LATE QUATERNARY GLACIATION IN WESTERN CENTRAL ASIA

The mountains of western Central Asia contain evidence of extensive late Pleistocene valley glaciations. Morphologic and soil-development data from 8 drainages in the Kunlun and the Kyrgyz Tien Shan indicate similar moraine sequences with large, overlapping lobes extending onto intermontane basin floors. Cosmogenic ¹⁰Be dating of boulders on the moraines indicates that the largest glaciers of the last cycle (~110-15 ka) predated the Last Glacial Maximum (LGM) in Europe (~20 ka), consistent with evidence developed by Lewis Owen and others near Nanga Parbat. The largest advances in the Kunlun and in the southern Kyrgyz Tien Shan occurred ~ 63 - 45 ka and again at ~30 ka, whereas in the Kyrgyz Front Range and Issyk-Kul basins to the north the largest glaciers were ~110 - 90 ka and ~60 - 45 ka. Smaller Holocene glaciers also appear to have advanced in western Central Asia. We have evidence for pre-Little Ice Age advances at ~10 ka in Aksayqin Basin (Tibet), ~ 7 and ~4.5 ka in the southern Tien Shan, and at ~3.4 ka in the northern Tien Shan. The "LGM" glaciers in western Central Asia seem to have been much smaller than the maximum advances, and were restricted to the vicinity of the modern glaciers. The only direct evidence we found for an LGM advance was a ¹⁰Be date of 16 ka measured for a cirque moraine in the Ala-Bash drainage south of Lake Issyk-Kul.

The dominant climatic forcing mechanisms in western Central Asia are probably insolation and cyclonic storms from the west. Current average annual temperatures at the ELA vary from ~0° C in the western Tien Shan to -8°C near the border between Kyrgyzstan and China, with over 250 days of snow cover. Monsoonal storms today have little effect in the rain shadow north of the Himalaya and east of the Karakoram. Snowfall in northwestern Tibet appears to arrive on large vortexes created in the lee of the Tien Shan when frontal systems pass through. It is not clear that insolation changes alone can explain the greater snowline depression early in the last glacial cycle. Therefore, the glaciers of western Central Asia, on both sides of the Karakoram, may have been precipitation-starved during the LGM, although the exact mechanism is speculative. These results highlight the sensitivity of glaciers in western Central Asia to the location and intensity of westerly storm tracks across the southern Russian steppes.