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

Abstract

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.