210Pb and 234Th in settling particles collected by time-series sediment traps in the Okinawa Trough

Settling particles were collected from two locations in the Okinawa Trough (Stn. SST-1: 29–23.3°N, 128–14.5°E; Stn. SST-2: 28–08.2°N, 127–11.4°E) in the East China Sea continental margin from March 1993 to February 1994 by using time-series sediment traps and then analyzed for 210Pb. A few samples also were analyzed for 234Th. Sediment samples collected at the same locations also were analyzed for 210Pb. There was a clear tendency for 210Pb concentrations in settling particles to increase with depth. 210Pb fluxes showed large seasonal variations and increased with depth, with an especially large increase near-bottom. 234Th fluxes also showed a general tendency to increase with depth. Ratios of the annual mean 210Pb flux to the 210Pb deficiency flux were only 0.14 at SST-1 and 0.13 at SST-2 in the 600 m traps and 0.53 at SST-1 and 0.59 at SST-2 in the 800 m traps, showing a strong 210Pb flux deficit attributable to advective export from these sites. The annual mean 210Pb fluxes in the near-bottom traps were 2.30 times higher at SST-1 and 1.47 times higher at SST-2 than the 210Pb deficiency flux. These results might be attributable to lateral transport of particles that slowly slid down on the continental slope nepheloid layer while scavenging 210Pb. The annual net rates of laterally transported 210Pb were estimated to be 78.8 dpm/m2/day at SST-1 and 37.5 dpm/m2/day at SST-2 in the near-bottom layer. These rates were 3.5 times higher at SST-1 and 1.5 times higher at SST-2 than 210Pb flux observed in upper traps, indicating that lateral transport processes play a significant role in material transport on the continental margin in the East China Sea.