Sedimentation of natural radionuclides on the seabed across the northern Japan Trench

Sediment cores were obtained at six stations in the region from the continental slope to the oceanic basin across the northern Japan Trench and analyzed for the natural radionuclides, 238U, 234U, 232Th, 230Th and 210Pb. The subsurface sediment was enriched in authigenic uranium-containing minerals and the layer with the maximum concentration tended to be deeper with increasing distance from the coast to the basin. The subsurface layer of the hemipelagic sediments seems to be a significant sink for the oceanic uranium. The activities of 210Pb did not decrease monotonically with depth, indicating the sliding of bottom sediments from the shallower seabed towards the trench. The sediment accumulation rate increased from the continental slope towards the trench and was the largest of 7.3 mm/yr at a station in the trench. The sedimentation fluxes of excess 210Pb and excess 230Th in the study area were also the largest in the trench and one to two orders of magnitude larger than their deposition fluxes predicted if all the nuclides are removed only from the overlying water column. This higher fluxes are partly due to the so-called boundary scavenging effect, namely the accelerated removal process at the boundary, but mostly due to the sporadic events of sliding of sediments once deposited on the shallower seabed.