Program

     The South China Sea (SCS) is a large marginal sea between the Asian continent and the Pacific Ocean and rimmed by a broad continental shelf.  Sea level in the SCS was about 100-120 m below present at the last Glacial Maximum. As a result, the ratio of land (in the form of exposed continental shelf) to sea was greater than present, leading to enhanced continentality of climate in the region and expansion of the area covered by terrestrial vegetation. However, previous studies were mainly based on pollen analysis and have generated inconsistent information about the form and composition of vegetation on land including exposed continental shelf around the northern SCS. Lipid biomarkers from sedimentary organic matter can provide important information about production, delivery and preservation of organic matter and thereby reflect past vegetation and environmental changes. Thus the content of n-alkanes, n-alkane proxies (L_23-/H₂₄₊, CPI, C₃₁/C₂₇, ACL) and stable carbon isotopes compositions of n-alkanes (δ¹³C_alk) from core STD235 (20º21.15'N, 118º22.56'E) in the lower slope of the northeastern SCS were reported, which provided evidence of organic matter composition, terrigenous sources contribution and environmental change history since ~17 ka. N-alkane proxies of L_23-/H₂₄₊  ratios indicated that the sediment organics were contributed from mixed marine and terrestrial sources and dominated by the marine contribution over the past 17ka. The L_23-/H₂₄₊ ratios decreased obviously during the late last LGM. The average total content of long-chain n-alkane (C₂₅~C₃₃), representing terrestrial high plants contribution, were higher by around 3.09±0.99 ug/g in glacial period than those in interglacial period. In addition, the past C₃/C₄ plants composition of terrestrial contribution was reconstructed using compound-specific stable carbon isotopes of long-chain n-alkanes combined with some other appraisal proxies (eg. C₃₁/C₂₇, ACL, and pollen data). The calculated results from a binary end-member model showed that the C₄% contribution ranges from 27.93 to 66.47%, and the average C₄% in glacial and interglacial period is 54.76±6.3% and 38.39±4.3%, respectively. During the glacial period, lower temperature and greater aridity relative to present resulted in an expansion of herbaceous/C₄ vegetation  over an enlarged extent of exposed continental shelf. Following Holocene, warming and enhanced summer monsoonal activity led to increased humidity and to an expansion of woody vegetation (mostly C₃ plants).