Late Quaternary glaciations in Highland Asia

Abstract

The mountains of Tibet, Himalaya, Pamir, Tien Shan, and Altai, comprising Highland Asia, constitute the most glaciated area outside the Polar Regions. Over the past few decades, due to the considerable 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, a complex pattern of Quaternary glaciation throughout the region is becoming apparent. These studies show that glaciers advanced at different times and to differing extents from one region to the next, and there is much variation even within regions. The geological evidence shows that as many as 29 glacier advances occurred during the Quaternary in Highland Asia. Glacier advances in most semi-arid regions were most extensive before the last glacial cycle (before ∼100ka), while in monsoon-influenced regions they were most extensive during the early part of the last glacial period (∼30–70ka). In contrast, some regions experienced their most extensive glaciation during the global last glacial maximum (∼18–24ka). During times of glacier advance, ice caps expanded, and valley glaciers advanced up to several to tens of kilometers beyond their present positions. Since the global last glacial maximum, glacier advances have been limited to within a few kilometers in most regions, with the maximum advance (kilometers to tens of kilometers) occurring mostly during the Early Holocene (∼9–8ka). There have been several minor glacier advances during the Mid-Holocene and latter part of the Holocene with the most recent, the Little Ice Age (LIA), starting several hundred years ago, and ending at approximately the beginning of the 20th century. Since the LIA, glaciers have retreated in most areas, but in some regions, there has been little change in extent. In regions of greater aridity such as the interior of Tibet, the extent of glaciation has become increasingly restricted throughout the Quaternary leading to the preservation of pre-last glacial cycle glacial landforms. In contrast, in regions that are very strongly influenced by the monsoon, mainly the southern slopes of the Himalayas, the preservation potential of pre-Late Glacial moraine successions is very poor. This suggests a complex pattern of glacier fluctuations in response to human-induced climate change in the coming years. The extreme temporal and spatial variation in glaciation makes it particularly difficult to attribute different climatic forcing factors to glaciation. Most research, in particular glacier modeling, highlight that the regional patterns and timing of glaciation reflect temporal and spatial variability in the South Asian monsoon and mid-latitude westerlies, and regional precipitation gradients.