Cold-Based Subglacial Deformation: Glacier-Permafrost Interactions
Numerical ice sheet models have substantially improved our understanding of ice sheet dynamics and environmental change. The assumptions made about the basal boundary conditions characterising large areas of an ice sheet are critical to the accuracy of the predictions made by an ice sheet model. But how valid are some of our most commonly held assumptions? Take for example the Laurentide Ice Sheet, our understanding of which is based primarily on studies of its southern margins, whose rapid oscillations are traditionally related to the establishing influence of active basal processes. In contrast, it is widely assumed that large areas of its northern margin experienced cold-based conditions and a zero basal velocity. Consequently, this part of the ice sheet is frequently taken to have been relatively slow moving, stable and therefore to have only a limited influence upon the behaviour of the whole ice mass, despite the presence of some substantial ice streams. Work is now emerging to suggest that some cold-based ice may be more dynamic than previously thought challenging conventional models of basal processes. This work poses fundamental questions. Do cold-based ice sheets interact with their bed? If so how and on what spatial scales? Are cold-based ice sheets slow moving, inactive and stable? It is these questions, fundamental to our understanding of ice sheet dynamics and landscape change, which this research seeks to address.Numerical ice sheet models have substantially improved our understanding of ice sheet dynamics and environmental change. The assumptions made about the basal boundary conditions characterising large areas of an ice sheet are critical to the accuracy of the predictions made by an ice sheet model. But how valid are some of our most commonly held assumptions? Take for example the Laurentide Ice Sheet, our understanding of which is based primarily on studies of its southern margins, whose rapid oscillations are traditionally related to the establishing influence of active basal processes. In contrast, it is widely assumed that large areas of its northern margin experienced cold-based conditions and a zero basal velocity. Consequently, this part of the ice sheet is frequently taken to have been relatively slow moving, stable and therefore to have only a limited influence upon the behaviour of the whole ice mass, despite the presence of some substantial ice streams. Work is now emerging to suggest that some cold-based ice may be more dynamic than previously thought challenging conventional models of basal processes. This work poses fundamental questions. Do cold-based ice sheets interact with their bed? If so how and on what spatial scales? Are cold-based ice sheets slow moving, inactive and stable? It is these questions, fundamental to our understanding of ice sheet dynamics and landscape change, which this research seeks to address.
In 2003 the research team led by Professor Bennett reported evidence of deformation at sub-freezing temperatures beneath Hagafellsjökull-Eystri, an Icelandic surge-type glacier. The bed of a piedmont lobe that advanced during the 1999 surge comprises deformed blocks of glacier ice set within frozen sediment, as shown below. This material has also been injected through overlying ice to form a network of crevasse-squeeze ridges. This layer contains evidence for two phases of deformation under contrasting rheological conditions: (1) deformation under relatively warm conditions, when the blocks of glacier ice acted as competent clasts within an unfrozen deforming matrix and (2) subsequent deformation at sub-freezing temperatures when the ice blocks were attenuated into the surrounding frozen matrix along fractures and planar shears enriched with excess ice. This suggests that the basal thermal regime of the advancing ice margin changed from warm-based to cold-based during the surge event. The persistence and potential prevalence of subglacial sediment deformation at sub-freezing temperatures has fundamental implications for our understanding of the dynamic behaviour, sediment flux and geomorphic ability of cold-based glaciers.
Selected Publications
BENNETT, M.R., WALLER, R.I, MIDGLEY, N.G., HUDDART, D., GONZALEZ, S., COOK, S.J. & TOMIO, A. 2003. Subglacial deformation at sub-freezing temperatures? Evidence from Hagafellsjökull-Eystri, Iceland. Quaternary Science Reviews, 22, 915-923.
For more details contact Professor Matthew Bennett
