HOLOCENE WATER BALANCE FLUCTUATIONS OF AHUNG CO, TIBET

We have developed a record of water-balance fluctuations for Ahung Co (Lake) in Tibet using geochemical proxies. Unlike previous lake sediment records from Tibet, the age model for Ahung Co is based on radiocarbon dates from terrestrial charcoal and is free from reservoir effects. Radiocarbon dates and geochemical proxies suggest four stages in the history of the lake. During stage 1 (~8000 to ~7200 calendar years BP), large amounts of detrital material, low d¹⁸O values, and several biological indicators suggest that the lake was relatively deep and overflowing. Following this (~7200 to ~3700 calendar years BP), d¹⁸O and d¹³C values increased, indicating a gradual transition to drier conditions with increased residence time for lake water. This time period is punctuated by several intervals with increased deposition of Potamogeton, probably the result of environmental disturbances. At ~3700 calendar years BP, a sudden increase in d¹⁸O values indicates an abrupt change towards drier conditions. This coincides with the fall of the Indus Civilization, which some researchers believe was caused by a decrease in precipitation. Following this, d¹⁸O values remain relatively high, suggesting these conditions persisted for some time. Sometime after ~3700 calendar years BP, and perhaps as late as ~2000 calendar years BP, no sediment was preserved. This suggests that the lake was no longer perennial in the late Holocene. Lake levels rose sometime within the last couple hundred years and Ahung Co is currently perennial. This pattern of climate change is similar to that observed from other Tibetan lakes, suggesting that the water-balance fluctuations of Ahung Co reflect regional events. This is the first study, however, to provide dates for these climate transitions that are free of the uncertainties associated with reservoir effects.