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′240Pu and 137Cs. The sediment accumulation rates estimated from 230Thxsand 231Paxsprofiles yielded 0.4 to 1 cm/ky. The particle mixing coefficient, DBfor 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′240puand 137Cs. The DBvalues based on 239,240puand 137Cs data and a continuous input model, however, are roughly equal to those based on 210Pbxsexcept for one core showing subsurface maxima for the fallout nuclides. Generally, the fallout nuclides exist in detectable amounts down to 10–20 cm 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. © 1986, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.