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This study explores for the first time the possibilities that the 233U/236U atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, 233U originates from global fallout while 236U carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the 233U/236U ratio provides a tool to identify water masses with distinct U sources. In this work, 233U and 236U were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their 233U/236U signature; and (b) Deep and Bottom Waters show extremely low 233U and 236U concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high 233U/236U ratios are interpreted as a relative increase of naturally occurring 233U and 236U and thus for gradually reaching natural 233U/236U levels in the deep Arctic Ocean. Our results set the basis for future studies using the 233U/236U ratio to distinguish anthropogenic and pre-anthropogenic U in the Arctic Ocean and beyond.

This study explores for the first time the possibilities that the U-233/U-236 atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, U-233 originates from global fallout while U-236 carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the U-233/U-236 ratio provides a tool to identify water masses with distinct U sources. In this work, U-233 and U-236 were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their U-233/U-236 signature; and (b) Deep and Bottom Waters show extremely low U-233 and U-236 concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high U-233/U-236 ratios are interpreted as a relative increase of naturally occurring U-233 and U-236 and thus for gradually reaching natural U-233/U-236 levels in the deep Arctic Ocean. Our results set the basis for future studies using the U-233/U-236 ratio to distinguish anthropogenic and pre-anthropogenic U in the Arctic Ocean and beyond., Funding Agencies|ETH Zurich Research GrantETH Zurich [ETH-06 16-1]; Swiss National Science FoundationSwiss National Science Foundation (SNSF)European Commission [PRIMA SNF PR00P2_193091]; Spanish Government (Ministerio de Ciencia, Innovacion y Universidades)Spanish Government [PGC2018-094546-B-I00]

Marine biogenic materials such as corals, shells, or seaweed have long been recognized as recorders of environmental conditions. Here, the bivalve Cerastoderma edule is used for the first time as a recorder of past seawater contamination with anthropogenic uranium, specifically 236U. Several studies have employed the authorized radioactive releases, including 236U, from nuclear reprocessing plants in La Hague, France, into the English Channel, and Sellafield, England, into the Irish Sea, to trace Atlantic waters and to understand recent climate induced circulation changes in the Arctic Ocean. Anthropogenic 236U has emerged over recent years as a new transient tracer to track these changes, but its application has been challenged owing to paucity of fundamental data on the input (timing and amount) of 236U from Sellafield. Here, we present 236U/238U data from bivalve shells collected close to La Hague and Sellafield from two unique shell collections that allow the reconstruction of the historical 236U contamination of seawater since the 1960s, mostly with bi-annual resolution. The novel archive is first validated by comparison with well-documented 236U discharges from La Hague. Then, shells from the Irish Sea are used to reconstruct the regional 236U contamination. Apart from defining new, observationally based 236U input functions that will allow more precise tracer studies in the Arctic Ocean, we find an unexpected peak of 236U releases to the Irish Sea in the 1970s. Using this peak, we provide evidence for a small, but significant recirculation of Irish Sea water into the English Channel. Tracing the 1970s peak should allow extending 236U tracer studies into the South Atlantic Ocean.

This study provides dissolved and particulate 230Th and 232Th results as well as particulate 234Th data collected during expeditions to the central Arctic Ocean (GEOTRACES, an international project to identify processes and quantify fluxes that control the distributions of trace elements; sections GN04 and GIPY11). Constructing a time series of dissolved 230Th from 1991 to 2015 enables the identification of processes that control the temporal development of 230Th distributions in the Amundsen Basin. After 2007, 230Th concentrations decreased significantly over the entire water column, particularly between 300 and 1500 m. This decrease is accompanied by a circulation change, evidenced by a concomitant increase in salinity. A potentially increased inflow of water of Atlantic origin with low dissolved 230Th concentrations leads to the observed depletion in dissolved 230Th in the central Arctic. Because atmospherically derived tracers (chlorofluorocarbon (CFC), sulfur hexafluoride (SF6)) do not reveal an increase in ventilation rate, it is suggested that these interior waters have undergone enhanced scavenging of Th during transit from Fram Strait and the Barents Sea to the central Amundsen Basin. The 230Th depletion propagates downward in the water column by settling particles and reversible scavenging.

In this study 129I and 236U concentrations in seawater samples collected onboard R/V Polarstern during the PS100 expedition in the Fram Strait in 2016 are presented. The overall aim of the study was to investigate the distribution of these long‐lived radionuclides along the transect located at 79°N. The combination of both radionuclides was used for the first time in the Fram Strait to trace ocean circulation pathways of Atlantic waters. Results show that both 129I and 236U concentrations as well as 236U/238U ratios are about two times higher (> 600 × 107 at kg(−1), > 20 × 106 at kg(−1), and 2.8 × 10−9, respectively) in the cold and fresh outflowing surface waters from the Arctic Ocean (Polar Surface Water, PSW) compared to inflowing Atlantic origin waters (300 × 107 at kg(−1) 129I, 12 × 106 at kg(−1) 236U, and 1.4 × 10−9 236U/238U). A comparison with the different 129I and 236U input functions for the Atlantic branches entering the Arctic Ocean reveals that the middepth Atlantic origin waters outflowing the Arctic Ocean show more influence of the Barents Sea Branch Water than the Fram Strait Branch Water. The high radionuclide concentrations observed in the PSW indicate substantial influence of the Norwegian Coastal Current. This current carries a significantly larger proportion of 129I and 236U releases from European reprocessing plants than the aforementioned Atlantic branches. We estimate surface water transit times from the northern Norwegian Coast through the Arctic to the PSW of 12–19 years, less than for the middepth Barents Sea Branch Water (16–23 years).

The extended multiple linear regression technique is used to determine changes in anthropogenic carbon in the intermediate layers of the Eurasian Basin based on occupations from four cruises between 1996 and 2015. The results show a significant increase in basin‐wide anthropogenic carbon storage in the Nansen Basin (0.44–0.73 ± 0.14 mol C·m−2·year−1 and the Amundsen Basin (0.63–1.04 ± 0.09 mol C·m−2·year−1). Over the last two decades, inferred changes in ocean acidification (0.020–0.055 pH units) and calcium carbonate desaturation (0.05–0.18 units) are pronounced and rapid. These results, together with results from carbonate‐dynamic box model simulations and 129I tracer distribution simulations, suggest that the accumulation of anthropogenic carbon in the intermediate layers of the Eurasian Basin are consistent with increasing concentrations of anthropogenic carbon in source waters of Atlantic origin entering the Arctic Ocean followed by interior transport. The dissimilar distributions of anthropogenic carbon in the interior Nansen and Amundsen Basins are likely due to differences in the lateral ventilation of the intermediate layers by the return flows and ramifications of the boundary current along the topographic boundaries in the Eurasian Basin.

This study presents the data on 129I and 236U concentrations in seawater samples and sea ice cores obtained during two expeditions to the Arctic Ocean that took place onboard R/V Polarstern (PS94) and R/V Lance (N‐ICE2015) in summer 2015. Carbon‐14 was also measured in the deep water samples from the Nansen, Amundsen, and Makarov Basins. The main goal was to investigate the distribution of 129I and 236U in a transect from the Norwegian Coast to the Makarov Basin to fully exploit the potential of combining 129I and 236U as a dual tracer to track Atlantic waters throughout the Arctic Ocean. The use of the 129I/236U and 236U/238U atom ratios allowed identifying a third Atlantic branch that enters the Arctic Ocean (the Arctic Shelf Break Branch) following the Norwegian Coastal Current that carries a larger proportion of the European reprocessing plants signal compared to Fram Strait Branch Water and Barents Sea Branch Water. The combination of 129I and 236U also allowed quantifying the different proportions of the La Hague stream, the Scottish stream, and Atlantic waters forming the three Atlantic branches of the Arctic Ocean Boundary Current. The results show that the 129I/236U atom ratio can now be used to identify the different Atlantic branches entering the Arctic Ocean. New input functions for 129I, 236U, and 129I/236U have also been described for each branch, which can be further used for calculation of transit time distributions of Atlantic waters.

This study presents the data on 129I and 236U concentrations in seawater samples and sea ice cores obtained during two expeditions to the Arctic Ocean that took place onboard R/V Polarstern (PS94) and R/V Lance (N-ICE2015) in summer 2015. Carbon-14 was also measured in the deep water samples from the Nansen, Amundsen, and Makarov Basins. The main goal was to investigate the distribution of 129I and 236U in a transect from the Norwegian Coast to the Makarov Basin to fully exploit the potential of combining 129I and 236U as a dual tracer to track Atlantic waters throughout the Arctic Ocean. The use of the 129I/236U and 236U/238U atom ratios allowed identifying a third Atlantic branch that enters the Arctic Ocean (the Arctic Shelf Break Branch) following the Norwegian Coastal Current that carries a larger proportion of the European reprocessing plants signal compared to Fram Strait Branch Water and Barents Sea Branch Water. The combination of 129I and 236U also allowed quantifying the different proportions of the La Hague stream, the Scottish stream, and Atlantic waters forming the three Atlantic branches of the Arctic Ocean Boundary Current. The results show that the 129I/236U atom ratio can now be used to identify the different Atlantic branches entering the Arctic Ocean. New input functions for 129I, 236U, and 129I/236U have also been described for each branch, which can be further used for calculation of transit time distributions of Atlantic waters.

The production of dicalcium phosphate (DCP) uses phosphate rock (PR) as a raw material. Sedimentary phosphate rocks are enriched with relevant concentrations of natural radionuclides from the 238U decay chain (around 103 Bq·kg-1), leading to the need of controlling potential exposures to radiation of workers and members of the public in accordance with IAEA safety standards. Indeed, phosphate industries are classified as Naturally Occurring Radioactive Material (NORM) industries. Thus, the aim of this work is to assess the radiological risk of the workers in a DCP production plant located in the Iberian Peninsula (South-West Europe), which digests PR with hydrochloric acid. In the present study 238U, 230Th, 222Rn, 210Pb and 210Po concentrations in aerosols (indoor and outdoor areas) are reported. Aerosols showed concentrations between 0.42–92 mBq·m-3 for 238U, 0.24–33 mBq·m-3 for 230Th, 0.67–147 mBq·m-3 for 210Pb and 0.09–34 mBq·m-3 for 210Po. Long-term exposure (four months) of passive 222Rn detectors provided concentrations that ranged from detection limit (< DL) to 121 Bq·m-3 in outdoor areas and from < DL to 211 Bq·m-3 in indoor areas, similar to concentrations obtained from short-term measurements with active detectors from < DL to 117 Bq·m-3 in outdoor areas and from < DL to 318 Bq·m-3 in indoor places. 226Ra accumulation in ebonite and pipe scales were the most important contributions to the ambient dose equivalent H*(10), resulting in 0.07 (background)–27 µSv·h-1 with a median value of 1.1 µSv·h-1. Average 222Rn air concentrations were lower than the 300 Bq·m-3 limit and therefore, according to European Directive 2013/59/EURATOM, 222Rn concentration is excluded from the worker operational annual effective dose. Thus, considering the inhalation of aerosols and the external dose sources, the total effective dose determined for plant operators was 0.37 mSv·y-1., Postprint (author's final draft)

The performance of the 1 MV AMS system at the CNA (Centro Nacional de Aceleradores, Seville, Spain) for 236U and 239,240Pu measurements has been extensively investigated. A very promising 236U/238U abundance sensitivity of about 3 × 10−11 has been recently achieved, and background figures for 239Pu of about 106 atoms were reported in the past. These promising results lead to the use of conventional low energy AMS systems for the analysis of 236U and 239Pu and its further application in environmental studies. First 236U results obtained on our AMS system for marine samples (sediments and water) are presented here. Results of two new IAEA reference materials (IAEA-410 and IAEA-412, marine sediments from Pacific Ocean) are reported. The obtained 236U/239Pu atom ratios, of 0.12 and 0.022, respectively, show a dependency with the contamination source (i.e. local fallout from the US tests performed at the Bikini Atoll and general fallout). The results obtained for a third IAEA reference material (IAEA-381, seawater from the Irish Sea), are also presented. In the following, the uranium and plutonium isotopic compositions obtained on a set of 5 intercomparison seawater samples from the Arctic Ocean provided by the ETH Zürich are discussed. By comparing them with the obtained results on the 600 kV AMS facility Tandy at the ETH Zürich, we demonstrate the solidity of the CNA technique for 236U/238U determinations at, at least, 7 × 10−10 level. Finally, these results are discussed in their environmental context.

The impact of the earthquake and tsunami on theeast coast of Japan on 11 March 2011 caused a loss of powerat the Fukushima Dai-ichi nuclear power plant (NPP) thatresulted in one of the most important releases of artificialradioactivity into the environment. Although several workswere devoted to evaluating the atmospheric dispersion of ra-dionuclides, the impact of the discharges to the ocean hasbeen less investigated. Here we evaluate the distribution ofFukushima-derived90Sr (n=57) and89Sr (n=19) through-out waters 30–600 km offshore in June 2011. Concentrationsof90Sr and89Sr in both surface waters and shallow profilesranged from 0.8±0.2 to 85±3 Bq m−3and from 19±6 to265±74 Bq m−3, respectively. Because of its short half-life,all measured89Sr was due to the accident, while the90Srconcentrations can be compared to the background levels inthe Pacific Ocean of about 1.2 Bq m−3. Fukushima-derivedradiostrontium was mainly detected north of Kuroshio Cur-rent, as this was acting as a southern boundary for transport.The highest activities were associated with near-shore ed-dies, and larger inventories were found in the closest stationsto Fukushima NPP. The data evidence a major influence ofdirect liquid discharges of radiostrontium compared to theatmospheric deposition. Existing137Cs data reported fromthe same samples allowed us to establish a90Sr /137Cs ratioof 0.0256±0.0006 in seawater off Fukushima, being signif-icantly different than that of the global atmospheric fallout(i.e., 0.63) and may be used in future studies to track waterscoming from the east coast of Japan. Liquid discharges of of90Sr to the ocean were estimated, resulting in an inventory of53±1 TBq of90Sr in the inshore study area in June 2011 andtotal releases of90Sr ranging from 90 to 900 TBq, dependingupon the reported estimates of137Cs releases that are consid-ered.

A numerical model consisting of a 3D advection/diffusion equation, including uptake/release reactions between water and sediments described in a dynamic way, has been applied to simulate the marine releases of 90Sr from the Fukushima power plant after the March 2011 tsunami. This is a relevant issue since 90Sr releases are still occurring. The model used here had been successfully applied to simulate 137Cs releases. Assuming that the temporal trend of 90Sr releases was the same as for 137Cs during the four months after the accident simulated here, the source term could be evaluated, resulting in a total release of 80 TBq of 90Sr until the end of June, which is in the lower range of previous estimates. Computed vertical profiles of 90Sr in the water column have been compared with measured ones. The 90Sr inventories within the model domain have also been calculated for the water column and for bed sediments. Maximum dissolved inventory (obtained for April 10th, 2011) within the model domain results in about 58 TBq. Inventories in bed sediments are 3 orders of magnitude lower than in the water column due to the low reactivity of this radionuclide. 90Sr/137Cs ratios in the ocean have also been calculated and compared with measured values, showing both spatial and temporal variations.

La roca de fosfato se utiliza como fuente para la producción de suplementos alimenticios, como en forma de fosfato dicálcico (DCP), que es un suministro de calcio y fósforo para animales domésticos.