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The Fram Strait is a key region for investigating the exchange of Atlantic water with the Arctic Ocean. Uranium-236 (236U) from the two European nuclear reprocessing plants (NRPs) at La Hague and Sellafield provides a unique fingerprint in Atlantic water which can be used for studying its circulation patterns in the Arctic Ocean. And NRPs-derived 236U (236U NRPs) can be identified by its 233U/236U signature. In this study, we use colored dissolved organic matter (CDOM) absorption to constrain the selection of three Atlantic branch waters that carried different inputs of 236U NRPs in Fram Strait. This can potentially provide better estimates of transit times of Atlantic waters in the Arctic Ocean. High CDOM levels (a 350 ≥ 0.35 m − 1) in Fram Strait reflect the passage of Atlantic water transported to the Arctic by the Norwegian Coastal Current (NCC) and its extension and subsequently along the Siberian continental slope and shelf where the Ob, Yenisei and Lena rivers supply terrestrial organic matter with significantly high CDOM content. Conversely, low CDOM water represents Atlantic water that has remained off the shelf. Based on CDOM absorption, potential temperature (Θ), potential density (σ Θ) and 236U concentration, the path of a given body of Atlantic water could be inferred and an appropriate NRPs input function constrained so that transit times could be estimated. Our results indicate that Arctic High CDOM Water (a 350 ≥ 0.35 m − 1) sourced from the NCC and Barents Sea Branch Water (BSBW) in the Barents Sea Opening has an average of 7–27 yrs transit time in the upper ∼200 m of the western shelf of the Fram Strait. Atlantic Low CDOM Water (a 350 < 0.35 m − 1 , Θ > 2 °C) sourced from the Fram Strait Branch Water (FSBW) has a short pathway from the eastern Fram Strait. Arctic Low CDOM / High 236U Water (a 350 < 0.35 m − 1 , Θ ≤ 2 °C, σ Θ ≤ 27.97 or 236U concentration ≥15 × 106 atom/L) sourced from the BSBW and the FSBW has an average of 22–28 yrs transit time. These findings demonstrate how combining measurements of CDOM with 236U NRPs can improve the robustness in estimation of transit times of different Atlantic water pathways in the Arctic Ocean. There are limited ways to empirically derive estimates and the values provided offer unique data for comparison with estimates from regional circulation models. • New approach defining water classes by combining CDOM and 236U. • 233U applied to isolate the reprocessing plant signal of 236U in the Fram Strait. • Atlantic water reaching Siberian shelf has 14 yrs transit time in the Arctic Ocean. • Siberian shelf water with longer transit time has higher CDOM content. • Atlantic water remaining off-shelf has a transit time of 22–28 yrs. [ABSTRACT FROM AUTHOR]

The pathway and transport time of Atlantic water passing northern Europe can be traced via anthropogenic radioisotopes released from reprocessing of spent nuclear fuels at Sellafield (SF) and La Hague (LH). These reprocessing derived radioisotopes, with extremely low natural background, are source specific and unique fingerprints for Atlantic water. This study explores a new approach using 99 Tc- 233 U- 236 U tracer to estimate the transit time of Atlantic water in the coast of Greenland. We isolate the reprocessing plants (RP) signal of 236 U ( 236 U RP ) by incorporating 233 U measurements and combine this with 99 Tc which solely originates from RP, to estimate the transit time of Atlantic water circulating from Sellafield to the coast of Greenland-Iceland-Faroe Islands. Both being conservative radioisotopes, the temporal variation of 99 Tc/ 236 U RP ratio in Atlantic water is only influenced by their historic discharges from RP, thus 99 Tc/ 236 U RP can potentially be a robust tracer to track the transport of Atlantic water in the North Atlantic-Arctic region. Based on our observation data of 99 Tc- 233 U- 236 U in seawater and the proposed 99 Tc/ 236 U RP tracer approach, Atlantic water transit times were estimated to be 16-22, 25 and 25 years in the coast of Greenland, Iceland and Faroe Island, respectively. Our estimates from northeast Greenland coastal waters agree with earlier results (17-22 years). Therefore, this work provides an independent approach to estimate Atlantic water transit time with which to compare estimates from ocean modelling and other radiotracer approaches., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

This work reports the first dataset of 236 U and 233 U in sediment cores taken from the Barents Sea, with the aim to better understand the source terms of anthropogenic uranium in the Arctic region. Concentrations of 236 U and 233 U along with 137 Cs, and 233 U/ 236 U atomic ratio were measured in six sediment profiles. The cumulative areal inventories of 236 U and 233 U obtained in this work are (3.50-12.7) × 10 11 atom/m 2 and (4.92-21.2) × 10 9 atom/m 2 , with averages values of (8.08 ± 2.93) × 10 11 atom/m 2 and (1.08 ± 0.56) × 10 10 atom/m 2 , respectively. The total quantities of 236 U and 233 U deposited in the Barents Sea bottom sediments were estimated to be 507 ± 184 g and 7 ± 3 g, respectively, which are negligible compared to the total direct deposition of 236 U (6000 g) and 233 U (40-90 g) from global fallout in the Barents Sea. The integrated atomic ratios of 233 U/ 236 U ranging in (0.98-1.57) × 10 -2 reflect the predominant global fallout signal of 236 U in the Barents Sea sediments and the highest reactor- 236 U contribution accounts for 30 ± 14 % among the six sediment cores. The reactor- 236 U input in the Barents Sea sediments is most likely transported from the European reprocessing plants rather than related to any local radioactive contamination. These results provide better understanding on the source term of anthropogenic 236 U in the Barents Sea, prompt the oceanic tracer application of 236 U for studying the dynamics of the Atlantic-Arctic Ocean and associated climate changes. The 236 U- 233 U benchmarked age-depth profiles seem to match reasonably well with the reported input function history of radioactive contamination in the Barents Sea, indicating the high potential of anthropogenic 236 U- 233 U pair as a useful tool for sediment dating., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

We report the first combined dataset of 236 U and 233 U in the Greenland marine environment during the period of 2012-2016. Results are discussed in terms of time evolution and spatial distribution of 236 U concentration, and atomic ratios of 236 U/ 238 U and 233 U/ 236 U. 236 U concentrations along the Greenland coast are distributed within a relatively narrow range of (0.7-12.9) × 10 7 atom/L, corresponding to 236 U/ 238 U atomic ratios of (1.1-15.5) × 10 -9 . The 233 U/ 236 U atomic ratios obtained vary from 0.12 × 10 -2 to 1.16 × 10 -2 , with the majority distributed in the range of (0.2-0.7) × 10 -2 . We applied 233 U/ 236 U and 236 U/ 238 U atomic ratios in a binary mixing model to identify possible 236 U source terms. The results indicate that anthropogenic 236 U and 233 U in Greenland surface seawater originated from the direct global fallout (DGF) and the Sellafield and La Hague reprocessing plants (RP) is diluted by a third endmember, mostly likely natural ocean water (NOW), containing marginal 236 U and 233 U. A preliminary estimation of the source terms of 236 U using 233 U/ 236 U atomic ratios indicate that, for both eastern and western Greenland seawater, contributions from global fallout (GF) constitute about 30% of 236 U. The dominating source for 236 U, i.e. 70 %, is associated to reactor 236 U including discharges from RP and local reactor input in the Arctic Ocean., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)