LATERAL TRANSPORT OF SEDIMENT PARTICLES IN THE OKINAWA TROUGH DETERMINED BY NATURAL RADIONUCLIDES

Settling particles at 640 m, 1390 m, and 1920 m depths in the Okinawa Trough and bottom sediments at six sites in the East China Sea were analyzed for U and Th isotopes, Pb-210, and Po-210. Concentrations of Th-232 in the settling particles were higher than those observed in the open ocean and were relatively constant with depth compared with other radiogenic nuclides. Fluxes and concentrations of these radiogenic nuclides increased with depth and the measured fluxes were larger than those calculated from water column inventories. While sinking through the water column of 640-1390 m, the increases in Pb-210 and unsupported Th-228 fluxes originating intensively in the shelf region were much larger than the unsupported Th-230 fluxes produced evenly in sea water. The Po-210/Pb-210 activity ratio in the settling particles was almost unity and the Th-228/Th-230 activity ratio of the radiogenic fraction was much smaller than the ratio of the production rates of these nuclides in the shelf water. These results suggest that a large fraction of the deeper settling particles, as well as the sediments in the trough zone originating in the continental shelf zone, are transported laterally and downward via the near bottom layer on a time scale of approximately 16 years on the average.