PALEOCEANOGRAPHY OF THE SOUTH CHINA
SEA
Despite the importance and interconnections of the East Asian and Indian
monsoons, few marine-based studies have compared the past monsoonal
variations of the two subsystems. Previous ODP studies have focused on
the Arabian Sea monsoon (Prell, Niitsuma, et al., 1991; Prell et al., 1992,
and references within) and on the Sulu Sea to the south of the South China
Sea. Many of the East Asian and South China Sea paleomonsoon studies
have used traditional and long piston cores to focus on the late Quaternary
climate changes. During the Last Glacial Maximum, sea-level lowering
greatly altered 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 [the Great Asian Bank], and Sahul Shelf [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
surface 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. Large decreases in the
winter SST in the western Pacific marginal seas and especially in the SCS
(Wang and Wang, 1990; Miao et al., 1994; Wang et al., 1995; Chen et al.,
1999) are interpreted to indicate that the winter monsoon strengthened,
the polar front shifted southward, and the Kuroshio Current migrated
eastward. Together with the negligible changes in the summer SST, the
South China Sea experienced a much higher SST seasonality during the LGM
(Wang et al., 1999). An important consequence of the glacial conditions in
the SCS region is the intensified aridity in China. The summer monsoon is
the main source of water vapor for rainfall in East China (Chen et al.,
1991), and changes in shelf emergence, SST decline, and land-sea heating
patterns must have led to a reduction of vapor transport to southern Asia.
A rough calculation suggests that the reduction in evaporation from the
SCS during the LGM could correspond to one-eighth to one-fourth of the
annual precipitation in all of China (Wang et al., 1997). The glacial
reduction in water vapor transport helps to explain the intensification of
aridity in the China hinterland as evidenced by the extensive distribution
of loess deposits. Moreover, the glacial increase of seasonality in the
marginal seas may help resolve 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;
Anderson and Webb, 1994).
Studies of the late Quaternary have demonstrated the great potential of
the SCS's hemipelagic sediments to provide high-resolution
paleoenvironment records. A core from the northern SCS (SONNE95-17940)
reveals a highly detailed transition from glacial to Holocene conditions
(Fig. 7; Wang et al., 1999). The LGM and isotope Stage 3 are characterized
by low fluvial clay content (50%-60%) and high modal grain size (10-25
µm), whereas the Holocene is marked by high clay content (>70%) and low
modal grain size (<6.3 µm). These data are interpreted to indicate a
strong winter monsoon and weak summer monsoon precipitation during the
glacial regime and a strong summer monsoon and weakened winter
monsoon during the Holocene regime. However, with lowered sea level
during glacials, a large subareal sediment source is exposed in the shelf
of the East China Sea. Deflation and transport of these sediments to the
South China Sea during glacials is another possible explanation for the
coarser particle sizes. The delta18O data from the mixed-layer
planktonic foraminifer Globigerinoides ruber reveal numerous short-term
light delta18O events superimposed on the main pattern of
glacial-postglacial change (Fig. 7). These events appear to reflect
increases in summer monsoon intensity (i.e., reduced sea-surface salinity
together with increased input of fluvial clay and decreased modal grain
size). The increases in summer monsoon intensity can be correlated with
Dansgaard-Oeschger Events 1-10 in the GISP2 ice core (Fig. 7). Also
observed in this SCS core are four periods of relatively heavy
delta18O associated with low fluvial clay content and larger grain
size (i.e., reduced summer monsoon rainfall and increased winter monsoon
wind, which correlate with the Heinrich Events 1-4 (Fig. 7). The early
Holocene/Preboreal summer monsoon maximum revealed by a broad
delta18O minimum and fluvial clay maximum has also been
reported from the Arabian Sea (Prell, 1984b; Sorocko et al., 1993). The
8.2-ka cooling event recorded in the GISP2 ice core appears to coincide
with a large increase in delta18O and, hence, a decrease in summer
monsoon precipitation in the SCS. Similar rapid events in the Bay of
Bengal and Andaman Sea have been related to North Atlantic climate
change (Colin et al., 1998). The Leg 184 cores, along with the recent cores
from the joint German-Chinese Monitor Monsoon expedition (Sarnthein et
al., 1994), and the 1997 IMAGES III Cruise for the first time provide
systematic and high-quality material for studying the long-term evolution
and variability of the monsoonal South China Sea.
Scientific Objectives
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