Abstract
We present 17 cosmogenic 10Be ages of glacial deposits in Coire an Lochain (Cairngorm Mountains), which demonstrate that glacial and nival deposits cover a longer timescale than previously recognised. Five ages provide the first evidence of a late-Holocene glacier in the British Isles. A previously unidentified moraine ridge was deposited after c. 2.8 kyr and defines a small slab-like glacier with an equilibrium line altitude (ELA) at c. 1047 m. The late-Holocene glacier was characterised by rapid firnification and a dominance of sliding, enabling the glacier to construct moraine ridges in a relatively short period. Isotopic inheritance means that the glacier may have existed as recently as the ‘Little Ice Age’ (LIA) of the 17th or 18th century ad, a view supported by glacier-climate modelling. Nine 10Be ages confirm a Younger Dryas Stadial (YDS) age for a cirque-floor boulder till, and date the glacier maximum to c. 12.3 kyr when the ELA was at c. 963 m altitude. Both glaciers existed because of enhanced accumulation from wind-blown snow, but the difference in ELA of only c. 84 m belies the YDS–LIA temperature difference of c. 7°C and emphasises the glacioclimatic contrast between the two periods. Three 10Be ages from till boulders originally deposited in the YDS yield ages <5.5 kyr and indicate snow-avalanche disturbance of older debris since the mid-Holocene, as climate deteriorated towards marginal glaciation.
Highlights
Scotland’s last glaciers are generally accepted to have vanished at the close of the Younger Dryas Stadial (YDS), around 11.5 kyr (Golledge et al, 2008)
Attention has focused on attempts to show that moraines ascribed to the YDS were late Holocene in age (Sugden, 1977), but 14C dating of cirque lake sediments (Batterbee et al, 2001; Rapson, 1985) does not support a Little Ice Age’ (LIA) age of cirque moraines in the northern Cairngorm Mountains
Exposure-age dating was used (1) to test the presumed YDS. Age for the latter, which was initially sampled with a view to investigating whether the whole deposit was of a single age, and (2) to investigate whether the Great Slab moraine represents evidence of a small Holocene glacier in the upper cirque
Summary
Scotland’s last glaciers are generally accepted to have vanished at the close of the Younger Dryas Stadial (YDS), around 11.5 kyr (Golledge et al, 2008). This paper reports cosmogenic 10Be ages and glaciological reconstructions of cirque moraines in Coire an Lochain (Figure 1) to examine the evidence for both YDS and subsequent glacial advances.
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