GEOLOGIC FRAMEWORK FOR THE SOUTH CHINA SEA
The opening of the SCS was genetically related to the deformation of Asia. The rhomboid-shaped Central Basin is the major deep-water oceanic crust feature of the SCS (Fig. 5A). Judging from the magnetic anomalies in the SCS Central Basin, seafloor spreading lasted from 32 Ma (magnetic Anomaly 11) to 16 Ma (Anomaly 5c), with a southward ridge jump at ~27 Ma (Anomaly 7/6b) (Briais et al., 1993). The opening of the SCS basin is thought to be linked with the Red River fault zone, which has at least 500 to 600 km of left-lateral displacement during the Oligocene and Miocene (Schärer et al., 1990; Briais et al., 1993). The slopes of the SCS contain numerous coral reef terrain systems that migrated during the SCS opening: the Nansha Terrain (Reed Bank and Dangerous Ground), the Xisha-Zhongsha Terrain (Macclesfield Bank and Paracel Island) and others (Jin, 1992).
The northern continental margin of the SCS has been extensively studied as part of oil exploration and geophysical studies to determine the amount of crustal extension during formation of the SCS (e.g., Hayes et al., 1995). The sedimentary basins of the northern shelf show a typical double-layer structure, with a lower section characterized by half grabens formed during Paleogene rifting and an upper section characterized by a wider distribution of deposits formed during the broad subsidence in the Neogene (Fig. 5B; Ru et al., 1994).
Modern sediments in the SCS consist mainly of terrigenous material, biogenic carbonate and opal, and a small portion of volcanic material. Clastic sediment is mainly discharged into the SCS from the Mekong River, Red River, and Pearl River. However, during the past glacial intervals, the paleo-Sunda River system provided a great amount of sediment into the SCS. Recent sediment trap studies in the northern SCS have shown that the highest particle-flux rates are correlated with high wind speed during the winter monsoon and, hence, the suspended matter from the East China Sea and the Pacific may exceed the amount of river input into the northern SCS (Jennerjahn et al., 1992). With the high terrigenous input and the location of the modern carbonate compensation depth (CCD) at 3500 m, the extensive continental slopes of the SCS are dominated by hemipelagic sediments; whereas, the deep-sea basin is covered by abyssal clay, and biogenic carbonates are found around coral reef islands. Two types of carbonate cycles are found in the late Quaternary SCS: the "Atlantic" type (above the lysocline), where the controlling factor is dilution by terrigenous clasts, and the "Pacific" type (below the lysocline), where deep-sea dissolution is the controlling factor (Wang et al., 1986; Bian et al., 1992; Thunell et al., 1992; Zheng et al., 1993; Miao et al., 1994; Wang et al., 1995b).
The shelf basins on the SCS contain thousands of meters of Cenozoic deposits that have been drilled by petroleum companies. The basins have nonmarine sequences underlying marine sediments that were deposited during the Miocene or late Oligocene (Fig. 6). Reworked Paleocene and Eocene marine microfossils were present in Neogene deposits from the northern shelf, and Paleocene deltaic and Eocene marine sediments were found in the southern part of the SCS (Fig. 6) such as the Liyue Bank (Reed Bank) Basin, where carbonate deposition started from the middle Oligocene (ASCOPE, 1981; Jin, 1989). Among the basins in the northern SCS, the Pearl River Mouth Basin is the most studied. Over 150 wells have been drilled there and a detailed stratigraphy established for the marine sequence from the uppermost Oligocene to Pleistocene on the basis of various groups of planktonic microfossils (Huang, 1997). Of particular interest are the boreholes drilled on the continental slope in water depths over 500 m, such as Well BY 7-1-1, where the marine sequence ranges from NP23/24 to NN20 (see fig. 3 in Huang, 1997). On the basis of recent publications (Jiang et al., 1994; Wu, 1994; Huang, 1997), the Cenozoic stratigraphy of the Pearl River Mouth Basin is summarized in Figure 6. Noticeable is the presence of nonmarine intercalations in the northern part of the basin, thinning out and decreasing in proportion southward toward the deeper part of the slope. Major depositional hiatuses have been observed at least in the lower part of the lower Miocene, near the end of the middle Miocene, and around the Plio Pleistocene boundary (L. Huang, pers. comm., 1998).
During the last glacial maximum (LGM), sea-level lowering caused remarkable alteration in the configuration and area of the western Pacific marginal seas. The three major shelf areas that emerged during the LGM (East China Sea Shelf, Sunda Shelf or the Great Asian Bank [Fig. 7A], and Sahul Shelf or the Great Australian Bank) amount to 3,900,000 km2, which is comparable in size to the Indian subcontinent. The SCS lost half of its area (>52%) as a result of shelf exposure, which changed its configuration into a semi-isolated basin (Wang et al., 1997). Moreover, the most extensive shelf area of the SCS is located in the modern Western Pacific Warm Pool bounded by the 28°C surface isotherm. The reduction in size must have profoundly influenced the thermodynamic role played by the Global Warm Pool.
The central portion of the SCS experienced a considerable decline in the sea-surface temperature (SST) during the LGM. The winter monsoon strengthened, the polar front shifted southward, and the Kuroshio Current migrated eastward. All of these changes caused a drastic decrease in the winter SST in the Western Pacific marginal seas in general and in the SCS particularly (Wang and Wang, 1990; Miao et al., 1994; Wang et al., 1995b). Together with the negligible changes in the summer SST, the decrease in winter SST resulted in a much more intensive seasonality during the LGM (Fig. 7B; Wang, in press). Among the important consequences of the glacial changes of the SCS is the intensified aridity in China. The SCS is the main source of water vapor for precipitation in East China (Chen et al., 1991). The above-described shelf emergence and SST decline must have led to a reduction in evaporation and vapor supply from the sea to the land. A very preliminary estimate shows that the reduction in evaporation from the SCS during the LGM could correspond to 1/8 to 1/4 of the annual precipitation in all of China (Wang et al., 1997). The glacial reduction in vapor supply from the sea at least partially explains the intensification of aridity in the China hinterland as evidenced, for example, by the extensive distribution of loess deposits. Moreover, the glacial increase of seasonality in the marginal seas may offer an alternate approach to the tropical paleoclimate enigma in the Pacific, that is, the discrepancy between marine and terrestrial indicators of paleotemperature during the LGM (Stuijts et al., 1988; Andersen and Webb, 1994).
The late Quaternary studies have demonstrated the great potential of the SCS's hemipelagic sediments to provide high-resolution paleoenvironment records. A core from the northern SCS (SONNE 17940) reveals a highly detailed transition from glacial to Holocene conditions (Wang et al., in press). On the basis of low fluvial clay content (50%-60%) and high modal grain size (10-25 µm) during the LGM and stage 3 and the transition to high clay content (>70%) and low modal grain size (<6.3 µm) in the Holocene (Fig. 8), a strong winter monsoon and weak summer monsoon precipitation are inferred for the glacial regime and a strong summer monsoon and weakened winter monsoon are inferred for the Holocene regime. The d18O data from the mixed layer planktonic foraminifer Globigerinoides ruber reveal numerous short-term light d18O events superimposed on the main pattern of glacial-postglacial change (Fig. 8). These events are interpreted to reflect increases in summer monsoon intensity, i.e., reduced sea-surface salinity together with increases of fluvial clay and decreases in modal grain size. The increases in summer monsoon intensity can be correlated with Dansgaard-Oeschger events 1-10 in the GISP2 ice core (Fig. 8). Also observed in this SCS core are four periods of relatively heavy d18O associated with low fluvial clay content and high grain size, i.e., reduced summer monsoon precipitation and increased winter monsoon wind, which correlate to the Heinrich events 1-4 (Fig. 8). The early Holocene/Pre Boreal (EHPB) summer monsoon maximum revealed by a broad d18O minimum and fluvial clay maximum has also been reported from the Arabian Sea (Prell, 1984b; Sorocko, et al., 1993). The 8.2k cooling event recorded in the GISP2 ice core appears to coincide with a large increase in d18O and hence decrease in summer monsoon precipitation in the SCS. The Leg 184 cores, along with the recent cores from the joint German-Chinese expedition Monitor Monsoon (Sarnthein et al., 1994), and the IMAGES III Cruise in 1997 provide for the first time systematic and high-quality material for paleomonsoon studies in the region (e.g., Wang et al., 1995a; Sun, 1996; Sun and Li, in press).
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