Ocean Drilling Program (ODP) Site 1144 was drilled on the northern margin of the SCS (20°3.18´N, 117°25.14´E) in ~2037 m water depth, with a maximum penetration of 452 meters below seafloor (mbsf). This site is at the same location as core 17940 (Sonne-95 cruise) (Fig. F1) and is considered particularly suitable for high-resolution paleoenvironmental reconstitutions of the last glacial period (pollen, siliciclastic grain size, etc.) (Sarnthein et al., 1994; Wang et al., 1999). Site 1144, located ~400 km off the Hong Kong shore and the Pearl River mouth, was drilled on a seamount in order to avoid turbidites. The lithology throughout the recovered section is quite homogeneous and is dominated by terrigenous silty clay with quartz silt and nannofossil carbonate ooze. Other minor lithologies including low proportions of sponge spicules and diatoms were also observed (Fig. F2B). Carbonate contents are quite low and range from 10 to 20 wt% (Shipboard Scientific Party, 2000).
The stratigraphy of Site 1144 was established using (1) biostratigraphic datums (Shipboard Scientific Party, 2000; Shyu et al., this volume), (2) nine radiocarbon 14C datings (five datings on mixed bulk planktonic foraminifers [Chen et al., this volume] and four datings based on samples of the planktonic foraminifers Globigerina ruber and Globigerina sacculifer [Buehring et al., this volume]), and (3) the high-resolution 
18O record from the planktonic foraminifer G. ruber, using two references: the 18O stack of Bassinot et al. (1994) for the upper 413 meters composite depth (mcd), and below (413-517 mcd), the ODP Site 677 18O record, as age reference. Site 1144 provides a sedimentary record extending back to marine isotopic Stage (MIS) 23 (Fig. F2A). This site presents small hiatuses during MIS 5.5 and MIS 11 as well as a 50-k.y. hiatus that comprises the lower part of MIS 7.5 and most of MIS 8.
The age (ka) vs. depth (mcd) diagram (Fig. F2B) shows a downcore decrease in sedimentation rates and higher sedimentation rates during glacial stages than interglacial stages. Such changes could be explained by (1) changes in the terrigenous supply to the SCS, (2) changes in the biogenic productivity, and/or (3) changes in the lithology that would provide differential compaction. The compaction has an important effect on the long-term downcore sedimentation rate decrease. However, as no important changes in lithology were observed between glacial and interglacial changes, it is unlikely that differential compaction has a major effect on the sediment between adjacent glacial and interglacial stages. Therefore, compaction cannot explain the important variations in sedimentation rates observed during glacial and interglacial stage changes. At Site 1144, glacial stages are characterized by an increase in sedimentation rates in agreement with the sedimentation rates calculated for several piston cores of the northern part of the SCS (Huang and Wang, 1998). This increase in sedimentation rates could be attributed to an increase in terrigenous supply, as the biogenic material (silica and carbonate) does not vary significantly between glacial and interglacial changes.
Site 1144 is characterized by high sedimentation rates (average = ~48 cm/k.y.) that are particularly suitable for high-resolution paleoenvironmental reconstitution. Hole 1144A was sampled at 150-cm intervals for grain size and mineralogical and geochemical investigations.
Grain-size distribution measurements of carbonate-free sediment were carried out on a Malvern Mastersizer X apparatus following the procedure described in detail by Trentesaux et al. (2001). A 100-µm lens was used, allowing an analytical grain size range of 1-160 µm. Bulk sediments were first suspended in deionized water and gently shaken to achieve disaggregation. Ultrasound was used before pouring the sediment into the laser grain sizer in order to decrease the degassing time of the water. The suspension was then gently poured into the fluid module of the granulometer. After a first run, hydrochloric acid in excess was injected to obtain the carbonate-free fraction grain size distribution. Sonication was not used to complete the sediment dispersion, as previous measurements have shown that the use of ultrasonic dispersion has a dramatic effect on some particles such as foraminifers or vesicular volcanic glass. Grain size distributions of the carbonate-free fraction still include a small portion of marine opal.
Clay mineralogy determinations were performed at the University of Orsay by standard X-ray diffraction (XRD) on the carbonate-free, <2-µm size fraction, following the procedure described by Holtzapfell (1985). The <2-µm clay fraction was isolated by gravitational settling. X-ray diffractograms were made on a Kristalloflex (Siemens) X-ray diffractometer from 3.5° to 30°2
using CuK
radiation. Three tests were performed on the oriented mounts: (1) untreated, (2) glycolated (12 hr in ethylene glycol), and (3) heated at 500°C for 2 hr. Diffractograms showed the presence of different minerals including quartz and feldspars. Clay minerals are composed of illite, chlorite, kaolinite, smectite, and complex mixed-layer minerals. These mixed-layer clays were mainly assigned to randomly mixed illite-smectite species, and they will be referred to as "smectites" in the text. The semiquantitative composition of the clay fraction was obtained by measuring the peak areas of basal reflections on XRD diagrams using the MacDiff software (Petschick, 1997).
Quartz and carbonate contents of the bulk fraction were determined by Fourier transform infrared (FTIR) spectroscopy (Pichard and Fröhlich, 1986). The samples were ground in acetone to a particle size of <2 µm with small agate balls in an agate vial and kept at 4°C to prevent heating and structural changes. The powder was then mixed with KBr in an agate mortar with a dilution factor of 0.25%. A 300-mg pellet, 13 mm in diameter, was pressed into a vacuum die with up to 8 t/cm2 of compression. For each sample, an infrared spectrum averaging 50 scans in the 4000- to 250-cm-1 energy range with a 2-cm-1 resolution was recorded using a Perkin-Elmer FT 16 PC spectrometer.
Major element content analyses were performed using an electron microprobe on glass samples obtained after fusion of the sediment (Colin et al., 1998). The carbonate fraction was removed by leaching 50 mg of sediment with 20% acetic acid in an ultrasonic bath, followed by rinsing several times and centrifuging to remove traces of carbonate solution. The carbonate-free fraction was subsequently carefully mixed by hand with 20% Li2CO3 SPMerck in an agate mortar. The mixture was fused in air on a platinum cell by radio frequency induction heating. The cell was heated to 900°C during 15 s to drive H2O and CO2 from the sample, and the temperature was then increased sufficiently above the mixture liquidus to ensure complete melting of the sample. Minimum temperatures required for rapid and complete fusion were established by trial and error and range from 1300° to 1450°C for the studied sediments. Similar techniques have been previously applied by Nicholls (1974) and Brown (1977) for major element content analysis.
Taking into account the sedimentation rates and the sampling intervals (one sample every 150 cm), grain size analyses were performed with a chronological resolution of 1-4 k.y. for the last 300 k.y. and 4-10 k.y. for the older period. Clay mineralogy determinations and major element content analyses were performed with the same chronological resolution only for the last 400 and 250 k.y., respectively.
