| Sediments from ODP Site 1148, ODP Site 1144, KNG7 and KNG5 in the northern South China Sea were used to trace sediment sources and reconstruct the sedimentary evolution of the South China Sea since Oligocene, obtain the paleoenvironmental evolution of the South China Sea since 1.1 Ma, ascertain sea level change, the evolution of East Asia Summer Monsoon, current transport and paleoclimatic change of the South China Sea since 18 ka BP based on a multi-proxy approach including a monomineralic quartz isolation procedure, identification of clay minerals by X-ray diffraction, grain-size analysis of isolated terrigenous materials, transmission electron microscope combined with energy dispersive X-ray spectrometer (TEM-EDS), dating of AMS14C, oxygen isotope ratios (δ18O) and scanning electron microscope (SEM) analysis of isolated quartz and rare earth elements analysis. The major conclusions are as follows:It was found that the evolution of South China Sea Basin could be divided into five stages, namely 34～28.5,28.5～23,23～16.5,16.5～5 and 5 Ma to present. The sediment source of ODP Site 1148 was mainly from Palawan during the early expansion period of the South China Sea, with the extensive expansion of the South China Sea since 30 Ma, Palawan block moving southward gradually, making the contents of the terrigenous components and terrigenous mass accumulation rate decreasing. Then, with Palawan block moving southward, the collision of Indian-Australian Plate and Eurasian Plate led to the intensified uplift of Qinghai-Tibet Plateau, and the ancient geography of high in the west and low in the east was gradually formed, and quartz oxygen isotope data showed that the 25.4-23.3 period was a source conversion phase of ODP Site 1148, South China block turning to be the main source of ODP Site 1148. Later, with the intensification of the Pearl River headward erosion, ancient sedimentary rocks of South China block have changed to be the major source of ODP Site 1148.During glaciations, quartz and feldspar contents, terrigenous mass accumulation rate and mean grain size of ODP Site 1144 were much higher, indicating a dry and cold climate, low sea level, and exposed continental shelf. During the interglaciations, quartz and feldspar contents, terrigenous mass accumulation rate and mean grain size of ODP Site 1144 were much lower, implying a warm and humid climate, high sea level and submerged continental shelf. The differences between glaciations and interglaciations significantly increased after 0.9 and 0.47 Ma, which was attributed to the strengthened monsoon system caused by gradual uplift of the Tibetan Plateau. Spectrum analyses of quartz content reveal the presence of Milankovitch cycles, including～92 ka (eccentricity),～20 ka (precession), indicating that the tropical low-latitude processes also have an impact on the climate change.Clay minerals of Core KNG5 mainly consist of illite, chlorite and kaolinite, with associated smectite. Sediment source studies indicate that kaolinite mainly from the Pearl River, smectite mainly from the Luzon River, and illite and chlorite were mainly offered by the Pearl River and Taiwan, respectively. Our results indicate that clay minerals are mainly from the Pearl River during stage 17,500~12,500 cal yr BP, and the South China Sea modern current system began to form since 12,500 cal yr BP, as a result, Taiwan turns to be the major contributor of clay minerals after 12,500 cal yr BP. Grain size, clay minerals and rare earth elements of Core KNG5 in the late glacial were controlled by relative changes of Sea Level. The 1-2.2μm grain-size population and rare earth elements record demonstrates that East Asian Summer Monsoon intensity generally follows changes in insolation and that the response is similar for a large area of China and other northern low-latitude records, implying the globality of the monsoon evolution since Holocene. The anomalous environmental conditions in the northern South China Sea may imply intensified ENSO activity during the late Holocene.
|
PDF Full Text Request