The broad scientific themes of Leg 184 were threefold:
To address these broad themes, Leg 184 had a number of shipboard and shore-based scientific objectives, as discussed in the following sections.
The summer monsoon is responsible for most of southern Asia's annual rainfall; its evolution and variability have a great impact on the region's climate, vegetation, erosion, weathering, and transport of sediments. In the South China Sea, potential proxies of the summer monsoon include isotopic and faunal indicators of lower salinity and freshwater from monsoon rains and runoff, tropical pollen carried by southerly winds, variation of clay minerals and rare-earth elements indicating provenance, the presence of upwelling/mixing faunas due to southerly winds and upwelling along Vietnam, variations of accumulation rates, and physical properties. Pollen analysis of core SONNE95-17940 suggests that an increase in herbaceous pollen and charcoal can indicate aridity and hence weakened summer monsoons; the appearance of alpine conifer pollen may be used as a proxy of the winter monsoon (Sun, 1996; Sun and Li, 1999). Although these properties are not exclusively monsoon in origin, we expect to find both long-term trends and orbital-scale (as well as millennial scale in some cases) variability in an inter-related array of these monsoon proxies.
Because the winter monsoon reflects cooling over northern Asia (a function of both precession and obliquity), it may exhibit a more complex response than the summer monsoon. Potential proxies of the winter monsoon include lower winter season SSTs, increased subtropical and subpolar index species, increased accumulation of loess, and enhanced transport from the East China Sea and the Pacific. Comparison of upper Neogene sections from the northern and southern part of the SCS will enable us to construct a history of the thermal gradient within the SCS. These paleotemperature data should provide information on when the winter monsoon began to develop large seasonality, especially in the northern SCS, and on the stability and variability of temperatures in the southern SCS, which lies within the Western Pacific Warm Pool. Although seasonality reflects a number of processes, intensification of monsoon circulation is one mechanism that could increase the seasonality of the region. The onset of glacial conditions would also increase seasonality within the SCS but would likely be related to a strengthening of the East Asian winter monsoon.
A major theme of Leg 184 was to evaluate potential relationships between the Tibetan Plateau uplift, monsoon evolution, and global cooling. A number of hypotheses have been proposed to explain various tectonic-climate relationships. These hypotheses come from a diverse set of disciplines including structural geology, micropaleontology, geophysics, geochemistry, stratigraphy of both terrestrial and marine sections, and climate modeling.
The Leg 184 drilling and logging program was designed to obtain long-term, high-resolution records of monsoonal proxies to establish the history of monsoon evolution in the SCS so that it could be compared to other marine and terrestrial records of tectonic and climatic change. Specifically, we sought to develop records of monsoon intensity, denudation/accumulation rates, and climate cooling in the SCS. However, the relationships between tectonics, erosion, and climate are complex and highly nonlinear (see papers in Ruddiman, 1997). The tectonic control of the Asian monsoons, for example, is by no means limited to the plateau uplift. Only recently has the influence of the evolution of surrounding marine basins on monsoon evolution been discussed, with only the role of the Paratethys considered (Ramstein et al., 1997). The western Pacific marginal seas, however, should have more direct impact on the evolution of the East Asian monsoon. The SCS cores should provide a new set of constraints on the pattern and timing of weathering/erosion and sediment transport related to tectonic uplift and climate.
Much of the previously identified monsoonal variability in tropical oceans is precessional (23 k.y.) in scale. Shipboard construction of initial splices and age models indicate strong primary orbital periodicity in the SCS sediments. By inference, strong precessional responses are likely related to monsoonal processes. One critical objective is to use this orbital-scale variability to establish the amplitude, coherency, and phase relationships of the East Asian monsoon with orbital and glacial forcing as well as internal feedbacks of the climate system (Clemens, 1999). Initial shipboard results should resolve whether the SCS monsoonal variations are consistent with orbital models of monsoonal variability. Postcruise research will be needed to expand and refine the sedimentary time series and perform more rigorous tests.
Stationarity is a
fundamental property of a time series and defines the temporal stability of a
variable's characteristics. A nonstationary property changes its mean value,
amplitude (or variance), and phase over some length of time. Clemens et al.
(1996) have shown that the Indian Southwest Monsoon is nonstationary over the
past 3.6 m.y. Specifically, monsoon proxies change their phase relative to ice
volume (
18O)
and dust content. Although the temporal changes are not large, they completely
change the phase relationships at the precessional scale. The time series
evolution of the SCS Neogene sections will provide a different monsoonal regime
to test the stationarity of the monsoon response.
One objective of Leg 184 was to recover high-accumulation-rate sediments that would allow paleoceanographic analyses on millennial, centennial, and even decadal time scales. This objective is feasible in the northern SCS. For example, a piston core near Site 1144 (SONNE95-17940) contains a Holocene section nearly 7 m thick, which equates to a temporal resolution of 15 yr/cm. The remaining core has rates equivalent to ~40-50 yr/cm (Sarnthein et al., 1994; Wang et al., 1999). However, the Sonne core reaches only 40 ka. Our objective is to analyze the sediments of Site 1144 to extend this high-resolution record over the past 1 m.y.
The paleoclimatic and paleoceanographic history of the SCS is paralleled by changes in the planktonic faunas and floras. Some changes are global ocean events whereas others, especially the population characteristics of groups, reflect the evolving oceanography of the SCS. Previous studies have found remarkable changes in relative abundances of planktonic foraminifer Pulleniatina obliquiloculata in the Pleistocene, in the size and abundance of nannoplankton reticulafenestrids in the Miocene/Pliocene, and in species of mangrove pollen Florschuetzia in the Miocene—all of which are tied to environmental events. Leg 184 data will enable us to systematically compare the biological and physical processes.
Extensive petroleum exploration and academic studies have produced much information on the Cenozoic paleoenvironmental history of mainland and offshore China (Zhou, 1984; Li et al., 1984; Ye et al., 1993). Because of the language barrier and commercial restrictions, however, few of these data have been available to the global scientific community. In addition, poor stratigraphic control of the mostly nonmarine deposits has made it difficult to correlate the sediment records with the global paleoenvironmental history. Leg 184 shipboard stratigraphy will provide the first direct calibration of open-marine stratigraphy to the local and regional land-based stratigraphies, thereby linking them with the record of global environmental changes. These correlations will provide a different perspective on the identification of the timing of changes in denudation/accumulation rates, the leads or lags between terrestrial and marine records, tectonic events, and monsoon intensification. A correlation between the loess/paleosol sequence and the deep-sea sediments in the SCS will be most relevant to this purpose.
A number of studies have suggested that the South China Sea accumulates eolian silts and clays (loess) during glacial climate phases (e.g., Wang et al., 1999). Leg 184 scientists seek to identify both the long-term evolution and orbital-scale variability of eolian sedimentation in the SCS, especially at the northern sites. This objective is difficult to address because of the terrigenous nature of the hemipelagic sediments that blanket the northern continental slope. We expect that combinations of the geochemical, mineralogical, and grain-size characteristics will help identify the eolian component of these sediments. Numerous volcanic ash layers should also provide additional indicators of paleowind directions when their sources are identified by postcruise analysis.
Another goal of Leg 184 is to compare the evolution of the East Asian monsoon in the SCS with the evolution of the Indian monsoon in the Arabian Sea to identify common sources of causality. Given the development of reliable chronology and monsoonal indices, the SCS records will be correlated with the ODP Arabian Sea sites, which primarily record the summer monsoon. We predict that the SCS and Arabian Sea summer monsoon responses will be similar in long-term trends and in their phase relative to ice volume. The winter monsoon response is expected to be stronger in the SCS.
Leg 184 scientists plan to continue to develop reliable proxies of monsoonal response in the SCS, as recorded in sediment properties, rates of sediment accumulation, chemical content, and species distribution of flora and fauna. On the basis of previous studies, we expect that numerous sediment properties along with faunal changes will exhibit variability related to monsoonal forcing (Figs. F4, F7). Shipboard measurements included core logging of magnetic susceptibility (MS), bulk density, color reflectance (CR), and natural gamma radiation (NGR). Postcruise work will measure and refine the time series of chemical, isotopic, and faunal variability to place additional constraints on the relationship between sediment proxies and monsoonal intensity.
The opening of the SCS basin together with crustal subsidence must have given rise to transgressive sequences during the late Oligocene and early Miocene. The formation of islands on the eastern and southern borders of the SCS resulted from collision with the Australian and Philippine Sea plates, reducing the water exchange between the SCS and the Pacific and Indian Oceans. The appearance of the modern Bashi Strait (sill depth ~2600 m) between Luzon and Taiwan resulted from the Luzon Arc collision begun ~6.5 m.y. ago. Before then, a free connection existed between the South China Sea and the western Pacific, as indicated by similarities in deep-water faunas and CCD. The evolution of the shape and borders of the SCS should also have changed the source areas of terrigenous material; hence, tectonic models can be partly tested by provenance analyses of sediments from different stages of basin evolution.
The formation of marginal seas in the western Pacific and their response to the glacial cycles must have played a crucial role in the global climate system. Because the connection between these marginal seas and the open Pacific is usually through narrow and/or shallow seaways, these seas are highly sensitive to any tectonic deformation or eustatic fluctuations. The SCS, the largest of the marginal seas, is a critical pathway for heat and vapor exchanges between Asia and the ocean; its geological evolution should have significant climatic impacts. The formation of its southern border was closely related to the Australia-Asia collision that caused the closure of the Indonesian Seaway and the enhancement of the Kuroshio Current ~10-12 Ma (Kennett, 1985). The rotation of the Philippine Sea plate created Luzon Island, which splits the west-flowing Equatorial Warm Current into the Kuroshio and Midanao Warm Currents essential to the supply of Western Pacific Warm Pool waters. The progressive closure of the SCS has increased its amplifying effect on glacial signals and the north-south contrast in climate (Wang, 1999).
