Paleoglaciology of the Tian Shan and Altai Mountains, Central Asia
Central Asia is home to some of the highest and most spectacular mountain ranges in the world, including the Tian Shan and Altai Mountains, and plays a major role in global and regional climate and hydrology. Understanding the glacial history of this vast region is important for several reasons, but in particular there is a general lack of paleoclimatic data from this highly continental region, at the confluence of major climate systems, and glaciers are sensitive monitors of climate change. This thesis examines the pattern and history of glacial deposition and erosion in the Tian Shan and Altai Mountains using a combined approach including 1) geomorphological mapping, 2) spatial analysis of glacial geomorphology, 3) hypsometry, 4) Equilibrium Line Altitudes (ELA), and 5) 10Be exposure dating of erratic boulders on glacial landforms. Preliminary mapping of the Altai Mountains suggests the area mainly experienced alpine style glaciations, with glacial centers as ice caps and ice fields located around the higher mountainous areas. This is consistent with previous work on the Tian Shan. For the Tian Shan we have new apparent minimum 10Be exposure ages from ~0.2 ka to ~ 180 ka, with large site-specific scatter. Most of our apparent exposure ages come from boulders with an age range between 30 ka to 0.2 ka. Although we recognize that more studies combining mapping, dating and modeling are needed to understand the full history of past glaciation in this region, our conclusions to date include: 1) The oldest recorded glacial event occurred in the Taragay Basin in the Tian Shan, dated to 92.1±11.4 ka, and two MIS 2 glacial advances have been recorded on opposite sides of the Ak-Shyrak Range, dated to 16.0±3.4 and 17.3±4.7 ka respectively. 2) Remote-sensing-based mapping and cosmogenic nuclide dating indicate that Pleistocene glaciations were restricted to the mountains and plateau areas of the Tian Shan. 3) Glaciation ages indicate that glacial events occurred during, MIS 2, 3 and 5. 4) There are no regional spatial trends in changes in ELA (ΔELA) however, when comparing the distribution of ELAs to the hypsometric signature (area elevation relationship) of individual catchments across the Tian Shan, there is a range of cases from valleys experiencing “typical” or “extensive” modes of glaciation, where paleo ELAs coincide with hypsometric maxima (peaks in area-elevation curves), to valleys with more complicated signatures, either reflecting “dynamic” or “limited” glaciations at H MAX. This suggests that in future work “typical” catchments should be targeted for geochronological studies and paleo-ELA reconstructions. 5) Finally, when comparing deglaciation ages to global and regional climate records we observe both northern hemispheric and monsoonal signatures as potential drivers behind glacial expansions in the Tian Shan.
Harbor, Purdue University.
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