This study presents the data on 129 I and 236 U 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 129 I and 236 U in a transect from the Norwegian Coast to the Makarov Basin to fully exploit the potential of combining 129 I and 236 U as a dual tracer to track Atlantic waters throughout the Arctic Ocean. The use of the 129 I/ 236 U and 236 U/ 238 U 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 129 I and 236 U 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 129 I/ 236 U atom ratio can now be used to identify the different Atlantic branches entering the Arctic Ocean. New input functions for 129 I, 236 U, and 129 I/ 236 U have also been described for each branch, which can be further used for calculation of transit time distributions of Atlantic waters. Plain Language SummaryIn 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 129 I and 236 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 1960s until today) located in France and United Kingdom. In particular, the input of 129 I 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 129 I/ 236 U was used as a new tool to identify the characteristics of these two Atlantic branches entering the Arctic Ocean and even prove the existance of a more surface branch (Arctic Shelf Break Branch) carrying a significant proportion of reprocessing plant-derived radionuclides.

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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).

Abstract. The inflow of Atlantic Water to the Arctic Ocean is a crucial determinant for the future trajectory of this ocean basin with regard to warming, loss of sea ice, and ocean acidification. Yet many details of the fate and circulation of these waters within the Arctic remain unclear. Here, we use the two long-lived anthropogenic radionuclides 129I and 236U together with two age models to constrain the pathways and circulation times of Atlantic Water in the surface (10–35 m depth) and in the mid-depth Atlantic layer (250–800 m depth). We thereby benefit from the unique time-dependent tagging of Atlantic Water by these two isotopes. In the surface layer, a binary mixing model yields tracer ages of Atlantic Water between 9–16 years in the Amundsen Basin, 12–17 years in the Fram Strait (East Greenland Current), and up to 20 years in the Canada Basin, reflecting the pathways of Atlantic Water through the Arctic and their exiting through the Fram Strait. In the mid-depth Atlantic layer (250–800 m), the transit time distribution (TTD) model yields mean ages in the central Arctic ranging between 15 and 55 years, while the mode ages representing the most probable ages of the TTD range between 3 and 30 years. The estimated mean ages are overall in good agreement with previous studies using artificial radionuclides or ventilation tracers. Although we find the overall flow to be dominated by advection, the shift in the mode age towards a younger age compared to the mean age also reflects the presence of a substantial amount of lateral mixing. For applications interested in how fast signals are transported into the Arctic's interior, the mode age appears to be a suitable measure. The short mode ages obtained in this study suggest that changes in the properties of Atlantic Water will quickly spread through the Arctic Ocean and can lead to relatively rapid changes throughout the upper water column in future years.

The Nd isotopic composition (eNd) of seawater and cold-water coral (CWC) samples from the Gulf of C adiz and the Alboran Sea, at a depth of 280-827 m were investigated in order to constrain middepth water mass dynamics within the Gulf of C adiz over the past 40 ka. eNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of C adiz reveals relatively constant values (28.6 to 29.0 and 29.5 to 210.4, respectively). Such values are similar to those of the surrounding present-day middepth waters from the Mediterranean Outflow Water (MOW; eNd 29.4) and Mediterranean Sea Water (MSW; eNd 29.9). In contrast, glacial eNd values for CWC collected at thermocline depth (550-827 m) in the southern Gulf of C adiz display a higher average value (28.9 6 0.4) compared to the present-day value (211.7 6 0.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic eNd values ( 28) at 19, 23-24, and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW eNd values remained stable during the last glacial period, it is suggested that these radiogenic eNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin.