Natural and man-made radionuclide distributions in Northwest Pacific deep-sea sediments: rates of sedimentation, bioturbation and ²²⁶Ra migration

Abstract

During the KH-80-2 Hakuho-Maru cruise in 1980, four sediment cores were obtained from abyssal basins of the western North Pacific. The cores were analyzed for U, Th, ²³¹Pa, ²²⁶Ra, ²¹⁰Pb, ^{239, 240}Pu and ¹³⁷Cs. The sediment accumulation rates estimated from ²³⁰Th_XS and ²³¹Pa_XS profiles yielded 0.4 to 1 cm/ky. The particle mixing coefficient, D_B for top ∼10cm was also estimated from excess ²¹⁰Pb profiles to range from 0.1 to 0.6cm²/y. Application of a mixing model with two different modes of input (pulse and continuous) showed limited success in reproducing the observed ^{239, 240}Pu and ¹³⁷Cs. The D_B values based on ^{239, 240}Pu and ¹³⁷Cs data and a continuous input model, however, are roughly equal to those based on ²¹⁰Pb_XS except for one core showing subsurface maxima for the fallout nuclides. Generally, the fallout nuclides exist in detectable amounts down to 10-20cm from the surface suggesting relatively rapid transport of these nuclides through burrows. These nuclides may be associated with surface derived, large particles of which food values are high, so that they are segregated and transported by benthic organisms to deeper depths causing separation from ²¹⁰Pb associated with fine particles. The ²¹⁰Pb inventory varies considerably from core to core, suggesting patchiness of ²¹⁰Pb deposition on the sea-floor. ²²⁶Ra fluxes are also highly variable depending on location, suggesting inhomogeneous supply from the ocean bottom to the overlying water. The input of ²²⁶Ra from Northwest Pacific sediments appears to be small compared to the radioactive decay of ²²⁶Ra in the overlying water column. This requires net lateral transport of ²²⁶Ra from other regions to the western North Pacific.