ABSTRACT

Sediment cores were obtained at six stations in the area across the northern Japan Trench including hemipelagic and oceanic basin zones, and analyzed for ²³¹Pa as well as ²³⁰Th. The concentration of ²³¹Pa in the surface sediments increased with both the distance from the coast and the increasing water depth in the slope and the trench zones except the oceanic basin. The vertical profiles varied widely from station to station, but they showed no distinct vertical trend at each station. Although the sedimentation rate of unsupported ²³¹Pa was more strongly controlled by the accumulation rate of bulk sediments than by its concentration, its sedimentation rates were about 200 times larger in the continental slope and the trench zones than in the oceanic basin. Furthermore, the ratios of ²³¹Pa/²³⁰Th decreased offshore. The boundary scavenging, therefore, is more pronounced for the removal of ²³¹Pa from the water column. The activity ratios of ²³¹Pa/²³⁰Th in the continental slope sediments were considerably greater than the production ratio in the water column of 0.092, and also remarkably larger in the western North Pacific than in the eastern North Pacific. The difference is probably due to biogenic opal, of which fraction is larger in the settling particles in the western North Pacific.