Reconstructing Changes In Sediment Composition And Productivity In Daya Bay Based On Isotopes And Geochemical Indicators

The three sediment cores collected from the inner bay,central bay and bay mouth in the Daya Bay were analysed for determining the mass accumulation rate and linear sedimentation rate via 210Pb and 137Cs datings.The contents of biogenic silica,organic carbon,total nitrogen,metals(Fe,Cu,Mn,Cd,Zn)and the isotopic compositions of carbon and nitrogen were also determined.The burial fluxes of biogenic silica and organic carbon were calculated based on the mass accumulation rate.The purpose of this study is to reveal the historical changes of sediment source,biological productivity and metal pollution in the Daya Bay.The range of mass accumulation rates and linear sedimentation rates in the Daya Bay were 0.01-0.95 g/cm2/yr and 0.01-1.92 cm/yr with the mean values of 0.25 g/cm2/yr and 0.35 cm/yr,respectively.The burial fluxes of biogenic silica and organic carbon were 0.01-0.12 mmol/cm2/yr and 0.01-0.26 mmol/cm2/yr with the mean values of 0.25 g/cm2/yr and 0.35 cm/yr,respectively.The positive relationships between the burial fluxes of biogenic silica and organic carbon and the mass accumulation rates were observed,indicating that mass accumulation rate was the major factor controling the burial of biogenic silica and organic carbon in the Daya Bay.The interannual variations of organic matter and stable carbon and nitrogen isotopes in the Daya Bay suggested that the increase of biological productivity began in 1960s in the northeastern bay,followed by the increase began in 1980s and 1990s in the eastern bay and the southeast bay mouth,respectively,which was resulted from the fertilizer utilization started in 1960s,and the increase of population and industrial development since 1980s and 1990s,respectively.The contents of Fe,Cu,Mn,and Cd were basically controlled by natural processes except Zn.The content of Zn showed a heavy pollution in the Daya Bay since 1860.The correlation coefficients between metal enrichment factors and terrestrial organic carbon and marine organic carbon showed spatial variations.The correlation coefficients between metal enrichment factors and terrestrial organic carbon were higher in core C1 collected from the inner bay were higher than those between metal enrichment factors and marine organic carbon,indicating the terrestrial input is more obvious in the inner bay.