Late Holocene environmental change on the NE Tibetan Plateau : a paleolimnological study of Lake Qinghai and Lake Gahai, China, based on stable isotopes
Climate driven hydrologic variability has direct socio-economic impacts on local, regional and global scales. Particularly vulnerable is the region that lies within the boundaries of the East Asian monsoon, which is one of the most pronounced and influential phenomena of Earth's climate system. People within the heavily populated Asian countries have adapted many aspects of their society to the subtleties of monsoon rains, and are thus highly susceptible to small changes in the timing and intensity of monsoon precipitation. This study aims to reconstruct high-resolution late Holocene climatic and hydrologic variability from Lake Qinghai, NE Tibetan Plateau, using stable isotope techniques. The thesis also examines the modern isotope systematics of Lake Qinghai through mass balance methods to understand the modern hydrology of the lake in order to determine which factors are important in influencing lake water isotope composition. The isotopic compositions of Lake Qinghai's modern waters indicate that they have been modified by evaporation, as they plot below the Global Meteoric Water Line (GMWL). Isotope mass balance models quantify this, and demonstrate that 29 to 35% of the waters entering the lake are from direct precipitation and surface runoff, while evaporation from the lake's surface accounts for 44 to 54% of the lakes water loss. This suggests that the balance of precipitation to evaporation (P/E) is an important control on the composition of lake waters and therefore carbonate incorporated into the sediment record will reflect this. Four cores were used in this study and they provide a palaeohydrological history displaying distinct changes in 518Oauth and 513Cauth over the past 1500 years. These are interpreted in terms of effective moisture and one particularly pronounced event, between 1600 and 1850 AD has been attributed to decreases in evaporation. This event, when compared to other regional palaeoenvironmental archives is coincident with cold temperatures, synonymous with the Northern Hemisphere Little Ice Age, tentatively suggested to be a result of solar variability.