Variations in organic matter composition in sediments from Lake Huguang Maar (Huguangyan), south China during the last 68 ka: implications for environmental and climatic change

This study presents a continuous and long-term palaeoenvironmental and palaeoclimatic record from a lacustrine sediment core (Lake Huguang Maar) at the northern coast of the South China Sea, extending back to about 68 cal ka BP. We provide a comprehensive ecosystem analysis from the opportunity to combine organic geochemical and palynological data. Several climatically induced changes to the flora and fauna have been recognised. From 68 to 58 and 48 to 40.5 cal ka BP, relatively depleted δ13CTOC (−31.3 to −25.0‰) and δ13C values of mid- and long-chain n-alkanes (−34.3 to −30.2‰), as well as high percentages of tropical pollen, indicate a vegetation dominated by subtropical/tropical forest (C3 plants). An expansion of a mixed C3/C4 open grassland between 58 and 48 and particularly after 40.5 cal ka BP mirrors drier climatic conditions, in conjunction with a lowering of atmospheric CO2 concentrations; δ13CTOC (−24.1 to −16.8‰) and δ13C values of n-alkyl lipids (−28.9 to −24.1‰) are heavier, higher percentages of non-arboreal pollen were observed and the wood/grass lignin index (WGLI) from open pyrolysis (Py–GC) indicates a higher input of grass lignin. Abundant oxidised terrigenous particles that resulted from increased fire activity and/or greater erosion rates are a further hint of enhanced dryness. The clear drop in moisture availability suggests that this site is sensitive in fluctuations of the summer and winter monsoon activity in accordance with other proxy records in south-east Asia. The response of the aquatic ecosystem is documented by an extremely large δ13C shift of algal-derived botryococcenes from −35.4 to −6.2‰, which records a change towards a bicarbonate carbon source at low dissolved CO2 concentrations.