Response Of Evolution Of The Salt Marsh In The Eastern Part Of Chongming Island To The Sediment Discharge From Yangtze River In Recent Six Decades

Chongming Island is the largest estuary alluvial island in China.Its formation and development relate closely with the fluxes of water and sediment from the Yangtze River.According to main seawalls constructed in the eastern part of Chongming Island during the last 60 years,the progradation rates and siltation area between seawalls of the eastern part of Chongming Island are calculated.The provenance and characteristics of sedimentary sequence of the salt marsh was studied,based on the analyses of grain sizes,environmental magnetism,organic matter and dating for six cores with sedimentary section of the salt marsh in the eastern part of Chongming Island in recent six decades.And in association with the change characteristics of the sediment discharge from Yangtze River since 1950s,we study on the response of evolution of the salt marsh to the sediment discharge from Yangtze River.Main seawalls constructed in the eastern part of Chongming Island during the last 60 years were determined,based on the Chongming county,Chongming water conservancy,Shanghai professional records Farm records Second series Reclamation,Coastal zone and coastal resources comprehensive survey report of Shanghai and other historical data,and combined with the field survey and consultation to the local people.Main seawalls in the eastern part of Chongming Island were constructed in1954,1964,1968,1992 and 1998,respectively,during the last 60 years.The seawalls were actually existing in the history and have the exact geographical location.It is feasible to study the shoreline change through the seawall.The method can provide a reference for studying the shoreline changes in long time scale.The shoreline progradation rate was great at first,then decreased markedly,and increased at last in the eastern part of Chongming Island during the last 60 years.The average annual increasing rate of siltation area was increased.If the area of Tuanjie sandbank was removed,the average annual increasing rate of siltation area was also great at first,then decreased markedly,and increased at last,consistent with the change trend of the shoreline progradation rate.The progradation rate that through study on the ratio of POC content to clay content was great at first,then decreased markedly,and increased at last in the eastern part of Chongming Island during the last60 years.This result was consistent with the progradation rate in the horizontal.The trend of progradation of the eastern part of Chongming Island is coincident with that of variation of the annual sediment discharge from the Yangtze River from the 1950s to the 1980s.This indicated that siltation of the salt marsh in the eastern part of Chongming Island was controlled by the sediment discharge from Yangtze River in this period.In the background of the sediment discharge reduction from the Yangtze River,there is a difference between both trends for the heavy reclamation period since1990s.This indicated that siltation of the salt marsh in the eastern part of Chongming Island was controlled by the sediment discharge from Yangtze River,but also influenced by the human activities such as reclamation period since 1990s.Ferromagnetic minerals dominates magnetic properties of the salt marsh in the eastern part of Chongming Island in recent six decades,and the grain size is mainly pseudo single domain?PSD?and multi-domain?MD?.Since the 1990s,the grain size of magnetic minerals tended to be thinner,the content of magnetic minerals and the content of incomplete antiferromagnetic minerals increased.The dating results of the CM-6 core are slightly better of salt marsh in the eastern part of Chongming Island in recent six decades.The results of ²¹⁰Pb and ¹³⁷Cs showed that the deposition rate of the modern salt marsh in the eastern part of Chongming Island was about 1.04-1.56cm/yr.The organic carbon stable isotope results showed that the organic matter of salt marsh in the eastern part of Chongming Island in recent six decades mainly came from suspended load carried by the Yangtze River runoff.Since the 1990s,the contents of organic matter?POC and TN?of salt marsh increased markedly,which may be related to the increase of clay content from the sediment discharge from Yangtze River.The material of the salt marsh in the eastern part of Chongming island is derived from the sediment discharge from the Yangtze River.The grain size of the salt marsh in the eastern part of Chongming Island in recent six decades was decreased.The results are coincident with the previous research results.In 1960s,the sediment discharge from Yangtze River was the most,and the sediment in different reaches of the Yangtze River mixed with each other.This leads to three poor correlations between the contents of organic matter?POC and TN?and contents of grains in different size fractions,between?¹³C and C/N ratios,and between?and SIRM,respectively.Subsequently,with the decrease of the sediment discharge from Yangtze River,the proportion of the sediment from the middle and lower reaches of the Yangtze River rises,and the material source of the sediment discharge from Yangtze River tends to be more consistent.There is a decline of both correlations between?¹³C and C/N ratios and between?and SIRM for the heavy reclamation period since1990s.This indicated that evolution of the salt marsh in the eastern part of Chongming Island was controlled by the sediment discharge from Yangtze River from the 1950s to the 1980s.Since the 1990s,evolution of the salt marsh in the eastern part of Chongming Island was controlled by the sediment discharge from Yangtze River,but also influenced by the human activities such as reclamation.