SCIENTIFIC
OBJECTIVES
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
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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
To 184 Proposed
Sites
To 184 Table of
Contents