Reduced early Holocene moisture availability in the Bayan Har Mountains, northeastern Tibetan Plateau, inferred from a multi-proxy lake record

A late glacial and Holocene lake sediment record from the northeastern Tibetan Plateau was investigated using ostracod and geochemical analysis. After Lake Koucha was established about 16 cal ka BP, oligohaline conditions gradually converted to mesohaline conditions about 10 cal ka BP clearly suggesting that the water body was a closed-basin lake. From about 10 cal ka BP, the salinity of the lake rose continuously until hyperhaline levels were reached about 7.5 cal ka BP. This increase in salinity was accompanied by a falling lake level. Extreme rainfall and flooding events in a generally dry climate hit the shallow saline lake around 8.5 cal ka BP causing a unique, thin gravel-containing layer within the continuous fine-grained lake sediments. The age control of our record does not allow us to correlate this event exactly to the widely recorded 8.2 cal ka BP event, however, the recovered sediments from Lake Koucha provide strong evidence that climate changes around 8.5 cal ka BP had significant environmental implications in northeastern Tibet. After about 7.3 cal ka BP, the available moisture significantly increased causing a continuous salinity decline until the present-day freshwater conditions of a through-flow lake were established about 6.5 cal ka BP in Lake Koucha. This major shift from a shallow saline to a deeper freshwater lake was probably accompanied by a fundamental change from a macrophyte-dominated to a phytoplankton-dominated lake. A slight reduction in available moisture in comparison to the periods before and afterward was recorded between 4.3 and 2.0 cal ka BP, in phase with numerous records from monsoonal Asia. The reconstructed overall change in moisture availability from dry conditions before about 7.3 cal ka BP to wetter conditions afterwards contradicts most palaeoclimate records from the Tibetan Plateau and its surroundings which show that wetter conditions in the early Holocene were replaced by drier and probably cooler conditions in the mid and late Holocene. However, in the semi-arid region of Lake Koucha with a mean annual precipitation of about 500 mm, the insolation-driven strengthened monsoon in the early Holocene may have resulted in a temperature rise and higher evaporation rates which outweighed the increase in precipitation and therefore led to significantly lower moisture availability. Our record provides further evidence for a regionally non-uniform response of the available moisture evolution to the overall monsoonal signal in the Holocene climate history of Central Asia.