Human impacts on vegetation exceeded the hydroclimate control 2 ka ago in the Qinghai Lake basin revealed by n-alkanes of loess

Vegetation is one of the most important components of the land ecosystem. Human activity and climate change are the two main factors that control the changes in regional vegetation distribution. The past vegetation changes have been extensively studied all over the world, while few had focused on the relative contributions of human activities and climate change on vegetation change. Due to its key geographic position, the Qinghai Lake basin (QLB), located on northeastern Qinghai-Tibetan Plateau, is an ideal locality to carry out such study as its vegetation dynamics are more sensitive to any natural and anthropogenic disturbances. In this study, we analyzed the chain length distribution and concentration of n-alkanes of a 3.6 m thick loess profile (QH-18) in the QLB to infer the information about past vegetation change and its controlling factors. The proxy for aquatic macrophytes (Paq) and carbon preference index (CPI) values have been applied to distinguish the relative con-tributions from aquatic or terrestrial higher plants to the n-alkanes. Combining with luminescence-based chro-nology and magnetic susceptibility data of the QH-18 section, the resulted showed that the n-alkane distribution was characterized with two remarkable variations at-11.5 ka and-2 ka, respectively. The first major variation in n-alkanes was considered to be correlate to the gradual climate warming and humidification during the last deglaciation period, suggesting the response of loess n-alkanes to natural environmental changes over the orbital scales. The second markedly change at-2 ka was mainly controlled by the enhanced human activities. We concluded that the hydroclimate has played the dominant role before 2 ka, but human impacts have over-whelmed the natural variabilities thereafter and become the major controlling factor of n-alkane variations and vegetation change in QLB.