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Climate and vegetation variations since the LGM recorded by biomarkers from a sediment core in the northern South China Sea



Variations in stable carbon isotopic compositions of n-alkanes (δ13Calk) and n-alkane proxies (CPI, C31/C27 and ACL) from core MD05-2905 (20° 08.17′ N, 117° 21.6′ E) in the northern South China Sea (SCS), provided evidence of biological responses on adjacent land areas in the form of changes in terrestrial vegetation and provenance of the remains of land plants during and since the Last Glacial Maximum (LGM). Decreased values of n-alkane proxies and δ13Calk over the last 28 ka marked the transition from glacial to interglacial conditions and indicated a greater extent of herbaceous vegetation at the LGM than present and an increased abundance in woody vegetation during the Holocene. Relatively larger δ13Calk values at the LGM indicated that n-alkanes of terrestrial herbs in the sediment core were not only from the leaf waxes of C3 plants, such as herbs of Artemisia, growing on proximate parts of the exposed continental shelf, but also from C4 grasses likely from subtropical parts of southeastern China and Taiwan Island. Reconstructed sea-surface temperatures (SSTs), based on equation image data from the same core, revealed that temperatures were ∼3°C lower than present during the LGM, with abrupt variations during both glacial and interglacial periods. During the LGM, lower temperature and sea level and greater aridity relative to present resulted in an expansion of herbaceous vegetation over an enlarged extent of exposed continental shelf. Following deglaciation, warming and enhanced summer monsoonal activity led to increased humidity and to an expansion of woody vegetation (mostly C3 plants) during the Holocene. Results also revealed rapid oscillations in C31/C27 ratios over millennial scales, with lower values associated with cold stages. These low C31/C27 ratios indicated increased input of woody plant material possibly from more extensive gallery forest and greater transport of their plant debris to the coring site, perhaps facilitated by enhanced runoff under a strengthened Northern Hemisphere winter monsoon during short-duration cold events. Copyright © 2012 John Wiley & Sons, Ltd.