First 236U data from the Arctic Ocean and use of 236U/238U and 129I/236U as a new dual tracer

The first dataset of U-236/U-238 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) x 10(-12). Surface samples and depth profiles were collected during two GEOTRACES expeditions in 2011-2012 and analyzed for the concentrations of U-236 and I-129, with the aim of investigating whether the combination of U-236/U-238 and I-129/U-236 can be used as a new oceanographic tool in the AO. Results show that the distributions of the U-236/U-238 and I-129/U-236 atomic ratios are consistent with the different water masses in the AO. High U-236/U-238 and I-129/U-236 ratios in the upper water column (> 2000 x 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 U-236 but more data are needed to confirm this. In a simple mixing model, the combination of U-236/U-238 and I-129/U-236 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. (C) 2016 Elsevier B.V. All rights reserved.