Origins of biomarker aliphatic hydrocarbons in sediments of alpine Lake Ximencuo, China

Aliphatic hydrocarbons in lake sediments are important lipid biomarkers that have been widely used in paleoenvironmental reconstructions on the dual premises that their sources are defined and that they resist degradation. As a test of these premises, we have analyzed the n-alkane and n-alkene distributions in the leaf waxes of the eight dominant plants and in the surface soil at five locations surrounding Lake Ximencuo, a typical alpine lake on the eastern Qinghai-Tibetan Plateau, to explore the origins, delivery, and deposition of these molecular biomarkers in the surface sediment of this restricted lake system. Distinctive distributions of n-alkanes and n-alkenes were found in the leaf waxes of the different plants. Unlike those of the plants, the n-alkane distributions in the soil are virtually identical at the locations around the lake, and the n-alkenes found in the plant waxes are rare in the surface soil. These differences imply extensive diagenetic degradation of the soil-hosted hydrocarbons. In contrast to the soils, the hydrocarbon distribution in the lake sediment is more like those of the plants. It contains n-alkanes that range from C15 to C33 with a maximum abundance at n-C31 and includes a suite of n-C21:1 to n-C28:1 alkenes with odd-carbon preference. Although they may originate from land-plant debris that is washed directly into the lake, the alkenes are postulated to originate principally from algae living in the lake because of their absence in the soils. The biomarker hydrocarbons in the lake sediments appear to integrate multiple inputs, including plants around the lake, glacial meltwaters, and algae, fungi, and microbes within the lake. The results of this study highlight that different groups of aliphatic hydrocarbons in lake sediments can have different origins and consequently are likely to reflect different aspects of past environmental conditions. Hence, consideration of the multiple possible origins of these biomarkers has the potential to expand and refine paleoenvironmental reconstructions that employ them.