Molecular Organic Geochemistry Approach For Reconstruction Of The Paleoenvironment, Nansha Area, South China Sea Since The Last 30 Ka Years

In the last two decades, remarkable success has been achieved in the researches of paleoceanography in the South China Sea. Most of the works are concentrated on northern South China Sea. Only a few studies have been done about southern South China Sea. The researches about bio-geochemistry, particularly molecular organic geochemistry are limited. However, the study of organic matter in sediments is important for researches of the carbon cycle and paleoclimate changes in the tropical ocean.314 Samples of core 17962(7 11 N, 11 5 ' E), located in west Pacific "warm pool", Nansha area of the South China Sea. A precise quantitative method applicable to high-resolution molecular stratigraphy was established. The organic compounds in the sediments of core 17962 were analyzed qualitatively and quantitatively by this procedure.w-alkanes, n-alkanols, n-alkanoic acids, l,15-C3o-C32 diols and long chain alkenones have been identified in extracts of 314 samples. The 613C values (-38.98?0.17 %o) and carbon distribution show that the marine microalgae Eustigmatophyceae are potential source of the C^-Cn diols. The relative ratio of CM over Ca2 diols (C3o diols / [Cw diols + €32 diols]), defined as the diol index, serves as a good indicator for changes in paleoceanography and paleoclimate in southern South China during the last 30.0 ka. The diol index is high in the warm phase and low in the cold phase in southern South China Sea. It corresponds well with the cold events recorded in the sediments of the North Atlantic, such as YD, HI, H2 and H3. The centennial-to millennial-scale climate events such as D/O are also presented between Heinrichs. The productivity of Eustigmatophytes and coccolithophorid calculated by the concentrations of Cao diol and long-chain ketones, respectively, indicate that the paleoceanographic environment and paleoclimate were unstable in Nansha area during the last glacial. The productivity of the two algae fluctuated greatly and was higher during the glacial than that in Holocene. The productivity ofEustigmatophyceae and coccolithophorid maximized at H3, which one hand was caused by the strengthen of upwelling induced by winter monsoon. And the other hand was caused by an increased terrigenous nutrient supply from the Sunda Shelf. Possibly the later is the main reason.Terrigenous proxies, for example, the accumulation rates of higher molecular weight lipids showed that terrestrial input in Nansha area is higher in the last glacial than that in Holocene. The specific stable carbon isotope of terrigenous long-chain compounds, the w-alkanes ( 13C= -34.15- -28.61 %o), the ?alkanols ( 13C= -35.79--26.56 ), and the H-alkanol acids (613C= -31.75- -27.12) evidence that they are derived from higher plants of C3, which implies that the surrounding area of southern South China Sea was not drought during the last glacial. The accumulation rates of terrigenous long-chain compounds peaked at H3, paralleling those of the algae biomarkers, demonstrate that the enhancement of Eustigmatophceae and coccolithophorid productivity at H3 is caused by an increased terrigenous nutrient supply from the Sunda Shelf. This interpretation coincides with Sunda Shelf laid bare, the low sea level and river flood due to the abundant monsoon precipitation in southern South China Sea in the last glacial.The fluctuations of the organic molecular compound proxies in sediments of core 17962 were correlated well with that of the 818O in GRIP ice core during the glacial, indicating the paleoclimate and paleoenvironment of the South China Sea were teleconnected with those of high-latitude area possibly by the monsoon activities.