Recent sedimentary records in the East China Sea inner shelf and their response to environmental change and human activities

摘要

The East China Sea continental shelf is a unique area for the study of land-sea interactions and paleoenvironmental change because it receives a large amount of terrestrial material inputs.In recent decades,human activities and global climate change have greatly affected river discharges into the sea.However,changes in the deposition process caused by these factors in the East China Sea continental shelf are unclear.We collected eight short sediment cores from the East China Sea inner shelf (ECSIS) using a box core sampler in 2012 and 2015.The grainsize,210Pb,and 137Cs of these cores were analyzed in order to reconstruct the deposition history since the 1950s,and to reveal human activity and climate change influences on sediment deposition in the ECSIS.Results indicated that sediment grain size became finer after 1969,turned coarser after 1987,and then further coarser since 2003,corresponding well to the three steps of sediment load drop in the Changjiang (Yangtze) River,which are mainly caused by human activities (particularly the closure of the Three Gorges Dam).Simultaneously,the East Asian Monsoon influenced the deposition process in the ECSIS by changing the intensity of coastal currents.Mean grain size variations in the fine-grained population (divided by grain size vs.standard deviation method) coincided with that of the East Asian Winter Monsoon strength and reflected its weakness in 1987.Abrupt changes in sediment grain size over a short time scale in these sediment cores were generally caused by floods and typhoons.Spectral analyses of the sediment cores showed periodicities of 10-11 and 20-22 years,corresponding to the periodicity of solar activity (Schwabe cycle and Hale cycle).Mean grain size time series also displayed a 3-8 year periodicity corresponding to El Ni(n)o Southern Oscillation periodic change.