Fukushima and ocean radioactivity

Author Posting. © The Oceanography Society, 2014. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 27, no. 1 (2014): 92–105, doi:10.5670/oceanog.2014.02.The triple disaster of the March 11, 2011, earthquake, tsunami, and subsequent radiation releases from Japan's Fukushima Dai-ichi nuclear power plant were unprecedented events for the ocean and society. In this article, the radioactive releases from this event are compared to natural and prior human sources, with particular attention to cesium-137 and -134 radioisotopes. Total releases from Fukushima are not well constrained, with estimates from atmospheric fallout and direct ocean discharge spanning 4 to 90 peta Becquerels (PBq), but are most likely in the 15–30 PBq range. This source is smaller than any 137Cs remaining in the North Pacific from global and close-in fallout from the 1960s. It is of similar magnitude to 137Cs released to the ocean from the Sellafield nuclear reprocessing site on the Irish Sea, though of greater magnitude than fallout that reached the ocean from the 1986 Chernobyl nuclear power plant disaster in the Ukraine. The fate of Cs is largely determined by its soluble nature in seawater, though uptake in sediments does occur via cesium's association with both detrital particles and biological uptake and sedimentation. A mass balance of Cs supply from rivers and ongoing leakage from nuclear power plants suggests that sediments will remain contaminated for decades. This may be one reason why Cs concentrations in benthic fish stay elevated over predictions, causing fisheries to remain closed near Fukushima and ongoing concern to the public.Support for our Fukushima studies at the Woods Hole Oceanographic Institution (WHOI) has come primarily from the Gordon and Betty Moore Foundation, with additional support from the Deerbrook Foundation, US National Science Foundation, and, for outreach activities, the Japan Center for Global Partnership and the Morss Colloquia Endowed Fund at WHOI

Metrics that matter for assessing the ocean biological carbon pump

© 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., Boyd, P. W., Black, E. E., & Siegel, D. A. Metrics that matter for assessing the ocean biological carbon pump. Proceedings of the National Academy of Sciences of the United States of America, (2020): 201918114, doi: 10.1073/pnas.1918114117.The biological carbon pump (BCP) comprises wide-ranging processes that set carbon supply, consumption, and storage in the oceans’ interior. It is becoming increasingly evident that small changes in the efficiency of the BCP can significantly alter ocean carbon sequestration and, thus, atmospheric CO2 and climate, as well as the functioning of midwater ecosystems. Earth system models, including those used by the United Nation’s Intergovernmental Panel on Climate Change, most often assess POC (particulate organic carbon) flux into the ocean interior at a fixed reference depth. The extrapolation of these fluxes to other depths, which defines the BCP efficiencies, is often executed using an idealized and empirically based flux-vs.-depth relationship, often referred to as the “Martin curve.” We use a new compilation of POC fluxes in the upper ocean to reveal very different patterns in BCP efficiencies depending upon whether the fluxes are assessed at a fixed reference depth or relative to the depth of the sunlit euphotic zone (Ez). We find that the fixed-depth approach underestimates BCP efficiencies when the Ez is shallow, and vice versa. This adjustment alters regional assessments of BCP efficiencies as well as global carbon budgets and the interpretation of prior BCP studies. With several international studies recently underway to study the ocean BCP, there are new and unique opportunities to improve our understanding of the mechanistic controls on BCP efficiencies. However, we will only be able to compare results between studies if we use a common set of Ez-based metrics.We thank the many scientists whose ideas and contributions over the years are the foundation of this paper. This includes A. Martin, who led the organization of the BIARRITZ group (now JETZON) workshop in July 2019, discussions at which helped to motivate this article. We thank D. Karl for pointing us in the right direction for this paper format at PNAS and two thoughtful reviewers who through their comments helped to improve this manuscript. Support for writing this piece is acknowledged from several sources, including the Woods Hole Oceanographic Institution’s Ocean Twilight Zone project (K.O.B.); NASA as part of the EXport Processes in the global Ocean from RemoTe Sensing (EXPORTS) program (K.O.B. and D.A.S.). E.E.B. was supported by a postdoctoral fellowship through the Ocean Frontier Institute at Dalhousie University. P.W.B. was supported by the Australian Research Council through a Laureate (FL160100131)

Plutonium isotopes in the North Atlantic

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1986The artificial radionuclide Plutonium (Pu) has been introduced into the environment primarily as fallout from atmospheric nuclear weapons testing during the 1950's and 1960's. Earlier studies of Pu geochemistry are generally based upon the measurement of the combined activities of 239Pu and 249Pu (detected by alpha-counting and written as 239,240Pu) and assume an identical geochemical behavior for Pu from any of its fallout sources. A major focus of this thesis is the development of a mass spectrometric (m.s.) technique for the analysis of Pu in marine sediments, pore waters, sediment trap material and sea water from the North Atlantic. With the m.s. technique, not only is the detection limit for 239,240Pu increased by over an order-of-magnitude, but the 240Pu and 239Pu isotopes can be separated as well. The increased sensitivity for Pu provided by m.s. allowed me to measure Pu in deep-sea pore waters for the first time. Pore water studies are sensitive indicators of early diagenetic reactions, and can be used to examine the unresolved question of the extent of Pu remobilization out of marine sediments. Along a transect of cores ranging from highly reducing muddy sediments on the shelf to more oxic and carbonate-rich sediments in the deep-sea, I have found that the solubility of Pu is predominantly controlled by the distribution of Pu in the solid phase. The calculated 239,240Pu distribution coefficients (Kd = dpm per kg on solids/dpm per kg in solution) range from 0.2-23 x l04, with some suggestion of a trend towards lower values in the deeper cores (Kd's 2500m). Diffusive flux calculations based upon the observed Pu pore water gradients suggest that since its introduction, negligible Pu has been remobilized out of the sediments at all of the sites. On a time scale of 102 -103 years however, Pu remobilization may be significant. A large suite of sedimentary Pu and 210Pbex inventory data are also examined from the Northwest Atlantic shelf, slope and deep-sea sediments. Comparisons between Pu and 210Pb are of interest since both isotopes are predominantly supplied by atmospheric delivery to coastal waters, and since both isotopes are used to study recent accumulation and mixing processes in marine sediments. Inventories of these tracers will reflect their source function, removal efficiencies, and lateral transport in water and particles. A major conclusion is that the sediment inventories decrease with increasing water depth, reflecting a decrease in the net scavenging of these elements off-shore. Pu sediment inventories drop-off with increasing water depth much more rapidly than 210Pbex inventories, due to either the shorter residence time of 210Pb compared to Pu with respect to water column removal processes, or due to comparisons between the naturally occurring 210Pb steady-state scenario and the more recently introduced fallout Pu. When Pu and 210Pbex inventories are summed over water depths out to 4000 m in the Northwest Atlantic, the sediments can account for roughly 24 ± 8% of the expected Pu and 83 ± 15% of the expected 210Pbex inputs. The 240Pu/239Pu ratio data provided by the m.s. provide a unique insight into the relationship between the specific sources of fallout Pu and its geochemical behavior in the oceans. I find a systematic decrease in the 240Pu/239Pu ratio in sediments from 0.18 on the shelf to 0.10 in deep-sea (4500- 5000 m) sediments from the Northwest Atlantic. This trend is consistent with a model whereby Pu from surface based testing at the Nevada Test Site (240Pu/239Pu = 0.035) is carried by tropospheric fallout particles of a distinct physical/chemical form which are rapidly removed from the water column at all depths, in contrast to global stratospheric fallout (240Pu/239Pu = 0.18) which is only efficiently deposited to the sediments in the shallower cores where scavenging is more intense. This two source model was chosen since there is no evidence for the present day fractionation of 239Pu from 240Pu in the water column and sediment trap data. This two source model is supported by the analysis of 240Pu/239Pu ratios in marine sediments from the 1950's and 1960's which show lower or equivalent 240Pu/239Pu ratios than present day samples from the same locations (Nevada fallout was confined to 1951-1958 while global fallout inputs peaked in 1961/62). Also, while all of the North Atlantic deep-sea sediments show some evidence of the Nevada inputs (i.e. 240Pu/239P < 0.18), the net inventory of Pu from the Nevada source can be shown to decrease with increasing distance away from the Nevada source. Using the observed sediment 240Pu/239Pu ratios and a two end-member mixing model, the Pu supplied by the Nevada source in deep Northwest Atlantic sediments (>4500 m) is shown to account for roughly 40% of the total sediment 239,240Pu inventory. The very low inventories of 239,240Pu in the deep-sea sediments in general serves to accentuate the Nevada fallout signal at these sites. A pronounced dis-equilibrium is observed between the solid phase 240Pu/239Pu ratios (which range from 0.10-0.18) and the pore water 240Pu/23pPu ratios (which are constant at ≈ 0.18 ) at all sites. The low ratio Nevada fallout Pu is apparently more tightly bound by its solid phase carrier than Pu from global fallout sources and is therefore not participating in the general solid/solution exchange reactions. Within an individual sediment profile, the 240Pu/239Pu ratios are relatively constant from core top to core bottom. Using a sediment mixing model which combines the Pu activity data and the resulting 240Pu/239Pu ratios given the two Pu sources, I have been able to constrain the input function of Pu to the slope and deep ocean sediments. The data are consistent with a model which suggests that the bulk of the Pu deposited to the deep ocean sediments arrived early-on in the fallout record.Financial support was provided primarily by the Education Office of the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program in Oceanography, by Department of Energy contract DE-FG02-85ER60358, and by the Oak Ridge Associated Universities program for travel support to the Savannah River Laboratory

Comparison of carbon and opal export rates between summer and spring bloom periods in the region of the Antarctic Polar Front, SE Atlantic.

Although primary production in the Antarctic Circumpolar Current is not above the world average and carbon burial rates are low, 70% of the world's opal burial occurs in this zone and it has been suggested that blooms of large diatoms are responsible for this extraordinary situation. Here we compare export fluxes during bloom and steady-state situations near the Antarctic Polar Front in the SE Atlantic.In a previous expedition during the austral spring, we observed the development of a bloom that led to the sudden export of particles (Rutgers van der Loeff et al., 1997). Here we report the results of a second expedition to the same area in summer (Dec-Jan), 3 years later. 234Th was monitored in the surface water and in Rosette casts down to a water depth of 500m as tracer of export production in an intensive sampling program within a box of 275 x 375 km.The distribution of particulate and dissolved 234Th was remarkably constant over time and location. Total (dissolved + particulate) 234Th activities were depleted relative to its parent 238U at the surface (234Th/238U activity ratio approximately 83%), reaching equilibrium at a depth of around 190m. This constant depletion corresponds to a 234Th export rate of 1115 dpm m-2 d-1, 35% of the value observed during the spring bloom

Biogeochemical responses to late-winter storms in the Sargasso Sea, III—Estimates of export production using 234Th:238U disequilibria and sediment traps

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): 875-891, doi:10.1016/j.dsr.2009.01.008.Direct measurements of new production and carbon export in the subtropical North Atlantic Ocean appear to be too low when compared to geochemical based estimates. It has been hypothesized that episodic inputs of new nutrients into surface water via the passage of mesoscale eddies or winter storms may resolve at least some of this discrepancy. Here, we investigated particulate organic carbon (POC), particulate organic nitrogen (PON), and biogenic silica (BSiO2) export using a combination of water column 234Th:238U disequilibria and free-floating sediment traps during and immediately following two weather systems encountered in February and March 2004. While these storms resulted in a 2-4 fold increase in mixed layer NO3 inventories, total chlorophyll a and an increase in diatom biomass, the systems was dominated by generally low 234Th:238U disequilibria, suggesting limited particle export. Several 234Th models were tested, with only those including non-steady state and vertical upwelling processes able to describe the observed 234Th activities. Although upwelling velocities were not measured directly in this study, the 234Th model suggests reasonable rates of 2.2 to 3.7 m d-1. Given the uncertainties associated with 234Th derived particle export rates and sediment traps, both were used to provide a range in sinking particle fluxes from the upper ocean during the study. 234Th particle fluxes were determined applying the more commonly used steady state, 1-dimensional model with element/234Th ratios measured in sediment traps. Export fluxes at 200 m ranged from 1.91 ± 0.20 to 4.92 ± 1.22 mmol C m-2 d-1, 0.25 ± 0.08 to 0.54 ± 0.09 mmol N m-2 d-1, and 0.22 ± 0.04 to 0.50 ± 0.06 mmol Si m-2 d-1. POC export efficiencies (Primary Production/Export) were not significantly different from the annual average or from time periods without storms, although absolute POC fluxes were elevated by 1-11%. This increase was not sufficient, however, to resolve the discrepancy between our observations and geochemical based estimates of particle export. Comparison of PON export rates with simultaneous measurements of NO3 - uptake derived new production rates, suggested that only a fraction, < 35%, of new production was exported as particles to deep waters during these events. Measured bSiO2 export rates were more than a factor of two higher (p < 0.01) than the annual average, with storm events contributing as much as 50% of annual bSiO2 export in the Sargasso Sea. Furthermore it appears that 65 - 95% (average 86 ± 14%) of the total POC export measured in this study was due to diatoms. Combined these results suggest that winter storms do not significantly increase POC and PON export to depth. Rather, these storms may play a role in the export of bSiO2 to deep waters. Given the slower remineralization rates of bSiO2 relative to POC and PON, this transport may, over time, slowly decrease water column silicate inventories, and further drive the Sargasso Sea towards increasing silica limitation. These storm events may further affect the quality of the POC and PON exported given the large association of this material with diatoms during these periods.This study was funded by the National Science Foundation (Chemical Oceanography Grants OCE-0244612 and OCE-0241645)

An improvement in the small-volume technique for determining thorium-234 in seawater

Author Posting. © The Authors, 2005. 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 Marine Chemistry 100 (2006): 282-288, doi:10.1016/j.marchem.2005.10.016.The recently developed 2‐5 L small‐volume MnO2 coprecipitation method for determining 234Th in seawater has provided a new way to substantially increase, both temporally and spatially, the sampling resolution of 234Th and 234Th‐based particulate organic carbon export estimates in the upper ocean. In this study, we further optimize the technique by reducing reagent quantities and the use of an additional water bath heating step. This optimization allows the filtration of the MnO2 precipitate onto a 25‐mm diameter, 1.0 μm pore size QMA filter to be completed within < 30 minutes for unfiltered waters from the South China Sea. In addition, we have modified the MnO2 purification procedure to allow for alpha spetrometric measurements of 234Th recoveries. Results from recovery experiments suggest that reagent amounts can be reduced to 0.0375 mg KMnO4 and 0.1 mg MnCl2∙4H2O per liter of sample, while still maintaining high 234Th recovery. This study further confirms that the addition of a yield monitor is necessary for the application of this small‐volume method.Support for this work came from the Natural Science Foundation of China through grants #40206011, #49825162, and #90211020. This study was also supported by the China Ministry of Education through a program for Changjiang Scholars and Innovative Research Teams in Universities (PRSIRT)

Variability in efficiency of particulate organic carbon export: A model study

The flux of organic carbon from the surface ocean to mesopelagic depths is a key component of the global carbon cycle and is ultimately derived from primary production (PP) by phytoplankton. Only a small fraction of organic carbon produced by PP is exported from the upper ocean, referred to as the export efficiency (herein e-ratio). Limited observations of the e-ratio are available and there is thus considerable interest in using remotely-sensed parameters such as sea surface temperature to extrapolate local estimates to global annual export flux. Currently, there are large discrepancies between export estimates derived in this way; one possible explanation is spatial or temporal sampling bias in the observations. Here we examine global patterns in the spatial and seasonal variability in e-ratio and the subsequent effect on export estimates using a high resolution global biogeochemical model. NEMO-MEDUSA represents export as separate slow and fast sinking detrital material whose remineralisation is respectively temperature dependent and a function of ballasting minerals. We find that both temperature and the fraction of export carried by slow sinking particles are factors in determining e-ratio, suggesting that current empirical algorithms for e-ratio that only consider temperature are overly simple. We quantify the temporal lag between PP and export, which is greatest in regions of strong variability in PP where seasonal decoupling can result in large e-ratio variability. Extrapolating global export estimates from instantaneous measurements of e-ratio is strongly affected by seasonal variability, and can result in errors in estimated export of up to ±60%

Childbirth in South Sudan: Preferences, practice and perceptions in the Kapoetas

Background: Focus group discussions (FGDs) were designed to better understand the community’s views and preferences around maternity care to design a communications campaign to increase facility deliveries and skilled attendance at birth in the three county catchment areas of Kapoeta Civil Hospital.Methods: Twelve FGDs were conducted in Kapoeta South, Kapoeta East, and Kapoeta North counties. Four South Sudanese facilitators (two women, two men) were hired and trained to conduct sex-segregated FGDs. Each had 8-10 participants. Participants were adult women of reproductive age (18-49 years) and adult men (18+ years) married to women of reproductive age.Results: The majority of participants’ most recent births took place at home, though most reportedly intended to give birth in a health facility and overwhelmingly desire a facility birth next time. Husbands and the couple’s mothers are the primary decision-makers about where a woman delivers. More men than women preferred home births, and they tend to have more negative opinions than women about health facility deliveries. Though participants acknowledge that health facilities can theoretically provide better care than home births, fear of surgical interventions, lack of privacy, and perceived poor quality of care remain barriers to facility deliveries.Recommendations: Interventions encouraging facility births should target the decision-makers—husbands and a couple’s mothers. Improvements in quality of care are needed in health facilities. Developing social network interventions that circulate positive experiences about delivering in health facilities may be effective in changing public perception and decision-making about facility deliveries. Additional research and pilot testing is needed to more fully inform effective social and behavioural change strategies around maternal health in the Kapoetas in South Sudan.Keywords: Maternal health, childbirth, facility deliveries, behaviour change, qualitativ

Particle fluxes associated with mesoscale eddies in the Sargasso Sea

Author Posting. © Elsevier B.V., 2008. 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 II: Topical Studies in Oceanography 55 (2008): 1426-1444, doi:10.1016/j.dsr2.2008.02.007.We examined the impact of a cyclonic eddy and mode-water eddy on particle flux in the Sargasso Sea. The primary method used to quantify flux was based upon measurements of the natural radionuclide, 234Th, and these flux estimates were compared to results from sediment traps in both eddies, and a 210Po/210Pb flux method in the mode-water eddy. Particulate organic carbon (POC) fluxes at 150m ranged from 1 to 4 mmol C m-2 d-1 and were comparable between methods, especially considering differences in integration times scales of each approach. Our main conclusion is that relative to summer mean conditions at the Bermuda Atlantic Time-series Study (BATS) site, eddy-driven changes in biogeochemistry did not enhance local POC fluxes during this later, more mature stage of the eddy life cycle (>6 months old). The absence of an enhancement in POC flux puts a constraint on the timing of higher POC flux events, which are thought to have caused the local O2 minima below each eddy, and must have taken place >2 months prior to our arrival. The mode-water eddy did enhance preferentially diatom biomass in its center where we estimated a factor of 3 times higher biogenic Si flux than the BATS summer average. An unexpected finding in the highly depth resolved 234Th data sets are narrow layers of particle export and remineralization within the eddy. In particular, a strong excess 234Th signal is seen below the deep chlorophyll maxima which we attribute to remineralization of 234Th bearing particles. At this depth below the euphotic zone, de novo particle production in the euphotic zone has stopped, yet particle remineralization continues via consumption of labile sinking material by bacteria and/or zooplankton. These data suggest that further study of processes in ocean layers is warranted not only within, but below the euphotic zone.The EDDIES project was funded by the National Science Foundation Chemical, Biological, and Physical Oceanography Programs. Additional support for HPLC pigment analysis (Dr. Charles Trees, CHORS) was provided by NASA

Spatial variability and the fate of cesium in coastal sediments near Fukushima, Japan

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 11 (2014): 5123-5137, doi:10.5194/bg-11-5123-2014.Quantifying the amount of cesium incorporated into marine sediments as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident has proven challenging due to the limited multi-core sampling from within the 30 km zone around the facility; the inherent spatial heterogeneities in ocean sediments; and the potential for inventory fluctuations due to physical, biological, and chemical processes. Using 210Pb, 234Th, 137Cs, and 134Cs profiles from 20 sediment cores, coastal sediment inventories were reevaluated. A 137Cs sediment inventory of 100 ± 50 TBq was found for an area of 55 000 km2 using cores from this study and a total of 130 ± 60 TBq using an additional 181 samples. These inventories represent less than 1% of the estimated 15–30 PBq of cesium released during the FDNPP disaster. The time needed for surface sediment activities (0 to 3 cm) at the 20 locations to be reduced by 50% via sediment mixing was estimated to range from 0.4 to 26 yr. Due to the observed variability in mixing rates, grain size, and inventories, additional cores are needed to improve these estimates and capture the full extent of cesium penetration into the shallow coastal sediments, which was deeper than 14 cm for all cores retrieved from water depths less than 150 m.The authors would also like to acknowledge the support of the Gordon and Betty Moore Foundation, Deerbrook Charitable Trust, Woods Hole Oceanographic Institution, and Massachusetts Institute of Technology