Site 1143

Site 1143 (proposed site SCS-9) is located 9°21.72'N, 113°17.11'E, at a water depth of 2772 m (Table 1), within a basin on the southern continental margin of the South China Sea (Figs. 8, 10). On the Admiralty charts, the site lies within the Nansha or Dangerous Grounds area, which is riddled with reefs, shoals, and small islands, some of which are within 20 to 30 miles of the site. Site 1143 was located in the southern SCS in order to provide a Neogene paleoceanographic record within the region of relatively stable sea-surface temperature, the Western Pacific Warm Pool. The sediment record of high and relatively stable sea-surface temperature will allow the reconstruction of SST gradients (with the northern sites) across the SCS and should reflect the development of climatic seasonality in this region. Climate records from Site 1143 will provide the basic data to identify and interpret the evolution of summer and winter monsoon circulation in the South China Sea. Also, the southern location of Site 1143 was chosen to capture the long-term records of sediment accumulation rates and lithologic variability associated with the Mekong and Sunda River systems that might be related to uplift and denudation of Tibetan and East Asian tectonic systems.

We cored three APC/XCB holes at Site 1143. Hole 1143A reached 400 mbsf with 95% recovery, Hole 1143B reached 258 mbsf with 95% recovery, and Hole 1143C (following PPSP approval to deepen the hole) reached 500 mbsf with 96% recovery (Table 1). We requested approval to deepen Hole1143C beyond the originally approved 400 m penetration to extend the paleoenvironmental record in time, after the sediment age turned out to be younger than expected. We had confirmed that no significant hydrocarbon concentrations occurred in the entire interval recovered in Hole 1143A. Wireline logging was accomplished in Hole 1143A with the triple combo tool suite (86-400 mbsf) and the FMS-sonic tool combination (158-380 mbsf). Hole conditions did not allow deployment of the GHMT string.

The sediments at Site 1143 represent continuous hemipelagic sedimentation of fine-grained terrigenous material and pelagic carbonate from the late Miocene (~10 Ma) to present (Fig. 11). The three holes at Site 1143 were combined into a composite (spliced) stratigraphic section that is continuous in mcd scale from 0 to 190.85 mcd, the interval of APC coring. Incomplete core recovery and decreased core quality precluded splice construction below this interval, but intervals of cores can be correlated over the 190-400 mcd interval. Overall, most of the Pliocene/Pleistocene interval has a continuous and reliable spliced record. In general, the long-term decrease in sedimentation rates observed at Site 1143 from the upper Miocene to the Pleistocene is caused by declining accumulation rates of both carbonate and noncarbonate components. The temperature gradient determined from five advanced hydraulic piston corer temperature tool (APCT) downhole measurements is ~86°C/km.

The Pleistocene-age sediments at Site 1143 consist mostly of olive, greenish, and light gray-green and greenish gray clayey nannofossil mixed sediment, clay with nannofossils, and clay. In general, bedding is not evident, and compositional changes are gradual throughout the site. Minor lithologies vary with depth and include ash layers, turbidites, and green clay layers. Carbonate content of this interval is variable but averages ~18%, with both calcareous nannofossils and planktonic foraminifers abundant and well preserved. Benthic foraminifers are generally rare throughout the site. Changes in color define lithic subunits and are mainly controlled by carbonate content. The CR measurements (lightness parameter L*) vary with the carbonate data and increase downcore. Core-logging data, especially CR, MS, and NGR, show an increasing trend within the Pleistocene, with superposed patterns of orbital-scale cyclicity, much of which can be correlated with glacial-interglacial cycles and marine oxygen isotope stages. Magnetic susceptibility data show a number of significant spikes that correspond to observed volcanic ash layers and are particularly abundant in the intervals 20-30 and 70-100 mcd. The interstitial waters of the Pleistocene-age sediments are characterized by sulfate reduction, which reaches downhole to the base of the Pliocene-age sediments (~200 mbsf). Organic carbon content decreases from ~0.9% at core top to 0.2% at the base of the Pleistocene section and remains low throughout the core. Methane concentration in the sediment is <10 ppm in the entire interval to 500 mbsf. The extended interval of sulfate reduction appears consistent with the low organic carbon and methane content in the sediments. The Brunhes/Matuyama polarity reversal was observed at ~42.5-43.8 mcd, and the Pleistocene/Pliocene boundary is located between 93.5 and 94.3 mcd. Over the Pleistocene interval, the linear sedimentation rate (LSR) averages 50 m/m.y., and the total and carbonate mass accumulation rate (MAR) (g/cm2/k.y.) are 3.6 and 0.6, respectively.

The Pliocene-age sediments at Site 1143 are characterized by steadily increasing carbonate content, ranging from 20% to 40% and averaging 28%. Calcareous nannofossils and planktonic foraminifers are abundant and generally well preserved. This interval exhibits a continued increase in L* and grain density, reflecting the increase in carbonate. The MS and NGR values reach a plateau in the mid-Pliocene and rapidly decrease in the lowermost Pliocene. Magnetic susceptibility spikes related to volcanic ashes occur between 120 and 190 mcd. Cyclicity of most properties continues as in the Pleistocene section but with reduced amplitude. Near the base of the Pliocene (~200 mbsf), increases in the concentrations of dissolved silica, strontium, and lithium as well as alkalinity are consistent with a lithology change observed at that depth. Change from APC to XCB coring in the lower Pliocene significantly affected the core-logging data, particularly the NGR and MS signals. The values decreased by half across the APC/XCB coring transition, although this also partly reflects the change in lithology at that depth. NGR data from wireline logging confirm the change in lithology. The Pliocene/Miocene boundary is located between 213 and 200 mcd. Over the Pliocene interval, the average LSR is 36 m/m.y., and the total and carbonate MAR (g/cm2/k.y.) are 3.7 and 1.0, respectively.

The Miocene-age sediments at Site 1143 are distinguished by high carbonate content (averaging 47%), which is the primary criterion for identifying lithologic Subunits I and II. The green clay layers are less frequent and turbidite layers are more frequent in the Miocene sediments. Sediments of this interval also exhibit higher CR and bulk density but lower MS and NGR. These variations probably reflect subtle compositional changes such as variations in clay content or abundance of foraminifer turbidite layers worked into the clay matrix by bioturbation. Calcareous nannofossils are abundant, but their preservation deteriorates through the Miocene, whereas planktonic foraminifers are abundant and have good preservation. Over the upper Miocene interval, the average LSR is 114 m/m.y., and the total and carbonate MAR (g/cm2/k.y.) are 14.0 and 6.6, respectively. These higher sedimentation rates are caused by frequent turbidites that were observed in the cores and clearly distinguishable both in logging data and FMS images. The sand-rich base of the turbidites is characterized by lower gamma-ray, density, and resistivity and higher porosity and P-wave velocity values. The opposite is true for the top (clayey part).

Overall, Site 1143 provides an excellent continuous record with moderate accumulation rates, especially over the past 6 m.y. The site should enable researchers to develop high-resolution orbital-scale time series of paleoceanographic proxies and to reconstruct the record of seasonality in the South China Sea.

Site Summaries - Site 1144

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