Late Quaternary water depth changes in Hala Lake, northeastern Tibetan Plateau, derived from ostracod assemblages and sediment properties in multiple sediment records

摘要

Late Pleistocene and Holocene climate dynamics along the marginal belt of the East Asian Summer Monsoon in China and their responses to hydrological cycles in lake basins of the Tibetan Plateau are still a matter of scientific discussion. Hala Lake, a closed 65 m deep lake basin in the western Qilian Mountains, Qinghai Province, is considered a monitor of climate-driven hydrological and environmental changes during the past 24 kyr BP. The distribution patterns of ostracod assemblages, stable isotopes, sedimentegeochemical properties in four sediment records from different water depths and their combination with the unique limnological setting enabled us to reconstruct four major phases of centennial-scale water depth fluctuations from the global Last Glacial Maximum (ca 24 kyr BP) to the Present. Our results show that Hala Lake experienced a very shallow and small water body during the LGM and Lateglacial under cold and dry climate conditions. Rapid increase of water depth and contemporaneous lake expansion started at around 14 kyr BP (Phase I), most likely as a result of glacier melt due to the onset of climate warming. The lake reached >45 m water depth at around 13.5 kyr BP. Reduced water depth during the Younger Dryas spell (ca 12 kyr BP) may be attributed to a short-term return to cooler and drier conditions. During the early Holocene (Phase II), water depth increased further toward lake highstands close to its present level, with a highest lake level of up to 9 m above the present lakestand at 8.0e7.8 kyr BP. Besides continued glacier melt supply, we assume that summer monsoon effective moisture contributed to the overall water budget, but remained relatively unstable, favoring water depth fluctuations. A pronounced lower water depth falls into the period between 9.2 and 8.1 kyr BP, perhaps the result of weak monsoon influence or its complete absence, although the warming trend continued toward its optimum at ca 8e7 kyr BP. A distinct mass flow, most likely triggered by an earthquake, occurred during a lake lowstand either at ca 7.0 kyr BP or at around 8.1 kyr BP. The mid-Holocene (Phase III) was characterized by fluctuating water depths between 7.8 and 4.5 kyr BP. Conflicting trends of stable isotope data limit the validity of water depth estimations, but may show higher lake levels between 5.5 and 4.5 kyr BP, coincident with dated lake sediments in a cliff position at the northern lake shore. This positive water balance can most likely be attributed to increased westerlyderived moisture supply during autumn and late winter, although summer monsoon influence could also be of significance. Coincident with the 4.2 ka event, the lake experienced shallow water at around 4.1 kyr BP, perhaps as a result of continued cooling and drier climate conditions, supporting the arguments of a general cooling trend throughout the Holocene. The Late Holocene (Phase IV) is characterized by extremely unstable hydrological conditions with rapid fluctuations in water depth, more frequently controlled by westerly-driven effective moisture supply. Since the lake lowstand at about 1.4e1.2 kyr BP, the lake has developed toward its present level. Our research underlines the necessity for comparing multiple proxy records from different lake sites to better evaluate centennial-scale climate-driven variations throughout the late Pleistocene and Holocene periods.