Vertical distribution of U in the North Pacific Ocean.

The first extensive study on U in the North Pacific Ocean has been conducted. The vertical distribution of U/U isotopic ratios and the U concentrations were analysed on seven depth profiles, and large variations with depth were found. The range of U/U isotopic ratios was from (0.09 ± 0.03) × 10 to (14.1 ± 2.2) × 10, which corresponds to U concentrations of (0.69 ± 0.24) × 10 atoms/kg and (119 ± 21) × 10 atoms/kg, respectively. The variations in U 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) × 10 atoms/m and (7.03 ± 0.50) × 10 atoms/m, which is lower than in former studies on comparable latitudes in the North Atlantic Ocean and the Sea of Japan. The low inventories of U 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 U 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.