The South China Sea (SCS) is one of the marginal seas found in the western Pacific Ocean. Sediments typically consist of terrigenous material, biogenic carbonates, and opal, as well as small amounts of volcanic material. The sea is mainly fed by discharges from the Mekong, Red, and Pearl Rivers. However, during past glacial stages, the paleo-Sunda River system may have contributed large amounts of sediment to the SCS (Shipboard Scientific Party, 2000). Our objective is to study the detrital input to hemipelagic sedimentation during the Pleistocene using clay minerals as tracers.
Clay minerals are thought to change through time because of climate modifications (Windom, 1976; Bouquillon et al., 1990; France-Lanord et al., 1993). Specifically, smectite, mixed-layer clay minerals, and kaolinite could be produced under high hydrolysis regimes in soils, whereas illite and chlorite are produced by physical erosion of igneous and other rocks. If these statements are correct on a long-term basis, very rapid changes in clay mineral content, such as those observed during the Pleistocene, cannot be a result of similarly rapid changes in hydrolysis conditions; instead, clay mineral assemblages could reflect changes in the sedimentation/erosion balance on the continent. Therefore, sea level changes could have caused a very efficient redistribution of clay minerals from the continent or shelf to the ocean.
During the Pleistocene, the Asian continent experienced many changes, especially because of glacial-interglacial climate oscillations. These changes in climate conditions could also have altered either the intensity or seasonality of the Asian monsoon. Such changes are likely to have been recorded in the sediments through their finest component, the clay minerals. While the history of the East Asian monsoon has been well studied from sediments of the Chinese loess plateau using different proxies such as grain size or clay minerals (An et al., 1990; Chen et al., 1997; Lu et al., 2000), very few similar studies of SCS sediments have been undertaken (Wang et al., 1999). Clay minerals are very sensitive to the inland hydrolysis regime. Our objective is to use the mineralogical-assemblage inclinations as tracers of the climactic changes.
The size of the exposed continental shelf has varied considerably during such climatic oscillations on similar timescales. Site 1146 is located ~400 km offshore from Hong Kong and the Pearl River mouth, at 2092 m water depth. At this location, 200 km separates the shore from the -100-m isobath contour line (Fig. F1). Accordingly, the drainage basin area of the Pearl River (and limited coastal rivers flowing into the SCS between the Hainan and Taiwan Islands) would have increased by ~45% when the sea level was located at around -100 m during glacial maxima.