Understanding the relationship between glacier meltwater and lake water temperature changes on the Tibetan Plateau (TP) is important for water resources management, environmental protection, and geological risk analysis. Summer temperature is one of the main factors affecting glacier evolution, yet we lack summer temperature records for high-altitude regions where glaciers are widely developed. In this study, we established a calibration between TEX₈₆, which measures the abundance of aquatic isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) of Group I.1a aquatic Thaumarchaeota, and mean annual lake water temperature (MLWT) for 17 surface sediment samples from 10 lakes on the TP (MLWT=4.1× TEX₈₆+2.6 ^oC, R²=0.77). Given that lakes freeze during the cold season, MLWT is a record of the warm season temperature, especially the summer-biased temperature. We applied this calibration to reconstruct a Holocene MLWT record for Bangdag Co, a high-altitude hydrologically-closed lake (4909 m a.s.l.) on the northwestern TP, where glaciers are widely developed. The record shows low temperatures during the early Holocene, high temperatures during the middle Holocene, and a cooling trend during the middle to late Holocene. The MLWT is negatively correlated with the glacial meltwater volume during the early Holocene, indicating a large glacial meltwater influx to the lake at this time which resulted in low lake-water temperatures. Following the decrease in summer insolation during the middle to late Holocene, summer temperatures decreased and thus glacier meltwater production decreased. The MLWT shows a significant positive correlation with the glacial meltwater volume during the middle to late Holocene, which indicates that glacier meltwater barely influenced the MLWT, which was mainly controlled by the summer air temperature. Overall, when the air temperature decreased, the MLWT was low and the glacier meltwater production decreased. Our MLWT record for Bangdag Co agrees well with the cooling trend evident in summer temperature records and model simulations for the TP during the middle to late Holocene, which can be primarily attributed to decreasing summer insolation.

Northwestern Tibetan Plateau,Holocene,Summer-biased temperature,TEX86 calibration,glacier variation