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Evaluating the timing of former glacier expansions in the Tian Shan: a key step towards robust spatial correlations
Stockholm University, Faculty of Science, Department of Physical Geography.ORCID iD: 0000-0003-0306-5291
Stockholm University, Faculty of Science, Department of Physical Geography.
Stockholm University, Faculty of Science, Department of Physical Geography.
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(English)In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457XArticle in journal (Refereed) Accepted
Abstract [en]

The timing of past glaciation across the Tian Shan provides a proxy for past climate change in this critical area. Correlating glacial stages across the region is difficult but cosmogenic exposure ages have considerable potential. A drawback is the large observed scatter in 10Be surface exposure data. To quantify the robustness of the dating, we compile, recalculate, and perform statistical analyses on sets of 10Be surface exposure ages from 25 moraines, consisting of 114 new and previously published ages. We assess boulder age scatter by dividing boulder groups into quality classes and rejecting boulder groups of poor quality. This allows us to distinguish and correlate robustly dated glacier limits, resulting in a more conservative chronology than advanced in previous publications. Our analysis shows that only one regional glacial stage can be reliably correlated across the Tian Shan, with glacier expansions occurring between 15 and 28 ka during marine oxygen isotope stage (MIS) 2. However, there are examples of older more extensive indicators of glacial stages between MIS 3 and MIS 6. Paleoglacier extent during MIS 2 was mainly restricted to valley glaciation. Local deviations occur: in the central Kyrgyz Tian Shan paleoglaciers were more extensive and we propose that the topographic context explains this pattern. Correlation between glacial stages prior to late MIS 2 is less reliable, because of the low number of samples and/or the poor resolution of the dating. With the current resolution and spatial coverage of robustly-dated glacier limits we advise that paleoclimatic implications for the Tian Shan glacial chronology beyond MIS 2 are speculative and that continued work toward robust glacial chronologies is needed to resolve questions regarding drivers of past glaciation in the Tian Shan and Central Asia. 

Keyword [en]
Paleoglaciation, Tian Shan, Glacial geomorphology, 10-Be surface exposure dating
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-134744OAI: oai:DiVA.org:su-134744DiVA: diva2:1037710
Projects
Central Asia Paleoglaciology Project (CAPP)
Funder
Swedish Research Council, No. 2011-4892
Available from: 2016-10-17 Created: 2016-10-17 Last updated: 2017-11-29
In thesis
1. Paleoglaciology of the Tian Shan and Altai Mountains, Central Asia
Open this publication in new window or tab >>Paleoglaciology of the Tian Shan and Altai Mountains, Central Asia
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The mountain-systems of Central Asia, act as barriers to atmospheric circulation patterns, which in turn impose striking climate gradients across the region. Glaciers are sensitive indicators of climate change and respond to changes in climate gradients over time by advancing during cold and wet periods and receding during warm and dry periods. The aim of this thesis is to investigate whether there are large-scale patterns in how past glaciers in the Tian Shan and the Altai Mountains of Central Asia responded to climate change. Multiple methods have been used, including: remote sensing, terrain analysis, field investigations, and cosmogenic nuclide (CN) dating. The glacial landform records indicate that the region experienced mainly alpine-style glaciations in the past. Large complexes of ice-marginal moraines in high elevation basins are evidence of outlet glaciers sourced from large valley glaciers, ice caps and ice-fields, and these moraine sequences, record the maximum extent of paleoglaciation. In the Ikh-Turgen Mountains, located in the continental, eastern Altai Mountains, deglaciation of these moraines occurred during marine oxygen isotope stage (MIS) 3 at ~45 ka. This is consistent with a colder and wetter climate during this time, inferred from ice core and lake level proxies. Another deglacial phase occurred during MIS 2 at ~23 ka, synchronous with the global Last Glacial Maximum. In the Russian Altai Mountains, lobate moraines in the Chuya Basin indicate deglaciation at ~19 ka, by a highly dynamic paleoglacier in the Chagan-Uzun catchment, which experienced surge-like behaviour. Furthermore, across the Tian Shan, an evaluation of new and existing CN glacial chronologies (25 dated moraines) indicates that only one regional glacial stage, between 15 and 28 ka (MIS 2), can be defined and spatially correlated across the region. These paleoglaciers were mainly restricted to valleys as a result of arid conditions during this time and variation in their extents is interpreted to reflect topographic modulation on regional climate. The ages of the oldest evidence for robust local glacial stages in the Tian Shan are not yet well constrained, however, moraines in the central Kyrgyz Tian Shan and the eastern Chinese Tian Shan have apparent minimum ages overlapping with MIS 5 and MIS 3 (with missing MIS 4 and 6 stages). However, different geological processes, such as inheritance and post-depositional shielding (e.g. deposition by surging glaciers or hummocky terrain deposition), have influenced the dating resolution, making several moraine ages inappropriate for regional comparison. Finally, to quantify regional patterns of paleoglaciation, the hypsometry (area-elevation distribution) of glacial landforms is used to estimate average paleo equilibrium line altitudes for the region. This analysis shows that while present-day ELAs mirror strong climate gradients, paleoglaciation patterns were characterised by more gentle ELA gradients. The paleo-ELA depressions across Central Asia were most prominent in the continental southern and eastern regions (500–700 m). Finally, the results from this thesis, show that Central Asia was repeatedly glaciated in the past, but underscore the importance of considering 1) catchment characteristics and styles of glaciation and 2) other non-climatic factors controlling glacier dynamics when interpreting CN chronologies to make paleoclimate inference.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography, Stockholm University, 2016. 34 p.
Series
Dissertations from the Department of Physical Geography, ISSN 1653-7211 ; 59
Keyword
Paleoglaciology, glacial geomorphological mapping, cosmogenic nuclide dating, Tian Shan, Altai Mountains
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-134748 (URN)978-91-7649-567-4 (ISBN)978-91-7649-568-1 (ISBN)
Public defence
2016-12-09, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Projects
Central Asia Paleoglaciology Project (CAPP)
Funder
Swedish Research Council, No. 2011-4892
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Accepted. Paper 5: Manuscript.

Available from: 2016-11-16 Created: 2016-10-17 Last updated: 2016-11-04Bibliographically approved

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