Later Pleistocene/Holocene climate conditions of Qinghai-Xizhang Plateau (Tibet) based on carbon and oxygen stable isotopes of Zabuye Lake sediments

摘要

We present a time series of carbon and oxygen stable isotope records of the last 30000 ~(14)C years throughout the last glacial-postglacial cycle from western Qinghai-Xizhang (Tibet) Plateau. A 20-m core drilled in the south basin of Zabuye Salt Lake was analyzed for inorganic and organic carbon and total sulfur contents, δ~(13)C and δ~(18)O values of carbonates. Our results indicate that climatic changes have led to a drastic negative shift of stable isotope ratios at the transition between the Last Full Glacial and the postglacial phase during Late Pleistocene times (～16.2 kyr BP), and a rapid positive shift at the transition from Pleistocene to Holocene (～10.6 kyr BP). The first shift is marked by the drop of δ~(18)O_(carb) values of about 10‰ (from +2 to -8‰) and δ~(13)C_(carb) values of about 3‰ (from 5 to 2‰). The second shift which occurred at the transition from Pleistocene to Holocene was of similar magnitude but in the opposite direction. Isotope data, combined with total organic and inorganic carbon contents and the lithological composition of the core, suggest this lake was an alluvial pre-lake environment prior to ca. 28 ~(14)C kyr BP. During ca. 28-16.2 ~(14)C kyr BP, this lake probably overflowed and received abundant recharge from melting glaciers when the deglaciation was in its full speed. A spike of markedly enhanced δ~(13)C_(carb) and δ~(18)O_(carb) is seen at ～11.5 kyr BP, probably due to the isotopic effects left behind by the short but severe Younger Dryas (YD) event. After ca. 10.6 ~(14)C kyr BP, Zabuye Lake probably closed its surface outflow, due to strong desiccation and drastic climate warming. The Early and Middle Holocene were characterized by unstable climatic conditions with alternating warmer/cooler episodes as indicated by the severe fluctuations of total organic carbon, δ~(13)C and δ~(18)O values. A hypersaline salt lake environment was finally formed at Zabuye after ～5 ~(14)C kyr BP when the mirabilite and halite concentrations steadily increased and became the dominant minerals in the sediments. Severe imbalance of inflow/outflow resulted in the drastic increase of total sulfur, δ~(13)C_(carb) and δ~(18)O_(carb) values and dominance of halite in the lake since ca. 3.8 kyr BP to present.