Understanding the history of the British and Irish Ice Sheet (BIIS) at the Last Glacial Maximum (LGM) has been advanced by new approaches, in particular, by cosmogenic nuclide surface-exposure dating, aminostratigraphy of 'shelly' glacial deposits, AMS radiocarbon dating, and the evidence from continental margin marine cores, all of which supersede the previously weak geochronologic control. It was formerly believed that Great Britain and Ireland was largely ice free between the last interglacial (oxygen isotope sub-stage 5e) and the Late Devensian, when the LGM occurred. As such the BIIS was effectively out of phase with Laurentide and Scandinavian ice sheets, as well as inferences of ice volume from oxygen isotope stratigraphy. The BIIS during the Late Devensian maximum was also perceived as having been relatively stationary. New evidence shows that the LGM was an important event during the evolution of an earlier BIIS when the extent of ice was greater. Repeated iceberg rafting events over the past 50 ka are shown by marine cores, while the derivative inference of numerous corresponding glacial advances is supported by several clusters of 36Cl ages on glaciated surfaces and glacial boulders, that are indicative of deglacial events between 40 and 12 ka. These appear to be associated with Heinrich events, the earliest being inferred as Heinrich 4 at about 40 ka. During this advance, the BIIS and Scandinavian Ice Sheet (SIS) were in contact and all of Ireland was glaciated. The ice sheet appears to have fluctuated several times between 40 and 25 ka, although evidence for this is poorly preserved. But the 36Cl and 14C evidence is clear that the BIIS reached its LGM maximum size about 22ka, soon after Heinrich Event 2, when the BIIS and SIS were not in contact. One cluster of 36Cl and 14C ages, at 21.4± 1.3 ka, records an initial pulse of deglaciation that was followed by extensive deglaciation about 17.4±0.4 ka just before Heinrich Event 1, when the ice sheet readvanced. Contrary to previous views, the BIIS probably existed throughout much of Devensian time as a mobile and sensitive ice sheet, during which the LGM advance was but one important event. In places, glacial deposits of the earlier Devensian glaciation have previously been incorrectly identified as products of the later LGM glaciation.
- Programområde 5: Natur og klima