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Geochemical Journal
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231Pa in hemipelagic sediments: Is it affected by boundary scavenging?

Che Abd. Rahim Mohamed, Shizuo Tsunogai
Geochemical Journal, Vol. 32, No. 1, P. 11-20, 1998

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.

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