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 ephemer al saline lakes and dry saline pans are present on the Qarhan Salt Pla in. Dabusun Lake, the largest (about 200 km(2)), contains high salinit y Na-Mg-Cl brines. Whereas it precipitates halite, it is fringed by a potash salt flat. The dominant inflow to Dabusun Lake, the Golmud Rive r, contains dilute Na+-HCO3--rich meteoric waters. Dabusun Lake brines fall on an evaporation trend given by delta D (parts per thousand) = 3.3 delta(18)O-43. Both delta D and delta(18)O values increase with sa linity which in turn varies considerably with flooding and evaporation . The isotope compositions of the fluid inclusion brines from modern h alite formed along the lake's edge are intermediate to those of Dabusu n 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 co ntain three dissolution surfaces and three mud partings. The delta(18) O values for fluid inclusions in 22 primary halite samples from this s ection show a record of episodic flooding (lower delta(18)O values) fo llowed by evaporation (gradual increase in delta(18)O values). Primary fluid inclusions in halite crystallized initially at the surface prov ide a geochemical record of surface brines. Their major element compos itions varied through time. More concentrated fluids indicate more ari d 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 H2O of fluid inclus ions in primary halite were used to determine isotope variations in re gional precipitation and hence paleoclimatic changes on the Qinghai-Ti bet Plateau during the past 50,000 years.