Mass balance, flow and subglacial processes of a modelled Younger Dryas ice cap in Scotland

Abstract

ABSTRACT. We use an empirically validated high-resolution three-dimensional ice-sheet model to investigate the mass-balance regime, flow mechanisms and subglacial characteristics of a simulated Younger Dryas Stadial ice cap in Scotland, and compare the resulting model forecasts with geological evidence. Input data for the model are basal topography, a temperature forcing derived from GRIP δ 18 O fluctuations and a precipitation distribution interpolated from modern data. The model employs a positive-degree-day scheme to calculate net mass balance within a domain of 112 500 km 2 , which, under the imposed climate, gives rise to an elongate ice cap along the axis of the western Scottish Highlands. At its maximum, the ice cap is dynamically and thermally zoned, reflecting topographic and climatic controls, respectively. In order to link these palaeoglaciological conditions to geological interpretations, we calculate the relative balance between sliding and creep within the simulated ice cap, forecast areas of the ice cap with the greatest capacity for basal erosion and predict the likely pattern of subglacial drainage. We conclude that ice flow in central areas of the ice cap is largely due to internal deformation, and is associated with geological evidence of landscape preservation. Conversely, the distribution of streamlined landforms is linked to faster-flowing ice whose velocity is predominantly the result of basal sliding. The geometry of the main ice mass focuses subglacial erosion in the mid-sections of topographic troughs, and produces glaciohydraulic gradients that favour subglacial drainage through low-order arterial routes.

Evolution of a Lateglacial mountain icecap in northern Scotland

“…In subsequent experiments, Golledge et al (2010) and Golledge (2010b) found that similar modelled glacier configurations across Scotland could equally be produced with doubled or trebled seasonality, combined with reduced total precipitation, and more relaxed precipitation gradients; thus demonstrating that different climatic regimes can produce similar overall ice mass dimensions, albeit with different glaciological conditions. For the purposes of this discussion, comparisons are made with the simulation of Golledge et al ( , 2009, which is most fully described. The numerical simulation produced an overall ice cap configuration, drained radially by valley outlets, with a summit (~900 m a.s.l.)…”

“…Preservation of landforms interpreted as pre-YD in age, combined with the profile of the ice cap, suggests that motion was predominantly by internal deformation over the plateau, and locally basal sliding (with perhaps some bed deformation) in outlet areas (c.f. Golledge et al 2009). Retreat of the YD ice cap left regular, closely-spaced recessional moraine assemblages in many outlet valleys, indicative of active, oscillatory retreat (e.g.…”

“…A similar overall configuration is empirically reconstructed here. The numerical model simulates negligible basal velocities and low (-2 to -4.5°C) basal temperatures over much of the plateau (Golledge et al 2009), with the exception of 'drawdown' zones which feed valley outlet glaciers. This is also consistent with the empirical evidence.…”

“…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.…”

Glaciation of Scotland during the Younger Dryas stadial: a review

“…While such substrates may well have facilitated the flow of topographically unconstrained portions of former ice sheets, the roughness of the glacier bed in the Western Highlands, and the deep outlet glens such as Loch Lomond, are hard to reconcile with such speculations for the Younger Dryas ice cap. Glaciological data from numerical modelling show that ice flow in central areas was dominated by internal (ice) deformation -creep -whereas basal sliding (possibly including flow over deforming basal sediments) was only prevalent nearer glacier margins (Golledge et al, 2009). This radial zonation of the ice cap favoured preservation or subtle remoulding of preexisting sediments and landforms in core areas, with fresh deposition of thick subglacial till only occurring around the periphery of the ice cap.…”