Cosmogenic surface exposure dating the last deglaciation in Denmark: Discrepancies with independent age constraints suggest delayed periglacial landform stabilisation |
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| Institution: | 1. Centre for Geo-Genetics, Natural History Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350 København, Denmark;2. Department of Earth Science, University of Bergen, and Bjerknes Centre for Climate Research, Norway;3. School of Geographical and Earth Sciences, University of Glasgow, Scotland, UK;4. NERC Cosmogenic Isotope Analysis Facility, Scottish Universities Environmental Research Centre, East Kilbride, Scotland, UK;5. Scottish Universities Environmental Research Centre AMS-Facility, East Kilbride, Scotland, UK;6. School of Geosciences, University of Edinburgh, Scotland, UK;1. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany;2. Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 4, 20126 Milano, Italy;1. College of Geography Sciences and Key Laboratory of Virtual Geographic Environment (Ministry of Education), Nanjing Normal University, Nanjing, Jiangsu, 210023, China;2. Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN, 47907, USA;3. Jiangsu Center for Collaborative Innovation in Geographic Information Resource Development and Application, Nanjing, Jiangsu, 210023, China;4. Key Laboratory of Western China''s Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, 730000, China;1. Department of Earth Science, University of Bergen and Bjerknes Centre for Climate Research, Postbox 7803, N-5020 Bergen, Norway;2. Department of Geology, University at Buffalo, Buffalo, NY 14260, USA;3. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA;1. Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Strasse 24/25, 14476, Potsdam, Germany;2. GFZ German Research Centre for Geosciences, Telegrafenberg, 14473, Potsdam, Germany;3. Departamento de Geología, Universidad Nacional de Salta and CONICET (CEGA-INSUGEO), 4400, Salta, Argentina |
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| Abstract: | Cosmogenic nuclide surface exposure ages are determined from in situ 10Be and 36Cl analysis of 38 rock surfaces found in different glacial landforms in Denmark. Dating of erratic boulders and adjacent ice-sculpted bedrock on the island of Bornholm in the western Baltic Sea reveals almost identical values. This suggests that little if any inherited nuclides are present in the sampled boulders. West of the Last Glacial Maximum (LGM) ice margin in Denmark ages reflect exposure from the Middle Weichselian. East of the LGM margin exposure ages from 35 samples show Late Weichselian ages in a range between 20.6–11.9 ka. To test to what extent these dates reflect the onset of deglaciation immediately after cessation of active glacier flow, surface exposure ages are evaluated against independent chronologies of Late Weichselian ice-sheet fluctuations in southwestern Scandinavia. The Bornholm dates agree with the independent age model, however, in the data set for eastern Denmark only less than half the surface exposure ages lie within the expected age envelope. This apparent mismatch is most likely due to post-glaciation shielding and delayed surface stabilisation compared to the timing of ice-margin retreat. Thus ages from boulders resting in dead-ice moraines and mass wasting landscapes underestimate deglaciation by 3–6 thousand years. The results quantify the impact of exhumation and landform stabilisation on cosmogenic surface exposure ages on millennial scales. We conclude, that interpretation of cosmogenic exposure ages should include careful evaluation of possible post-depositional landform transformation in attempts to fine tune ages of e.g. end moraine features. With reference to independent age models we critically evaluate glacier advance – retreat scenarios from regions around the southern Baltic that alone are based on weighted average ages of cosmogenic exposure dating. |
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