Evaporative enrichment of stable isotopes (δ18O and δD) in lake water and the relation to lake-level change of Lake Qinghai, Northeast Tibetan Plateau of China

Stable isotopic compositions (δ18O and δD) have been utilized as a useful indicator for evaluating the current and historical climatic and environmental changes. Therefore, it is vital to understand the relationship between the stable isotopic contents in lake water and the variations of lake level, particularly in Lake Qinghai, China. In this study, we analyzed the variations of isotope compositions (δ18O, δD and d-excess) in lake water and precipitation by using the samples that were collected from Lake Qinghai region during the period from 2009 to 2012. The results showed that the average isotopic contents of δ18O and δD in lake water were higher than those in precipitation, which were contrary to the variations of d-excess. The linear regression correlations between δ18O and δD in lake water and precipitation showed that the local evaporative line (LEL) in lake water (δD=5.88δ18O-2.41) deviated significantly from the local meteoric water line (LMWL) in precipitation (δD=8.26δ18O+16.91), indicating that evaporative enrichment had a significant impact on isotopic contents in lake water. Moreover, we also quantified the E/I ratio (evaporation-to-input ratio) in Lake Qinghai based on the lake water isotopic enrichment model derived from the Rayleigh equation. The changes of E/I ratios (ranging from 0.29 to 0.36 between 2009 and 2012) clearly revealed the shifts of lake levels in Lake Qinghai in recent years. The average E/I ratio of 0.40 reflected that water budget in Lake Qinghai was positive, and consistent with the rising lake levels and the increasing lake areas in many lakes of the Tibetan Plateau. These findings provide some evidences for studying the hydrological balance or water budget by using δ18O values of lake sedimentary materials and contribute to the reconstruction of paleolake water level and paleoclimate from an isotopic enrichment model in Lake Qinghai.