Effects of ocean acidification on the biochemistry, physiology and parental transfer of Ampelisca brevicornis (Costa, 1853)

1. Introduction Since the 1980s, a variety of chemical tracers have shed light on the circulation patterns and mixing processes of Pacific Waters (PW) and Atlantic Waters (AW) in the Arctic Ocean (AO). Natural tracers such as nitrate and phosphate concentrations (E. Peter Jones etal.,1998), and new tracers like Ga (Whitmore etal.,2020), are used to distinguish the interface between PW and AW in the upper water column of the AO. Artificial tracers such as chlorofluorocarbons (CFCs) and 3H (T1/2=12.33years) have been used to understand the lateral transport of AW in the AO. CFCs have been introduced into the surface ocean via gas exchange since the 1950s with mostly increasing input functions (Bullister,2014; Tanhua etal.,2009), while a 3H pulse originating from atmospheric nuclear bomb testing (i.e., from 1945 to 1980, peaking in the 1960s) has rained down on the ocean's surface Abstract 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. Plain Language Summary We study the presence of 233U and 236U in seawater samples taken in the Arctic Ocean from the GN01 GEOTRACES expedition in 2015. Both long-lived manmade U isotopes were introduced worldwide during the open-air testing of nuclear weapons (mainly in the 1960s). 236U is also linked to the civil uses of nuclear energy, particularly to the liquid effluents from Sellafield (United Kingdom) and La Hague (France) reprocessing plants (starting in the 1950s). Atlantic Waters flowing into the Arctic Ocean carry both the bomb-tests and the reprocessing plants signals. Pacific Waters entering the Arctic Ocean carry the bomb-tests signal only. Thus, they show different 233U and 236U compositions. We demonstrate that the 233U/ 236U ratio and the 236U composition can be used to identify water masses of Pacific and Atlantic origin in the upper waters of the western Arctic Ocean. Deep and Bottom Waters, where an anthropogenic influence is not expected, exhibit extremely low 233U and 236U concentrations but 233U/ 236U ratios above the ones observed in the upper Arctic Ocean. These features can be explained by the presence of naturally produced uranium isotopes of lithogenic origin. This study demonstrates the potential of the 233U/ 236U ratio to distinguish anthropogenic and natural U in the Arctic Ocean and beyond. CHAMIZO ET AL. © 2022 The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. The Potential of 233U/ 236U as a Water Mass Tracer in the Arctic Ocean E. Chamizo1 , M. Christl2 , M. López-Lora1,3 , N. Casacuberta2,4 , A.-M. Wefing2,4 , and T. C. Kenna5 1Centro Nacional de Aceleradores (CNA), Universidad de Sevilla, Junta de Andalucía, Consejo Superior de Investigaciones Científicas, Sevilla, Spain, 2Laboratory of Ion Beam Physics, Institute for Particle Physics and Astrophysics, ETH Zürich, Zürich, Switzerland, 3Department of Health, Medicine and Caring Sciences (HMV), Linköping University, Linköping, Sweden, 4Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Zurich, Switzerland, 5Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA Key Points: • The 233U/ 236U ratio and the 236U composition unravel Pacific and Atlantic waters in Polar Surface Waters of the western Arctic Ocean • The Atlantic Layer exhibit distinct 233U and 236U signals from Polar Surface Waters • Deep and Bottom Waters show extremely low 233U and 236U concentrations with isotopic ratios distinct from known anthropogenic U sources Supporting Information: Supporting Information may be found in the online version of this article. Correspondence to: E. Chamizo and M. Christl, [email protected]; [email protected] Citation: Chamizo, E., Christl, M., López-Lora, M., Casacuberta, N., Wefing, A.-M., & Kenna, T. C. (2022). The potential of 233U/ 236U as a water mass tracer in the Arctic Ocean. Journal of Geophysical Research: Oceans, 127, e2021JC017790. https://doi.org/10.1029/2021JC017790 Received 16 JUL 2021 Accepted 2 MAR 2022 Author Contributions: Conceptualization: E. Chamizo, M. Christl, M. López-Lora, N. Casacuberta, A.-M. Wefing Data curation: E. Chamizo Formal analysis: E. Chamizo, M. Christl 10.1029/2021JC017790 Special Section: Uncovering the hidden links between dynamics, chemical, biogeochemical and biological processes under the changing Arctic RESEARCH ARTICLE 1 of 12 Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 2 of 12 (Dorsey & Peterson,1976; Weiss & Roether,1980). Both above tracers have been applied in the North Atlantic to estimate ventilation rates of deep waters (Doney etal.,1997). In addition to the global release of radionuclides during the atmospheric bomb tests (global fallout, GF), the nuclear reprocessing plants (RPs) of Sellafield and La Hague have fingerprinted AW and added artificial radionuclides to the AO. For example, the fission by-prod- ucts 137Cs (T1/2=30.07years) and 129I (T1/2=15.7 My) have been used to get insights into circulation patterns and transit times of AW in the AO (Smith etal.,2011; Tanhua etal.,2009). Today, the AO is experiencing critical changes that are rapidly affecting its unique properties such as ice-cover and density structure of the water column. PW and hence freshwater export from the AO is subject to temporal variations and plays an important role for deep-water formation in the subpolar North Atlantic (Holliday etal.,2020; Rahmstorf etal.,2015). Conventional tracers used to study PW in the AO are subject to ongoing discussion, since the resulting PW fractions show a large variation (Alkire etal.,2015,2019). Exploring new tracers that provide novel or complementary information on PW fractions and the general role of PW and AW in influencing or even driving changes in the AO water column is a key aspect for understanding and predicting the role of the AO in global climate change (Årthun etal.,2019; Woodgate etal.,2010). 236U (T1/2=23.4 My) has been used in the last few years to track pathways and timescales of AW within the AO. As a chemical tracer, 236U has three key properties: it can be assumed to be conservative in seawater as the naturally occurring isotope 238U (Aya Sakaguchi etal.,2012; Dunk etal.,2002); its radioactive decay is negligible in the time frame of the oceanic processes of interest in the AO; and its main source terms to the AO are traceable and time-dependent, which can be used to estimate pathways and circulation timescales. 236U is produced by the interaction of 235U ( 238U) with thermal (fast) neutrons (Sakaguchi etal.,2009). Thus, it is present globally as a consequence of the GF, and regionally in the North Atlantic Ocean and AO mostly due to the liquid releases from Sellafield and La Hague RPs (Casacuberta etal.,2016,2018). It has been estimated that RPs have released about 250kg of 236U to the Irish and North Seas since the 1950s (Castrillejo etal.,2020). The worldwide budget of GF 236U is roughly 1,000kg (Sakaguchi etal.,2009; Winkler etal.,2012). Yet, only small fractions of those budgets have entered the AO. About 8kg of GF 236U (assuming an average atmospheric deposition of 1.13×10 12 at/m 2 for the 70–90° latitude range (Hardy etal.,1973; Sakaguchi etal.,2009)) was directly deposited on the surface of the AO during the nuclear testing period. That budget has been significantly increased since then due to the inflow of AW and PW (i.e., waters from mid latitudes that have seen a much higher direct atmospheric deposition of bomb-tests nuclides), continental runoff and precipitation. 236U has been widely redistributed in the AO through the complex water masses currents system. Therefore, studying 236U concentrations alone does not provide clear information on its sources. To overcome this problem, in the last few years, 236U has been used in combination with 129I in a dual tracer approach (Wefing etal.,2021). Recently, the study of another minor U isotope, 233U (T1/2=0.159 My), in Greenland Sea surface waters, has set the basis to the use of 233U/ 236U as a tool to disentangle anthropogenic U sources and thus trace water masses in the AO (Qiao, Hain, & Steier,2020). Unlike 236U, 233U (T1/2=0.159 My) is not produced in significant amounts in conventional nuclear reactors running on uranium fuel and providing a thermal neutron spectrum (Ho etal.,2019). Theoretical estimations point to 233U/ 236U atom ratios at the level of 10 −6 in burned nuclear fuel (Naegeli,2004), and values ranging from about 6×10 −8 to 5×10 −7 have been documented for La Hague RP releases (“HELCOM MORS Discharge database,” n.d.). This implies that releases from Sellafield and La Hague RPs would account for a total of less than 0.5g of anthropogenic 233U. On the other hand, significant amounts of 233U were released worldwide during the atmospheric nuclear testing period due to a series of weapons with specific designs (i.e., by those either using 233U as the fissile material or by those thermonuclear devices containing a tamper of 235U in which 233U is produced in 235U(n, 3n) reactions with fast neutrons (Hain etal.,2020)). To date, the most accurate estimation of the GF 233U/ 236U atom ratio based on a peatbog core from Germany is (1.40±0.15)×10 −2 (Hain etal.,2020). Assuming this ratio can be extrapolated to other latitudes, about 14kg of 233U would have been globally dispersed during the atmospheric testing of nuclear weapons. A minor fraction of this GF 233U would be present today in the AO, mostly due to its transport away from the Arctic and by dilution with inflowing AW and PW. Former Soviet Union thermonuclear tests at Novaya Zemlya, among them the largest one ever conducted, of 50Mt in 1961 (UNSCEAR.,2000), could have also contributed to the regional 233U budget in the AO. Although most of that signal would have already left the AO with outflowing waters (Wefing etal.,2021), a part of it might still be present in ice sheets or in the AO river catchments, thus potentially influencing the current 233U (and 236U) budget in the AO. Due to a lack of information on the 233U content of the above Funding acquisition: T. C. Kenna Investigation: E. Chamizo, M. Christl, M. López-Lora Project Administration: N. Casacuberta Resources: E. Chamizo, M. Christl, N. Casacuberta, T. C. Kenna Supervision: M. Christl, N. Casacuberta Validation: E. Chamizo, M. López-Lora Visualization: M. López-Lora Writing – original draft: E. Chamizo Writing – review & editing: M. Christl, M. López-Lora, N. Casacuberta, A.-M. Wefing Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 3 of 12 sources we considered them included in the GF source term. Thus, the 233U/ 236U ratio, involving two radionuclides with distinct sources, could be used to get more insights into the presence of anthropogenic U in the AO. With respect to the use of this ratio as a water mass tracer, a further advantage is represented by the fact that two isotopes of the same element behave geochemically identical (and conservatively in the U case) in seawater. Therefore, 233U/ 236U has the potential to become a robust water mass tracer that is less sensitive to many mixing processes with tracer-free waters. A direct application would be the study of the mixing between PW and AW: while nuclear testing period ratios should be characteristic of North Pacific Ocean waters (Eigl etal.,2017; Hain etal.,2020), low ratios (i.e., below 1×10 −2 according to the Greenland Sea results reported in (Qiao, Hain, & Steier,2020)) should be characteristic of AW since they have been in contact with RPs sources. The obtained results using the 233U/ 236U signature could complement the most recently reported information based on Ga concentration results (Whitmore etal.,2020) and the most conventional approach based on nitrate and phosphate concentration relationships (Peter Jones etal.,1998). In a different context, another application would be the study of the U fingerprint in Deep and Bottom Waters (DBW) in the AO in which the anthropogenic influence can be considered negligible (E. P. Jones etal.,1995). 233U and 236U are naturally produced on the surface of rocks mainly from cosmic radiation induced neutrons (i.e., thermal neutron activation of 232Th and 235U, respectively). During surface wheathering some U is dissolved by oxygenated waters and the isotopic signal is eventually transported into the oceans (Dunk etal.,2002). Thanks to the long residence time of U in oceanic waters (i.e., 0.32–0.56 My (Dunk etal.,2002)), this isotopic signal should be preserved in the oldest water masses that can be found, for example, in the AO. The natural 233U and 236U inprint in these waters is expected to be characterized by extremely low 233U and 236U concentrations and 233U/ 236U ratios above that of anthropogenic sources (i.e., above 150×10 −2, see SI for further details). On the other hand, anthropogenic radionuclides might be present in DBW from either top-down diffusion and particle bound transport as observed for other GF radionuclides in the AO, thus altering the natural U signal (Macdonald & Carmack,1990). The study of the 233U and 236U signatures in DBW in the AO could shed light on such processes and provide new insights into the U isotopic signature in natural waters in the AO. In this work, we examine the potential of the 233U/ 236U atom ratio as a tool to disentangle water masses fingerprinted with different U sources in the AO. The study site is the western AO, sampled in 2015 during the GN01 US GEOTRACES expedition (Figure1). Seawater samples collected at full-depth profiles in the Amerasian Basin allowed us to cover the widest possible range of 233U and 236U concentrations in the AO and to explore the 233U/ 236U signature for both natural and artificial U sources. Figure 1. (a) Situation of the GEOTRACES GN01 transect with the main pathways of Pacific Waters (PW, in green) and Atlantic Waters (AW, in red) currents circulating at surface in the Arctic Ocean, and (b) sampling stations grouped by geographical areas. Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 4 of 12 2. Materials and Methods 2.1. Sampling and Hydrographic Parameters This study is based on samples taken during the GEOTRACES expedition onboard the US Coast Guard icebreaker Healy, which covered two main sections in the Amerasian Basin (GN01 section, Figure1). In this work, 140 seawater samples from 22 stations were selected for the study of both 233U and 236U (Table S1 in Supporting InformationS1). The selection of stations is based on profiles containing different water masses. Seawater sampling was carried out with 12 Niskin bottles of 30L each, and standard conductivity, temperature, and pressure sensors. Ice stations were sampled from small boat casts and Niskin bottles paired with McLane pumps. Samples were filtered by gravity through Teflon-lined Tygon tubing and Supor Acropak 500 capsule filters (0.8/0.4μm pore size), immediately acidified to pH 2 with distilled 6M HCl and shipped to the Lamont Doherty Earth Observatory (LDEO) premises in New York (GEOTRACES cruise report,2015). The samples are further characterized according to their potential temperature and salinity (T-S plot, Figure2). Bering Strait samples represent pure PW entering the AO, with salinities below 33 PSU and temperatures above Figure 2. Temperature and Salinity properties of the studied samples grouped by geographical areas (Figure1b) and color coded according to the distribution of water masses considered in this work: Pacific Waters, PW, in gray, corresponding to Bering Strait samples; Polar Surface Waters, PSW, in blue, representing the 0–100 depth interval; Atlantic Layer, AL, in red, corresponding to the 200–800m depth range in the Chukchi Sea and Canada and Makarov Basins (Amerasian Basin); and Deep and Bottom Waters, DBW, in purple, for those samples below 2,000 m depth. Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 5 of 12 3°C. Polar Surface Waters (PSW) represent the layer where PW and AW, the latter circulating in the upper water column of the Eurasian Basin, might mix with melted sea-ice, river run-off and precipitation. Here all samples from the 0–100m depth range were classified as PSW, exhibiting a wide range of salinities (i.e., between 22.8 and 33.2 PSU) and temperatures (i.e., −1.6°C–1.9°C). At the Amerasian Basin and the Chukchi Sea slope, the increase in salinity and potential temperature around 34.5 PSU and 0.5°C, respectively, indicates the presence of the core of AW circulating within the whole AO at middepth layers (i.e., Atlantic Layer, AL), assumed here to cover the 200–800m depth and salinities above 34 PSU following the approximation given in (Wefing etal.,2021). Deep and Bottom Waters, DBW, with salinities close to 35 PSU and temperatures down to −0.5°C, are present at the deepest stations in the Amerasian Basin (i.e., below 2,000 m depth). 2.2. Radiochemistry and Measurements The analysis of 236U and 233U concentrations was performed on 5L samples. Radiochemical sample preparation and AMS measurements were carried out as a collaborative effort between LDEO and the AMS facilities at ETH Zürich, Switzerland, and at the Centro Nacional de Aceleradores (CNA) in Seville, Spain. At the LDEO, U was concentrated by Fe(OH)3 co-precipitation. The Fe precipitates were sent to CNA where the U was chemically purified based on the method from (López-Lora etal.,2018). Following the same working methodology adopted in (Villa-Alfageme etal.,2019), samples were divided in two groups for the analysis of the 233U/ 238U and 236U/ 238U atom ratios either at CNA on a 1MV AMS system or at ETH on a 600kV AMS facility. Details on the 236U AMS measurement techniques at the ETH can be found in (Christl, Casacuberta, Lachner, etal.,2015) and at the CNA in (Chamizo & López-Lora,2019). At both facilities U isotopes were measured using the oxide method, He gas was used as stripper, and U 3+ ions were analyzed on the high-energy side with about 35% overall transmissions. The deepest samples from the Amerasian Basin, assumed to contain the lowest 236U/ 238U and 233U/ 238U atom ratios, were analyzed at ETH, where abundance sensitivities can reach the 10 −13 level or below thanks to its optimized ion-beam design. The 1MV CNA AMS system features a compact design which imposes restrictions to the minimum U atom ratios that can be measured (i.e., estimated at the 10 −10 and 10 −11 levels for 236U/ 238U and 233U/ 238U in seawater, respectively). Overall, 51 and 89 samples were prepared for their analysis at the CNA and ETH, respectively. Additionally, one procedural blank for every 10 processed samples (15 altogether) was prepared to control possible 233U and 236U contamination issues. New glassware and reagents of the highest purity were used during the sample processing to avoid background sources. The AMS analysis of 233U in 5L seawater samples is challenging because the expected 233U/ 238U atom ratios are at the 10 −11 level and below, leading to very low 233U count rates in all the samples. For example, even when assuming 100% chemical recovery and a quoted detection efficiency of 2×10 −4 will lead to less than 100 registered 233U counts from a 5L sample with a 233U/ 238U ratio of 10 −11, resulting in 1 sigma counting uncertainties of more than 10% for the measured 233U/ 238U ratio. Since instrumental instabilities are at the level of 1% and 2% at the ETH and CNA facilities, respectively, final uncertainties are dominated by counting statistics. To minimize and control the 233U and 236U AMS-related background problems, CNA and ETH AMS analyses were performed under clean ion-source conditions, and several instrumental blanks (i.e., unprocessed blank samples) were integrated in the measurement sequences. In-house standard materials were used to normalize the resulting 233U/ 238U and 236U/ 238U atom ratios (i.e., the ETH ZUTRI (Christl etal.,2013) and the CNA-U236 (Chamizo etal.,2015) standards). In general, procedural and instrumental blanks produced similar and reproducible 233U and 236U count rates. Background correction of the samples was performed by subtracting the number of 233U ( 236U) counts, given by the average number of counts of 233U ( 236U) of all blanks, from the total number of measured 233U ( 236U) counts (i.e., the integration time was the same for blanks and seawater samples). While the background correction for 236U did not have a significant effect on most 236U/ 238U results, the net (background corrected) number of 233U counts ranged between 1 and 40, which implies final relative 1 sigma counting uncertainties between 100% and 15% for the measured 233U/ 238U ratios, respectively. For 17 samples, only upper limits (on a 2 sigma level) for the 233U/ 238U ratios are provided because background corrected 233U/ 238U ratios were indistinguishable from zero. Additionally, from each sample 5ml seawater aliquots were collected by LDEO for 238U analyses by Inductively Coupled Mass Spectrometry (ICP-MS) using the isotope dilution method at the CITIUS (Centro de Investigación Tecnológica e Innovación Universidad de Sevilla) premises, in Seville, Spain. The 238U ICP-MS results were validated through the analysis a series of samples with well-known U isotopic compositions during the same Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 6 of 12 sample batch. A comparison between the obtained ICP-MS results and the values predicted by salinity data using the formula given in (Owens etal.,2011) is presented in Figure S1 in Supporting InformationS1. Only three samples produced unexpectedly high 238U ICP-MS concentrations, which could be attributed to a contamination problem during the handling of the ICP-MS aliquots. For those three samples the results predicted by the salinity data were used instead of the ICP-MS data. The 238U concentrations were used to calculate the 233U and 236U concentrations in the seawater samples using the 236U/ 238U and 233U/ 238U ratios from the AMS measurements (Table S1 in Supporting InformationS1). 3. Results The results in this work are grouped in three geographical regions: (a) Bering Sea and Bering Strait, (b) Chukchi Sea, and (c) Amerasian Basin, which includes the Makarov and the Canada Basins (including ice stations; Figure1). The 236U and 233U concentration profiles and the calculated 233U/ 236U atom ratios are presented in Figure3. Overall, 236U concentrations range from (0.035±0.017)×10 6 to (34.8±1.2)×10 6 at/kg, and 233U from (0.016±0.007)×10 6 to (0.257±0.065)×10 6 at/kg. Lowest concentrations of 236U and 233U may indicate the relative increase of the natural (pre-anthropogenic) U signal, with estimated, but never measured, values below 10 4 at/kg for both radionuclides (Peppard etal.,1952; Steier etal.,2008). The highest values clearly reflect the presence of anthropogenic U, with 236U and 233U concentrations at levels of 10 7 and 10 5 at/kg, respectively (Qiao, Hain, & Steier,2020; Qiao, Zhang, etal.,2020). 233U/ 236U atom ratios show a very large variation, moving from (0.34±0.16)×10 −2 to values above 10×10 −2. The lowest ratio is still a factor of 3 higher compared to the Figure 3. Obtained 236U (a,b,c) and the 233U (d,f,e) concentration depth profiles grouped by the geographical regions considered in this work (Figure1b), including the results for the ice-stations (IS) in the Amerasian Basin. Plots d, e and g also include the obtained 233U/ 236U atom ratios (plotted in gray). Amerasian Basin 236U plot (c) comprises also a depth profile from the Eurasian Basin for the purpose of data comparison (Stat. 117 from the GN04 GEOTRACES transect, red lines and symbols, from (Casacuberta etal.,2018)), and plot f includes an inset showing the 233U/ 236U ratios for the deepest samples. 236U concentration results for Stat. 56 were reported in (Wefing etal.,2021). Part of the 236U results for Stats. 43 and 48 at the Amerasian were published in (Mercedes López-Lora etal.,2021). Numerical values for the 233U and 236U concentrations can be found in Table S1 in Supporting InformationS1. Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 7 of 12 weighted average of both Irish Sea seawater and sediment impacted by Sellafield RP discharges reported in (Hain etal.,2020), of (0.12±0.01)×10 −2, and is 20% lower than the average value for Greenland Sea surface seawater in the 2012–2016 time period reported in (Qiao, Hain, & Steier,2020), of (0.42±0.08)×10 −2. The highest measured ratios, of (8±6)×10 −2 and (61±61)×10 −2, with large uncertainties due to the extremely low 233U and 236U involved concentrations, exceed all so far reported values in seawater samples. The distribution of 236U in seawater profiles (Figure 3a–3c) shows a different trend compared to 233U (Figure3d–3f). In the Bering Sea and Bering Strait concentrations of 236U are relatively low (below 10×10 6 at/kg) and constant throughout all depths. In the Chukchi Sea, 236U concentrations increase below 100m depth. Deep profiles taken in the Canada and Makarov Basins (Amerasian Basin) show a peak of 236U between 100 and 1,000m depth, and a decreasing trend for greater depths. Profiles of 233U similarly show higher values between 100 and 1,000m depth at Amerasian Basin stations despite their large uncertainties. 233U/ 236U atom ratios (Figure3d–3f) also show a trend toward lowest values at such depth range, but more remarkable is the increasing trend seen in the deepest samples of the basin stations (Figure3f). To investigate this further, samples were clustered by water masses according to the definitions given in Section2.1, and the results expressed using boxplots in Figure4. Bering Strait samples, representing PW inflowing into the AO, are characterized by median 236U and 233U concentrations of 6×10 6 and 0.1×10 6 at/kg, respectively, meaning a median 233U/ 236U atom ratio of 1.7×10 −2. In the Chukchi Sea, PSW exhibit a slightly higher median 236U concentration, of 7.6×10 6 at/kg, but a similar 233U value of 0.1×10 6 at/kg, giving a lower 233U/ 236U atom ratio, of 1.2×10 −2. At the Amerasian Basin, PSW show increasing and more scattered 236U concentrations, reaching a median value of 14×10 6 at/kg, but still similar 233U levels, going down the median 233U/ 236U atom ratio to a value of 0.9×10 −2 (Figure4c and4d). The AL shows similar 236U and 233U median values both in the Chukchi Sea Shelf and in the Amerasian Basin, of about 24×10 6 and 0.15×10 6 at/kg, respectively, with a median 233U/ 236U atom ratio of 0.6×10 −2. The highest 236U concentration observed in the western AO ((34.8±1.2)×10 6 at/kg for the sample at 305m depth at Stat. 56 in the Canada Basin (see also Wefing etal.,2021)) exceeds the maximum concentrations reported for the entire eastern AO in 2015 ((24±1)×10 6 at/ kg (Casacuberta etal.,2018)). DBW in the Amerasian Basin exhibit the lowest 236U and 233U concentrations, with median values of 0.25×10 6 and 0.03×10 6 at/kg, respectively. The minimum measured 236U concentration, (0.03±0.02)×10 6 at/kg, is 30 Figure 4. Boxplots showing the distribution of the obtained 233U, 236U and 233U/ 236U results for Bering Sea and Bering Strait samples (in gray), Polar Surface Waters (in blue), Atlantic Layer (in red) and Deep and Bottom Waters (in purple). Plot c includes the estimated 233U/ 236U atomic ratio in this work for pre-anthropogenic oceans or lithogenic background (LB), of about 150×10 −2 (SI). Plot d includes the reported 233U/ 236U atomic ratios for global fallout, of (1.40±0.15)×10 −2, and as for the Irish Sea the weighted average for both seawater and sediment samples, of (0.12±0.01)×10 −2, has been considered (Hain etal.,2020). Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 8 of 12 times below the lowest documented value for the Eurasian Basin in (Casacuberta etal.,2018), of (0.9±0.1)×10 6 at/kg. The 233U/ 236U atom ratios measured in DBW, with a median value of 7×10 −2, are higher compared to those of both PSW and the AL. 4. Discussion The average 236U concentrations are plotted versus the average 233U concentrations (Figure5a) and versus mean 233U/ 236U atom ratios (Figure5b) for every studied water mass and geographical area according to the description in Figure1b. Based on our results three main conclusions are drawn and discussed in the following: (a) In PSW, PW can be unraveled from AW using the 233U/ 236U atomic ratio: (b) the AL has a distinct isotopic signal from PSW; and (c) the unexpectedly high 233U/ 236U atomic ratios in DBW reflect the relative increase of natural (pre-anthropogenic) U in the deep AO. 4.1. Polar Surface Waters In the surface layer of the Amerasian Basin, PW flowing through the Bering Strait meet AW traveling through the eastern basin and reaching the western basin. Information on their mixing can be obtained if the results in this work are interpreted using the corresponding endmember values for those two water masses. Pure PW in this work is represented by Bering Strait samples, with average 236U and 233U concentrations of (6.25±0.10)×10 6 and (0.10±0.02)×10 6 at/kg, respectively, and an average 233U/ 236U atom ratio of (1.62±0.25)×10 −2 (Table1). Surface Atlantic waters in the Eurasian Basin are mostly formed of AW entering the AO through the Fram Strait and the Barents Sea (Figure1a). Using the 236U concentration reported in (Casacuberta etal.,2018) for the surface layer (i.e., 10–35m depth range) of the Eurasian Basin for also the 2015 sampling expedition, an average concentration of (17±4)×10 6 at/kg is assigned to this water mass. Note that the 236U concentration in inflowing surface Atlantic Waters to the AO is time dependent (Christl, Casacuberta, Vockenhuber, etal.,2015), but in this work the above mentioned value was considered as a first approach to the problem. Due to a lack of experimental 233U data for the Eurasian Basin, we assume that 233U concentrations are like the values we measured in the Amerasian Basin, with a weighted average of (0.10±0.02)×10 6 at/kg. This assumption is supported by the fact that the measured 233U concentrations in PSW in this work are in reasonable agreement with the reported concentrations for the West and East Greenland Sea in the period 2012–2016 in (Qiao, Hain, & Steier,2020), with an average value of (0.07±0.01)×10 6 at/kg, representing outflowing surface waters from the AO. Thus, the characteristic 233U/ 236U for the surface Atlantic layer in the Eurasian Basin is assumed to be (0.60±0.15)×10 −2 (Table1). Note that the Eurasian Basin surface waters values are just estimations we consider in this work to explore the potential of 233U/ 236U as a water mass tracer in the AO. Future measurements of 233U in seawater samples from the Eurasian Basin will help to better constrain this endmember for mixing calculations. When looking at the concentrations of 233U and 236U (Figure5a), results show that samples at the Chukchi Sea plot closer to the PW endmember values, whereas Makarov Basin values approach the endmember concentrations for the surface Atlantic Layer in the Eurasian Basin. 236U average concentrations at the Canada Basin are roughly 30% higher compared to the Chukchi Sea, pointing to the presence of AW in the Canada Basin surface waters. The 233U/ 236U ratio decreases consistently from the Chukchi Sea to the Makarov Basin (Figure5b). Canada Basin results plot in between the Makarov Basin and the Chukchi Sea. Drawing the mixing line between a hypothetical water mass of Pacific origin and another one representing pure surface AW in the Eurasian Basin, it is observed an increasing presence of AW in PSW from the Chukchi Sea to the Makarov Basin (Figure5b). This estimation Figure 5. Average 236U concentration versus average 233U concentration (a), and average 236U concentration versus average 233U/ 236U atomic ratio (b) by water mass and geographical area according to the description in Figure1b. Endmember values for the most representative water masses have been also included (gray symbols, Table1). In every case, error bars represent the standard deviation of the mean. Plot b includes the binary mixing line between Pacific and Atlantic Waters in Polar Surface Waters. In this case, error bars result from the error propagation of the individual endmember values. Journal of Geophysical Research: Oceans CHAMIZO ET AL. 10.1029/2021JC017790 9 of 12 excludes the essentially tracer free waters (meteoric waters and sea ice melt), which could go up to 10%–20% (Paffrath etal.,2021). These results contrast with those worked out in (Whitmore etal.,2020): using the Ga concentration alone in samples corresponding also to the GN01 transect, almost negligible fractions of AW were identified at the Chukchi Sea and at the Canada Basin. The conclusions of our study represent a first best estimate using the 233U/ 236U atomic ratio as a fingerprint of water masses. Uncertainties underlying the characterization of the involved endmember values in both approaches might explain the different results. It is important to underline the solidity of the 233U/ 236U atomic ratio as a water mass tracer thanks to the long residence time of U in seawater (i.e., about 0.4 My (Dunk etal.,2002), two orders of magnitude longer than the one of Ga (Whitmore etal.,2020)) and the fact that isotopic ratios are not influenced by biogeochemical processes. 4.2. Atlantic Layer Samples from the mid-depth AL in the Chukchi Sea, Canada Basin and Makarov Basin show about 3.5, 2.5 and 1.5 times, respectively, enhanced average 236U concentrations compared to PSW. Overall AL 233U values are about 30% above the measured ones in PSW (Figure5a). The 233U/ 236U isotopic signals in the AL are thus distinct (Figure5b). It can be observed that the obtained average results for the three investigated regions agree within uncertainties (Figures5a and5b). This means that the data obtained in this work can be used to establish representative values for the Atlantic Layer in the Amerasian Basin. Endmember values for the 236U and 233U concentrations are fixed to (24±2)×10 6 and (0.13±0.01)×10 6 at/kg, respectively, resulting in a 233U/ 236U atom ratio of (0.53±0.06)×10 −2 (Table1). This atomic ratio would mean that roughly 60% of the 236U in the AL in the western AO might come from RPs. The 30% enhanced 233U signal in the AL compared to PSW could be explained considering the circulation time scales of AW in the AO. According to the simulations in (Christl, Casacuberta, Vockenhuber, etal.,2015), the GF 236U and 233U signals in the AW entering the AO would have peaked in the 1960s with a subsequent decaying trend reaching a steady level in the 1990s. Longer circulation times of AW in the AL compared to surface waters (i.e., of about 10years, according to the results in (Wefing etal.,2021)) could explain the different 233U signal. The higher 236U levels in the AL compared to PSW might indicate either a more advective flow of AW reaching this basin compared to PSW, or releases of higher 236U from Sellafield and La Hague RPs (Wefing etal.,2021). It is interesting to mention that the measured 233U/ 236U atomic ratios for the AL in this work, agree with the ones that can be modeled assuming the mixing of water streams upon entering the AO worked out in (Casacuberta etal.,2018; see Section1 and Figure S2 in Supporting InformationS1 for further details). 4.3. Deep and Bottom Waters The extremely low 233U and 236U concentrations in DBW in the western basin (Figure5-a) corroborates the long residence times of these waters and their isolation from the above layers (i.e., absence of convection of dense waters produced on the shelves; Macdonald & Carmack,1990). More remarkable is, however, the behavior of Water mass 236U [×10 6 at/kg] 233U [×10 6 at/kg] 233U/ 236U [×10 −2 at/at] Mean Uncert. Mean Uncert. Mean Uncert. Pacific Waters Bering Strait results in this work 6.25 0.10 0.10 0.02 1.62 0.25 Surface Atlantic Layer in the Eurasian Basin 236U result from the surface layer (0–35m deep) in the Eurasian Basin in (Casacuberta etal.,2018). 233U results from Polar Surface Waters in the Amerasian Basin in this work 17 4 0.60 0.15 Middepth Atlantic Layer in the Amerasian Basin Results from this work 24 2 0.13 0.01 0.53 0.06 Deep and Bottom Waters (pre-anthropogenic oceans) 236U data from (Steier etal.,2008). 233U/ 236U atom ratio estimated in this work assuming cosmogenic production in the surface of rocks (Lithogenic Background) (section SI) 5×10 −3 5×10 −3 8×10 −3 8×10 −3 150 (lower limit) Note. Uncertainties are expressed as standard deviations. Table 1 Estimated Endmembers Values for the 236U and 233U Concentrations and 233U/ 236U Atomic Ratio for the Water Masses in the Amerasian Basin