Extent of the last ice sheet in northern Scotland tested with cosmogenic ¹⁰Be exposure ages

Abstract

The extent of the last British-Irish Ice Sheet (BIIS) in northern Scotland is disputed. A restricted ice sheet model holds that at the global Last Glacial Maximum (LGM; ca. 23-19 ka) the BIIS terminated on land in northern Scotland, leaving Buchan, Caithness and the Orkney Islands ice-free. An alternative model implies that these three areas were ice-covered at the LGM, with the BIIS extending offshore onto the adjacent shelves. We test the two models using cosmogenic 10 Be surface exposure dating of erratic boulders and glacially eroded bedrock from the three areas. Our results indicate that the last BIIS covered all of northern Scotland during the LGM, but that widespread deglaciation of Caithness and Orkney occurred prior to rapid warming at ca. 14.5 ka.

Exposure dating and reinterpretation of coarse debris accumulations (‘rock glaciers’) in the Cairngorm Mountains, Scotland

“…Collectively, the four sets of ages indicate widespread deglaciation of the Cairngorms and adjacent valleys before 15 ka and possibly 16 ka, which in turn suggests that most of NE Scotland was ice free before the former date and possibly the latter. This conclusion appears consistent with seven 10 Be deglaciation ages averaging 18.5 AE 1.0 ka (16.5-20.5 ka at 95% confidence) obtained for sites in Buchan (Phillips et al, 2008) and with deglacial radiocarbon ages obtained from fossiliferous marine muds at sites along the east coast of Scotland (Hall and Jarvis, 1989;McCabe et al, 2007). It implies extensive retreat of the last ice sheet in the area before the rapid warming that commenced at ca.…”

The last glaciation in Caithness, Scotland: revised till stratigraphy and ice-flow paths indicate multiple ice flow phases

“…Hill summits on Hoy were first deglaciated at 17.2 ± 1.6 ka cal. k yr yet other cosmogenic exposure ages indicate that low ground on west Mainland only became ice-free at 15-14 ka (Ballantyne, 2010;Phillips et al, 2008). These dates probably bracket the period of deposition of the Quendale Till.…”

“…The minimum extent of each advance is indicated by the areal and vertical distribution of individual till units and associated striae. The timing of the main events is constrained, where possible, by reference to existing dating information from radiocarbon dates and amino acid racemisation (AAR) ratios for marine shells from shelly till units (Bowen et al, 1989;Bowen and Sykes, 1988) and from cosmogenic exposure ages (Ballantyne, 2010;Phillips et al, 2008). Ice flow paths on land are shown mainly by sub-glacial bedforms, till fabrics, striae and local erratic carry.…”

“…Unconstrained ice flow thus required an ice sheet that was >~250 m thick at the present eastern coastline of the Moray Firth. Thicker ice, such as that needed to scour rock surfaces at elevations of up to 470 m on Hoy (Phillips et al, 2008) must have extended far onto the North Atlantic shelf.…”

Dating constraints on the last British-Irish Ice Sheet: a map and database

“…Where there are multiple dates at a single location, it is common in cosmogenic exposure dating, though not always preferred, to take the oldest date as the most reliable (Phillips et al, 2008). This considers the possibility of inheritance and/or postglacial shielding and interprets the oldest date as a minimum limiting age (Heyman et al, 2011).…”