PALAEOENVIRONMENTAL EVOLUTION OF SUBAQUEOUS YANGTZE DELTA INFERRED FROM SEDIMENTARY RECORDS

A~60 m long core HYZK5 was taken from the subaqueous Yangtze River delta. It consists of fluvial, coastal marsh, estuarine-shallow marine (prodelta)and distributary facies deposited since the latest glacial maximum. Peat was formed during the last glaciation when the climate was cold and wet. Foraminifera abundance shows that sea level had risen rapidly since 12 000 cal.aBP, but paused during 11 200~10 000 cal.aBP. The data of grain size, magnetic susceptibility, organic carbon and nitrogen show obvious variations in sedimentary facies in the core. Magnetic susceptibility can reflect hydrodynamics to certain extent, and indirectly suggest the evolution of sedimentary environment. But it is not a good indicator of climatic change. Parameters including grain size, magnetic susceptibility, organic carbon and nitrogen in the estuarine-shallow marine (prodelta)facies are very stable, reflecting a relatively stable environment. By contrast, coarser particle size, higher magnetic susceptibility and lower organic elements as well as intensive fluctuations occurred at the top layer of the core indicate a high-energy environment under the joint action of tidal current, runoff and other dynamics. The above phenomena have also provided important evidence for the siltation of the North Branch of Yangtze River and the decline of its mouth bar. Owing to the intensive land-sea interaction and complicated post-depositional changes in the Yangtze River Estuary, it is necessary to seek other sensitive and reliable proxies for postglacial climatic evolution.