The long-term goals of Leg 184 are to determine the evolution and variability of the East Asian monsoon during the late Cenozoic and to improve our knowledge of the links between climate and tectonics. To meet these goals, Leg 184 has a number of major cruise and shore-based scientific objectives, including the following

  1. Obtain continuous sequences of hemipelagic sediments that record the East Asian climate history during the late Cenozoic.
    The drilling plan takes advantage of the high hemipelagic sedimentation rates in the SCS to recover sections that are suitable for high-resolution stratigraphy. The seismic surveys reveal that different areas of the northern slope contain expanded sections of different ages. Our shipboard objective (see
    Proposed Sites and Operational/Drilling Plan sections) is to recover complete, high accumulation rate sections for each age interval. For example, sedimentation rates on the continental slope vary from 0.7 to 15 cm/k.y. for the Holocene and from 1.3 to 31 cm/k.y. for the LGM, with the maximum values found near the mouth of the Pearl River and the paleo-Sunda River (Wang et al., 1995b). Recently, Core 17940 (20°07ŽN, 117°23ŽE, in a water depth of 1727 m; Sarnthein et al., 1994) in the northeast SCS, revealed a Holocene section nearly 7 m in thickness, which enables a temporal resolution of less than 15 yr (Fig. 8). The proposed drilling during Leg 184 will provide continuous records of monsoon variations back to the late Paleogene, enabling a comparison with the Indian monsoon records.

  2. Establish records of monsoonal proxies for the SCS, including the variability of sediment properties, the rates of sediment accumulation, and the chemical, isotopic, and species variability of flora and fauna.
    On the basis of previous studies we anticipate that a number of sediment properties will exhibit variability related to monsoonal forcing (
    Figs. 3, 7, 8). Shipboard measurements will include core logging of magnetic susceptibility, bulk density, color reflectance, and natural gamma radiation, which along with faunal variations, can be related to monsoonal climates. Much of the previously identified monsoonal variability in tropical oceans is precessional (23 k.y.) in scale. We will construct initial splices and age models to identify the primary periodicity of the SCS records. By inference, strong precessional responses are likely related to monsoonal processes. Postcruise work will measure and refine the time series of chemical, isotopic, and faunal variability to give additional constraints on the relationship between sediment proxies and monsoonal variability.
  3. Establish stratigraphic ties between the SCS marine record and the terrestrial record of China.
    Petroleum exploration and academic studies have accumulated a tremendous amount of Cenozoic paleoenvironmental information, particularly for on land and offshore China (
    Fig. 6). Because of the language barrier and commercial restrictions, little of these data have been available to the global scientific community. In addition, the poor stratigraphic control of the mostly nonmarine deposits has made it difficult to correlate the sediment records with the global paleoenvironmental history. The shipboard stratigraphy of the proposed sites 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. Special attention will be paid to the timing of drastic changes in denudation/accumulation, monsoon intensification, seasonal cooling, and to the leads or lags between terrestrial and marine records.
  4. Establish the relationship of East Asian monsoon variability with orbital and glacial forcing, and internal feedbacks of the climate system.
    The variability of monsoonal proxies and sedimentary characteristics identified on shipboard and in postcruise studies will be compared directly to time series of orbital changes to establish their coherency and phase (
    Fig. 3). Initial shipboard results should establish if 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.
  5. Compare the evolution of the East Asian monsoon in the SCS with the Indian monsoon in the Arabian Sea to identify common sources of causality.
    Given the identification of monsoonal indices in the SCS, especially for the winter monsoon, the SCS records will be compared to records of the summer monsoon from the ODP Arabian Sea sites. We anticipate that the summer monsoon signals should be similar (in phase) and that the winter monsoon will be stronger in the SCS. Since the winter monsoon reflects cooling over northern Asia, which is a function of both precession and obliquity, it may exhibit a more complex response than the summer monsoon. These studies will be initiated on shipboard, but most detailed comparisons will be made only after the final time series are established by postcruise research.
  6. Test scenarios for the relationship between the Tibetan Plateau uplift, monsoon evolution, and global cooling.
    Land-based studies in China and marine-based ODP studies have postulated a variety of models for monsoon evolution (
    Table 1; Figs. 3, 4). The proposed drilling and logging program will calibrate the terrestrial records with those of the global ocean and make use of monsoonal proxies to establish the history of monsoon evolution in the SCS. Because uplift of the Tibetan Plateau is proposed to be responsible for both the late Cenozoic global cooling and for the intensification of the Asian monsoon, a comparison between records of monsoon intensity, denudation/accumulation rates, and climate cooling in the SCS will help test these hypotheses.
    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 marine factor for monsoon evolution been discussed, but then only the role of the Paratethys was considered (Ramstein et al., 1997); whereas, the Western Pacific marginal seas should have more direct impact on the evolution of the East Asian monsoon. Drilling in the SCS will allow insights into the mechanisms of monsoon variation and will provide a new set of constraints concerning the links between tectonic uplift, weathering/erosion, and climate.
    The shipboard identification of sediment characteristics and accumulation rates in the Miocene to Pleistocene sections of the SCS will likely distinguish between some models of monsoon evolution but will also raise questions or present new patterns to be deciphered. Significant postcruise research will be directed toward determining how the various sedimentary records are related to the models of HTC uplift and global cooling.
  7. Improve our understanding of seasonality in the low-latitude SCS and how it relates to the strength and evolution of the winter monsoon.
    Late 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 (
    Fig. 7B). These paleotemperature data will provide information on when the winter monsoon began to develop large seasonality in the SCS and on the stability/variability of temperatures in the southern SCS, which lies within the Western Pacific Warm Pool.

Although seasonality is not necessarily related to monsoon circulation, intensification of monsoon circulation can trigger an increase in seasonality. The glacial increase in seasonality within the SCS is at least partly attributed to the strengthening of the East Asian winter monsoon. Aside from SST estimates, seasonality can also be recognized through abundance of index species in planktonic fauna.

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