Hydrological variations in the Yellow River Source area (NE Tibetan Plateau) during the last 7.4 ka inferred from stable isotopes of ostracod valves

The availability of water resources in the Yellow River Source Area (YRSA) on the NE Tibetan Plateau (TP) is of great significance for the hydrology, ecology, and socioeconomics of the Yellow River Valley downstream. To better understand hydrological variations in the YRSA in the past, we investigated a 170-cm long sediment core from Xingxinghai (XXH), a lake in the YRSA, with robust chronology. The stable oxygen and carbon isotopes of ostracod valves, element concentrations of the bulk sediments, and magnetic susceptibility were analyzed to infer lake-level changes and their driving factors during the last 7.4 cal ka BP. The results show that relatively high lake levels existed in the initial period of the record from 7.4 to 7.0 cal ka BP, and lower levels afterwards until 6.2 cal ka BP. During 6.2-3.5 cal ka BP, highest lake levels prevailed at XXH, followed by lowest levels afterwards. A slight lake-level recovery is inferred at 2.4 cal ka BP. Comparison with other studies from the NE TP shows that the period of highest lake levels at XXH during the second half of the middle Holocene is a distinct phenomenon of wetter conditions in the YRSA. These higher lake levels across the region apparently resulted from higher effective moisture due to insolation-driven cooling and reduced evaporation. Our study shows that ongoing and future global warming may lead to a significant decrease of lake levels and surfaces in the YRSA, and to a reduction in Yellow River discharge and water availability downstream.