GRAIN SIZE RECORD OF TERRIGENOUS CLAST DURING MID-PLEISTOCENE TRANSITION (0.78~1.0 Ma) IN THE NORTHERN SOUTH CHINA SEA

High accumulation-rate sediments at Ocean Drilling Program (ODP) Site 1144 in the northern slope of the South China Sea provide unique materials for a high-resolution study on paleoenvironment. A high-resolution (200 a) record of the East Asian summer monsoon evolution in the northern South China Sea during the Mid-Pleistocene Transition (0.78~1.0 Ma) was revealed by grain size analysis of terrigenous clast at Site 1144. The result indicates that the terrigenous grain-size ratio of 3.5~8.2 μm/9~23 μm can be used as a proxy of the evolution of the East Asia summer monsoon, with relatively high values presenting strengthened summer monsoon. The ratio increased rapidly at 920 ka, indicating the beginning of the Mid-Pleistocene Transition with an association of the enhanced summer monsoon. Since then, the summer monsoon strengthened and weakened seasonally, showing a gradual change process of the Mid-Pleistocene Transition. The increase of East Asian summer monsoon intensity correlated well to the enhancement of the Northern Hemisphere insolation, demonstrating the orbital forcing of the East Asian monsoon evolution. The spectral analysis of terrigenous grain-size ratio (3.5~8.2 μm)/(9-23 μm) reveals millennial-scale climate fluctuations at intervals of 3.3 and 1.2 ka. The millennial-scale periods varied from pre-to post-920 ka, suggesting that the Mid-Pleistocene Transition behaved not only on orbital scale climate changes, but also on millennial-scale climate fluctuations.