Extent and timing of the Last Glacial Maximum (LGM) in Britain and Ireland: a review

Abstract

The Last Glacial Maximum (LGM) of Marine Isotope Stage (MIS) 2 is defined here as the period either of maximum global ice volume or in which major ice sheets reached their maximum extent. Both definitions lack clarity as there is no reason to suppose that maximum volumes or limits were reached simultaneously at either all ice sheets or around the margins of individual ice sheets. Here we review the traditional terrestrial limits of the BIIS (British-Irish Ice Sheet), examine developments in understanding the offshore record, and attempt a redefinition of the extent and timing of the MIS 2 maximum. As a working chronological model the build-up of the BIIS is assigned to after 35-32 ka, the maximum to the period 27-21 ka and a more limited readvance after 19 ka. As marginal oscillations, driven by ice sheet dynamics, appear to have characterised much of the advance, and especially the retreat of the BIIS, caution should be exercised in assigning climatic significance to the many identified ice limits or still-stands. While much of the terrestrial limit in Britain remains relatively unaltered, significant revision is required offshore around Britain and Ireland, to portray an extensive ice advance to the shelf edge and a period when the BIIS was conjoined with the adjacent Fenno-Scandinavian Ice Sheet. Although there appears to be a broad synchrony between the BIIS and other major ice sheets, local controls have produced some variation in the timing of advance to and retreat from maximum limits for particular ice streams or lobes, but much of the offshore limit remains very poorly age constrained. Consequently, a key objective in future research should be to improve and refine the chronological control for all portions of the maximum margin of the BIIS and its subsequent retreat stages.

Middle to Late Devensian glaciation of north‐east Scotland: implications for the north‐eastern quadrant of the last British–Irish ice sheet

“…Indeed, peak IRD flux coincident with this event is a feature of all records of the BIIS on its western margin (Scourse et al, 2009) and there is a convergence of evidence suggesting that the BIIS underwent a period of large-scale re-organization and overall reduction in ice volume of about 80 % at about this time (Bowen et al, 2002;Chiverrell and Thomas, 2010;Finlayson et al, 2010;Clark et al, 2012;Hughes et al, 2014. Rising sea level alone may not have been the trigger as ice retreated from the shelf edge during warmer periods coinciding with influxes of Atlantic Water towards the Norwegian Sea (Brendryen et al, 2015) ), such as during GI 2 at c. 23 ka.…”

“…Evidence now suggests that Scandinavian and Scottish ice did indeed coalesce within the NSB at least during an early stage in the last glaciation, during the local Last Glacial Maximum (LGM) (Figure 2), when the BIIS was thick (Sejrup et al, 1994Chiverrell and Thomas, 2010). This 'Atlantic Shelf Glaciation' at c. 32-25k cal a BP (Sejrup et al, , 2005, or 32-27k cal a BP (Sejrup, 2015), was followed by the growth of more restricted ice sheets after c. 22k cal a BP.…”

“…However, doubts surround the published geochronometry and it is difficult to explain why early deglaciation of the central NSB occurred during a period including the global LGM when other sectors of the BIIS were expanding Sejrup et al, 2016). It is generally agreed that the northern sector of the last BIIS built up towards the end of MIS 3, after c. 32k a BP, when ice extended to the continental shelf margin (Stoker and Holmes, 1991;Whittington and Hall, 2002;Wilson et al, 2002;Bradwell et al, 2008;Jacobi et al, 2009;Chiverrell and Thomas, 2010;Gibbard and Clark, 2011;Graham et al, 2011;Clark et al, 2012;Everest et al, 2013;Hughes et al, 2014Hughes et al, , 2016Hiemstra et al, 2015;Bradwell and Stoker, 2015a). It is unlikely that ice-free areas existed anywhere in northern Scotland during the global LGM, when the BIIS was thick (Ballantyne, 2010;Fabel et al, 2012) (Figure 2).…”

“…The BIIS was confluent with Scandinavian ice in the central North Sea basin prior to 22k 14 C a BP (Graham et al, 2011) or possibly as early as 27k cal a BP (Sejrup et al, 1994(Sejrup et al, , 2014) when a major shelf-edge-parallel ice sheet became established from Cape Wrath to Norway (Sejrup et al, 2005). Ice appears to have been most extensive and thickest early in the development of the last BIIS, during the local LGM Chiverrell and Thomas, 2010;Clark et al, 2012;Hughes et al, 2014Hughes et al, , 2016 (Figure 1). Importantly, Scourse et al (2009) report that the deep-sea ice-rafted detritus record from the continental slope adjacent to the last BIIS indicates that the ice sheet reached its maximum extent at Heinrich Event H2 (24 ka) (cf.…”

Ice sheet extension to the Celtic Sea shelf edge at the Last Glacial Maximum

“…The southernmost extent of the last BritisheIrish Ice Sheet (BIIS) has long been disputed (e.g. Mitchell et al, 1973;Scourse, 1991;Scourse and Furze, 2001;Bowen et al, 2002), but it is now agreed that onshore glacigenic deposits in Ireland and southern Britain provide evidence of an advance of the Irish Sea Ice Stream into the Celtic Sea during the Last Glacial Maximum (LGM), around 25e23 ka BP 1 (Scourse, 1991;Evans, 2001, 2007;Greenwood and Clark, 2009;Chiverrell and Thomas, 2010;Clark et al, 2010;McCarroll et al, 2010;O Cofaigh et al, 2012;Chiverrell et al, 2013). The extent of this advance across the continental shelf has been constrained by a dozen vibrocores acquired in the late 1970s that penetrated surficial sand and gravel to reach sediments of glacial character ( Fig.…”