The primary objectives of Site 1143 (proposed site SCS-9) in the southern South China Sea (SCS) were to (1) reconstruct the development of sea-surface temperature (SST) seasonality and its relation to the stability of the Western Pacific Warm Pool; (2) compare long-term records of accumulation rates and sediment variability in the southern SCS to records of uplift, denudation, and climate change; and (3) identify and interpret climate change in the southern SCS in the context of summer and winter monsoon evolution.
Site 1143 is located at 9°21.72´N, 113°17.11´E, at a water depth of 2772 m (Fig. F8 in the "Leg 184 Summary" chapter). The site lies within the northwestern part of the Dangerous Grounds or Nansha Islands area, a region of poorly charted islands and reefs on the southern continental slope of the SCS. Because the initial sites proposed in this area were close to a hydrocarbon province, discussions with the Pollution Prevention and Safety Panel (PPSP) resulted in locating SCS-9 farther downslope (to the north) and limiting penetration to 400 m (note that Hole 1143C was deepened to 500 meters below seafloor (mbsf); see "Operations"). The final location of Site 1143 lies between the high accumulation rate (100-300 m/m.y.) terrigenous deposits of the paleo-Sunda and Mekong Rivers to the south and the carbonate-rich but low sedimentation rate (10-20 m/m.y.) region of the northernmost southern margin (Sarnthein et al., 1994; Huang and Wang, 1998). We expected that the location of Site 1143 would have an accumulation rate of ~50 m/m.y. and would be a sensitive indicator of both the pelagic and terrigenous sources in the southern SCS. Given these accumulation rates, we expected to reach the upper Miocene (~8 Ma) with 400 m penetration.
The basement structure and sediment cover near Site 1143 are complex, as indicated by the seismic lines (NS95-240 and NSL95-160) provided by Guangzhou Marine Geological Survey, Ministry of Land and Resources of China (Figs. F1 and F4, in the "Seismic Stratigraphy" chapter). The site lies near the cross point of Lines NSL160 at common depth point (CDP) 1812 and NS240 at CDP 3617. This area is characterized by basement faulting on a scale of a few kilometers, overlain by sediments ranging from 700 to 1400 m in thickness. A series of seismic reflectors in this area, mapped by the Guangzhou Marine Geological Survey, indicated that the 400-m penetration should reach midway between Reflectors T1 (Miocene/Pliocene boundary; 5.2 Ma) and T2 (top of the middle Miocene; 10.2 Ma). Note that the latter reflector designation was uncertain. Site 1143 was approached along seismic Line NSL160 (from southwest to northeast) using the 3.5-kHz recorder to verify the bathymetry. The 3.5-kHz surface reflections and near-surface reflectors were parabolic, suggesting an irregular surface on a scale of meters. Site 1143 is located in a slight depression within a fault basin that contains ~1.25 s (~1250 m) of sediment. Because of interference by observing vessels, a more complete survey could not be conducted; the prospectus site location was confirmed by identification of the depth gradient leading to the site and the water depth.
Site 1143 provides a record of low-latitude climate that contrasts with the northern SCS sites and will enable us to construct a history of the thermal gradient within the SCS. The site lies within the Western Pacific Warm Pool, which has average annual temperatures >28°C and occupies much of the near-equatorial region of the western Pacific and eastern Indian Ocean. These warm waters provide latent heat to the summer monsoon circulation and are the source of convection that drives the Walker Circulation. The SST estimates from faunal, isotopic, and biomarker data at Site 1143 will provide information on both the long- and short-term variability of temperatures in the southern SCS and thus on the stability of the Warm Pool. The SST contrast with the northern sites will identify when the SCS began to develop large thermal gradients, primarily caused by lower winter SSTs associated with the development of the winter monsoon. Although seasonality is not uniquely related to monsoon circulation, an increase in seasonality during the past glacial interval is attributed, at least in part, to the strengthening of the East Asian winter monsoon.
Site 1143 is also thought to record terrigenous flux from the Mekong River system, which drains a large area of southeastern Asia and portions of the Tibetan Plateau. Therefore, the pattern of Neogene terrigenous accumulation rates at this site should provide a new record of sediment erosion, weathering, and transport related to Himalayan-Tibetan uplift; this record may then be compared with relevant records from the Bengal Fan (Leg 116) and Arabian Sea (Leg 117).
In addition to its geological complexity, Site 1143 had its share of logistical and political complexity (see "Operations"). As noted earlier, Site 1143 lies within the Dangerous Grounds area in the southern part of the South China Sea. Before navigation in this area, a port call was required to repair the shipboard 10-cm radar. In addition, transit to and from the site was made only during daylight and at reduced speeds. Finally, this region of the South China Sea is claimed by multiple jurisdictions, and diplomatic clearances were required from all parties. Besides waiting on final clearances, incidents of piracy near the Spratly Islands raised safety concerns that were addressed by contacting all jurisdictional parties to confirm the ship's location and its times of operation. Despite multiple obstacles, the Leg 184 shipboard party recovered an excellent sediment record from Site 1143.
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