Sediment sources and the evolution of the East Asian monsoon in the eastern South China Sea basin: geochemical and clay mineral records from IODP Site 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. At present, most of the studies on sediment sources and monsoon climate in the South China Sea focus on shallow basins. Studies on deep basins are relatively lack, which constrains our 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 Site U1431 in the eastern basin of the South China Sea was used as the main research object. The well was drilled for the first time into a long-time continuous sedimentary sequence in the eastern deep-sea basin. We selected 34 and 31 mudstone samples for mineralogical analyses of major elements and clay fractions (＜2 μm), respectively, to explore the sediment source and the evolution of the deep-sea plains in the eastern South China Sea Basin and its response to the evolution of the East Asian monsoon. In addition, by comparing the material sources between the Pearl River Estuary Basin at the northern margin and the eastern basin, we revealed the spatial and temporal differences in sediment supply from different tectonic units in the South China Sea. The integrated principal elements, Sr-Nd isotopes, and detrital zircon U-Pb age data indicated that the early sediments in the Pearl River Mouth Basin at the northern margin of the South China Sea were the proximal sources of eroded continental margin rocks surrounding the South China; since ~25 Ma, the coastal water systems of South China, such as the Pearl River, gradually took inland rivers of South China, and subsequently the eroded continental margin became the main source of material to the Pearl River Estuary Basin in relatively simple composition. 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 due to the expansion of the South China Sea at 12.8-6.5 Ma, and the eastern part of Vietnam and the Pearl River contributed partial terrestrial detritus to the abyssal plain. Since ~6.5 Ma, the sediments at station U1431 were mainly from the Pearl River, eastern Vietnam, and Taiwan, and the sources were more complicated. Clay mineral analyses showed that the clay mineral contents in the mudstone of U1431core are dominated by illite and smectite, and less abundant 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 were strengthened at the same time. From 6 to 3 Ma, the East Asian monsoon was weakened. The winter winds were more weakened than the summer winds, while the summer winds remained relatively stable. Since ~3 Ma, the East Asian monsoon was gradually strengthened again; the summer winds weakened, while winter winds were significantly strengthened in comparison to the summer winds.