Dating recent sediments from the subaqueous Yangtze Delta and adjacent continental shelf, China

摘要

In this study we analyzed sediment lithology, fallout of 210Pb and137Cs, and spheroidal carbonaceous particles (SCPs) for two short cores, YZE and CX38, obtained by gravity corer from the Yangtze River mouth offshore and adjacent continental shelf, to com-pare geochronological methods on the recent sediments of this area. Lithology and grain size changes in YZE suggested the re-discharging of the North Channel of the Yangtze River mouth by lfood events during 1949-1954 and associated accretion in the offshore area. This event was validated by a remarkable zone of declination in both 137Cs and210Pb activities and the absolute ages derived from the137Cs and SCPs. In contrast,210Pb results show obvious disturbance of grain size by sediment mixing and cannot be interpreted above 100 cm. In CX38, absolute ages for the early- and mid-1950s were derived by the137Cs and the SCP proifle respectively, which occurred in a reasonable sequence. The excess210Pb distribution shows exponentially decreasing activities with depth, and the mean sedimentation rate agrees roughly with the one inferred from the SCP proifle. We suggest that the limitation of the210Pb method needs consideration while the SCP proifle has the potential to provide a useful and independent dating method for recent Yangtze offshore and adjacent shelf sediments.