VARIATION OF ELEMENTS IN SEDIMENTS FROM THE HYPOXIA ZONE OF THE YANGTZE ESTUARY AND ITS RESPONSE TO SEDIMENTARY ENVIRONMENT OVER THE LAST 100 YEARS

We selected two cores in the Yangtze estuary, one within the region of seasonal hypoxia (named CJ43 core) and the other nearby but outside the hypoxia region (named CJ56 core). The grain size and elements, including ten kinds of major elements and thirteen kinds of trace elements, were determined on the ²¹⁰Pb-dated sediment cores. The two core sediments had different resources and represented different element distribution characteristics. Major sources of the CJ56 core sediment were from the old Yellow River estuary, and the distribution of most elements was controlled by "grain size effects". The sources of the CJ43 core sediment were mainly from the riverine input of the Yangtze River in summer and the burial fluxes of the dead marine organisms. The distribution of some redox sensitive elements (RSE) and biogenic elements did not conform with "grain size effects". Mo, Cd and As. have been enriched significantly, whose concentrations increased by about 83%, 73% and 50% respectively from 1970s. Conversely, Mn has been depleted since 1970s. The results indicate the redox environment of the bottom water-sediment interface has been changed, which might be caused by the eutrophication and hypoxia in the Yangtze estuary. Concentrations of biogenic elements, as well as Ca, Sr and P, have also increased by about 129%, 65% and 38% respectively since 1970s, which reflected the increasing of productivity and biomass influenced by the anthropogenic activities such as the fertilizer consumption during the last forty years.