5. Site 1144¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

The primary objectives of Site 1144 (proposed site SCS-1) on the northern margin of the South China Sea (SCS) were to (1) recover a continuous sequence of high-sedimentation-rate hemipelagic sediments that will allow reconstruction of the paleomonsoon history on millennial, centennial, or higher resolution time scales over the last million years; (2) compare these records with orbital-scale and higher frequency records from ice cores, other nearby marginal seas, and terrestrial deposits; and (3) establish a high-resolution record of faunal, isotopic, and sea-surface temperature (SST) changes associated with variability in the winter monsoon.

Site 1144 (SCS-1) is located at 20°3.18´N, 117°25.14´E, at a water depth of ~2037 m, which is above the sill depth of the Bashi Strait (2600 m) (see Figs. F2 and F9, both in the "Leg 184 Summary" chapter). The location of Site 1144 is distinguished by extremely high sedimentation rates. The Holocene deposits in the nearby core SONNE95-17940 have sedimentation rates of ~700 m/m.y., whereas glacial sediments accumulate at ~400 m/m.y. Similarly, nearby core MD 97-2-146 reaches only to the marine oxygen isotope Stage (MIS) 4/5 boundary at 38 m, indicating a sedimentation rate of ~500 m/m.y. (C. Huang, pers. comm., 1997). These high sedimentation rates have allowed reconstruction of centennial-scale summer monsoon variability in the Holocene and millennial-scale dry/humid cyclicity in the uppermost Pleistocene (Wang et al., 1995; Sun, 1996), indicating the great potential of this site to deliver high-resolution records of the Quaternary climate. The target penetration of 450 m was expected to recover Quaternary sediments (~1 m.y.). We planned a standard drilling program at Site 1144: Hole 1144A was to be cored to refusal using the advanced hydraulic piston corer (APC), deepened with the extended core barrel (XCB) to 450 meters below seafloor (mbsf), and wireline logged (using the triple combination tool, the geological high-resolution magnetic tool (GHMT), and the Formation MicroScanner (FMS)-sonic tool suite). Holes 1144B and 1144C were to be APC cored to refusal. However, Hole 1144B was deepened with the XCB to 450 mbsf (see "Operations").

Site 1144 is located on a thick sediment drift at the intersection of seismic Lines SO95-20 (common depth point [CDP] 3482, 04:02) and SO95-10 (CDP 9600, 04:20) (Figs. F5, F16A, and F16B, all in the "Seismic Stratigraphy" chapter). The drift nature of the sediment is indicated by high sedimentation rates, the geometry/morphology of the deposit, and its surface and internal structure (Figs. F7 and F8, both in the "Seismic Stratigraphy" chapter). The JOIDES Resolution 3.5-kHz depth recorder data and the Sonne PARASOUND data (Sarnthein et al., 1994) show that the surface near Site 1144 is irregular although locally smoothed by sediment accumulation in lower areas (Fig. F5 in the "Seismic Stratigraphy" chapter). Parabolic hills on the order of 50-100 m in height were observed near the site. The JOIDES Resolution data show that Site 1144 is located near the crest of a large hill with a thick, uniform sediment drape. The sediment thickness above a prominent basinward dipping reflector is ~0.75 s, or ~650 m. The seismic structure of the sediment column is characterized by an upper reflector series (0-0.5 s) that has distinct, closely spaced reflectors and a lower, more diffuse zone (0.5-0.7 s) with less-distinct reflectors. The northwest-southeast dip line (SO95-10) reveals a wedge of sediment ranging from ~1725 to ~2400 m in water depth. Internal reflectors indicate that the beds thin and pinch out with increasing water depth toward the toe of the deposit. The strike line (SO95-20) reveals wavy reflectors, suggesting possible drift-type transport or redeposition of suspended sediments. The conforming nature of the draped sediments and the lack of horizontally filled depressions imply that they are not bottom-transported, gravity-driven deposits.

Site 1144 is located near the center of the deposit, where reflectors are relatively uniform and evenly structured in both lines. We expected that the 450-mbsf penetration at Site 1144 would recover only the upper reflector series.

The high accumulation rate sediments at Site 1144 should offer an unprecedented opportunity to reconstruct late Quaternary monsoonal variations on centennial to orbital scales. Studies of the past 40 k.y. (Wang et al., 1999; Pflaumann and Jian, 1999; Pelejero et al., 1999; Sun and Li, 1999) have used a number of monsoonal and paleoclimate proxies, including the composition of planktonic faunas (tropical/subtropical nature and SST), the ¹⁸O of mixed-layer-dwelling foraminifers (salinity and SST), biomarker U (SST and productivity), pollen and charcoal (vegetation and aridity), clay content (fluvial/eolian source), and grain size (fluvial/eolian transport). These proxies have been used to infer that the summer monsoon rainfall was weaker and the winter monsoon eolian transport was stronger during the past glacial interval and, conversely, that the summer monsoon rainfall was stronger and the winter monsoon eolian transport weaker during the Holocene interglacial interval (Wang et al., 1999; Sun and Li, 1999). Superimposed on these glacial-interglacial-scale changes are short-term (10²-10³ yr) increases in monsoonal intensity that can be correlated with the first 10 Dansgaard-Oeschger Events in the GISP2 ice core (Wang et. al., 1999). Similarly, four periods of reduced summer monsoon precipitation and increased winter monsoon winds are observed and correlative with Heinrich Events 1-4.

On the basis of a broad ¹⁸O minimum and a fluvial clay maximum, the summer monsoon is thought to have reached its Holocene maximum in the South China Sea at ~10 ka. Similar timing is observed in the Arabian Sea (Prell, 1984; Sirocko et al., 1993).

We expected the multi-APC holes at Site 1144 to provide a high-quality spliced record that will enable sample intervals as small as 20 yr and thus enable centennial- to millennial-scale studies of the East Asian monsoon history.