Quaternary paleoenvironmental change in Tibet and the bordering mountains

Abstract

The mountains of Tibet, Himalaya, Pamir, Tien Shan, and Altai, comprising Highland Asia, constitute the most glaciated area outside of the Polar Regions. Until recently, the vastness, inaccessibility, and the political constraints of the region have hindered the advancement of the understanding of the nature of Late Quaternary glaciation. During the last decade, however, there has been a considerable increase in the number of studies in the region, many involving detailed fieldwork, the application of newly developing remote sensing technologies, and extensive programs of terrestrial cosmogenic surface exposure and optically stimulated luminescence dating. These studies are helping to accurately identify the extent of glaciation and are serving to determine the ages of previously undatable glacial and associated landforms and sediments to help define the timing of glaciation. As a result, a complex pattern of Quaternary glaciation throughout the region is becoming apparent. In particular, the new studies are showing how extremely challenging it is to correlate glacial advances between and within mountain ranges that constitute Highland Asia. Moreover, the extreme temporal and spatial variation in glaciation makes it particularly difficult to attribute different climatic forcing factors to glaciation. To generalize, most studies provide abundant evidence for significant glacial advances throughout the last glacial cycle. In the Pamir, Tien Shan, and most of the high Himalayan and Tibetan regions, glaciers reached their maximum extent early in the last glacial cycle, with true Last Glacial Maximum glacier advances being significantly less extensive. In most regions, other notable glacier advances occurred during the Late Glacial and the early Holocene, with minor advances in some regions during the mid-Holocene. Throughout the region, there is abundant evidence for multiple glacial advances during the latter part of the Holocene, although these are generally very poorly defined and were less extensive than the early Holocene glacier advances. In some areas, such as Muztag Ata and Kongur Shan in the Pamir, and the Khumbu Himal, the Late Glacial and Holocene glacial record is particularly well preserved and has the potential to yield high-resolution records of glaciation. Most research suggests that the regional patterns and timing of glaciation reflect temporal and spatial variability in the south Asian monsoon and mid-latitude westerlies, and in particular, regional precipitation gradients. In regions of greater aridity such as the interior of Tibet, the extent of glaciation has become increasingly restricted throughout the Late Quaternary leading to the preservation of prelast glacial cycle glacial landforms. In contrast, in regions that are very strongly influenced by the monsoon, mainly the southern slopes of the Himalaya, the preservation potential of pre-Late Glacial moraine successions is very poor. This is possibly because Late Glacial and Holocene glacial advances may have been most extensive and hence may have destroyed any evidence of earlier glaciations. While there has been a progressive retreat of glaciers during the twentieth century, and there is concern that this will continue into the twenty-first century as a consequence of human-induced global warming, glaciers in some regions such as the Karakoram have been thickening and surging during the last decade.