Cosmogenic surface exposure dating the last deglaciation in Denmark: Discrepancies with independent age constraints suggest delayed periglacial landform stabilisation

Cosmogenic nuclide surface exposure ages are determined from in situ10Be 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.

Apparent mismatch between independent Late Weichselian deglaciation chronologies and cosmogenic nuclide surface exposure ages was most likely caused by shielding and delayed surface stabilisation compared to the timing of active ice-margin retreat. Figure optionsDownload as PowerPoint slideHighlights
► Diverse 36Cl and 10Be ages suggest insufficient erosion or higher 36Cl production.
► 10Be ages from boulders and bedrock suggest a simple erosion and exposure history.
► Down wasting may cause exposure ages of erratics to lag ice sheet retreat up to 6 ka.
► Exposure ages used to date circum Baltic glacier fluctuations requires rethinking.