A first transect of 236U in the North Atlantic Ocean

In this study the first comprehensive dataset of 236U in the North Atlantic Ocean is presented. 236U/238U ratios and 236U concentrations are determined in about 70 seawater samples collected from 9 depth profiles along a transect through the Northwest Atlantic Ocean during the first two legs of the GEOTRACES cruise GA02 in 2010. The cruise track was designed to follow the path of the North Atlantic Deep Water (NADW) from its formation region to the deep parts of the western Atlantic Ocean basins. Our results show that there is a strong North to South and surface to deep ocean gradient spanning a broad range of 236U/238U, from (44 ± 15) × 10−12 to (1477 ± 91) × 10−12 that correlates well with the distribution of water masses in this region. All measured ratios are above the theoretical pre-anthropogenic levels of 10−13 to 10−14, showing that the whole transect is dominated by anthropogenic 236U. Based on the calculated 236U inventories the 236U inputs of global fallout and direct releases from nuclear reprocessing plants could be constrained to a range from 1000 to 1400 kg and from 115 to 250 kg of 236U, from the two sources, respectively. Our results show that the 236U/238U atomic ratio can be used as a marker of North Atlantic water masses: with high ratios (>1000 × 10−12) characterizing northern water masses (LSW and DSOW); and lowest ratios (50-400 × 10−12) corresponding to the increasing influence of AABW and AAIW.