HIMALAYAN AND TIBETAN GLACIAL CHRONOLOGIES: PROBLEMS, APPROACHES AND CURRENT STATE OF KNOWLEDGE

Tibet and the bordering mountains are the most extensively glaciated tract outside the polar regions, exerting an important influence on regional and global environmental change. The glacial system provides water for hundreds of millions of people in central Asia, and variations in the glacial and associated hydrological systems have profound socio-economic and political consequences. Yet despite the regional and global importance of glaciation in High Asia, the dynamics, extent and timing of Quaternary glaciation in this region are poorly understood and defined. This is partially because of the inaccessibility and vastness of the region but also because of the misinterpretation of glacial and non-glacial landforms and the lack of numerical dating to define the ages of landforms to establish regional chronologies. During recent years, however, new glacial geomorphic mapping, sedimentology, and cosmogenic radionuclide surface exposure and luminescence dating techniques have helped to define the timing and extent of Quaternary glaciation in distant regions throughout the Himalaya and Tibet. These studies suggest that the regional patterns and timing of glaciation throughout the region reflect temporal and spatial variability in the south Asian monsoon and, in particular, regional precipitation gradients. As a consequence, old (pre-Last Glacial) moraines and/or tills are more readily preserved in regions of greater aridity, such as central and westernmost Tibet. In contrast, within regions with greater rainfall as a result of the strong influence of the monsoon, such as the southern slopes of the Himalaya and eastern Tibet, the preservation potential of pre-Lateglacial moraine successions is extremely poor because of the associated high erosion rates. Furthermore, glaciation in such regions during the early Holocene insolation maximum was probably more extensive than earlier in the last glacial cycle (and specifically at the global LGM), and thus any evidence of older moraines has been destroyed by subsequent glacial advances. It is therefore likely that glaciation throughout Tibet and the Himalaya is strongly influenced by orography that, in turn, strongly influences climate. Understanding these factors and patterns are essential for helping to establish regional glacial chronologies.