Distribution and inventories of 90Sr, 137Cs, 241Am and Pu isotopes in sediments of the Northwest Pacific Ocean

Bottom sediments collected in the Northwest (NW) Pacific Ocean in 1997 were analysed for Sr-90, Cs-137, Pu-239,Pu-240 and Am-241 contents to determine their distribution patterns, inventories and sources. Enhanced inventories of Pu-239,Pu-240 and Am-241 were observed in the latitudinal belts of 10-20 degrees N and 30-40 degrees N, which correspond to major areas of local (tropospheric) and global (stratospheric) fallout (with a contribution from local fallout), respectively. The sediment inventory of Pu-239,Pu-240 near the Bikini Atoll exceeded its overlying water inventory, however, in the mid-latitudes, more than 70% of Pu-239,Pu-240 still remains in the water column. Am-241 inventories in sediments exceeded that of the water column for the entire NW Pacific Ocean. Higher Cs-137 and Sr-90 sediment inventories in the latitudinal belt of 30-40 degrees N are due to global fallout, and they account for about 10% and less than 5% of the water column inventories, respectively. The observed activity ratios of Cs-137/Sr-90, Pu-238/Pu-239,Pu-240 and Am-241/Pu-239,Pu-240 in sediment were at some stations higher than the global fallout ratios due to contributions from local fallout and due to specific processes in the water column. Two end-member mixing model based on the Pu-240/Pu-239 atom ratios observed in global and local fallout yielded similar to 60% contribution of the local fallout in the bottom sediments near the Bikini Atoll. The upward decrease in the Pu-240/Pu-239 atom ratios in the sediment column indicates a decrease in the contribution of local fallout to the Pu inventory with time. Am-241 and Pu-241 dating of sediment layers was utilized to explain a hiatus in sediment accumulation in the deep seafloor. (c) 2005 Elsevier B.V. All rights reserved.