Was the Younger Dryas (Loch Lomond Stadial) icefield on Rannoch Moor, western Scotland, deglaciated as early as c.12.5 cal ka BP?

Abstract

The concept that Rannoch Moor, the centre of the Younger Dryas (YD), West Highland Icefield, was deglaciated as early as 12.5calka BP is discussed in the light of radiocarbon dates and varve sequences from outlet glaciers of this icefield, and climate change during the YD. The maximum positions of three YD glaciers were reached after 11.6–11.8calka BP (Lomond), and after 11.8–11.9calka BP (Spean and Treig) indicating that ice remained on Rannoch Moor until long after c.12.5calka BP, and possibly until the YD/Holocene transition at c.11.7calka BP. Further, the Spean glacier dammed a proglacial lake in Lochaber for at least 495 varve years over a period that included the deposition of the Vedde Ash (c.12.1calka BP) and a late YD ash layer (c. 11.7–11.2calka BP), a thesis at variance with supposed early YD deglaciation. Recent examination of this issue using 10Be exposure age determinations from Rannoch Moor is equivocal. In view of the presence of hard water algae at the sampling site on Rannoch Moor it is recommended that the ‘early’ 14C dates from Rannoch Moor need to be further reassessed using chronological constraints provided by dated microtephra, and a collaborative radiocarbon dating programme.