Sediment Records Of Climatic And Environmental Changes In A Mangrove Wetland Of Shenzhen Bay During Past 60 Years

The Futian mangrove wetland of Shenzhen Bay is located in the subtropical western Pacific and subject to the influences of ENSO?El Ni?o-Southern Oscillation?and the urbanization of Shenzhen City and Hong Kong City,and therefore its sediments can record past climate change and human activities under the strong interaction of land and sea.In this study,a representative sediment cores?HS?were collected in the Futian Mangrove wetland.To deduce human activity and climate change over the past 60 years,geochemical elements?Cr,Co,Cu,Ni,Zn,Pb,Hg,As,Ti,Sr,Rb and Zr?,organic geochemical indicators(?¹³C,TOC,TN,TP and C/N)and granularity indicator were measured and analyzed.Subsequently,time series analysis and positive matrix factorization?PMF?methods were conducted to assess the relative contribution of climate change and human activities to the wetland environmental changes.The main conclusions drawn from this study are as follows:1.Positive matrix factorization?PMF?receptor model results revealed three sediment sources,i.e.strong hydrodynamic sources due to extreme climate events?Factor 1?,in situ source?Factor 2?,and tidal source?Factor 3?,which account for 43%,24%and 33%of heavy metals and grain size components in core HS in the past 60years,respectively.2.Through organic geochemical analysis(C/N ratio and?¹³C),it was found that the sediment organic matter in the Futian Mangroves wetland mainly come from freshwater algae,marine algae and mangrove,of which the most majority are freshwater algae with less marine contribution.3.The time series of sediment cores HS were analyzed with wavelet analysis?WT?and singular spectrum analysis-multi taper method?SSA-MTM?.The result of MTM showed that the periods of 2,3,4,5,11 and 13 years were commonly detected in trace elements?Cr,Co,Cu,Ni,Zn,Pb?,organic chemical indicators?TN,TP,TOC,C/N?,climate indicators?ENSO,sunspot activity?and Factor 1 representing extreme events.Therefore,periodic change in the sediment indicators can be ascribed to ENSO and sunspot activity.In addition,the WT results showed that periodic oscillations in element indicator and Factor 1 occurred interannually?4-8 years?and decadally?11-13years?from 1955 to 2015,which is consistent with the interannual oscillation?2-6 years?and decadal oscillation?11-13 years?of ENSO,and the decadally oscillations of sunspot activity?11-13 years?.Therefore,the periodic changes in sediment element contents over the last 60 years were caused by ENSO and sunspot activity,which jointly influence climate changes such as precipitation and moon pattern.4.The concentration of elements?Co,Cu,Ni,Zn,Pb and TP?increased rapidly during 1959 to 1987,declined from 1988 to 2002 year,and then increased slowly.The pattern of temporal variation coincides with Hong Kong's industrial progress from1950s through 1980s.In contrast,it is seemingly unrelated to Shenzhen's urbanization soared after 1985.The result indicated that the element content in HS core sediment of Shenzhen Bay are mainly sourced from the industrial development in Hong Kong.However,the increase of heavy metals and nutrition?TP?concentration in sediments after 2002 are influenced by the urbanization development of both two cities?Shenzhen and Hong Kong?.