6. Site 1145¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

The primary objectives of Site 1145 (SCS-2) on the northern margin of the South China Sea were to (1) recover a continuous sequence of deep-water, hemipelagic sediments to reconstruct the middle Pliocene to Pleistocene/Holocene paleomonsoon history; (2) establish whether the South China Sea records an intensification of the winter monsoon during the Pliocene-Pleistocene, coincident with the development of loess in China, and whether the summer monsoon is intensifying or weakening during the Pliocene-Pleistocene; and (3) reconstruct (in conjunction with Site 1144) the water-mass depth gradients and ventilation history of the South China Sea during the Quaternary.

Site 1145 (SCS-2) is located at 19°35.04´N, 117°37.86´E (Fig. F9 in the "Leg 184 Summary" chapter). The site lies downslope from Site 1144 at a water depth of ~3175.5 m, which is below the sill depth of the Bashi Strait (2600 m). Based on piston core data, sedimentation rates were expected to be ~100 to ~200 m/m.y. or about one-half to one-third of those at Site 1144.

Site 1145 is located at shotpoint 4704 (11:10) on the seismic Line SO95-10 (Figs. F5 and F10, both in the "Seismic Stratigraphy" chapter) on the lower of a series of terraces that form the continental slope off southern China. The thick, conformable sediments extend almost 1 second below seafloor (sbsf) to a strong wavy reflector, presumed to be prerift sediments. Using the interval velocities provided by Hamburg and Tongji Universities, this two-way time is equivalent to ~760 m of sediment cover. The 200 meters below seafloor (mbsf) penetration expected at Site 1145 should reach 0.28 sbsf. The seismic structure within the upper 200 mbsf contains an upper zone of multiple (about six) strong reflectors (0-0.12 sbsf) and a lower, more diffuse zone (0.12-0.28 sbsf) that is divided by a prominent reflector at ~0.21 sbsf (Fig. F6 in the "Seismic Stratigraphy" chapter). The northwest-southeast dip line (SO95-10) reveals sediments that are generally smooth and slightly onlapping onto the continental slope from the adjacent deep basin. The JOIDES Resolution 3.5-kHz data and SO-95 PARASOUND data (Sarnthein et al., 1994) reveal relatively smooth, parallel, conformable sediment cover with no distinct surface features (Fig. F7 in the "Seismic Stratigraphy" chapter).

We expected Site 1145 to have hemipelagic clays, similar to Site 1144 but with less carbonate and organic carbon. We predicted that the multiple advanced hydraulic piston corer (APC) holes at Site 1145 would provide a high-quality spliced record from the late Pliocene to Pleistocene (Holocene). These records should allow us to apply a wide range of monsoonal and paleoceanographic proxies to reconstruct the orbital-scale climate changes in the Quaternary and late Pliocene and the water-mass and ventilation changes across the Bashi Strait sill, which is the only deep-water connection between the Pacific and the South China Sea.

Given scientific priorities, time constraints, and shipboard operational limitations, we adjusted our plan to triple APC core to refusal and to deepen all three holes to 200 mbsf with the extended core barrel (XCB) if the APC did not reach 200 mbsf.