Vertical distribution of 236U in the North Pacific Ocean

The first extensive study on 236U in the North Pacific Ocean has been conducted. The vertical distribution of 236U/238U isotopic ratios and the 236U concentrations were analysed on seven depth profiles, and large variations with depth were found. The range of 236U/238U isotopic ratios was from (0.09 ± 0.03) × 10−10 to (14.1 ± 2.2) × 10−10, which corresponds to 236U concentrations of (0.69 ± 0.24) × 105 atoms/kg and (119 ± 21) × 105 atoms/kg, respectively. The variations in 236U concentrations could mainly be attributed to the different water masses in the North Pacific Ocean and their formation processes. Uranium-236 inventories on the water column of each sampling station were calculated and varied between (3.89 ± 0.08) × 1012 atoms/m2 and (7.03 ± 0.50) × 1012 atoms/m2, which is lower than in former studies on comparable latitudes in the North Atlantic Ocean and the Sea of Japan. The low inventories of 236U found for the North Pacific Ocean in this study can be explained by the lack of additional input sources of artificial radionuclides, apart from global and regional/local fallout. This study expands the use of 236U as oceanographic circulation tracer to yet another ocean basin and shows that this isotope can be used for tracing circulation patterns of water masses in the Pacific Ocean.