Recent Changes In Detrital Supply And Organic Matter Burial Inferred From Elemental And Isotopic Geochemistry Of Dated Short Sediment Cores From The Changjiang Distal Mud Belt,East China Sea

Since the modern industrial revolution,human activities have dramatically affected the natural environment,especially the topic of global change with climate warming as the main cause of widespread concern.Carbon burial and carbon cycle are important factors affecting global warming,and related river processes,especially sediment transport and burial from the world's major rivers to the sea,are also closely related to climate change.In the relevant process,in addition to the direct impact of human industrial and agricultural activities on climate,there is still insufficient discussion on the impact of a large number of reservoir dams built in the basin on the corresponding climate change mechanism.The land-sea material cycle of the Changjiang and the East China Sea is one of the most important processes of carbon burial and cycling in the marginal sea of the continental shelf in the world.In the past 50 years,more than 5,000 reservoir dams have been built in the Changjiang Basin,and the Three Gorges Dam,the largest water conservancy project in the world,has been built.Reservoir dams regulate runoff in the Changjiang basin and intercept some coarse sediment while reservoirs store water.On the one hand,water storage and sediment storage make sed:iment transport into the sea,material transport and burial in the river process have a certain impact on carbon cycle and climate change mechanism;on the other hand,it also weakens the reliability of using climate-precipitation-river transport-carbon burial-climate feedback process to retrieve and reconstruct past climate change.Although the inversion of climate change in the relevant regions has been over the scale of thousands of years and ten thousand years,and the related disturbances caused in recent decades are not obvious,considering the drastic impact of modern human beings on climate,the short-term related processes deserve further discussion.Based on the analysis of geochemical characteristics of two short sedimentary cores,DH6-1 and DH7-3,in the Changjiang distal mud belt formed on the inner continental shelf of the East China Sea,the possible impacts of the reduction of sediment transport caused by reservoir dams in the Changjiang Basin on the sediment deposition in this mud belt,as well as the relationship between the Zhejiang-Fujian coastal current and the East Asian winter monsoon associated with sediment transport and burial Changes.Firstly,according to the related grain size characteristics in the sediment cores,and the changing trend of Al/Ti ratio and organic carbon content related to particle size,it is found that the proportion of coarse sediment particles decreases while the proportion of fine sediment particles increases in recent decades.At the same time,combined with indicators such as stable carbon and nitrogen isotopes in sediments,it is found that DH6-1 and DH7-3 of the two sedimentary cores are affected by different sedimentary mechanisms due to their different locations and water depths:the sedimentary characteristics of DH6-1 in shallow waters near the coast are more affected by coastal currents;while DH7-1 in the periphery of muddy areas in deep waters is less disturbed by coastal currents,which more reflects the changes in the transport of the Changjiang.For example,the construction of the Three Gorges Dam makes the proportion of coarse sediment particles decrease significantly,while the proportion of fine sediment particles increases.Finally,through in-depth study of DH6-1,which is greatly affected by the coastal currents in Zhejiang and Fujian,and comparing with the previous research results,it is found that the East Asian winter monsoon has been weakening since about 1975.