QUATERNARY GLACIAL HISTORY OF THE QILIAN SHAN, NORTHEASTERN TIBETAN PLATEAU, CONSTRAINED BY 10BE SURFACE EXPOSURE DATING

The Qilian Shan (Shan=mountain), along the northeast boundary of the Tibetan Plateau, is located in a transition region between the mid-latitude westerlies and the Asian monsoonal regions. During the glacial-interglacial cycles of the Quaternary, the mountains were extensively and repeatedly glaciated. However, few ¹⁰Be surface exposure dating studies of glacial landforms have been conducted from the western and middle parts of the Qilian Shan, and available ¹⁰Be ages from its eastern region have just provided evidence for glacial events during the global Last Glacial Maximum (LGM) and Lateglacial. Here, we present 37 new ¹⁰Be surface exposure ages from glacial landforms in the Qiyi Glacier valley of its western part, the Bailang River valley of its Middle part and the Laolongwan valley of its Eastern part. The moraine groups identified from field and remote sensing imagery mapping record seven glacial events at 107.27±10.88 ~ 123.82±12.59 ka (MIS6), 35.25±3.52 ~ 47.07±4.71 ka (MIS3), 22.09±2.15 ~ 30.49±3.01 ka (global LGM), 17.85±1.78 ~ 19.71±1.91 ka (Heinrich event 1), 12.21±1.20 ~ 14.48±1.42 ka (Younger Dryas), 9.87±1.02 ~ 10.44±1.07 ka (early Holocene) and 0.23±0.03 ~ 0.40±0.05 ka (Little Ice Age). However, it was not possible to define the numerical ages of the oldest moraines in this region, because there were no boulders suitable for surface exposure dating. Based on the principles of geomorphology and stratigraphy, and previously published ESR ages, the oldest glacial event in the Qilian Shan appear to have occurred at older than MIS6, probably during MIS12. The extent of glaciation in the Qilian Shan became increasingly more restricted over time, likely indicating the decreased precipitation due to the blocking effect from the uplift of the Tibetan Plateau. Furthermore, the local LGM in the Qilian Shan occurred during MIS3, which appears consistent with the timing and extent of the glacial advance in the monsoon-dominated Tibetan Plateau, but asynchronous with the MIS2 global LGM.