The Asian monsoon is one of the major components of the global climate system and its evolution plays a significant role in our understanding of global climates (Fig. 1) (Hastenrath, 1991; Hastenrath and Greischar, 1993; Webster, 1987; Webster, 1994; Webster et al., 1998). The Asian summer and winter monsoons dominate the seasonal winds, precipitation and runoff patterns, and the character of land vegetation over southern and eastern Asia. The winter monsoon is characterized by high pressure over northern Asia, northeast winds across the South China Sea (SCS) (which intensify during cold surges), and enhanced precipitation in the Austral-Asian equatorial zone (Fig. 1A, 1C). The summer monsoon circulation is characterized by low pressure over Tibet, strong southwesterly winds, upwelling in the Arabian Sea, and high precipitation over southern and eastern Asia (Fig. 1B, 1D). The SCS is ideally located to record the paleoceanographic responses to both winter and summer monsoons (Figs. 1, 2). Evolution of the Asian monsoon system is thought to reflect at least four types of large-scale climate forcing or boundary conditions: (1) the tectonic development of the Himalayan-Tibetan orography, (2) changes in the atmospheric CO2 concentration, (3) changes in the Earth's orbital parameters and the resulting variations in seasonal solar radiation, and (4) changes in the extent of glacial climates. These factors act to amplify or dampen the seasonal development of land-sea heating and pressure gradients, latent heat transport, and moisture convergence over the Asian continent.
The effects of tectonically induced orographic changes on the monsoon system provide an explanation for its initiation, intensification, and long-term (106 yr) evolution (see the papers and references in Ruddiman, 1997). Prior to the collision of India with Asia, the Himalayas and Tibet did not exist in their present state, and the Asian continent was not as large. The smaller size and lower elevations of the pre-collision continent might be expected to support a lower land-sea heating contrast because of the important role of sensible heating over the plateau and the condensational heating over and on the flanks of the Tibetan Plateau. In general, the modern monsoon circulation would not exist if the Himalayas and Tibet were not at their present location and elevation. For the summer monsoon, the thermal effects, both sensible and latent heating, of the Himalayan and Tibetan Plateau complex (HTC) are the major impact of the orographic forcing. During the winter monsoon, the thermal effects of the HTC are thought to be small, but the mechanical effects, such as blocking and directing low-level winds and the development of cold surges, are the major impacts of the orography (Murakami, 1987). The SCS should provide an especially good record of the winter monsoon evolution and its relationship to the evolving orography of Asia.
A variety of observations have suggested that CO2 levels were higher during the Tertiary and may have been equivalent to double the present CO2 levels at ~20 Ma (see Kump and Arthur, 1997 and other papers in Ruddiman, 1997). Higher CO2 levels might be expected to strengthen the summer monsoon through increased land-sea contrasts and more active hydrologic budgets but might also weaken the winter monsoon through warmer continents. Lowered CO2 levels are thought to have caused global changes in vegetation from C3 to C4 ~7 Ma (see Fig. 3), which also has implications for monsoonal processes related to soil moisture, albedo, and carbon cycling (Cerling, 1997). Hence, the strength of winter monsoons in the SCS may covary with both increased orography and decreased CO2.
Changes in the Earth's orbit result in redistribution of solar energy over the surface of the Earth. For example, during high eccentricity intervals the precessional-driven summer season radiation budget over the Tibetan Plateau can vary as much as +12.5% (relative to modern values of 450 W/m2). Numerous studies of Indian Ocean and western Pacific sediments reveal that a number of monsoon indicators (upwelling fauna, productivity, dust particle size, and vegetation types) vary coherently with orbital periodicities (Prell, 1984a, 1984b; Clemens et al., 1991; Morley and Heusser, 1997). However, the monsoonal indices are not always in direct proportion or in phase with the apparent solar forcing. The phase of monsoonal responses in the SCS should provide additional constraints on the relative importance of orbital forcing and internal feedbacks on monsoonal variability.
Glacial Climate Forcing
The extent of glacial-age surface boundary conditions also affects the monsoon system (Prell and Kutzbach, 1987, 1992). Numerous studies have shown that more extensive glacial climates tend to weaken the summer monsoon (Clemens et al., 1996), although glacial intervals do contain strong monsoons (Clemens and Prell, 1991). These responses result from the lower sea surface temperature, lower sea level, higher albedo of the land surface, and the extent and elevation of large ice masses (CLIMAP, 1976, 1981). However, more extensive glacial climates may strengthen the winter monsoon, especially as recorded in the SCS.
Given these potential "causal" factors, our goal is to understand their relative importance in the initiation, evolution, and variability of the Asian monsoon system. Hence, one of the long-term goals of Leg 184 is to decipher how the tectonic development of Asia, the Asian monsoon circulation, and global climate have co-evolved during the Neogene.
Despite the importance and interconnections of the two monsoonal subsystems (East Asian and Indian), previous marine-based studies of past monsoonal variations have concentrated on the Indian monsoon (Prell, Niitsuma, et al., 1991; Prell et al., 1992 and references within). The East Asian paleomonsoon studies have been restricted mainly to land-based work, with monsoon information commonly being obtained from the Chinese loess. Less attention has been paid to the marine aspects of the East Asian monsoon until recently. Extensive hydrocarbon exploration in China and its surrounding offshore areas has accumulated extensive geological data that are rich in Cenozoic paleomonsoon information. Together with recent progress in Quaternary science for East Asia and the western Pacific (e.g., Liu and Ding, 1993; Wang, 1990), the data have led to the development of a four-stage model of East Asian monsoon evolution: a premonsoon stage (Paleocene and early Eocene), a transitional stage (late Eocene to Oligocene), a monsoon Stage I (Miocene and Pliocene), and a monsoon Stage II (late Pliocene [2.4 Ma] to present) (Table 1; Wang, 1997).
Palynologic, paleobotanic, and lithologic data (Fig. 4C) indicate that the climate pattern in China underwent a profound reorganization around the beginning of the Neogene (Wang, 1990; Sun and Wang, pers. comm., 1998). The Paleocene in China inherited the Late Cretaceous environmental pattern ("Pre-monsoon Stage"), with a broad arid zone traversing the whole country from west to east (Fig. 4A). The middle-late Eocene and Oligocene climate in China ("Transitional Stage") was characterized by variable, weak summer monsoons that brought moisture to the otherwise dry areas, which created the most favorable conditions for nonmarine oil accumulation in China. During the Miocene, the arid zone retreated to northwest China, and eastern China became more humid (Fig. 4B; "Monsoon Stage I") as the southeast summer monsoon strengthened and brought moisture
Table 1. Evolution stages of the East Asian monsoon in the Cenozoic, based on land studies from China.
|Absent or insignificant||Smaller Asia without Tibetan Plateau|
|Transitional||late Eocene to Oligocene||Weak||India joined with Asia|
|Monsoon I||Miocene to Pliocene||Summer monsoon developed||Plateau uplift started|
|Monsoon II||late Pliocene to Pleistocene||Summer and winter monsoon developed||Intensive plateau uplift|
from the sea. This general regime has existed from the Miocene to the Holocene. The intensification of the winter monsoon in eastern Asia is thought to occur much later, and to have marked the beginning of deposition of the Chinese loess deposits at about 2.4 Ma ("Monsoon Stage II"; Liu and Ding, 1982). The loess deposits are a joint product of winter and summer monsoons and thus imply that both systems are active. However, some recent studies suggest that the eolian component of the loess red-clay sequence may have began as early as 7 Ma (Ding et al., 1998), which would have implications for the tectonic vs. glacial initiation and intensification of the winter monsoon.
Among the continents of the world, Asia has been subjected to the most significant Cenozoic deformation. The Cretaceous-Paleocene topography of China was generally tilted to the west, with the coastal areas of the Tethys in the west and relatively high land and endorheic basins in the east. This paleogeographic pattern lasted until the late Eocene when India collided with Asia, thereby bringing the maritime conditions in western China to an end. The uplift of the Tibetan Plateau may have started about 21-20 Ma (Copeland et al., 1987; Harrison et al., 1991) and was accompanied by a general subsidence of East China. Other studies (Molnar et al., 1993) suggest that the Tibetan Plateau was uplifted rapidly about 10 Ma and has been subsiding since the middle Miocene. These tectonic changes led to a reversal of the topographic trend in China from west tilting to east tilting, with the west-east gradient in altitude increasing continuously since then. In addition, the early Miocene was also the time of formation for many of the western Pacific marginal seas. The radical changes that occurred in the topography of Asia during the Cenozoic must have had a profound impact on climate, including the onset or strengthening of the monsoon circulation in East Asia. The further development of east-sloping topography and monsoon precipitation has brought about the large river systems, which discharge enormous amounts of sediments into the newly formed marginal seas along the East Asian coast and build extensive coastal plains and continental shelves.
Because the Indian and East Asian monsoons share the same tectonic factor in their evolution, we would expect to find synchronous evolutionary stages. Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) core studies have revealed the onset of clastic deep-sea fan sedimentation at about 20 Ma, which may be related to monsoon runoff (Cochran, 1990). Both marine and terrestrial data indicate a major intensification of the Indian monsoon around 8 Ma (Fig. 3) (Prell, et al., 1992 and references within; Prell and Kutzbach, 1992). If the East Asian monsoon record in the SCS supports this timing, it will be a strong argument in support of tectonic forcing as a cause for intensification of both Asian monsoons.
The accelerating uplift of the Tibetan Plateau is thought to be responsible not only for the intensification of the Asian monsoon, but also for late Cenozoic global cooling (e.g., Ruddiman and Kutzbach, 1989). Raymo et al. (1988) proposed that long-term increased chemical erosion in rapidly uplifted areas could reduce atmospheric CO2 (Raymo, 1994) and thereby cool the planet and enable widespread glaciation. If this is the case, the evolution of Asian monsoon and global cooling should be correlated with each other. The proposed drilling in the SCS will test for causal relationships between the three major environmental features in the late Cenozoic (1) global cooling, (2) enhanced chemical and physical weathering, and (3) onset/intensification of the Asian monsoon.
To 184 Geologic Framework for the South China Sea
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