PA-231 IN HEMIPELAGIC SEDIMENTS - IS IT AFFECTED BY BOUNDARY SCAVENGING

Sediment cores were obtained at six stations in the area across the no rthern Japan Trench including hemipelagic and oceanic basin zones, and analyzed for Pa-231 as well as Th-230. The concentration of Pa-231 in the surface sediments increased with both the distance from the coast and the increasing water depth in the slope and the trench zones exce pt the oceanic basin. The vertical profiles varied widely from station to station, but they showed no distinct vertical trend at each statio n. Although the sedimentation rate of unsupported Pa-231 was more stro ngly 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 Pa-231/Th-230 decreased offshore. The boun dary scavenging, therefore, is more pronounced for the removal of Pa-2 31 from the water column. The activity ratios of Pa-231/Th-230 in the continental slope sediments were considerably greater than the product ion ratio in the water column of 0.092, and also remarkably larger in the western North Pacific than in the eastern North Pacific. The diffe rence is probably due to biogenic opal, of which fraction is larger in the settling particles in the western North Pacific.