Lingering radioactivity at the Bikini and Enewetak Atolls

© 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.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.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

Radium-based estimates of cesium isotope transport and total direct ocean discharges from the Fukushima Nuclear Power Plant accident

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 10 (2013): 2159-2167, doi:10.5194/bg-10-2159-2013.Radium has four naturally occurring isotopes that have proven useful in constraining water mass source, age, and mixing rates in the coastal and open ocean. In this study, we used radium isotopes to determine the fate and flux of runoff-derived cesium from the Fukushima Dai-ichi Nuclear Power Plant (FNPP). During a June 2011 cruise, the highest cesium (Cs) concentrations were found along the eastern shelf of northern Japan, from Fukushima south, to the edge of the Kuroshio Current, and in an eddy ~ 130 km from the FNPP site. Locations with the highest cesium also had some of the highest radium activities, suggesting much of the direct ocean discharges of Cs remained in the coastal zone 2–3 months after the accident. We used a short-lived Ra isotope (223Ra, t1/2 = 11.4 d) to derive an average water mass age (Tr) in the coastal zone of 32 days. To ground-truth the Ra age model, we conducted a direct, station-by-station comparison of water mass ages with a numerical oceanographic model and found them to be in excellent agreement (model avg. Tr = 27 days). From these independent Tr values and the inventory of Cs within the water column at the time of our cruise, we were able to calculate an offshore 134Cs flux of 3.9–4.6 × 1013 Bq d−1. Radium-228 (t1/2 = 5.75 yr) was used to derive a vertical eddy diffusivity (Kz) of 0.7 m2 d−1 (0.1 cm2 s−1); from this Kz and 134Cs inventory, we estimated a 134Cs flux across the pycnocline of 1.8 × 104 Bq d−1 for the same time period. On average, our results show that horizontal mixing loss of Cs from the coastal zone was ~ 109 greater than vertical exchange below the surface mixed layer. Finally, a mixing/dilution model that utilized our Ra-based and oceanographic model water mass ages produced a direct ocean discharge of 134Cs from the FNPP of 11–16 PBq at the time of the peak release in early April 2011. Our results can be used to calculate discharge of other water-soluble radionuclides that were released to the ocean directly from the Fukushima NPP.The authors thank the Gordon and Betty Moore Foundation for funding this effort

Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 56 (2009):1143-1167, doi:10.1016/j.dsr.2009.04.001.An extensive 234Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport in the Global Ocean) study. Total 234Th:238U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while 234Th:238U ~0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites- even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in 234Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for 234Th and/or with sediment traps. Also, the application of 234Th as POC flux tracer relies on accurate sampling of particulate POC/234Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50 μm at ALOHA and >5–350 μm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of 234Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these 234Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10’s to 100’s km at a resolution that is difficult to obtain with other methods.Funding for VERTIGO in the US was provided primarily by research grants from the US National Science Foundation Programs in Chemical and Biological Oceanography with additional support by the US Department of Energy (DAS). For TWT, support came from the Australian Cooperative Research Centres program

Review of the analysis of Th-234 in small volume (2-4 L) seawater samples: improvements and recommendations

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Clevenger, S. J., Benitez-Nelson, C. R., Drysdale, J., Pike, S., Puigcorbe, V., & Buesseler, K. O. Review of the analysis of Th-234 in small volume (2-4 L) seawater samples: improvements and recommendations. Journal of Radioanalytical and Nuclear Chemistry, 329(1), (2021): 1–13, https://doi.org/10.1007/s10967-021-07772-2.The short-lived radionuclide 234Th is widely used to study particle scavenging and transport from the upper ocean to deeper waters. This manuscript optimizes, reviews and validates the collection, processing and analyses of total 234Th in seawater and suggests areas of further improvements. The standard 234Th protocol method consists of scavenging 234Th from seawater via a MnO2 precipitate, beta counting, and using chemical recoveries determined by adding 230Th. The revised protocol decreases sample volumes to 2 L, shortens wait times between steps, and simplifies the chemical recovery process, expanding the ability to more rapidly and safely apply the 234Th method.The authors would like to acknowledge support from the National Aeronautics and Space Administration (NASA) as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program awards 80NSSC17K0555; and the Woods Hole Oceanographic Institution’s Ocean Twilight Zone study for KOB and SJC

Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Estapa, M., Buesseler, K., Durkin, C. A., Omand, M., Benitez-Nelson, C. R., Roca-Marti, M., Breves, E., Kelly, R. P., & Pike, S. Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa. Elementa: Science of the Anthropocene, 9(1), (2021): 00122, https://doi.org/10.1525/elementa.2020.00122.Comprehensive field observations characterizing the biological carbon pump (BCP) provide the foundation needed to constrain mechanistic models of downward particulate organic carbon (POC) flux in the ocean. Sediment traps were deployed three times during the EXport Processes in the Ocean from RemoTe Sensing campaign at Ocean Station Papa in August–September 2018. We propose a new method to correct sediment trap sample contamination by zooplankton “swimmers.” We consider the advantages of polyacrylamide gel collectors to constrain swimmer influence and estimate the magnitude of possible trap biases. Measured sediment trap fluxes of thorium-234 are compared to water column measurements to assess trap performance and estimate the possible magnitude of fluxes by vertically migrating zooplankton that bypassed traps. We found generally low fluxes of sinking POC (1.38 ± 0.77 mmol C m–2 d–1 at 100 m, n = 9) that included high and variable contributions by rare, large particles. Sinking particle sizes generally decreased between 100 and 335 m. Measured 234Th fluxes were smaller than water column 234Th fluxes by a factor of approximately 3. Much of this difference was consistent with trap undersampling of both small (1 mm) and with zooplankton active migrant fluxes. The fraction of net primary production exported below the euphotic zone (0.1% light level; Ez-ratio = 0.10 ± 0.06; ratio uncertainties are propagated from measurements with n = 7–9) was consistent with prior, late summer studies at Station P, as was the fraction of material exported to 100 m below the base of the euphotic zone (T100, 0.55 ± 0.35). While both the Ez-ratio and T100 parameters varied weekly, their product, which we interpret as overall BCP efficiency, was remarkably stable (0.055 ± 0.010), suggesting a tight coupling between production and recycling at Station P.The authors would like to acknowledge funding support from the NASA EXPORTS program (Award 80NSSC17K0662) for all sediment trap data presented here. Net primary production data collection was supported by EXPORTS (Award 80NSSC17K568) to Oregon State University. Thorium data collection was supported by EXPORTS (Award 80NSSC17K0555) to KB, CRBN, and L. Resplandy

High-resolution spatial and temporal measurements of particulate organic carbon flux using thorium-234 in the northeast Pacific Ocean during the EXport processes in the ocean from RemoTe sensing field campaign

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Buesseler, K. O., Benitez-Nelson, C. R., Roca-Marti, M., Wyatt, A. M., Resplandy, L., Clevenger, S. J., Drysdale, J. A., Estapa, M. L., Pike, S., & Umhau, B. P. High-resolution spatial and temporal measurements of particulate organic carbon flux using thorium-234 in the northeast Pacific Ocean during the EXport processes in the ocean from RemoTe sensing field campaign. Elementa: Science of the Anthropocene, 8(1), (2020): 030, https://doi.org/10.1525/elementa.030.The EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program of National Aeronautics and Space Administration focuses on linking remotely sensed properties from satellites to the mechanisms that control the transfer of carbon from surface waters to depth. Here, the naturally occurring radionuclide thorium-234 was used as a tracer of sinking particle flux. More than 950 234Th measurements were made during August–September 2018 at Ocean Station Papa in the northeast Pacific Ocean. High-resolution vertical sampling enabled observations of the spatial and temporal evolution of particle flux in Lagrangian fashion. Thorium-234 profiles were remarkably consistent, with steady-state (SS) 234Th fluxes reaching 1,450 ± 300 dpm m−2 d−1 at 100 m. Nonetheless, 234Th increased by 6%–10% in the upper 60 m during the cruise, leading to consideration of a non-steady-state (NSS) model and/or horizontal transport, with NSS having the largest impact by decreasing SS 234Th fluxes by 30%. Below 100 m, NSS and SS models overlapped. Particulate organic carbon (POC)/234Th ratios decreased with depth in small (1–5 μm) and mid-sized (5–51 μm) particles, while large particle (>51 μm) ratios remained relatively constant, likely influenced by swimmer contamination. Using an average SS and NSS 234Th flux and the POC/234Th ratio of mid-sized particles, we determined a best estimate of POC flux. Maximum POC flux was 5.5 ± 1.7 mmol C m−2 d−1 at 50 m, decreasing by 70% at the base of the primary production zone (117 m). These results support earlier studies that this site is characterized by a modest biological carbon pump, with an export efficiency of 13% ± 5% (POC flux/net primary production at 120 m) and 39% flux attenuation in the subsequent 100 m (POC flux 220 m/POC flux 120m). This work sets the foundation for understanding controls on the biological carbon pump during this EXPORTS campaign.The authors would like to acknowledge support from the National Aeronautics and Space Administration (NASA) as part of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program awards 80NSSC17K0555 and 80NSSC17K0662; the Woods Hole Oceanographic Institution’s Ocean Twilight Zone study for KOB and MRM, and the National Science Foundation Graduate Research Fellowship Program (NSF-GRFP) for funding and support of AW

Concentrations, ratios, and sinking fluxes of major bioelements at Ocean Station Papa

© 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.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.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

Particle export during the Southern Ocean Iron Experiment (SOFeX)

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of American Society of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 50 (2005): 311-327, doi:10.4319/lo.2005.50.1.0311.We studied the effect of iron addition on particle export in the Southern Ocean by measuring changes in the distribution of thorium-234 during a 4 week Fe enrichment experiment conducted in the high-silicate high-nitrate waters just south of the Southern Antarctic Circumpolar Current Front at 172.5°W. Decreases in 234Th activity with time in the fertilized mixed layer (0-50m) exceeded those in unfertilized waters, indicating enhanced export of 234Th on sinking particles after Fe enrichment. The addition of Fe also affected export below the fertilized patch by increasing the efficiency of particle export through the 100 m depth horizon. Extensive temporal and vertical Lagrangian sampling allowed us to make a detailed examination of the 234Th flux model, which was used to quantify the fluxes of particulate organic carbon (POC) and biogenic silica (bSiO2). Iron addition increased the flux of both POC and bSiO2 out of the mixed layer by about 300%. The flux at 100 m increased by more than 700% and 600% for POC and bSiO2, respectively. The absolute magnitude of the POC and bSiO2 fluxes were not large relative to natural blooms at these latitudes, or to those found in association with the termination of blooms in other ocean regions. Our results support the hypothesis that Fe addition leads directly to significant particle export and sequestration of C in the deep ocean. This is a key link between ocean Fe inputs and past changes in atmospheric CO2 and climate.This work was supported by the Division of Ocean Sciences at the U.S. National Science Foundation, the Biological and Environmental Research Office, Office of Science, U.S. Department of Energy, and for RIB Polar Star logistical support, Polar Programs at the U.S. National Science Foundation

Carbonate-Templated Self-Assembly of an Alkylthiolate-Bridged Cadmium Macrocycle

In the presence of Cd(ClO4)2 and a base, a new mixed N,S-donor alkylthiolate ligand supported both carbonate formation from atmospheric CO2 and the self-assembly of a novel bicapped puckered (CdS)6 molecular wheel. The remarkable stability of the complex was demonstrated by slow intermolecular ligand exchange on the 2J(HH) and J(111/113Cd1H) time scales at elevated temperature. Both CO2 and the base were required to convert amorphous “CdLClO4” precipitated in the absence of air to the carbonate complex. The complex shares structural features with the ζ-carbonic anhydrase class associating cadmium(II) with the biogeochemical cycling of carbon and is the first structurally characterized carbonate complex of any metal involving an alkylthiolate ligand