Sediment sources and the evolution of the East Asian monsoon in the eastern South China Sea basin: geochemical and clay mineral records from IODP station U1431

The thick Cenozoic sediments in the South China Sea are the key carriers for the study of the “source-sink” process and monsoon climate evolution in the eastern margin of the Tibetan Plateau. Currently, most of the studies on sediment sources and monsoon climate in the South China Sea focus on shallow basins, and there is a relative lack of studies on deep basins, which constrains the overall understanding of the sediment transport system and monsoon climate evolution in the South China Sea. In this study, the deep-sea drill core from IODP station U1431 in the eastern basin of the South China Sea, which was drilled for the first time into a long-time continuous sedimentary sequence in the eastern deep-sea basin, was used as the main research object. We selected 34 and 31 mudstone samples for mineralogical analyses of major elements and clay fractions (＜2 μm), respectively, to explore the sediment source evolution of the deep-sea plains in the eastern South China Sea Basin and its response to the evolution of the East Asian monsoon. The comparison of the material sources between the Pearl River Estuary Basin at the northern margin and the eastern basin reveals the spatial and temporal differences in sediment supply from different tectonic units in the South China Sea. The results of integrated principal elements, Sr-Nd isotopes and detrital zircon U-Pb age data indicate that the early sediments in the Pearl River Mouth Basin at the northern margin of the South China Sea were proximal sources of denudation of the South China continental margin rocks. Since ～25 Ma, the coastal water systems of South China, such as the Pearl River, gradually assaulted inland South China and became the main source of material in the Pearl River Estuary Basin, with a relatively single source of material. On the other hand, the sediment source at station U1431 in the abyssal plain of the eastern basin was mainly affected by the magmatic activity after the expansion of the South China Sea at 12.8-6.5 Ma, and the eastern part of Vietnam and the Pearl River provided part of the terrestrial source of detritus for the abyssal plain. Since ～6.5 Ma, the sediments at station U1431 are mainly from the Pearl River, eastern Vietnam and Taiwan, and the sources are more complicated. Clay mineral analyses show that the clay mineral contents in the mudstone of Borehole U1431 are dominated by illite and smectite, with less abundance of chlorite and kaolinite. The sediment chemical alteration index (CIA) and smectite/(illite + chlorite) ratio indicate that the East Asian monsoon was relatively strong from 8 to 6 Ma, and the winter and summer winds strengthened at the same time. from 6 to 3 Ma, the strength of the East Asian monsoon weakened. The winter winds weaken relative to the summer winds, and the summer winds remain relatively stable. Since ～3 Ma, the East Asian monsoon has been gradually strengthening again, with the summer winds weakening and the winter winds significantly strengthening relative to the summer winds.