Isotopic determinations on water samples from the northeastern Tibetan Plateau

On the Tibetan Plateau, applications of dD and d18O values in paleoclimate studies tend to be complicated due to multiple processes influencing isotopic compositions in paleoclimatic archives. In this study, isotopic compositions of modern waters in the eastern Qaidam Basin on the northeastern Tibetan Plateau, and dD values of n-fatty acids (n-FA dD) from a sediment core at Hurleg Lake were systematically analyzed to infer hydroclimate controls during the Holocene. The modern water isotopic results show a major contribution of snowmelt water originating from high-elevation mountains to the north of the Qaidam Basin via river and groundwater discharge, and the importance of evaporation in affecting lake water budget in this region. n-C26 FA dD values tend to be more negative at millennial-scale warm-dry periods during the Holocene, and vice versa, opposite to what is commonly expected. Assisted with modern water isotopic results, we infer amplified contribution of snowmelt water to the soil water around this open lake system at warm-dry periods. Meanwhile, changes in n-C16 FA dD values at Hurleg Lake reflect the evolution of isotopic compositions of lake water, thus we use the isotopic difference between n-C26 and n-C16 FA (dDC16-C26) to infer hydroclimate and evaporation variations in this region. Based on our data, relatively low n-C26 FA dD and n-C16 FA dD values at 10-6 cal ka BP indicate large contribution of snowmelt water into the lake during the Holocene Climate Optimum. After 6 cal ka BP, changes in evaporation became the major control on lake hydrology and led to larger fluctuations of dDC16-C26. Our study highlights the importance of systematic analysis on modern processes before using stable isotopes for paleoclimate reconstructions, and demonstrates that dD difference between long-chain and short-chain n-FA might be an effective way to better understand the controlling factor of hydrological variations in a climatic complex region like the Tibetan Plateau