239,240Pu and137Cs in the East China and the Yellow Seas

The distribution and inventory of artificial radionuclides,^239,240Pu and¹³⁷Cs were determined in the East China and the Yellow Seas in 1987. Almost all of^239,240Pu and 50 to 80% of¹³⁷Cs in the continental shelf area are retained in the sediment column.^239,240Pu sediment inventory in the sea area is larger than the fallout input and tends to increase southwardly. This excess^239,240Pu and the lateral distribution are attributable to the supply of^239,240Pu by the Yangtze River discharge. On the contrary,¹³⁷Cs sediment inventory shows a decrease to the south, and the fact can be accounted for by the southward dispersion of fine silt particles discharged from the Yellow River. Total¹³⁷Cs inventory is smaller than the estimated fallout input, and the fact seems to indicate dispersion of¹³⁷Cs out of the shelf region. Vertical profiles of^239,240Pu and¹³⁷Cs contents in sediments differ from that of natural²¹⁰Pb, implying the effect of varied accumulation rates of the artificial radionuclides over the sediment particle mixing by benthic organisms. Apparent maximum sediment particle mixing coefficient (D_B) calculated from the excess²¹⁰Pb profiles in stations located between the inner and outer shelves ranged from 1.4 to 8.3 cm²y^–1. ThisD_B value is higher than that in the Okinawa Trough (1.0 cm²y^–1), but lower than previously estimatedD_B value (26 cm²y^–1) in the outer shelf mud.