THE DEGLACIATION OF ICELAND: MODELLING THE MAXIMUM EXTENT AND SUBSEQUENT RAPID FLUCTUATIONS OF THE LATE WEICHSELIAN ICE-SHEET INTO THE HOLOCENE

A 3D thermomechanical model is used to investigate the dynamics of the Icelandic ice sheet from the LGM at ~21 ka through the vicissitudes of lateglacial times to ~10 ka. The model enables the variables of ice thickness, flow and temperature to interact freely and through the computation of longitudinal stresses accounts for the complex interaction of ice-shelves, ice-calving and thermally-coupled basal sliding with ice dynamics. The model is applied at 2 km resolution and realistic topographic, thermal and mass-balance boundary conditions are employed. The coupled model is perturbed by relative changes in sea-level, precipitation and temperature and yields space-time distributions of ice thickness, basal and surface velocity, melt-water flux and other palaeo-environmental indices.

The model successfully replicates the present-day ice distribution across Iceland with a temperature forcing of -2°C and a suite of sensitivity experiments are implemented to identify an ‘optimal’ spin-up to the LGM. These experiments, integrated over 5,000 years using a scaled GRIP ¹⁸O record, testify that once the ice sheet has advanced beyond the coastline it is relatively independent of climate and largely controlled by relative sea level through ice-calving. Correspondence with field evidence indicates that the optimum LGM ice sheet extended significantly offshore with an area of 3.29 x 10⁵ km², but has a relatively low volume of 2.38 x 10⁵ km³ yielding a mean ice thickness of 720 m with a net mass flux of ~900 km³ w.e. per year. The experiments highlight the central role played by basal thermodynamics in activating extensive areas of fast flow throughout the neovolcanic zone yielding a highly dynamic, low aspect ratio ice sheet with a maximum plateau elevation of 1,700 m breached throughout by numerous nunataks. The modelled ice sheet is subsequently forced into deglaciation using an optimally scaled GRIP record and is marked by a collapse of ~200,000 km³ of ice from offshore shelves at 17.5-16 ka, a period coinciding with the Heinrich-1 discharge event. A smaller collapse at 13.9 ka reflects the rapid withdrawal of ice from northern and western coast during the Bølling interstadial which is followed by the rapid cooling of the Younger Dryas which results in an independent icecap in the northwest and advances to topographic pinning points around the coast in most sectors.