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Boulder cosmogenic exposure ages as constraints for glacial chronologies
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
Department of Earth and Atmospheric Sciences, Purdue University.
Department of Physics, Purdue Rare Isotope Measurement Laboratory, Purdue University.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Cosmogenic exposure dating has greatly enhanced our ability to define glacial chronologies spanning several global cold periods, and glacial boulder exposure ages are now routinely used to constrain deglaciation ages. However, exposure dating involves assumptions about the geological history of the sample that are difficult to test and yet may have a profound effect on the inferred age. Two principal geological factors yield erroneous inferred ages: exposure prior to glaciation (yielding exposure ages that are too old) and post-glacial shielding (yielding exposure ages that are too young). Here we show that post-glacial shielding is more important than prior exposure, using datasets of glacial boulder 10Be exposure ages from the Tibetan Plateau (1123 boulders), Northern Hemisphere palaeo-ice sheets (615 boulders), and present-day glaciers (186 boulders). No boulders from present-day glaciers and very few boulders from the palaeo-ice sheets have exposure ages significantly older than independently known deglaciation ages, indicating that prior exposure is of limited significance. Further, the exposure age distribution of boulders from the Tibetan Plateau agrees with the distribution produced by a simple post-glacial landform degradation model, indicating that post-glacial shielding is important. The large global dataset demonstrates that, in the absence of other evidence, glacial boulder exposure ages should be viewed as minimum limiting deglaciation ages.

Keyword [en]
cosmogenic nuclides, exposure age interpretation, cosmogenic inheritance, post-glacial shielding, glacial chronology, deglaciation
National Category
Physical Geography Geology
Research subject
Physical Geography; Quarternary Geology
URN: urn:nbn:se:su:diva-38686OAI: diva2:312532
Available from: 2010-04-26 Created: 2010-04-26 Last updated: 2010-10-07Bibliographically approved
In thesis
1. Palaeoglaciology of the northeastern Tibetan Plateau
Open this publication in new window or tab >>Palaeoglaciology of the northeastern Tibetan Plateau
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This study concerns the palaeoglaciation of the northeastern Tibetan Plateau, with emphasis on the Bayan Har Shan (Shan = Mountain) in the headwaters of Huang He (Yellow River). To reconstruct past glacier development multiple techniques, including remote sensing, field investigations, cosmogenic exposure dating, and numerical modelling have been employed. Analysis of the large-scale geomorphology indicates that glacial erosion has been dominant in the elevated mountain areas on the low-relief plateau, whereas fluvial erosion outpaces glacial erosion along the plateau margin. Landform and sediment records yield evidence for multiple local glaciations, restricted to the highest mountain areas, and a maximum glaciation beyond the mountain front. Absence of data supporting the former presence of proposed ice sheets, plateau-wide or regional, tentatively indicates that no ice sheet glaciation occurred on the northeastern Tibetan Plateau. Cosmogenic exposure dating of boulders, surface pebbles, and sediment sections in central Bayan Har Shan indicates that its record of past glaciations predates the global Last Glacial Maximum (LGM). Based on a world-wide analysis, yielding that wide age disparity within apparent exposure age datasets is most likely caused by post-glacial shielding processes, the Bayan Har Shan exposure ages constrain four periods of glaciation with minimum ages of 40-65 ka, 60-100 ka, 95-165 ka, and undetermined oldest stage. Similar to Bayan Har Shan, the plateau-wide distribution of boulders with pre-LGM exposure ages close to present-day glaciers shows that its LGM glaciers were generally not much larger than today. The results of a high resolution glacier model applied to nine regions across the plateau indicates that temperature depressions of 2-4 K are enough to expand glaciers beyond their global LGM extent, implying that during periods of Northern Hemisphere glaciation the Tibetan Plateau was not much colder than today or became exceedingly dry.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography and Quaternary Geology, Stockholm University, 2010. 15 p.
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 21
Tibetan Plateau, palaeoglaciology
National Category
Physical Geography Geology
Research subject
Physical Geography
urn:nbn:se:su:diva-38689 (URN)978-91-7447-074-1 (ISBN)
Public defence
2010-06-02, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
At the time of doctoral defence the following publications were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.Available from: 2010-05-11 Created: 2010-04-26 Last updated: 2010-05-13Bibliographically approved

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Heyman, JakobStroeven, Arjen P
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