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 inferred from mineralogical and isotopic analyses of lacustrine Sediments from the Sumxi-Longmu Co basin. The interpretation is based on isotope data from the modem system. Carbon isotope chemistry is complex because of the presence of old limestones on the watershed. Sediments contain detrital and authigenic calcites, the proportions of which are quantitatively estimated by X-ray diffractometry. Stable isotope contents of the authigenic calcite are reconstructed from mass-balance equations. The high C-13 contents (up to +6 part per thousand) are attributed to an enhancement of calcite precipitation from water surface during 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 paleobiological data from the same material, shows the sudden establishment of wet/warm conditions at almost-equal-to 10,000 BP which culminated at almost-equal-to 7500-6000 BP after a brief reversal event at almost-equal-to 7900 BP. Maximum aridity for the whole Holocene is recorded at almost-equal-to 4300 BP.