Extent and deglacial chronology of the last British-Irish Ice Sheet: Implications of exposure dating using cosmogenic isotopes

Abstract

Over 160 10Be and 36Cl exposure ages pertaining to the extent and chronology of the last British-Irish Ice Sheet (BIIS) are critically reviewed. Despite uncertainties inherent in the use of exposure ages, this approach has demonstrated that at the Last Glacial Maximum (LGM; ca. 26-21 ka) the BIIS extended over all low ground in Scotland and all (or almost all) of Ireland, consistent with recent models depicting extension of the last BIIS to the Atlantic shelf edge. All exposure ages obtained for high-level sites above trimlines on the mountains of northwestern Scotland and Ireland pre-date the LGM. This finding confirms that high plateaux in these areas escaped significant glacial erosion, probably under a protective cover of cold-based glacier ice that remained frozen to the underlying substrate. In terms of deglaciation chronology, the southern Irish Sea basin was probably deglaciated by 19-18 ka, with extensive deglaciation of low ground in Ireland and northeastern Scotland prior to ca. 14.5 ka. Ice cover on low ground in northwestern Scotland apparently persisted after 14.0 ka, suggesting survival of ice in favourable locations during the Lateglacial Interstade (ca. 14.5-12.9 ka). Attempts to date readvances have had mixed success. The Wester Ross Readvance in northwestern Scotland was initially dated to ca. 16.3 ka, but more recent research implies a much younger age (ca. 14.0-13.5 ka). Exposure ages indicating a prolonged stillstand during ice sheet retreat in the Cairngorms at ca. 14.5-14.0 ka conflict with ages obtained on postglacial rockslides and basal radiocarbon dates, which indicate deglaciation before ca. 15.5-15.0 ka. Exposure dating has been successfully employed to confirm a Loch Lomond Stadial (ca. 12.9-11.5 ka) age for moraines in several mountain areas, and to constrain the minimum altitude of the ice cap that extended over the Western Grampians at that time. Copyright © 2009 John Wiley & Sons, Ltd.