Tracing the Three Atlantic Branches Entering the Arctic Ocean With 129I and 236U

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. Plain Language Summary In this work we present results of artificial radionuclides that were measured in seawater samples collected in the Arctic Ocean. 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 1960s until today) located in France and United Kingdom. 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. In this study, we collected about 150 seawater samples from 20 different stations in the Arctic Ocean in summer 2015, onboard the R/V Polarstern. Our results reveal that the mixing ratios of the two reprocessing plant effluents are different for the Fram Strait Branch and the Barents Sea Branch, contrasting with previous studies. The 129I/236U was used as a new tool to identify the char