Pb-210 fluxes and sedimentation rates on the lower continental slope between Taiwan and the South Okinawa Trough

Abstract

Pb-210 and Ra-226 have been measured on 11 cores taken from the continental slope between northern Taiwan and the western South Okinawa Trough. Ra-226 activities generally fall between 0.5 and 2 dpm g −1. Pb-210 activities are widely variable in these cores: negligible excess Pb-210 (or no Pb-210 flux) is observed on the upper and middle slope while large excess Pb-210 values (50–90 dpm g −1) are present on the lower slope. Sedimentation rates estimated from the excess Pb-210 profiles of the cores range from 0.09 to 0.52 cm y −1 (0.08 – 0.42 g cm −2 y −1) for the lower slope and decrease toward the deeper (eastern) slope. Excess Pb-210 inventories and the Pb-210 fluxes calculated from the lower slope are 320–840 dpm cm −2 and 10–26 dpm cm −2 y −1 , respectively, assuming a steady-state input at each site for the past 100 years. Atmospheric input of Pb-210 and local production from Ra-226 do not account for the observed fluxes. Two alternative explanations are proposed: (1) boundary scavenging coupled with the Kuroshio water flowing through the area; or (2) downslope transport of high Pb-210 activity fine-grained particles from the shelf and upper/middle slope to the lower slope where they are deposited.