CHANGES IN DISTRIBUTION AND COMPOUND-SPECIFIC CARBON ISOTOPE COMPOSITIONS OF n-ALKANES AS RECORDED IN LUGU LAKE SEDIMENTS FROM SOUTHWESTERN CHINA SINCE LAST GLACIAL MAXIMUM AND IMPLICATIONS FOR PALEOVEGETATION EVOLUTION
Lugu Lake is a semi-closed lake lied on Yungui Plateau in typical southwest monsoonal region. A 18. 3m long sediment core was collected at water depth of 69.3m in Lugu Lake ( 27°43'08.4"N, 100°46'33.9"E ) . Core lithology changed from grayish yellow fine silty clays in the lower part ( 10.0 ~ 8.1m) to grey brown muddy silty sand (8.0~4.1m),grey muddy silty sand between 4.1m and 1.4m and dark grey fine silty clays above 1.4m. A total of 22 AMS ~(14)C dates were obtained above the depth of 10m, composed of 15 from bulk sediments and 7 from terrestrial plant remains. The ~(14)C dates have been calibrated to establish calendar ages using CALIB 5.1 and the CalPal program. 15 samples taken from the sediment above 7.5m depth ( corresponding to 20.5ka) at a 50-cm interval were used in this study. n-alkanes distribution and compound-specific ~(13)C/~(12)C ratios of the n-alkanes from terrestrial higher plants were measured on those samples with objective to decipher the changes in trees/grasses and C_3/C_4 vegetation over the Last Glacial Maximum ( LGM) and discuss the potential factors controlling C_3/C_4 variations. During the period from LGM to Early Holocene, n-alkanes concentration and ( C_(27) + C_(29)) /2C_(31) ratio increased gradually whereas average chain length of n-alkanes (>C_(25) ) ( ACL) decreased accordingly, indicative of a higher portion of woody plants with a trend of increase, which reflect the climate developed towards warmer and more humid little by little. Meanwhile, the delta~(13)C values of the ( C_(27),C_(29) and C_(31)) n-alkanes showed an increasing trend, which cannot be explained by climate changes and thus represent variations in C_3/C_4 vegetation. The calculated portion of C_4 plants based on two end-members model increased from 19.6% to 31.9%. The increases in C_4 plants were mainly attributed to a gradual increase in temperature during this period and the narrowly-focused rainfall in summer also enhanced C_4 biomass. In Middle Holocene, the characteristics of n-alkanes distribution demonstrated the portion of woody plants remained high, indicating a warm and humid climate. The delta~(13)C values of the n-alkanes became more negative due to the combined influence of increased rainfall and decreased C_4 biomass. During Late Holocene,the characteristics of n-alkanes distribution suggested the portion of grassy plants increased while the delta~(13)C values of the n-alkanes turned to slightly more positive,which was caused by relatively cold and dry climate. The changes in tree/grass as indicated by the characteristics of higher plants n-alkanes distribution are consistent with that revealed by pollen record in the region of Lugu Lake. This study further confirms that temperature was the main controlling factor for C_4 plants incidence whereas the increased rainfall would largely limit the advantage of C_4 plants over C_3 plants even if the temperatures were favorable to C_4 plants.