Sedimentary Evolution In The Western South Yellow Sea And Its Environmental Response Since 3.5Ma

Reconstructing the Plio-Pleistocene sedimentary evolution of the South Yellow Sea and chemical weathering history of its source region is important for understanding the long-term influence of the Yellow River(Huanghe)and/or Yangtze River(Changjiang)on the Asian marginal seas and possible links between river development,regional paleoenvironmental change,tectonic deformation and/or global climate change.In this study,we analyzed a long sedimentary Core CSDP-1 drilled from the western of the South Yellow Sea by the Continental Shelf Drilling Program of China.Based on the sediment grain size,clay mineralogy including composition,crystallographic characteristics and micro-morphology,mass accumulation rate,Sr-Nd isotopes(<63 ?m)as well as major and trace element geochemistry of clay-size fraction,we studied the sedimentary evolution and its control mechanism since the late Pliocene in the western South Yellow Sea.We tried to reveal the evolution of the Yangtze River and Yellow River since late Pliocene,the paleoenvirnomental evolution of Yellow and Bohai Seas,and the history of the chemical weathering in the Yangtze and/or Yellow River basins.Moreover,we discussed the combined effects of geomorphology,tectonic and climate change on evolution of river system and East Asian marginal sea.Our study suggests that large-scale transgression in the Yellow Sea occurred after ?0.8 Ma,possibly related to tectonic subsidence of eastern China coast and the Min-Zhe Uplift.In contrast,the sedimentary environment of Bohai and Yellow Seas was mainly dominated by fluvial/lacustrine deposits at 3.5�0.8 Ma.Provenance analysis suggests a major change in sediment provenance from the Yangtze River to the Yellow River at ?0.8 Ma,which corresponds to the timing of the final integration of the Yellow River in its present form.At the same time the major channels of the Yangtze River migrated from the South Yellow Sea to the modern Yangtze River delta due to the subsidence of east China coast.The consistence in timing of the provenance transition and large-scale regional marine transgression suggests that tectonic deformation,rather than climate change,is the first-order control on the evolution of the Yellow and Yangtze Rivers and sedimentary environmental change in the Bohai and Yellow Seas.In comparison with modern Southeast Asian rivers,the chemical weathering intensity as revealed by chemical index of alteration(CIA)in Yangtze River basin during 3.5-0.8 Ma is much higher than the modern Yangtze River basin,the chemical weathering intensity of the old Yangtze River basin is even as strong as the modern tropical region(e.g.,Thailand and Borneo).During 3.5-0.8 Ma,the variation of the chemical weathering intensity of the old Yangtze River basin is highly fluctuated,likely affected by both tectonic and climate change.Since 0.8 Ma,the degree of chemical weathering in Yellow River basin is medium with small variablity.The chemical weathering intensity of the Yellow River basin since 0.8 Ma is similar to the Chinese loess deposited since the last glacial,suggesting the major control of global cooling since the middle Pleistocene transition.In general,the gradual weakening of chemical weathering intensity in East Asia since the late Pliocene is a response to the global climate cooling.Chemical weathering in East Asia increased during 3.5-2.6 Ma,corresponding to the enhancement of East Asian summer monsoon and warm period of the late Pliocene.