STABLE-ISOTOPE AND RADIOCARBON BALANCES OF 2 TIBETAN LAKES (SUMXI CO,LONGMU CO) FROM 13,000-BP

Hydrologic and climatic changes in western Tibet since 13,000 BP are i nferred from mineralogical and isotopic analyses of lacustrine Sedimen ts from the Sumxi-Longmu Co basin. The interpretation is based on isot ope data from the modem system. Carbon isotope chemistry is complex be cause of the presence of old limestones on the watershed. Sediments co ntain detrital and authigenic calcites, the proportions of which are q uantitatively estimated by X-ray diffractometry. Stable isotope conten ts of the authigenic calcite are reconstructed from mass-balance equat ions. The high C-13 contents (up to +6 part per thousand) are attribut ed to an enhancement of calcite precipitation from water surface durin g freezing periods. The O-18 content is related to residence time and evaporation. The validity of C-14 dates obtained from different types of material is discussed. Our record, in good agreement with paleobiol ogical data from the same material, shows the sudden establishment of wet/warm conditions at almost-equal-to 10,000 BP which culminated at a lmost-equal-to 7500-6000 BP after a brief reversal event at almost-equ al-to 7900 BP. Maximum aridity for the whole Holocene is recorded at a lmost-equal-to 4300 BP.