Brown University, United States

Measurement of clay mineralogy at ODP Site 1146 in the northern South China Sea (SCS) indicates that illite, chlorite, and kaolinite content increased during glacial periods and smectite content increased during interglacial periods. The smectite/(illite+chlorite) ratio and smectite abundance were determined to be mineralogical indicators of East Asian monsoon evolution. At a 10 ka timescale, the prevailing southeasterly surface oceanic currents during interglacial periods transported more smectite from the south and east areas to the north, indicating a strengthened summer monsoon circulation, whereas counter-clockwise surface currents that dominated during glacial periods carried more illite and chlorite from Taiwan as well as from the Yangtze River via the Luzon Strait to the northern SCS, indicating a strongly intensified winter monsoon. Based on a 100 ka timescale, a linear correlation between the smectite/(illite+chlorite) ratio and the sedimentation rate reflects that the winter monsoon has prevailed in the northern SCS in the intervals of 2000-1200 ka and 400-0 ka and the summer monsoon has prevailed in the interval of 1200-400 ka. The evolution of the summer monsoon provides an almost linear response to the summer insolation of the Northern Hemisphere, implying an astronomical forcing of the East Asian monsoon.