Accumulation of anthropogenic and natural radionuclides in bottom sediments of the Northwest Pacific Ocean

Activities of anthropogenic radionuclides in bottom sediments of the Northwest-Central and West Caroline basins of the Northwest Pacific Ocean are reported. Down-core distributions of natural radionuclides were also investigated in order to elucidate the sedimentation processes of anthropogenic radionuclides in the seafloor. In the Northwest-Central Basin, the inventories of 239+240Pu and 241Am decrease from south to north while the opposite trend is observed for 137Cs and 90Sr. This latitudinal gradient in the inventories of artificial radionuclides reflects higher fluxes of 239+240Pu and 241Am near the weapons test sites and transport of 137Cs and 90Sr over longer distances, as well as the regional differences in sediment-mixing and accumulation rates. In the West Caroline Basin, relatively low inventories of 239+240Pu and 241Am were observed due to the topographic isolation of deep water. Sediment-mixing rates were calculated using down-core distributions of 239+240Pu, 241Am, 137Cs and 90Sr based on two distinct models (continuous and constant) of tracer input and with a third model with pulsed input during the peak fallout period. The continuous input model yielded mixing coefficients comparable to those derived from excess 210Pb profiles within an order of magnitude.