwave Limb Sounder (MLS) aboard the Upper Atmospheric Research Satellite. Trajectory tracing was used to select MLS measurements in the 70°–80°S latitude band that sampled air observed by the GBMS during passage over the Pole. When temperatures were near the HNO3 condensation range, additional screening was performed to select MLS measurements that sampled air parcels within 1.5 K of the temperature they experienced over the Pole. Quantitative comparisons are given at 7 different potential temperature levels spanning the range ∼19–30 km. Agreement between the data sets is quite good between 465 and 655 K (∼20–25 km) during a large fraction of the year. Agreement is best during winter and spring, when seasonally averaged differences are generally within 1 ppbv below ∼25 km. At higher altitudes, and during summer and fall, the agreement becomes worse, and GBMS measurements can exceed MLS values by more than 3 ppbv. We provide evidence that differences occurring in the lower stratosphere during fall are due to lack of colocation between the two data sets during a period of strong poleward gradients in HNO3. Remaining discrepancies between GBMS and MLS V5 HNO3 measurements are thought to be due to instrumental or retrieval biases.
    Description: Published
    Description: id 4809
    Description: 5A. Ricerche polari e paleoclima
    Description: JCR Journal
    Keywords: MLS ; Nitric acid ; polar stratosphere ; 01.01. Atmosphere
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 42
    Publication Date: 2020-09-28
    Description: Author Posting. © American Geophysical Union, 2020. 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 47 (2020): e2019GL086813, doi: 10.1029/2019GL086813.
    Description: Mixing efficiency is an important turbulent flow property in fluid dynamics, whose variability potentially affects the large‐scale ocean circulation. However, there are several confusing definitions. Here we compare and contrast patch‐wise versus bulk estimates of mixing efficiency in the abyss by revisiting data from previous extensive field surveys in the Brazil Basin. Observed patch‐wise efficiency is highly variable over a wide range of turbulence intensity. Bulk efficiency is dominated by rare extreme turbulence events. In the case where enhanced near‐bottom turbulence is thought to be driven by breaking of small‐scale internal tides, the estimated bulk efficiency is 20%, close to the conventional value of 17%. On the other hand, where enhanced near‐bottom turbulence appears to be convectively driven by hydraulic overflows, bulk efficiency is suggested to be as large as 45%, which has implications for a further significant role of overflow mixing on deep‐water mass transformation.
    Description: TI is a JSPS Overseas Research Fellow. LS, KLP, and JMT acknowledge support from the U.S. National Science Foundation and Office of Naval Research. The authors express their gratitude to Ali Mashayek and an anonymous reviewer for their useful comments on the original manuscript. Data used in this study is available from the Woods Hole Open Access Server (https://hdl.handle.net/1912/25456).
    Description: 2020-09-28
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  • 43
    Publication Date: 2020-09-28
    Description: Author Posting. © American Geophysical Union, 2020. 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 47(7), (2020): e2020GL087405, doi:10.1029/2020GL087405.
    Description: The origin and distribution of the gabbroic bodies provide crucial information to understand the formation and evolution processes of the oceanic core complexes (OCCs). Nevertheless, images of the shape of the gabbroic bodies across the domes and gabbroic intrusion into the mantle have remained elusive. High‐resolution acoustic early‐arrival full waveform inversion tomography models obtained along and across the Kane OCC characterize the detailed lateral variability in structure and composition of the upper ~2 km of this well‐developed OCC. Reverse time migration images show the gabbroic plutons embedded in mantle rocks are seismically transparent, while more reflective sections correspond to the layered magmatic crust. Lithological interpretation shows heterogeneous distribution of gabbroic bodies within the Kane OCC, indicating strong spatial and temporal variability in magmatism during fault exhumation. Our results will also be of high value for future scientific ocean drilling efforts in the area.
    Description: Seismic data acquisition was funded by NSF Grant OCE99‐87004. Data files can be obtained from Interdisciplinary Earth Data Alliance (IEDA) (https://doi.org/10.1594/IEDA/314508) (Tucholke & Collins, 2014). The velocity models and migrated seismic sections shown in the paper are freely available for download from 4TU. Centre for Research Data (doi:10.4121/uuid:3ef55160-4a5a-4d1a-b734-fe2b8d2871ae). Full waveform inversion was performed with the software TomoPlus (GeoTomo LLC) licensed to SCSIO. This research was supported by the National Natural Science Foundation of China (41676044 and 91858207) and Special Foundation for National Science and Technology Basic Research Program of China (2018FY100505). M. X. acknowledges supports from Guangdong NSF research team project (2017A030312002), K. C. Wong Education Foundation (GJTD‐2018‐13), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (GML2019ZD0205), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA13010105). J. P. C. acknowledges support from the Independent Research and Development Program at WHOI. J. P. Wang and X. R. Mu from China University of Petroleum are thanked for helping with the RTM setup.
    Description: 2020-09-28
    Keywords: oceanic core complex ; detachment faulting ; seismic structure ; full waveform inversion ; reverse time migration ; lithology
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  • 44
    Publication Date: 2020-09-27
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(4), (2020): e2019JC015544, doi:10.1029/2019JC015544.
    Description: The Radium Delayed Coincidence Counter (RaDeCC) is one of the most extensively used equipment for measuring 223Ra and 224Ra activities in water and sediment samples. Samples are placed in a closed He‐circulation system that carries the Rn produced by the decay of Ra to a scintillation cell. Each alpha decay recorded in the cell is routed to an electronic delayed coincidence system which enables the discrimination of 223Ra and 224Ra. In this study, the measurement and quantification methods using the RaDeCC system are assessed through analyses of registered data in different RaDeCC systems worldwide and a set of simulations. Results of this work indicate that the equations used to correct for 223Ra and 224Ra cross‐talk interferences are only valid for a given range of activities and ratios between isotopes. Above certain limits that are specified in this study, these corrections may significantly overestimate the quantification of 223Ra and 224Ra activities (up to ~40% and 30%, respectively), as well as the quantification of their parents 227Ac and 228Th. High activities of 226Ra may also produce an overestimation of 224Ra activities due to the buildup of 222Rn, especially when long measurements with low activities of 224Ra are performed. An improved method to quantify 226Ra activities from the buildup of 222Rn with the RaDeCC system is also developed in this study. Wethus provide a new set of guidelines for the appropriate quantification of 223Ra, 224Ra, 227Ac, 228Th, and 226Ra with the RaDeCC system.
    Description: The authors acknowledge the support from the Generalitat de Catalunya autonomous government through its funding schema to excellence research groups (grants 2017 SGR 1588 and 2014 SGR 1356) and the support from Spanish Government (projects CGL2013‐48869‐C2‐1‐R/2‐R and CGL2016‐77122‐C2‐1‐576 R/2‐R). We would like to thank all the people who contributed to this work sharing the data of their RaDeCC systems, including J. Scholten, C. Claude, M.A. Charette, J.K. Cochran, and R. Neuholz. We want to express our gratitude to our colleagues from the Laboratori de Radioactivitat Ambiental (Universitat Autònoma de Barcelona) and Dr. W. Geibert (AWI) for improving the quality of this work. A. Alorda‐Kleinglass acknowledges financial support from ICTA “Unit of Excellence” (MinECo, MDM2015‐0552‐17‐1)PhD fellowship, BES‐2017‐080740. Dr. V. Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya autonomous government (2017‐BP‐00334). P. van Beek thanks the support from ANR (MED‐SGD project, ANR‐15‐CE01‐0004). M. Diego‐Feliu acknowledges the economic support from the FI‐2017 fellowships of the Generalitat de Catalunya autonomous government (2017FI_B_00365). Compliance with AGU's DataPolicy: All the https://data.mendeley.com/datasets/jtct7mt8zr/2 codes and spreadsheets used in this article are provided online (supplementary material).
    Description: 2020-09-27
    Keywords: RaDeCC ; Ra isotopes ; quantification ; U/Th series ; submarine groundwater discharge
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  • 45
    Publication Date: 2020-10-08
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(5), (2020): e2019JC015920, doi:10.1029/2019JC015920.
    Description: A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river‐influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high‐resolution pan‐Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25–50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle‐reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv (106 m3 s−1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.
    Description: Funding for Arctic GEOTRACES was provided by the U.S. National Science Foundation, Swedish Research Council Formas, French Agence Nationale de la Recherche and LabexMER, Netherlands Organization for Scientific Research, and Independent Research Fund Denmark. Data from GEOTRACES cruises GN01 (HLY1502) and GN04 (PS94) have been archived at the Biological and Chemical Oceanography Data Management Office (Biological and Chemical Oceanography Data Management Office (BCO‐DMO); https://www.bco-dmo.org/deployment/638807) and PANGAEA (https://www.pangaea.de/?q=PS94&f.campaign%5B%5D=PS94) websites, respectively. The inorganic carbon data are available at the NOAA Ocean Carbon Data System (OCADS; doi:10.3334/CDIAC/OTG.CLIVAR_ARC01_33HQ20150809).
    Description: 2020-10-08
    Keywords: Arctic Ocean ; Transpolar Drift ; trace elements ; carbon ; nutrients ; GEOTRACES]
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  • 46
    Publication Date: 2020-10-04
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(5), (2020): e2019JC015989, doi:10.1029/2019JC015989.
    Description: Relatively minor amounts of methane, a potent greenhouse gas, are currently emitted from the oceans to the atmosphere, but such methane emissions have been hypothesized to increase as oceans warm. Here, we investigate the source, distribution, and fate of methane released from the upper continental slope of the U.S. Mid‐Atlantic Bight, where hundreds of gas seeps have been discovered between the shelf break and ~1,600 m water depth. Using physical, chemical, and isotopic analyses, we identify two main sources of methane in the water column: seafloor gas seeps and in situ aerobic methanogenesis which primarily occurs at 100–200 m depth in the water column. Stable isotopic analyses reveal that water samples collected at all depths were significantly impacted by aerobic methane oxidation, the dominant methane sink in this region, with the average fraction of methane oxidized being 50%. Due to methane oxidation in the deeper water column, below 200 m depth, surface concentrations of methane are influenced more by methane sources found near the surface (0–10 m depth) and in the subsurface (10–200 m depth), rather than seafloor emissions at greater depths.
    Description: This research was supported by DOE Grant (DE‐FE0028980) to J. K. and by DOE‐USGS Interagency Agreement DE‐FE0026195.
    Description: 2020-10-04
    Keywords: methane ; ocean ; isotopes ; gas seeps ; mid Atlantic bight ; oxidation
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  • 47
    Publication Date: 2020-10-04
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(4), (2020): e2019JG005158, doi:10.1029/2019JG005158.
    Description: Long‐term soil warming can decrease soil organic matter (SOM), resulting in self‐reinforcing feedback to the global climate system. We investigated additional consequences of SOM reduction for soil water holding capacity (WHC) and soil thermal and hydrological buffering. At a long‐term soil warming experiment in a temperate forest in the northeastern United States, we suspended the warming treatment for 104 days during the summer of 2017. The formerly heated plot remained warmer (+0.39 °C) and drier (−0.024 cm3 H2O cm−3 soil) than the control plot throughout the suspension. We measured decreased SOM content (−0.184 g SOM g−1 for O horizon soil, −0.010 g SOM g−1 for A horizon soil) and WHC (−0.82 g H2O g−1 for O horizon soil, −0.18 g H2O g−1 for A horizon soil) in the formerly heated plot relative to the control plot. Reduced SOM content accounted for 62% of the WHC reduction in the O horizon and 22% in the A horizon. We investigated differences in SOM composition as a possible explanation for the remaining reductions with Fourier transform infrared (FTIR) spectra. We found FTIR spectra that correlated more strongly with WHC than SOM, but those particular spectra did not differ between the heated and control plots, suggesting that SOM composition affects WHC but does not explain treatment differences in this study. We conclude that SOM reductions due to soil warming can reduce WHC and hydrological and thermal buffering, further warming soil and decreasing SOM. This feedback may operate in parallel, and perhaps synergistically, with carbon cycle feedbacks to climate change.
    Description: We would like to acknowledge Jeffery Blanchard, Priya Chowdhury, Kristen DeAngelis, Luiz Dominguez‐Horta, Kevin Geyer, Rachelle Lacroix, Xaiojun Liu, William Rodriguez, and Alexander Truchonand and for assistance with field sampling. We would like to acknowledge Michael Bernard for assistance with field sampling and lab work. We would like to acknowledge Aaron Ellison for statistical consultation. This research was financially supported by the U.S. National Science Foundation's Long Term Ecological Research Program (NSF‐DEB‐0620443 and NSF‐DEB‐1237491), the Long Term Research in Environmental Biology Program (NSF DEB‐1456528) , and the U.S. Department of Energy (DOE‐DE‐SC0005421 and DOE‐DE‐SC0010740). Data used in this study are available from the Harvard Forest Data Archive (Datasets HF018‐03, HF018‐04, and HF018‐13), accessible at https://harvardforest.fas.harvard.edu/harvard‐forest‐data‐archive.
    Description: 2020-10-04
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  • 48
    Publication Date: 2020-10-06
    Description: Author Posting. © American Geophysical Union, 2020. 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 21(4), (2020): e2020GC008953, doi:10.1029/2020GC008953.
    Description: Earthquakes near oceanic trenches are important for studying incoming plate bending and updip thrust zone seismogenesis, yet are poorly constrained using seismographs on land. We use an ocean bottom seismograph (OBS) deployment spanning both the incoming Pacific Plate and the forearc to study seismicity near the Mariana Trench. The yearlong deployment in 2012–2013 consisted of 20 broadband OBSs and 5 suspended hydrophones, with an additional 59 short period OBSs and hydrophones recording for 1 month. We locate 1,692 earthquakes using a nonlinear method with a 3D velocity model constructed from active source profiles and surface wave tomography results. Events occurring seaward of the trench occur to depths of ~35 km below the seafloor, and focal mechanisms of the larger events indicate normal faulting corresponding to plate bending. Significant seismicity emerges about 70 km seaward from the trench, and the seismicity rate increases continuously towards the trench, indicating that the largest bending deformation occurs near the trench axis. These plate‐bending earthquakes occur along faults that facilitate the hydration of the subducting plate, and the lateral and depth distribution of earthquakes is consistent with low‐velocity regions imaged in previous studies. The forearc is marked by a heterogeneous distribution of low magnitude (〈5 Mw) thrust zone seismicity, possibly due to the rough incoming plate topography and/or serpentinization of the forearc. A sequence of thrust earthquakes occurs at depths ~10 km below seafloor and within 20 km of the trench axis, demonstrating that the megathrust is seismically active nearly to the trench.
    Description: We thank the captains, crew, and science teams on the R/V Thompson, Langseth and Melville, Dr. Patrick Shore for providing data management and technical support, and Ivan Komarov and Zhengyang Zhou for assistance with data analysis. We thank Ingo Grevemeyer and an anonymous reviewer for their comments to improve the manuscript. Instrumentation and technical support was provided by the PASSCAL program of the Incorporated Research Institutions in Seismology (IRIS) and the Woods Hole, Lamont‐Doherty, and Scripps facilities of the Ocean Bottom Seismograph Instrumentation Pool (OBSIP). Funding was provided by the MARGINS/GeoPRISMS program through NSF grant OCE‐0841074 (D.A.W.) and the Spencer T. and Ann W. Olin Fellowship program at Washington University in Saint Louis. Raw seismic data used in this study are available through the Data Management Center of the Incorporated Research Institutions for Seismology (http://www.iris.edu/dms/nodes/dmc) under network IDs XF and MI.
    Description: 2020-10-06
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  • 49
    Publication Date: 2020-10-06
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(5), (2020): e2019JB018632, doi:10.1029/2019JB018632.
    Description: Carbonate‐altered peridotite are common in continental and oceanic settings and it has been suggested that peridotite‐hosted carbonate represent a significant component of the carbon‐cycle and provide an important link in the CO2 dynamics between the atmosphere, hydrosphere, and lithosphere. The ability to constrain the timing of carbonate and accessory phase growth is key to interpreting the mechanisms that contribute to carbonate alteration, veining, and mineralization in ultramafic rocks. Here we examine a mantle section of the Samail ophiolite exposed in Wadi Fins in southeastern Oman where the peridotite is unconformably overlain by Late Cretaceous‐Paleogene limestone and crosscut by an extensive network of carbonate veins and fracture‐controlled alteration. Three previous 87Sr/86Sr measurements on carbonate vein material in the peridotite produce results consistent with vein formation involving Cretaceous to Eocene seawater (de Obeso & Kelemen, 2018, https://doi.org/10.1098/rsta.2018.0433). We employ (U‐Th)/He chronometry to constrain the timing of hydrothermal magnetite in the calcite veins in the peridotite. Magnetite (U‐Th)/He ages of crystal sizes ranging from 1 cm to 200 μm record Miocene growth at 15 ± 4 Ma, which may indicate (1) fluid–rock interaction and carbonate precipitation in the Miocene, or (2) magnetite (re)crystallization within pre‐existing veins. Taken together with published Sr‐isotope values, these results suggest that carbonate veining at Wadi Fins started as early as the Cretaceous, and continued in the Miocene associated with magnetite growth. The timing of hydrothermal magnetite growth is coeval with Neogene shortening and faulting in southern Oman, which points to a tectonic driver for vein (re)opening and fluid‐rock alteration.
    Description: This research was supported by a National Science Foundation (NSF) Graduate Research Fellowship to E.H.G. Cooperdock, the UTChron Laboratory at The University of Texas at Austin, the Chevron (Gulf) Centennial Professorship to D.F. Stockli, and by a Sloan Foundation grant awarded to P.B. Kelemen. We are grateful to Desmond Patterson for assistance and training with He measurements and data reduction, to Jessie Maisano for technical support with the X‐Ray Computed Tomography. These data and images were produced at the High‐Resolution X‐ray Computed Tomography Facility of the University of Texas at Austin. EHGC is grateful to Jaime Barnes, Richard Ketcham, Frieder Klein and Othmar Müntener for helpful comments on an earlier version of this manuscript. Thank you to Fin Stuart and Uwe Ring for their helpful reviews, and Stephen Parman for feedback and editorial handling of the manuscript. The (U‐Th)/He data in this manuscript are available in the GeoChron repository (https://www.geochron.org) and sample IGSNs are in the SESAR database (http://www.geosamples.org).
    Description: 2020-10-06
    Keywords: magnetite ; U‐Th/He thermochronology ; ophicarbonate ; Oman ; Wadi Fins ; serpentinite
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  • 50
    Publication Date: 2020-10-21
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(5), (2020): e2019JC015377, doi:10.1029/2019JC015377.
    Description: Internal waves strongly influence the physical and chemical environment of coastal ecosystems worldwide. We report novel observations from a distributed temperature sensing (DTS) system that tracked the transformation of internal waves from the shelf break to the surf zone over a narrow shelf slope region in the South China Sea. The spatially continuous view of temperature fields provides a perspective of physical processes commonly available only in laboratory settings or numerical models, including internal wave reflection off a natural slope, shoreward transport of dense fluid within trapped cores, and observations of internal rundown (near‐bed, offshore‐directed jets of water preceding a breaking internal wave). Analysis shows that the fate of internal waves on this shelf—whether transmitted into shallow waters or reflected back offshore—is mediated by local water column density structure and background currents set by the previous shoaling internal waves, highlighting the importance of wave‐wave interactions in nearshore internal wave dynamics.
    Description: We are grateful for the support of the Dongsha Atoll Research Station (DARS) and the Dongsha Atoll Marine National Park, whose efforts made this research possible. The authors would also like to thank A. Hall, S. Tyler, and J. Selker from the Center for Transformative Environmental Monitoring Programs (CTEMPs) funded by the National Science Foundation (EAR awards 1440596 and 1440506), G. Lohmann from WHOI, A. Safaie from UC Irvine, G. Wong, L. Hou, F. Shiah, and K. Lee from Academia Sinica for providing logistical and field support, as well as E. Pawlak, S. Lentz, B. Sanders, and S. Grant for equipment, and B. Raubenheimer, S. Elgar, R. Walter and D. Lucas for informative discussions that improved this work. We acknowledge the US Army Research Laboratory DoD Supercomputing Resource Center for computer time on Excalibur, which was used for the numerical simulations in this work. Funding for this work supported by Academia Sinica and for K.D. and E.R. from NSF‐OCE 1753317 and for O.F., J.R., and R.A. from ONR Grant 1182789‐1‐TDZZM. A portion of this work (R.A.) was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE‐AC52‐07NA27344.
    Description: 2020-10-21
    Keywords: internal waves ; distributed temperature sensing ; coral reef ; internal wave reflection
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  • 51
    Publication Date: 2020-10-23
    Description: Author Posting. © American Geophysical Union, 2020. 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 47 (2020): e2020GL087669, doi:10.1029/2020GL087669.
    Description: We present a year‐round time series of dissolved methane (CH4), along with targeted observations during ice melt of CH4 and carbon dioxide (CO2) in a river and estuary adjacent to Cambridge Bay, Nunavut, Canada. During the freshet, CH4 concentrations in the river and ice‐covered estuary were up to 240,000% saturation and 19,000% saturation, respectively, but quickly dropped by 〉100‐fold following ice melt. Observations with a robotic kayak revealed that river‐derived CH4 and CO2 were transported to the estuary and rapidly ventilated to the atmosphere once ice cover retreated. We estimate that river discharge accounts for 〉95% of annual CH4 sea‐to‐air emissions from the estuary. These results demonstrate the importance of resolving seasonal dynamics in order to estimate greenhouse gas emissions from polar systems.
    Description: All data generated by the authors that were used in this article are available on PANGAEA (https://doi.org/10.1594/PANGAEA.907159) and model code for estimating CH4 transport is available on GitHub (https://doi.org/10.5281/zenodo.3785893). We acknowledge the use of imagery from the NASA Worldview application (https://worldview.earthdata.nasa.gov), part of the NASA Earth Observing System Data and Information System (EOSDIS), and data from Ocean Networks Canada, and Environment Canada. We thank everyone involved in the fieldwork including C. Amegainik, Y. Bernard, A. Cranch, F. Emingak, S. Marriott, and A. Pedersen. Laboratory analysis and experiments were performed by A. Cranch, R. McCulloch, A. Morrison, and Z. Zheng. We thank J. Brinckerhoff, the Arctic Research Foundation, and the staff of the Canadian High Arctic Research Station for support with field logistics. Funding for the work was provided by MEOPAR NCE funding to B. Else, a WHOI Interdisciplinary Award to A. Michel., D. Nicholson. and S. Wankel, and Canadian NSERC grants to P. Tortell. and B. Else. Authors received fellowships, scholarships, and travel grants including an NSERC postdoctoral fellowship to C. Manning, an NDSEG fellowship to V. Preston, NSERC PGS‐D and Izaak Walton Killam Pre‐Doctoral scholarships to S. Jones, and Northern Scientific Training Program funds (Polar Knowledge Canada, administered by the Arctic Institute of North America, University of Calgary) to S. Jones and P. Duke. We also thank Polar Knowledge Canada (POLAR) and Nunavut Arctic College for laboratory space and field logistics support.
    Description: 2020-10-23
    Keywords: greenhouse gases ; biogeochemistry ; Arctic coastal waters ; biogeochemical sensing ; seasonal cycles ; methane
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  • 52
    Publication Date: 2020-10-23
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(5), (2020): e2020JC016123, doi:10.1029/2020JC016123.
    Description: The processes underlying the strong Kuroshio encountering a cape at the southernmost tip of Taiwan are examined with satellite‐derived chlorophyll and temperature maps, a drifter trajectory, and realistic model simulations. The interaction spurs the formation of submesoscale cyclonic eddies that trap cold and high‐chlorophyll water and the formation of frontal waves between the free stream and the wake flow. An observed train of eddies, which have relative vorticity about one to four times the planetary vorticity (f), is shed from the recirculation that occurs in the immediate lee of the cape as a result of flow separation. These propagate downstream at a speed of 0.5–0.6 m s−1. Farther downstream, the corotation and merging of two or three adjacent eddies are common owing to the topography‐induced slowdown of eddy propagation farther downstream. It is found that the relative vorticity of a corotating system (1.2f) is 70% weaker than that of a single eddy due to the increase of eddy diameter from ~16 to ~33 km, in agreement with Kelvin's circulation theorem. The shedding period of the submesoscale eddies is strongly modulated by either diurnal or semidiurnal tidal flows, which typically reach 0.2–0.5 m s−1, whereas its intrinsic shedding period is insignificant. The frontal waves predominate in the horizontal free shear layer emitted from the cape, as well as a density front. Energetics analysis suggests that the wavy features result primarily from the growth of barotropic instability in the free shear layer, which may play a secondary process in the headland wake.
    Description: Yu‐Hsin Cheng was supported by the CWB of Taiwan through Grant 1062076C. Ming‐Huei Chang was supported by the Ministry of Science and Technology of Taiwan (MOST) under Grants 103‐2611‐M‐002‐018, 105‐2611‐M‐002‐012, and 107‐2611‐M‐002‐015. Sen Jan was supported with MOST Grants 101‐2611‐M‐002‐018‐MY3, 103‐2611‐M‐002‐011, and 105‐2119‐M‐002‐042. Magdalena Andres was supported by the U.S. Office of Naval Research Grant N000141613069.
    Description: 2020-10-23
    Keywords: Kuroshio ; submesoscale eddy ; headland ; recirculation ; eddy corotation ; barotropic instability
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  • 53
    Publication Date: 2020-10-20
    Description: Author Posting. © American Geophysical Union, 2020. 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: Space Physics 125(7), (2020): e2019JA027160, doi:10.1029/2019JA027160.
    Description: This paper reveals unprecedented periodicity in the voltage series of relative ionospheric opacity meters (riometers) of the Canadian Riometer Array (CRA). In quiet times, the riometer voltage series is accurately modeled by a stochastic process whose components include both a six term expansion in harmonic functions and some amplitude modulated modes of lower signal to noise ratio (SNR). In units of cycles per sidereal day (cpsd), the frequencies of the six harmonic functions lie within 0.01 cpsd of an integer. Earth's rotation induces a splitting of the low SNR components, resulting in the appearance of nine multiplets in standardized power spectrum estimates of the considered CRA voltage series. A second feature of these spectrum estimates is a 6 min periodic element appearing in both the CRA voltage series and the proton mass density series of the Advanced Composition Explorer (ACE). Spectral peak frequencies have been detected, which lie near established solar mode frequency estimates. In addition, some of these peak frequency estimates are coincident with peak frequency estimates of the standardized power spectra for the time series of proton mass density and interplanetary magnetic field strength (IMF) at ACE.
    Description: “Marshall_Francois_Supporting_Information_JGR_2019.pdf” contains a summary of the supporting information. The 1 hr sampled F10.7 series was obtained from DRAO (National Research Council, 2017). The three MAG time series of IMF strength were acquired from The ACE Science Center (2007), while the SWEPAM time series of proton mass density was acquired from Space Weather Prediction Center, National Oceanic and Atmospheric Administration (2018). The relevant data sets for the analysis of this paper are included in Marshall (2019). This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canadian Statistical Sciences Institute (CANSSI), Bonneyville Power Authority, and Queen's University. David J. Thomson, the official holder of the grants and contracts, provided research and conference funding to advance this project. Special thanks to Ken F. Tapping (DRAO of NRCan) for his guidance in finding the data sets relevant to solar radio emissions. Glen Takahara, of the Department of Mathematics and Statistics at Queen's University, suggested exploring different data sets to confirm the modal origin of spectral peaks observed in the Ottawa riometer of the CRA. Alessandra A. Pacini of the Arecibo Observatory recommended checking to see if some of the modes could have been driven by the harmonics of Earth's rotation. Frank Vernon of the Institute of Geophysics and Planetary Physics at Scripps Institution of Oceanography confirmed how seismic data could be expected to reveal coincident spectral peaks at the detected frequencies in the riometer standardized spectra.
    Description: 2020-10-20
    Keywords: periodic elements ; riometer ; multitaper spectral analysis ; cosmic noise ; sidereal day ; solar modes
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  • 54
    Publication Date: 2020-10-29
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(5), (2020): e2019JC015348, doi:10.1029/2019JC015348.
    Description: Here we present an assessment of eddy activity in a 3,500 × 2,000 km region of the North Pacific. Eddies were identified and tracked within a numerical simulation that used the Massachusetts Institute of Technology general circulation model and an eddy characterization algorithm. Spatially, eddy births were more frequent: (1) nearshore (cyclones) and offshore (anticyclones) on the windward side of the main Hawai‘ian Islands; (2) in patches of cyclones and anticyclones that resembled the dipole structure of wind stress curl along the islands’ leeward side; and (3) in zonal patches of eddies of both polarities west and north of the islands. Temporally, high eddy activities occurred in spring. There was a meridional distribution of eddy lifespans, which increased northward. Cyclones were more abundant, longer‐lived, smaller, and more nonlinear. Reef fish spawning locations in Hawai‘i coincide with the regions of high eddy activity, with nonlinear eddies responsible for high larval retention.
    Description: This work was supported by the National Ocean and Atmospheric Administration (NOAA) Fisheries And The Environment (FATE) Award WE133F17SE1020. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation Grant NSF‐OCE170005.
    Description: 2020-10-29
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  • 55
    Publication Date: 2020-10-27
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(5), (2020): e2019JC016007, doi:10.1029/2019JC016007.
    Description: Benthic inputs of nutrients help support primary production in the Chukchi Sea and produce nutrient‐rich water masses that ventilate the halocline of the western Arctic Ocean. However, the complex biological and redox cycling of nutrients and trace metals make it difficult to directly monitor their benthic fluxes. In this study, we use radium‐228, which is a soluble radionuclide produced in sediments, and a numerical model of an inert, generic sediment‐derived tracer to study variability in sediment inputs to the Chukchi Sea. The 228Ra observations and modeling results are in general agreement and provide evidence of strong benthic inputs to the southern Chukchi Sea during the winter, while the northern shelf receives higher concentrations of sediment‐sourced materials in the spring and summer due to continued sediment‐water exchange as the water mass traverses the shelf. The highest tracer concentrations are observed near the shelfbreak and southeast of Hanna Shoal, a region known for high biological productivity and enhanced benthic biomass.
    Description: This study presents data from multiple Arctic expeditions over the past two decades, and we are indebted to the captains, crews, and scientific parties that made this data collection possible. This work was funded by NSF awards OCE‐1458305 to M. Charette, OCE‐1458424 to W. Moore, OCE‐1434085 to D. Kadko, PLR‐1504333 to R. Pickart, and OPP‐1822334 to M. Spall. Funding was also provided by National Oceanic and Atmospheric Administration Grant NA14‐OAR4320158 to R. Pickart. L. Kipp was supported by an Ocean Frontier Institute Postdoctoral Fellowship. Radium data used in this manuscript are available in Table S1.
    Description: 2020-10-27
    Keywords: Chukchi Sea ; benthic flux ; radium‐228 ; GEOTRACES
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  • 56
    Publication Date: 2020-11-11
    Description: Author Posting. © American Geophysical Union, 2020. 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 21(6), (2020): e2020GC008957, doi:10.1029/2020GC008957.
    Description: At the Galapagos triple junction in the equatorial Pacific Ocean, the Cocos‐Nazca spreading center does not meet the East Pacific Rise (EPR) but, instead, rifts into 0.4 Myr‐old lithosphere on the EPR flank. Westward propagation of Cocos‐Nazca spreading forms the V‐shaped Galapagos gore. Since ~1.4 Ma, opening at the active gore tip has been within the Cocos‐Galapagos microplate spreading regime. In this paper, bathymetry, magnetic, and gravity data collected over the first 400 km east of the gore tip are used to examine rifting of young lithosphere and transition to magmatic spreading segments. From inception, the axis shows structural segmentation consisting of rifted basins whose bounding faults eventually mark the gore edges. Rifting progresses to magmatic spreading over the first three segments (s1–s3), which open between Cocos‐Galapagos microplate at the presently slow rates of ~19–29 mm/year. Segments s4–s9 originated in the faster‐spreading (~48 mm/year) Cocos‐Nazca regime, and well‐defined magnetic anomalies and abyssal hill fabric close to the gore edges show the transition to full magmatic spreading was more rapid than at present time. Magnetic lineations show a 20% increase in the Cocos‐Nazca spreading rate after 1.1 Ma. The near‐axis Mantle Bouguer gravity anomaly decreases eastward and becomes more circular, suggesting mantle upwelling, increasing temperatures, and perhaps progression to a developed melt supply beneath segments. Westward propagation of individual Cocos‐Nazca segments is common with rates ranging between 12 and 54 mm/year. Segment lengths and lateral offsets between segments increase, in general, with distance from the tip of the gore.
    Description: E. M. and H. S. are grateful to the National Science Foundation for funding this work and to InterRidge and the University of Leeds for providing support for a number of the international students and scholars who were able to participate on the cruise. We are also grateful for the extraordinary work of the Captain and crew of R/V Sally Ride , whose efficiency and good cheer made the cruise such a success. We thank M. Ligi and two anonymous reviewers for their comments which greatly improved the manuscript. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
    Description: 2020-11-11
    Keywords: Galapagos triple junction ; mid‐ocean ridges ; seafloor spreading ; Galapagos microplate ; plate boundaries
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  • 57
    Publication Date: 2020-11-28
    Description: Author Posting. © American Geophysical Union, 2020. 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 125(6), (2020): e2019JB019239, doi:10.1029/2019JB019239.
    Description: P‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vs structure of Cascadia Basin sediments. We used P‐to‐S waves converted at the basement to derive an empirical function describing the average Vp/Vs of Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vs functions from semblance velocity analysis of S‐converted intrasediment and basement reflections, which we used to define an empirical Vp/Vs versus burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vs structure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs. These zones with elevated Vp/Vs are likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vs functions of Cascadia Basin sediments.
    Description: This research was funded by National Science Foundation (NSF) Grant OCE‐1657237 to J. P. C, OCE‐1657839 to A. F. A. and S. H., and OCE‐1657737 to S. M. C. Data used in this study were acquired with funding from NSF Grants OCE‐1029305 and OCE‐1249353. Data used in this research were provided by instruments from the Ocean Bottom Seismic Instrument Center (http://obsic.whoi.edu, formerly OBSIP), which is funded by the NSF. OBSIC/OBSIP data are archived at the IRIS Data Management Center (http://www.iris.edu) under network code X6 (https://doi.org/10.7914/SN/X6_2012). Data processing was conducted with Emerson‐Paradigm Software package Echos licensed to Woods Hole Oceanographic Institution under Paradigm Academic Software Program and MATLAB package SeismicLab of the University of Alberta, Canada (http://seismic-lab.physics.ualberta.ca), under GNU General Public License (MATLAB® is a registered trademark of MathWorks).
    Description: 2020-11-28
    Keywords: Vp/Vs ; sediments ; ocean bottom seismometer ; Juan de Fuca plate ; Cascadia
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  • 58
    Publication Date: 2020-05-01
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
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