Optically stimulated luminescence dating of a moraine and a terrace in Laohugou valley, western Qilian Shan, northeastern Tibet

The Qilian Shan is one of the most important mountain ranges for understanding Quaternary glaciation in the northeastern Tibetan Plateau. However, its western part is not well studied. A moraine and an outwash terrace in the Laohugou valley were mapped, using geomorpho-stratigraphy, and dated by optically stimulated luminescence. After analyzing the minimum age and central age of coarse- and fine-grained quartz, most of the minimum ages for coarse grains are consistent with geomorpho-stratigraphic sequence, indicating they are closest to the true ages of the sediments. The lateral moraine was initially formed at 20.2 ± 2.6 ka, during the Last Glacial Maximum, and was modified between 11.6 ± 2.0 and 12.9 ± 1.3 ka, in the Late Glacial. The outwash terrace was formed between 10.0 ± 1.7 and 5.5 ± 0.7 ka, and coincides with temperature rise in the early Holocene and the Holocene Optimum. At its Last Glacial Maximum, the palaeo-glacier was ∼13.6 km long with an area of 45.3 km2, 4.3 km longer and 34% larger than the contemporary glacier. The reconstructed palaeo-glacier equilibrium line altitude (ELA) was 4600 ± 40 m, or 140 m lower than the present ELA. The glacier advance in the Laohugou valley was less extensive and ELA depression was less than found in the eastern Qilian Shan during LGM and Late Glacial, suggesting a drier climate during the Last Glacial.