Sedimentary Source Area And Paleoenvironment Analysis Since Late Miocene In The Southern South China Sea

The South China Sea,as the largest marginal sea of the Western Pacific.The continuous deep sea sedimentary sequences from late Miocene to Holocene were provided by Ocean Drilling Program?ODP?Leg 184 Site 1143 in the southern South China Sea,which were divided into two lithologic subunits?unit IA and unit IB?.In this paper,36 sediment samples?45.77-251.36 mcd?collected from the core of Site1143 were analyzed.The paleoclimate,paleoproductivity and provenance were discussed,paleoenvironment of the sediments at Site 1143 were reconstructed.Meanwhile,the proxy indicators of the East Asian monsoon and paleoproductivity in the southern South China Sea has been established,and compared with the deposition rate of sediments and clay mineral records.To restore the evolution of the East Asian monsoon in the southern South China Sea during 6-0.8 Ma,and to discuss the influence of the tectonic activities in Vietnam and the uplift of the Tibet Plateau on it.The contents of Fe,Al,Mg,Ca,Ti and Rb were obviously lower than that of the Upper Continental Crust?UCC?,while K and Na contents were higher than that of UCC.In addition to the significant negative correlation between Ca and Al,Fe,K,Mg,Ti and Rb all show strong positive correlation with Al,indicating that they were related to mica or clay minerals in sediments.The chemical index of alteration?CIA?in the sediment core were between 54 and76,with an average value of 60,which also reflects the moderate weathering intensity in the source area.During the period of 6-5.2Ma,the chemical weathering intensity of the sediment source area were relatively higher.Due to the tectonic uplift in Vietnam,the increase of physical erosion and summer monsoon in the source area increase the input of terrigenous materials,while the increase of summer monsoon intensity drives the increase of upwelling activity and precipitation,and the significant increase of CO₂contents in the atmosphere led to the increase of phytoplankton photosynthesis and paleoproductivity higher.At 5.2-1.7Ma,the climate in the sediment source area became cold,which led to the weakening of marine biological activities and the low productivity of the ocean.Among them,at 3.6-2.6Ma,it is suggested that the East Asian monsoon simultaneously strengthened and the climate was becoming cold and dry gradually.It was a response to rapidly tectonic uplift of the Tibet Plateau and the global sea level decline caused by the formation of the ice sheet.It shows that the increase of the input of terrestrial nutrients also corresponds to the increase of the productivity of the sea surface.Since 1.7Ma,the winter monsoon has enhanced and the climate has become cold,and the input of aeolian dust minerals brought by winter monsoon has promoted,the water cycle in the basin has gradually increased and the nutrients produced at the bottom can be transported to the surface.therefore,the photosynthesis and paleoproductivity of marine organic matter on the surface have increased.Based on the analysis of element geochemistry and regional geological background,it is concluded that the tectonic environment of the provenance area of the sediments was continental island arc,and the source rock of the sediments may contain more felsic component,which is presumed to be mainly granite.The Mekong River may be the dominant potential provenance area of sediments at Site 1143.