Evolution of a Lateglacial mountain icecap in northern Scotland

Abstract

Detailed geomorphological mapping of the Beinn Dearg massif in northern Scotland, was conducted to examine the maximum (Younger Dryas) extent, and earlier interstadial evolution, of an ice cap that existed during the Lateglacial period (14.7 -11.7 cal. ka BP). Landform evidence indicates a plateau ice cap configuration, with radial outlet glaciers, during the Younger Dryas. The interpreted age is supported by new cosmogenic exposure ages, and previously reported interstadial sediments beyond the ice cap margin. The reconstructed Younger Dryas Beinn Dearg ice cap covered 176 km 2 , with its summit positioned over the western side of the massif. Area-altitude balance ratio (AABR) equilibrium line altitudes (ELAs) of 570 -580 m were calculated for the ice cap as a whole. The empirically reconstructed ice cap is compared with recent numerical model simulations; both methods produce an ice cap with a similar configuration. However, differences are apparent in the extent of eastern and western outlets (±1-5 km), and in the spatial variation of ELAs. Results suggest that the numerical simulation over-estimates the extent of western ice cap sectors, and under-estimates the extent of eastern ice cap sectors. We attempt to quantify these differences in terms of ice cap mass balance and assess their possible causes. Geomorphological evidence for pre-Younger Dryas ice cap configuration indicates that the Beinn Dearg massif remained an important source during earlier deglaciation. In contrast, the neighbouring Fannich mountains acted as an 'unzipping' zone, and were ice free on their northern side by the Allerød (Greenland Interstadial 1c to 1a). Deglaciation continued over western parts of the Beinn Dearg plateau, with the possibility that glaciers remained in some central and eastern catchments, prior to (Younger Dryas) ice cap (re)growth.Andrew Finlayson (e-mail: afin@bgs.ac.uk), British Geological Survey, Edinburgh, UK and Institute of Geography, University of Edinburgh, UK; Nick Golledge (e-mail: nick.golledge@vuw.ac.nz), Antarctic Research Centre, Victoria University of Wellington, New Zealand; Tom Bradwell (e-mail: tbrad@bgs.ac.uk), British Geological Survey, Edinburgh, UK; Derek Fabel (e-mail: Derek.Fabel@ges.gla.ac.uk ), Department of Geographical and Earth Sciences, University of Glasgow, UK.Reconstructions of palaeo-, or formerly more extensive, ice masses in northwest Europe have enabled inference of past glacier mass balance and climate, and allowed the causes of ice mass fluctuations to be assessed (e.g. Ballantyne 1989;Dahl & Nesje 1992;Carr 2001;Rea & Evans 2007;Golledge et al. 2009;Nesje 2009). In the Scottish Highlands, the last decade has seen a renewed focus of research into the extent and behaviour of ice masses during the Lateglacial Younger Dryas (YD), or Loch Lomond Stadial (Greenland Stadial 1 (GS-1)) (12.9 -11.7 cal. ka BP (Lowe et al. 2008))(e.g. Ballantyne 2002Ballantyne , 2007aBenn & Ballantyne 2005;Finlayson 2006;Golledge 2007;Lukas & Bradwell 2010). Key outcomes of this research have been the...

A glacial geomorphological map of the Monadhliath Mountains, Central Scottish Highlands

“…This spatial pattern has been observed in many other upland areas of Scotland (e.g. Benn, 1992;Benn & Ballantyne, 2005;Benn, Lowe, & Walker, 1992;Bennett & Boulton, 1993a, 1993bFinlayson, Golledge, Bradwell, & Fabel, 2011;Lukas, 2005;Lukas & Benn, 2006;Lukas & Bradwell, 2010) and supports an interpretation that the moraines represent former ice-marginal positions of actively retreating glaciers. Figure 3.…”

The glacial geomorphology of the Loch Lomond Stadial in Britain: a map and geographic information system resource of published evidence

“…In several cases, landforms originally attributed to ice limits that pre-date the LLS are also included in the original maps (Ballantyne, 2007;Benn, 1990;Finlayson, Golledge, Bradwell, & Fabel, 2011). In such cases, only features within or intersecting the authors' proposed glacier limits were included in the database.…”

A review of topographic controls on moraine distribution

“…This link between moraines and palaeoclimate is usually made in one of three ways: (i) moraine positions are used to infer variations in the areal extent of glaciers, and to provide a qualitative understanding of palaeoclimatic variation between periods (e.g., Lasalle and Elson, 1975). (ii) Moraines (and other landforms) are used to generate three-dimensional (3D) palaeoglacier reconstructions, from which steady-state palaeo equilibrium-line altitude (ELA) estimates can be derived (e.g., Benn and Ballantyne, 2005;Benn and Lukas, 2006;Rea and Evans, 2007;Finlayson et al, 2011). This approach is based on the assumption that the ELA can be linked to climate because it marks the point on a glacier where net annual accumulation and ablation are exactly equal (i.e., it is governed by variations in temperature and precipitation) (see Braithwaite et al, 2006;Hughes and Braithwaite, 2008;Golledge et al, 2010).…”

“…However, in theory, much of the uncertainty introduced by topographic factors can be mitigated by only obtaining palaeoclimatic data from three-dimensional reconstructions of palaeoglacier form. This approach has validity in that glacial geomorphology and land surface topography are taken into direct consideration to yield estimates of palaeo-ELA (e.g., Benn and Ballantyne, 2005;Rea and Evans, 2007;Finlayson et al, 2011). However, in reality, even when such reconstructions are generated, topographic factors continue to introduce uncertainty and limit the validity of resulting palaeoclimatic data.…”

“…However, in reality, even when such reconstructions are generated, topographic factors continue to introduce uncertainty and limit the validity of resulting palaeoclimatic data. One key limitation stems from the fact that palaeoglacier reconstructions often rely on directly dating a small number of moraines and extrapolating these age-estimates to a wider population of landforms (e.g., Finlayson et al, 2011;Bendle and Glasser, 2012;Nawaz Ali et al, 2013). This approach relies on chronologically grouping moraines that are assumed to have been deposited synchronously.…”