The impact of elevated orography on atmospheric circulation provides an explanation for the initiation, intensification, and long-term (106 yr) evolution of the Asian monsoon system (Ruddiman, 1997). Before the collision of India with Asia at ~55-50 Ma, the Himalayas and Tibetan Plateau were not the dominant orographic features of Asia, and the continent was not as large. The smaller size and lower elevations of precollision Asia and the greater extent of epicontinental seas would have resulted in lower land-sea heating contrasts, especially because of the reduced role of sensible heating over the plateau and the condensational heating over and on the flanks of the Himalayan-Tibetan Plateau Complex (HTC). Model studies suggest that the plateau must be at least half its present elevation to induce a strong monsoon circulation (Prell and Kutzbach, 1992). During the summer monsoon, the major orographic impact is thermal and results from the tendency of higher elevation to increase both sensible and latent heating of the mid-troposphere, leading to stronger monsoon circulation. During the winter monsoon, the mechanical effects of high orography, such as blocking and directing low-level winds and the development of cold surges, are the major orographic impacts of the HTC. The thermal impacts are thought to be relatively small (Murakami, 1987). The location of the South China Sea, with active winter and summer monsoons, provides an ideal site to study the seasonal monsoon system, especially the evolution of the winter monsoon and its relationship to the development of the loess plateau in central China (Ding et al., 1998).
The coincidence of high relief, southerly and easterly sloping topography, and abundant monsoon rainfall has resulted in large river systems, which discharge enormous amounts of sediment onto the coastal plains and the continental shelves and slopes of the Southeast Asian marginal seas. Prior Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites on the Ganges and Indus Fans have found clastic deep-sea fan sedimentation in the lower Miocene and Oligocene, which gives a minimum age for Himalayan-type erosion and transport. Although these fan sediments may be related to monsoon runoff (Cochran, 1990), they do not reflect the onset of clastic deposition from the Asian continent, which is likely to have begun with the earliest collisional arcs. In the upper Miocene, both marine and terrestrial data indicate a major intensification of the Indian monsoon around 8 Ma (Prell et al., 1992, and references within; Molnar et al., 1993; Prell and Kutzbach, 1992). However, the significance of clastic accumulation rates at this time is unclear, and various explanations invoke changes in tectonic, climatic, and sea-level conditions (Rea, 1992; Derry and France-Lanord, 1997, Prell and Kutzbach, 1997). The SCS sediments provide additional combinations of these processes and should help constrain the monsoon-related responses to tectonic forcing.
A variety of observations has 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. The Neogene decrease in CO2 has also been linked to uplift of the Tibetan Plateau and late Cenozoic global cooling (e.g., Ruddiman and Kutzbach, 1989; Raymo et al., 1988) through long-term increased chemical erosion in rapidly uplifted areas that reduce atmospheric CO2 (Raymo, 1994) and thereby cool the planet, enabling widespread glaciation. Other studies have linked the decreased CO2 to uplift, erosion, and carbon burial in clastic deep-sea fans (Derry and France-Lanord, 1997). In either case, evolution of the Asian monsoon and global cooling may well be related. Lower atmospheric CO2 is also proposed as the cause of global-scale changes from C3- to C4-type vegetation at ~7 Ma (Cerling, 1997) (see Fig. 4). This vegetation shift also has implications for monsoonal processes related to soil moisture, albedo, and carbon cycling (Cerling, 1997). Hence, the late Neogene trend toward lower CO2 may covary with stronger winter monsoons and weaker summer monsoon hydrologic cycles.
Superposed on the tectonic and CO2 trends are orbitally induced, periodic variations in the seasonal and meridional distribution of solar energy over the Earth's surface. The variations associated with obliquity and precessional cycles can change the seasonal radiation budget over the Tibetan Plateau by as much as +12.5% (relative to modern values of 450 W/m2) (Laskar et al., 1993; Berger and Loutre, 1991). Numerous studies of Indian Ocean and western Pacific sediments have documented that certain monsoon indicators (upwelling fauna, productivity, dust particle size, and vegetation types) vary coherently with these orbital periodicities, especially the 23 k.y. (Prell, 1984a, 1984b; Clemens and Prell, 1990, 1991a, 1991b; Clemens et al., 1991, 1996; Morley and Heusser, 1997; Schultz et al., 1998). However, the variation of monsoonal indices is not always in direct proportion or in phase with the apparent solar forcing. These phase differences indicate that other processes within the climate system are influencing the timing and amplitude of maximum monsoon responses. We expected that the phase of monsoonal responses in the SCS, especially Sites 1143,1144, and 1146, would provide high-quality records of orbital-scale variations and new constraints on the relative importance of orbital forcing and internal feedbacks to East Asian monsoonal variability.
The monsoon system is also affected by the general state of the Earth's climate, especially the extent of glacial-age surface boundary conditions, which include lowered sea levels, continental-scale terrestrial ice sheets and large areas of sea ice, lowered sea-surface temperatures, lowered CO2, and differing vegetation and land-surface characteristics (CLIMAP, 1981; Prell and Kutzbach, 1987, 1992). In general, more extensive glacial conditions tend to weaken the summer monsoon circulation (Clemens et al., 1996), although certain glacial intervals do have strong monsoons (Clemens and Prell, 1991a, 1991b). In the SCS, studies indicate that increased upwelling during the Last Glacial Maximum (LGM) may reflect a stronger winter monsoon circulation. Also, the emergence of the maritime continent during intervals of lower sea level may affect the regional dynamics of the East Asian monsoon.
A synthesis of Cenozoic terrestrial data from China (e.g., Liu and Ding, 1993; Wang, 1990) has led to the development of a four-stage model of East Asian monsoon evolution: (1) a premonsoon stage (Paleocene and early Eocene), (2) a transitional stage (middle Eocene to Oligocene), (3) a monsoon Stage I (Miocene and Pliocene), and (4) a monsoon Stage II (late Pliocene [2.4 Ma] to present) (Fig. 5; Wang, 1997).
The Paleocene (premonsoon stage) is characterized by a broad east-west-trending arid zone traversing all of China (Fig. 5). This is similar to the Late Cretaceous environmental pattern, when evaporitic endoreic basins accumulated thousands of meters of halite- and gypsum-bearing deposits in the middle and lower reaches of the Changjiang (Yangtze) River, a region that is humid today. The subsequent middle-late Eocene and Oligocene transitional stage was characterized by variable, weak summer monsoons that brought moisture to the otherwise dry areas, which created favorable conditions for nonmarine oil accumulation in China. Beginning in the Neogene, however, palynologic, paleobotanic, and lithologic data indicate that the climate pattern in China underwent a profound reorganization (Wang, 1990; Sun and Wang, pers. comm., 1998). During the Miocene monsoon Stage I, the arid zone retreated to northwest China, and eastern China became more humid (Fig. 5) as the southeast summer monsoon strengthened and brought moisture 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 have occurred much later and to have marked the beginning of deposition of the Chinese loess deposits at ~2.4 Ma (monsoon Stage II; An et al., 1990). However, some recent studies suggest that the eolian component of the loess red-clay sequence may be as old as 7 Ma (Ding et al., 1998), which would have implications for the tectonic vs. glacial initiation and intensification of the winter monsoon.
A primary goal of Leg 184 was to understand the relative importance of these complex "causal" factors in the initiation, evolution, and variability of the Asian monsoon system. In short, we sought to decipher the coevolution of tectonic uplift of the HTC, the Neogene changes in global climate, and the development and variability of the Asian monsoon circulation.
Sediments of the South China Sea
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