Stratigraphic records and inventories of anthropogenic ²³³U and ²³⁶U in Baltic Sea sediments

This work reports the first comprehensive observation for the distribution of U-233 and U-236 in sediment cores collected from the major basins across the Baltic Sea. In addition to U-233 and U-236, this study encompassed the analysis of Cs-137, Pu-239 and Pu-240 in the same sediment core samples. Depth profiles of U-233 and U-236 in Baltic Sea sediments indicate that U-233 or global fallout (GF)-derived U-236 is a useful time marker, in addition to Pu239+240, to identify the onset of the Anthropocene epoch. Based on the cumulative areal inventories reported previously for water columns and investigated herein for sediment cores, our estimated overall inventories of U-233 and U-236 in the present Baltic Sea water and sediment are about 18 +/- 6 g and 2010 +/- 547 g, respectively, with >80 % accumulated in the Baltic seabed (i.e., 16 +/- 6 g of U-233 and 1694 +/- 522 g of U-236, respectively). The total budget for nuclear reactor (NR)-derived U-236 in the Baltic seabed is estimated to be 683 +/- 223 g. Despite covering only 13 % of the total area, the hypoxic regions (325 +/- 64 g) reserve a nearly equivalent amount of NR-derived U-236 as the oxygenated regions (358 +/- 159 g), highlighting the significance of the hypoxic regions in retaining NR-derived U-236. With a multi-radionuclide approach, this work contributes to a more thorough understanding on distribution and accumulation of anthropogenic uranium in the Baltic Sea.