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.