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  • 1
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    Kinetics of thorium and particle cycling along the U.S. GEOTRACES North Atlantic Transect (2017)
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 125 (2017): 106-128, doi:10.1016/j.dsr.2017.05.003.
    Description: The high particle reactivity of thorium has resulted in its widespread use in tracing processes impacting marine particles and their chemical constituents. The use of thorium isotopes as tracers of particle dynamics, however, largely relies on our understanding of how the element scavenges onto particles. Here, we estimate apparent rate constants of Th adsorption (k1), Th desorption (k−1), bulk particle degradation (β-1), and bulk particle sinking speed (w) along the water column at 11 open-ocean stations occupied during the GEOTRACES North Atlantic Section (GA03). First, we provide evidence that the budgets of Th isotopes and particles at these stations appear to be generally dominated by radioactive production and decay sorption reactions, particle degradation, and particle sinking. Rate parameters are then estimated by fitting a Th and particle cycling model to data of dissolved and particulate 228,230,234Th, 228Ra, particle concentrations, and 234,238U estimates based on salinity, using a nonlinear programming technique. We find that the adsorption rate constant (k1) generally decreases with depth across the section: broadly, the time scale 1/k1 averages 1.0 yr in the upper 1000 m and (1.4–1.5) yr below. A positive relationship between k1 and particle concentration (P) is found, i.e., , k1 ∝ Pb where b ≥ 1, consistent with the notion that k1 increases with the number of surface sites available for adsorption. The rate constant ratio, K = k1/(k-1 + β-1), which measures the collective influence of rate parameters on Th scavenging, averages 0.2 for most stations and most depths. We clarify the conditions under which K/P is equivalent to the distribution coefficient, KD, test that the conditions are met at the stations, and find that decreases with P, in line with a particle concentration effect (dKD/dP 〈 0). In contrast to the influence of colloids as envisioned by the Brownian pumping hypothesis, we provide evidence that the particle concentration effect arises from the joint effect of P on the rate constants for thorium attachment to, and detachment from, particles.
    Description: We acknowledge the U.S. National Science Foundation for providing funding for this study (grant OCE-1232578) and for U.S. GEOTRACES North Atlantic section ship time, sampling, and data analysis. The U.S. NSF also supported the generation of 230Th data (OCE-0927064 to LDEO, OCE-O092860 to WHOI, and OCE-0927754 to UMN) and 228,234Th data (OCE-0925158 to WHOI).
    Keywords: GEOTRACES ; Thorium ; Particle Concentration Effect ; Single-particle class model ; Inverse method
    Repository Name: Woods Hole Open Access Server
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  • 2
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    The GEOTRACES Intermediate Data Product 2017 (2018)
    Elsevier
    Details
    Publication Date: 2022-05-25
    Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Chemical Geology 493 (2018): 210-223, doi:10.1016/j.chemgeo.2018.05.040.
    Description: The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or along ridges. The IDP2017 is the result of a truly international effort involving 326 researchers from 25 countries. This publication provides the critical reference for unpublished data, as well as for studies that make use of a large cross-section of data from the IDP2017. This article is part of a special issue entitled: Conway GEOTRACES - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.
    Description: We gratefully acknowledge financial support by the Scientific Committee on Oceanic Research (SCOR) through grants from the U.S. National Science Foundation, including grants OCE-0608600, OCE-0938349, OCE-1243377, and OCE-1546580. Financial support was also provided by the UK Natural Environment Research Council (NERC), the Ministry of Earth Science of India, the Centre National de Recherche Scientifique, l'Université Paul Sabatier de Toulouse, the Observatoire Midi-Pyrénées Toulouse, the Universitat Autònoma de Barcelona, the Kiel Excellence Cluster The Future Ocean, the Swedish Museum of Natural History, The University of Tokyo, The University of British Columbia, The Royal Netherlands Institute for Sea Research, the GEOMAR-Helmholtz Centre for Ocean Research Kiel, and the Alfred Wegener Institute.
    Keywords: GEOTRACES ; Trace elements ; Isotopes ; Electronic atlas ; IDP2017
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  • 3
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    Fukushima 137Cs at the base of planktonic food webs off Japan (2016)
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    Publication Date: 2022-05-25
    Description: © The Author(s), 2015. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 106 (2015): 9-16, doi:10.1016/j.dsr.2015.09.006.
    Description: The potential bioaccumulation of 137Cs in marine food webs off Japan became a concern following the release of radioactive contaminants from the damaged Fukushima nuclear power plant into the coastal ocean. Previous studies suggest that 137Cs activities increase with trophic level in pelagic food webs, however, the bioaccumulation of 137Cs from seawater to primary producers, to zooplankton has not been evaluated in the field. Since phytoplankton are frequently the largest component of SPM (suspended particulate matter) we used SPM concentrations and particle-associated 137Cs to understand bioaccumulation of 137Cs in through trophic pathways in the field. We determined particle-associated 137Cs for samples collected at 20 m depth from six stations off Japan three months after the initial release from the Fukushima nuclear power plant. At 20 m SPM ranged from 0.65 to 1.60 mg L-1 and rapidly declined with depth. The ratios of particulate organic carbon to chlorophyll a suggested that phytoplankton comprised much of the SPM in these samples. 137Cs activities on particles accounted for on average 0.04% of the total 137Cs in seawater samples, and measured concentration factors of 137Cs on small suspended particles were comparatively low (~102). However, when 137Cs in crustacean zooplankton was derived based only on modeling dietary 137Cs uptake, we found predicted and measured 137Cs concentrations in good agreement. We therefore postulate the possibility that the dietary route of 137Cs bioaccumulation (i.e., phytoplankton ingestion) could be largely responsible for the measured levels in the copepod-dominated (%) zooplankton assemblages in Japanese coastal waters. Finally, our data did not support the notion that zooplankton grazing on phytoplankton results in a biomagnification of 137Cs.
    Description: This project was funded by the Gordon and Betty Moore Foundation through Grants GBMF3007 and GBMF 3423, and JSPS KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas Grant Number 24110005.
    Description: 2016-09-25
    Keywords: Fukushima ; Cesium ; Trophic transfer ; Phytoplankton ; Zooplankton
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Flux of particulate elements in the North Atlantic Ocean constrained by multiple radionuclides (2019)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    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 Global Biogeochemical Cycles 32(12), (2019): 1738-1758, doi:10.1029/2018GB005994.
    Description: Sinking particles strongly regulate the distribution of reactive chemical substances in the ocean, including particulate organic carbon and other elements (e.g., P, Cd, Mn, Cu, Co, Fe, Al, and 232Th). Yet, the sinking fluxes of trace elements have not been well described in the global ocean. The U.S. GEOTRACES campaign in the North Atlantic (GA03) offers the first data set in which the sinking flux of carbon and trace elements can be derived using four different radionuclide pairs (238U:234Th ;210Pb:210Po; 228Ra:228Th; and 234U:230Th) at stations co‐located with sediment trap fluxes for comparison. Particulate organic carbon, particulate P, and particulate Cd fluxes all decrease sharply with depth below the euphotic zone. Particulate Mn, Cu, and Co flux profiles display mixed behavior, some cases reflecting biotic remineralization, and other cases showing increased flux with depth. The latter may be related to either lateral input of lithogenic material or increased scavenging onto particles. Lastly, particulate Fe fluxes resemble fluxes of Al and 232Th, which all have increasing flux with depth, indicating a dominance of lithogenic flux at depth by resuspended sediment transported laterally to the study site. In comparing flux estimates derived using different isotope pairs, differences result from different timescales of integration and particle size fractionation effects. The range in flux estimates produced by different methods provides a robust constraint on the true removal fluxes, taking into consideration the independent uncertainties associated with each method. These estimates will be valuable targets for biogeochemical modeling and may also offer insight into particle sinking processes.
    Description: This study grew out of a synthesis workshop at the Lamont‐Doherty Earth Observatory of Columbia University in August 2016. This workshop was sponsored by the U.S. GEOTRACES Project Office (NSF 1536294) and the Ocean Carbon and Biogeochemistry (OCP) Project Office (NSF 1558412 and NASA NNX17AB17G). The U.S. National Science Foundation supported all of the analytical work on GA03. Kuanbo Zhou measured 228Th in the large size class particles (NSF 0925158 to WHOI). NSF 1061128 to Stony Brook University supported the BaRFlux project, for which Chistina Heilbrun is acknowledged for laboratory and field work. The lead author acknowledges support from a start‐up grant from the University of Southern Mississippi. Two anonymous reviewers are thanked for their constructive comments. All GEOTRACES GA03 data used in this study are accessible through the Biological and Chemical Oceanography Data Management Office (http://data.bco‐dmo.org/jg/dir/BCO/GEOTRACES/NorthAtlanticTransect/), and derived parameters are reported in the supporting information.
    Description: 2019-05-22
    Keywords: Biological carbon pump ; Trace metals ; North Atlantic ; Export ; GEOTRACES
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  • 5
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    Ironing out Fe residence time in the dynamic upper ocean (2020)
    American Geophysical Union
    Details
    Publication Date: 2022-10-26
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Black, E. E., Kienast, S. S., Lemaitre, N., Lam, P. J., Anderson, R. F., Planquette, H., Planchon, F., & Buesseler, K. O. Ironing out Fe residence time in the dynamic upper ocean. Global Biogeochemical Cycles, 34(9), (2020): e2020GB006592, doi:10.1029/2020GB006592.
    Description: Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234Th‐ and sediment‐trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation‐based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0–250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub‐annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond.
    Description: We would like to thank S. Albani for providing the dust model results (Community Atmosphere Model, C4fn) and the three anonymous reviewers for their constructive comments. The U.S. GEOTRACES work was supported by the National Science Foundation (OCE‐1232669 and OCE‐1518110) and E. Black was also funded by a NASA Earth and Space Science Graduate Fellowship (NNX13AP31H) and the Ocean Frontier Institute. The GEOVIDE work was funded by the Flanders Research Foundation (G071512N), the Vrije Universiteit Brussel (SRP‐2), the French ANR Blanc GEOVIDE (ANR‐13‐BS06‐0014), ANR RPDOC BITMAP (ANR‐12‐PDOC‐0025‐01), IFREMER, CNRS‐INSU (programme LEFE), INSU OPTIMISP, and Labex‐Mer (ANR‐10‐LABX‐19).
    Keywords: Thorium‐234 ; Iron ; Export ; GEOTRACES ; Residence time
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  • 6
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    Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa (2021)
    University of California Press
    Details
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Roca-Marti, M., Benitez-Nelson, C. R., Umhau, B. P., Wyatt, A. M., Clevenger, S. J., Pike, S., Horner, T. J., Estapa, M. L., Resplandy, L., & Buesseler, K. O. Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa. Elementa: Science of the Anthropocene, 9(1), (2021): 00166, https://doi.org/10.1525/elementa.2020.00166.
    Description: Fluxes of major bioelements associated with sinking particles were quantified in late summer 2018 as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign near Ocean Station Papa in the subarctic northeast Pacific. The thorium-234 method was used in conjunction with size-fractionated (1–5, 5–51, and 〉51 μm) concentrations of particulate nitrogen (PN), total particulate phosphorus (TPP), biogenic silica (bSi), and particulate inorganic carbon (PIC) collected using large volume filtration via in situ pumps. We build upon recent work quantifying POC fluxes during EXPORTS. Similar remineralization length scales were observed for both POC and PN across all particle size classes from depths of 50–500 m. Unlike bSi and PIC, the soft tissue–associated POC, PN, and TPP fluxes strongly attenuated from 50 m to the base of the euphotic zone (approximately 120 m). Cruise-average thorium-234-derived fluxes (mmol m–2 d–1) at 120 m were 1.7 ± 0.6 for POC, 0.22 ± 0.07 for PN, 0.019 ± 0.007 for TPP, 0.69 ± 0.26 for bSi, and 0.055 ± 0.022 for PIC. These bioelement fluxes were similar to previous observations at this site, with the exception of PIC, which was 1 to 2 orders of magnitude lower. Transfer efficiencies within the upper twilight zone (flux 220 m/flux 120 m) were highest for PIC (84%) and bSi (79%), followed by POC (61%), PN (58%), and TPP (49%). These differences indicate preferential remineralization of TPP relative to POC or PN and larger losses of soft tissue relative to biominerals in sinking particles below the euphotic zone. Comprehensive characterization of the particulate bioelement fluxes obtained here will support future efforts linking phytoplankton community composition and food-web dynamics to the composition, magnitude, and attenuation of material that sinks to deeper waters.
    Description: The authors would like to acknowledge support from the National Aeronautics and Space Administration as part of the EXport Processes in the Ocean from RemoTe Sensing program awards 80NSSC17K0555 and 80NSSC17K0662. They also acknowledge the funding from the Woods Hole Oceanographic Institution’s Ocean Twilight Zone study for MRM and KOB, the National Science Foundation Graduate Research Fellowship Program for AMW, and the Ocean Frontier Institute for MRM.
    Keywords: Biological pump ; Bioelements ; Particulate fluxes ; Transfer efficiency ; Size-fractionated particles ; EXPORTS
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  • 7
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    The GEOTRACES Intermediate Data Product 2014 (2016)
    Elsevier
    Details
    Publication Date: 2022-05-26
    Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marine Chemistry 177 (2015): 1-8, doi:10.1016/j.marchem.2015.04.005.
    Description: The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes.
    Description: We gratefully acknowledge financial support by the Scientific Committee on Oceanic Research (SCOR) through grants from the U.S. National Science Foundation, including grants OCE-0608600, OCE-0938349, and OCE-1243377. Financial support was also provided by the UK Natural Environment Research Council, the Ministry of Earth Science of India, the Centre National de Recherche Scientifique, l'Université Paul Sabatier de Toulouse, the Observatoire Midi-Pyrénées Toulouse, the Universitat Autònoma de Barcelona, the Kiel Excellence Cluster The Future Ocean, the Swedish Museum of Natural History, The University of Tokyo, The University of British Columbia, The Royal Netherlands Institute for Sea Research, the GEOMAR-Helmholtz Centre for Ocean Research Kiel, and the Alfred Wegener Institute.
    Keywords: GEOTRACES ; Trace elements ; Isotopes ; Electronic atlas
    Repository Name: Woods Hole Open Access Server
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  • 8
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    Contrasting biogeochemical cycles of cobalt in the surface western Atlantic Ocean (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 28 (2014): 1387–1412, doi:10.1002/2014GB004903.
    Description: Dissolved cobalt (DCo; 〈0.2 µm; 14 to 93 pM) and the apparent particulate cobalt (PCo; 〉0.2 µm; 〈1 to 15 pM) were determined in the upper water column (〈1000 m) of the western Atlantic Ocean along the GEOTRACES-A02 section (64°N to 50°S). The lowest DCo concentrations, typical of a nutrient-type distribution were observed in surface waters of the subtropical domains. Strong linear relationships between DCo and phosphate (P) as well as meridional gradients of decreasing DCo from high latitudes were characterized and both linked to the Co biological requirement. External sources such as the Amazon and the atmospheric deposition were found to contribute significantly (〉10%) to the DCo stock of the mixed layer in the equatorial and north subtropical domains. Biotic and abiotic processes as well as the physical terms involved in the biogeochemical cycle of Co were defined and estimated. This allowed establishing the first global budget of DCo for the upper 100 m in the western Atlantic. The biological DCo uptake flux was the dominant sink along the section, as reflected by the overall nutrient-type behavior of DCo. The regeneration varied widely within the different biogeochemical domains, accounting for 10% of the DCo-uptake rate in the subarctic gyre and for up to 85% in southern subtropical domain. These findings demonstrated that the regeneration is likely the prevailing source of DCo in the surface waters of the western Atlantic, except in the subpolar domains where physically driven sources can sustain the DCo biological requirement.
    Description: This investigation was supported by the GEOTRACES-GEOSECS revisited in the West Atlantic project coordinated by M. Boye and funded by the French LEFE-CYBER National Program of the Institut National des Sciences de l'Univers (INSU). We also acknowledge the European COST-Action ES801 for funding a short-term Scientific Mission to G. Dulaquais to join the last cruise of the GEOTRACES-A02 section. The Université de Bretagne Occidentale (UBO) and the Région Bretagne (ARED) are supporting the PhD fellowship of G. Dulaquais. P.M. was supported in part by a Gledden Visiting Fellowship awarded by the Institute of Advanced Studies at The University of Western Australia.
    Description: 2015-06-05
    Keywords: Cobalt ; Biogeochemistry ; Atlantic Ocean ; Chemical Oceanography ; GEOTRACES
    Repository Name: Woods Hole Open Access Server
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  • 9
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    Unexpected source of Fukushima-derived radiocesium to the coastal ocean of Japan (2017)
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    Publication Date: 2022-05-26
    Description: Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 114 (2017): 11092-11096, doi:10.1073/pnas.1708659114.
    Description: There are 440 operational nuclear reactors in the world, with approximately half situated along the coastline. This includes the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which experienced multiple reactor meltdowns in March 2011 followed by the release of radioactivity to the marine environment. While surface inputs to the ocean via atmospheric deposition and rivers are usually well monitored after a nuclear accident, no study has focused on subterranean pathways. During our study period, we found the highest cesium-137 (137Cs) levels (up to 23,000 Bq m-3) outside of the FDNPP site not in the ocean, rivers or potable groundwater, but in groundwater beneath sand beaches over tens of kilometers away from the FDNPP. Here, we present evidence of a previously unknown, ongoing source of Fukushima-derived 137Cs to the coastal ocean. We postulate that these beach sands were contaminated in 2011 through wave and tide driven exchange and sorption of highly radioactive Cs from seawater. Subsequent desorption of 137Cs and fluid exchange from the beach sands was quantified using naturally occurring radium isotopes. This estimated ocean 137Cs source (0.6 TBq y-1) is of similar magnitude as the ongoing releases of 137Cs from the FDNPP site for 2013-2016, as well as the input of Fukushima-derived dissolved 137Cs via rivers. Though this ongoing source is not at present a public health issue for Japan, the release of Cs of this type and scale needs to be considered in NPP monitoring and scenarios involving future accidents.
    Description: V. Sanial was supported by a Postdoctoral Scholarship from the Center for Marine and Environmental Radioactivity (CMER). Funding for this work was provided by the Gordon and Betty Moore Foundation, the Deerbrook Charitable Trust, as well as the EC 7th Framework project COMET-FRAME (COordination and iMplementation of a pan-Europe instrumenT for radioecology) and KAKENHI (24110008).
    Keywords: Fukushima Dai-ichi Nuclear Power Plant accident ; Cesium ; Submarine groundwater discharge
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  • 10
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    Lingering radioactivity at the Bikini and Enewetak Atolls (2018)
    Elsevier
    Details
    Publication Date: 2022-05-26
    Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science of The Total Environment 621 (2018): 1185-1198, doi:10.1016/j.scitotenv.2017.10.109.
    Description: We made an assessment of the levels of radionuclides in the ocean waters, seafloor and groundwater at Bikini and Enewetak Atolls where the US conducted nuclear weapons tests in the 1940's and 50's. This included the first estimates of submarine groundwater discharge (SGD) derived from radium isotopes that can be used here to calculate radionuclide fluxes in to the lagoon waters. While there is significant variability between sites and sample types, levels of plutonium (239,240Pu) remain several orders of magnitude higher in lagoon seawater and sediments than what is found in rest of the world's oceans. In contrast, levels of cesium-137 (137Cs) while relatively elevated in brackish groundwater are only slightly higher in the lagoon water relative to North Pacific surface waters. Of special interest was the Runit dome, a nuclear waste repository created in the 1970's within the Enewetak Atoll. Low seawater ratios of 240Pu/239Pu suggest that this area is the source of about half of the Pu in the Enewetak lagoon water column, yet radium isotopes suggest that SGD from below the dome is not a significant Pu source. SGD fluxes of Pu and Cs at Bikini were also relatively low. Thus radioactivity associated with seafloor sediments remains the largest source and long term repository for radioactive contamination. Overall, Bikini and Enewetak Atolls are an ongoing source of Pu and Cs to the North Pacific, but at annual rates that are orders of magnitude smaller than delivered via close-in fallout to the same area.
    Description: Finally, none of this would have been possible without the generous financial support from the Dalio Explore Fund (WHOI #25531513) for the vessel and our post cruise analyses that together resulted in this unique and successful research program.
    Keywords: Marshall Islands ; Runit dome ; Plutonium ; Cesium ; Radium ; Nuclear weapons tests
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