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Tracing Atlantic Waters Using
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 I and
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 U in the Fram Strait in 2016

In this study I and U 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 I and U concentrations as well as U/U ratios are about two times higher (> 600 × 10 at kg, > 20 × 10 at kg, and 2.8 × 10, respectively) in the cold and fresh outflowing surface waters from the Arctic Ocean (Polar Surface Water, PSW) compared to inflowing Atlantic origin waters (300 × 10 at kg I, 12 × 10 at kg U, and 1.4 × 10 U/U). A comparison with the different I and U input functions for the Atlantic branches entering the Arctic Ocean reveals that themiddepth Atlantic origin waters outflowing the Arctic Ocean showmore 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 I and U 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 I and U 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 I 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 I and U in the waters outflowing the Arctic Ocean compared to those entering the polar region through the Fram Strait. The combination of I and U 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 I and U have a great potential as tracers to understand ocean circulation and travel times in the Arctic and North Atlantic oceans.