Stable isotopes of lake and fluid inclusion brines, Dabusun Lake, Qaidam Basin, western China: Hydrology and paleoclimatology in arid environments

The Qaidam Basin, underlain by salt, is the largest (120,000 km 2) on the Qinghai-Tibet Plateau, western China. Numerous shallow to ephemeral saline lakes and dry saline pans are present on the Qarhan Salt Plain. Dabusun Lake, the largest (about 200 km 2), contains high salinity NaMgCl brines. Whereas it precipitates halite, it is fringed by a potash salt flat. The dominant inflow to Dabusun Lake, the Golmud River, contains dilute Na +HCO − 3-rich meteoric waters. Dabusun Lake brines fall on an evaporation trend given by δD(‰) = 3.3δ 18 O−43 . Both δD and δ 18O values increase with salinity which in turn varies considerably with flooding and evaporation. The isotope compositions of the fluid inclusion brines from modern halite formed along the lake's edge are intermediate to those of Dabusun Lake brines and those from the salt flat. Shallow sediments beneath Qarhan consist of interbedded salts and mud. A short core section (1.3–1.7 m depth) from the northern edge of Dabusun Lake, was found to contain three dissolution surfaces and three mud partings. The δ 18O values for fluid inclusions in 22 primary halite samples from this section show a record of episodic flooding (lower δ 18O values) followed by evaporation (gradual increase in δ 18O values). Primary fluid inclusions in halite crystallized initially at the surface provide a geochemical record of surface brines. Their major element compositions varied through time. More concentrated fluids indicate more arid conditions in the basin, whereas wetter conditions prevailed during intervals of non-salt deposition when laminated muds accumulated. The isotope compositions together with the activity of H 2O of fluid inclusions in primary halite were used to determine isotope variations in regional precipitation and hence paleoclimatic changes on the Qinghai-Tibet Plateau during the past 50,000 years.