Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea

Abstract

Radiochemical measurements of 234Th (t 1/2 = 24days), 137Cs (bomb-produced), and 210Pb (t 1/2 = 22y) have been used to characterize rates of mixing, deposition, and accumulation on 100-day and 100-y time scales in East China Sea sediments. In the inner-shelf mud deposit near the mouth of the Changjiang (Yangtze River), 234Th data indicate deposition rates as rapid as 4.4 cm month −1 on a 100-day time scale. 210Pb data indicate that on a 100-y time scale accumulation rates are an order of magnitude slower (typically 1 to 5 cm y −1) than the short-term deposition rates. Most of the sediment deposited near the mouth of the Changjiang on a 100-day time scale is transported southward along the dispersal system on a 100-y time scale, probably as a result of winter storms and a strong coastal current. 210Pb accumulation rates from the inner-shelf mud deposit indicate that approximately 40% of the sediment discharged by the Changjiang can be accounted for in the sediments north of 30°N. The offshore mud deposit in the East China Sea is associated with the Huanghe (Yellow River) dispersal system. The dominant process affecting radionuclide profiles within this deposit is particle mixing (not sediment accumulation). In this area the upper 5 cm of the seabed are intensely reworked ( D b = 26cm 2 y −1) relative to the zone between 5 and 25 cm ( D b = 2cm 2 y −1). The maximum accumulation rate in the offshore mud deposit is 0.3 cm y −1. Less than 2% of the sediment discharged by the Huanghe can be accounted for in the offshore mud deposit. The relative intensity of mixing and accumulation is different for the proximal deposits of the Changjiang (where accumulation dominates) relative to the distal deposits of the Huanghe (where mixing dominates). Near the Changjiang mouth radiographs show distinct horizontal stratification, and the value of G (mixing rate/accumulation rate) is near zero. In the off-shore mud deposit radiographs reveal nearly homogeneous sedimentary structure, and the G value is ⩾18.