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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). Plain Language Summary: In this work we reconstructed the circulation of Atlantic waters in‐ and outflowing the Arctic Ocean through one of the main gates connecting these two oceans: the Fram Strait, located between Greenland and Svalbard. We measured the long‐lived artificial radionuclides 129I and 236U to track the different water masses. These two radionuclides are present in the marine environment after the nuclear weapon tests (1950s‐1960s) and from two European nuclear reprocessing plants (from 1960's until today). In particular the input of 129I from these two reprocessing plants changed over time and can therefore also be used to estimate travel times of water masses. We collected 140 seawater samples at various depths from the Fram Strait in summer 2016. Our results depict higher concentrations of 129I and 236U in the waters outflowing the Arctic Ocean compared to those entering the polar region through the Fram Strait. The combination of 129I and 236U allowed us to distinguish between three main branches of Atlantic origin waters outflowing the Arctic Ocean having different travel times through the Arctic Ocean. We proved that 129I and 236U have a great potential as tracers to understand ocean circulation and travel times in the Arctic and North Atlantic oceans. Key Points: In 2016, inflowing Atlantic waters to the Arctic Ocean have lower 129I and 236U concentrations than outflowing Arctic watersHigh 129I and 236U in outflowing surface Arctic waters indicate substantial influence from the Norwegian Coastal CurrentCombination of 129I and 236U allowed for an estimation of transit times for Atlantic branches through the Arctic Ocean to the Fram Strait [ABSTRACT FROM AUTHOR]

The first dataset of 236 U/ 238 U in the water column of the Arctic Ocean (AO) is presented and shows the widest range of ratios reported so far in the open ocean, from ( 5 ± 5 ) to ( 3840 ± 260 ) × 10 − 12 . Surface samples and depth profiles were collected during two GEOTRACES expeditions in 2011–2012 and analyzed for the concentrations of 236 U and 129 I, with the aim of investigating whether the combination of 236 U/ 238 U and 129 I/ 236 U can be used as a new oceanographic tool in the AO. Results show that the distributions of the 236 U/ 238 U and 129 I/ 236 U atomic ratios are consistent with the different water masses in the AO. High 236 U/ 238 U and 129 I/ 236 U ratios in the upper water column ( > 2000 × 10 − 12 and >200, respectively) illustrate the penetration of Atlantic waters (AW) into the AO. Lower values were found in Pacific waters (PW) and deep waters of the AO. Rivers seem to represent a temporally and spatially-constrained third anthropogenic source of 236 U but more data are needed to confirm this. In a simple mixing model, the combination of 236 U/ 238 U and 129 I/ 236 U reveals a high contribution (>99%) of natural background waters (pre-nuclear era) in the deep and bottom waters of the Amerasian basin, indicating an apparent water mass renewal time of >1000 years. Despite the relatively high apparent age of the Amerasian Basin deep waters, this work shows the potential of using the dual-tracer approach as a new oceanographic tool in the Arctic Ocean. [ABSTRACT FROM AUTHOR]