GLACIAL ASYNCHRONY IN THE KUNLUN SHAN, NORTHWESTERN TIBET

Glaciation in much of interior Central Asia appears to have been controlled by the deep rain shadow of the western ranges – the Karakorum and the Pamir – that blocked moisture-bearing frontal systems during the height of the last glaciation, at the global Last Glacial Maximum (LGM). Cosmogenic ¹⁰Be and ²⁶Al exposure dates from tillstones exposed on moraines show the local “LGM” in the Karakax Valley (36°N, 78°E) of the Kunlun Shan occurred at least 65-70 ka. We find no evidence of large marine isotope stage (MIS) 2 glaciers in the Karakax, only Little Ice Age moraines (<1 ka) and active glaciers upvalley of the ~70 ka moraines. In contrast to the moraine record, exposure ages on adjoining alluvial fans in the Karakax Valley indicate that they formed during periods of maximum insolation (~6 ka and ~110 ka; Ryerson et al., 1999) rather than maximum glaciation.

Moraines that date to MIS 2 or the late glacial in the Aksayqin Basin (35°N, 80°E) on the northwestern Tibetan Plateau are within 1 km of the modern glacier termini. An older, much larger, and more weathered set of moraines lie well outside of the MIS 2 moraines, and appear to be equivalent in age to the early Wisconsinan moraines below the Karakax glaciers. This pattern of large early-Wisconsinan glaciers also occurs in parts of the Tien Shan to the north: cosmogenic dates from several locations (e.g., DieHanJeLe Gou, 43°N, 88°E; Muzart He, 42°N, 81°E) suggest that glaciers there attained maxima during both the early part of the Wisconsinan (~40-75 ka), but also during MIS 2 (13-28 ka), with the former advance being slightly larger. The pattern suggested by these findings is that, in the hyperarid Kunlun Shan, during the LGM it may simply have been too dry to support large glaciers despite low temperatures. During somewhat warmer, less “glacial” climates (e.g., early Wisconsinan), the local glaciers throughout the region were able to grow because of increased delivery of precipitation. Thus, local glacial maxima were asynchronous with glacial maxima in Europe and western North America, and even in the Himalaya, such that the local LGM occurred early in the 10⁵-year-long glacial cycle phase rather than near its end.