Leg 184 recovered a sequence of hemipelagic sediments that record the past 31 m.y. of environmental history of the South China Sea. For the first time in the lower latitude western Pacific, these cores provide a high-resolution continuous record of relatively carbonate-rich fine-grained sediments and a possibility of sea-land correlation of the upper Cenozoic stratigraphy for the region.

The lithologies, microfossils, and physical properties of the hemipelagic sequence reveal significant trends, clear cyclicities, and abrupt changes that provide detailed records of environmental transformations, including provenance and volcanism. The most prominent are variations in CR, MS, and NGR, which exhibit obvious orbital-scale and finer fluctuations in monsoonal climate.

The discovery of high accumulation rate (5-20 g/cm2/k.y.) Oligocene hemipelagic sediments on the lower continental slope near the continent crust margin probably reflects active downslope transport of terrigenous sediment during the early stage of seafloor spreading of the South China Sea basin. This "deep-water" sequence continues below the "acoustic basement," which might be a thick marine Paleogene section.

The Oligocene/Miocene boundary in the northern SCS is marked by sedimentary deformation, abrupt lithologic changes, and a stratigraphic hiatus. These related features will help resolve the nature and timing of one of the most significant Cenozoic tectonic and climate changes of the region.

Cores from all drill sites show a high carbonate content for the Miocene and lower Pliocene in both the northern and southern South China Sea. The low terrigenous input and partly high carbonate production resulted in a sediment environment in the northern slope similar to that near the reef areas in the southern part of the sea in the Miocene; accumulation rates, however, are significantly different in the northern and southern SCS after the Miocene.

A general increase of noncarbonate sediment accumulation after 2-3 Ma was found at all drill sites; for the northern sites, the increase has become even more significant in the latter part of the last million years. A site with exceptionally high rates of hemipelagic fine-grained sedimentation (Site 1144, 450 m of sediment for the last 1 m.y.) offers a unique opportunity for fine-resolution paleoenvironmental studies at decadal scale.


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