The Early Pleistocene Climate Change Recorded In The Northern South China Sea Sediments

The South China Sea(SCS) as the largest marginal sea of western Pacific is adjacent to the western Pacific warm pool. Since the SCS is located in typical East Asian Monsoon region, the sediments can not only record global climate change, but also document significant regional changes in paleoclimate and paleoceanography. In this study, a total of 367 samples spanning the early Pleistocene(about 2.5~1.5 Ma) were selected from the Ocean Drilling Program Site 1145 in the northern SCS. We analyzed carbonate contents, content and grain size of terrigenous components, magnetic susceptibility, and elemental concentrations to reconstruct the early Pleistocene paleoclimate/paleoceanography changes in this region. Moreover, we compared carbonate content、grain size and magnetic susceptibility records from Lingtai loess-paleosol sequences and ODP 1145 sediments to investigate the similarities and differences between marine and continental archives. The major findings are as follows:(1) Variation of carbonate content of the ODP 1145 sediments belongs to the "Atlantic type" cycle. The terrigenous content is mainly affected by the dilution of biogenic substance. Grain size of the terrigenous materials is controlled by multi-factors such as changes in sea level and fluvial input. Magnetic susceptibility variation mainly related to oceanic oxidation/reduction environment.(2) Principal component analyses of carbonate-free basis element composition and Al-normalized elemental ratios show that elemental geochemistry of ODP 1145 sediment is controlled mainly by three factors: productivity of biogenic substances(such as Ca, Si), fluvial input of terrigenous material(such as Al, Ti and Zr), marine authigenic conditions(Mg and Mn). Al-normalized elemental ratios can be used to reflect relative contribution of the biogenic/terrigenous materials(Ca/Al), chemical weathering in the source areas(K/Al), grain-size effect(Zr/Al), and oxidation / reduction transformation of marine environment(Mn/Al).(3) Variations of elemental contents and Al-normalized elemenatal ratios reveal that productivity displays stepwise decreases at about 2 Ma and 1.7 Ma, terrigenous input increased obviously since ~2 Ma, and the amplitude of marine environmental change increased since ~1.7 Ma. Both physical and chemical proxies show significant shifts at about 2 Ma and 1.7 Ma, indicating stepwise strengthening(weakending) of the East Asian Winter(Summer) Monsoon during the early Pleistocene. Spectral results of these proxies show that oceanic productivity and grain size variation may be mainly controlled by ice volume-induced sea level change, whereas chemical weathering in sorce areas and marine environmental changes are subject to the combined effects of high-latitude ice volume and low-latitude radiation.(4) Comparison between carbonate content, grain size and magnetic susceptibility of ODP 1145 sediment and Lingtai loess-paleosol seuqnece suggests that carbonate content variation of the SCS sediments differs from that of Chinese loess, magnetic susceptibility records show similar decreasing trend, and variations of mean grain size of SCS terrestrial component and Lingtai loess exhibit similar glacial-interglacial variability. Different trend of the mean grain size variations indicate that the grain size of marine sediment may be affected by other factors in addition to the winter monsoon and sea level changes.