The University of Sheffield
Department of Geography

The origin and glaciological significance of the overdeepening of ice-sheet beds

Supervisors: Dr Darrel SwiftProfessor Chris Clark, Dr Robert Bingham (University of Aberdeen)


Overdeepenings are large-scale glacial erosional landforms that are ubiquitous in contemporary and formerly glaciated settings. Notably, overdeepenings are present beneath the interiors and outlet glaciers of contemporary ice sheets (eg Vaughan et al, 2006; Bo et al, 2009). For marine-terminating outlet glaciers and ice sheets, theoretical studies show that overdeepenings will play a significant role in determining future sea level change because grounding-line retreat in the presence of an overdeepening is inherently unstable (eg Joughin and Alley, 2011). Field and theoretical studies also indicate significant ice-dynamic implications for ice that is grounded above sea level, mainly because ice must traverse a closed subglacial basin – leading to strong changes in longitudinal stress gradients (eg O'Neel et al, 2010) – and basal water must navigate an adverse bed-slope – which raises subglacial water pressures (eg Creyts and Clarke, 2010). However, the processes that cause overdeepening are enigmatic; the origin and distribution of overdeepenings beneath large contemporary ice masses is poorly known; and the significance of overdeepenings for glacier and ice sheet dynamics has been subject to only limited investigation.

Together with the increasing availability of high-quality remotely-sensed imagery, the acquisition of comprehensive ice-thickness and bed-elevation datasets for large ice sheets (such as BEDMAP2) means that there is now potential to answer key questions regarding the origin, distribution, context and significance of overdeepenings in contemporary ice sheet and outlet glacier systems. Notably, theoretical explanations and numerical simulations of overdeepening development indicate contrasting overdeepening distributions and morphologies. Hence, spatial and morphometric analyses of ice-sheet beds can be used to identify overdeepenings and group them according to their likely mechanism of formation. Further, advances in remote-sensing mean that, once the location of overdeepenings is known, their ice-dynamic implications can be assessed using analyses of ice-surface velocity, crevasse patterning, surface-gradient, and ice-margin (or grounding-line) retreat. This project will therefore seek to acquire and analyse available datasets to test existing theories of overdeepening origin and further our understanding of their distribution and significance at ice sheet scales. The project will significantly advance current understanding that has been gained largely in formerly glaciated environments where: (1) postglacial sedimentation precludes detailed morphometric analysis; and (2) the relationship to other important glaciological processes is lost.

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