ABSTRACT



The Asian monsoon system is a major component of both regional and global climate. Evolution of monsoonal climates in southern Asia is linked to the growth of the Himalayan-Tibetan orogen, the opening and closing of marginal seas, and changes in global climate. The location of the South China Sea (SCS) between East Asia and the "maritime continent" is ideal to record the paleoceanographic responses to both winter and summer monsoons. The broad scientific themes of Leg 184 were (1) to document the Cenozoic history of the SCS, including its biostratigraphy, lithostratigraphy, chronology, paleoclimatology, and paleoceanography; (2) to reconstruct the evolution and variability of the East Asian monsoon during the late Cenozoic on millennial, orbital, and tectonic time scales; and (3) to identify and better understand the links between tectonic uplift, erosion and weathering, hemipelagic deposition, and climate change, including the evolution of the Asian monsoon and Neogene global cooling. The Leg 184 shipboard party cored 17 holes at Sites 1143-1148 in the SCS and recovered 5463 m of sediment that will enable the study of these themes, meeting the mission objectives beyond expectations. Core recovery averaged 83%-101%; all sites (except 1148) were triple cored with the advanced hydraulic piston corer and partially double or triple cored with the extended core barrel to construct continuous stratigraphic sections at the meters composite depth (mcd) scale. At all sites the hemipelagic deposits are rich in calcareous microfossils. The suite of sites yields an almost continuous record of the environmental history of the South China Sea during the last 30 m.y.

Site 1143 is located in the Spratly Islands area of the southern SCS at a water depth of 2772 m. It yielded a 516-m-long composite section recording the depositional history of the last ~10 m.y., with linear sedimentation rates (LSRs) of 3-7 cm/k.y. and mass accumulation rates (MARs) of 3-10 g/cm2/k.y. Sites 1144 through 1148 are located on the northeast continental slope of the SCS. Site 1144 provided a 522-m-long composite section from a sediment drift at a water depth of 2037 m, spanning the last 1 m.y. and yielding a very high LSR (30-110 cm/k.y.) and MAR (25-140 g/cm2/k.y.). At Site 1145, a 213-m-long composite section representing the last 3 m.y. was recovered from a water depth of 3175 m, with LSRs of 4-25 cm/k.y. and MARs of 4-19 g/cm2/k.y. Site 1146, at a water depth of 2092 m, recovered a 645-m-long composite section representing a 19-m.y. record and yielding LSRs of 2-36 cm/k.y. and MARs of 2-23 g/cm2/k.y. Site 1147 is a short record (85 mcd) recovered only 0.4 nautical miles from Site 1148 and designed to recover the top interval missing at Site 1148. Site 1148 recovered a 861-m-long composite section from a water depth of 3294 m, spanning the last 30 m.y. The LSR is 1-2 cm/k.y. for the Miocene to mid-Pliocene, up to 20 cm/k.y. for the late Pliocene and Pleistocene, and up to 30 cm/k.y. for the Oligocene. The high accumulation rate (5-20 g/cm2/k.y.) of Oligocene sediments on the lower continental slope near the continent crust margin probably reflects active downslope transport of terrigenous sediments during the early stage of seafloor spreading of the South China Sea basin. If the acoustic basement was penetrated at ~800 meters below seafloor (mbsf), the sediments indicate that a deep-water sequence continues below the bottom of the hole and might be a thick marine Paleogene section. A second possibility is that the acoustic reflector was not penetrated in the cored interval and the nature of the acoustic basement is unknown.

The depositional history of the late Cenozoic in the northern slope had three important stages: an Oligocene interval with extremely high sedimentation rates, a Miocene and early Pliocene interval with lower sedimentation rates and high carbonate content, and an interval of the last 3 m.y. with high clastic sediment accumulation rates. A different trend of depositional history is indicated at the southern Site 1143, where carbonate accumulation decreased from the late Miocene toward the late Pleistocene and the noncarbonate accumulation has risen again for the last 3 m.y. However, the upper Miocene sediments were similar in composition between the northern and southern sites, containing more than 50% of carbonate. The Oligocene/Miocene boundary in the northern SCS (Site 1148) is marked by sedimentary deformation, abrupt lithologic changes, and a stratigraphic hiatus (~24-27 Ma). These related features will help resolve the nature and timing of one of the most significant Cenozoic tectonic and climatic changes of the region. A general increase of noncarbonate sediment accumulation after 2-3 Ma was found at all drill sites, and for the northern sites the increase becomes even more significant in the latter part of the last million years. Excellent orbital-scale cyclicity is displayed in color reflectance, natural gamma radiation, magnetic susceptibility, and bulk density, particularly for the Pliocene-Pleistocene intervals.

Introduction

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