Implications of till deformation on glacier dynamics

Abstract

The dynamics of glacier motion is governed, to a large extent, by the properties of the basal interface. In this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of Black Rapids Glacier, Alaska. We also investigate whether or not a till layer has a clear surface-observable signature in the dynamics of the glacier. Towards this end we use a finite element ice-flow model with a Coulomb failure criterion within the basal till layer. We find that simple 'till physics' can be used to describe decadal, seasonal, and short term (hours to days) velocity variations, and possibly uplift events. Mechanisms for each of these variations involve an increase in the extent of till at failure, a transfer of shear stress across the bed, and a consequent increase in ice deformation. 'Effective shape factors' are calculated to permit a simple incorporation of this boundary condition into glacier response models. Our analyses, however, have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of the glacier.

Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland

“…interpretation is supplied to predictive models of glacier flow, given the sensitive relationship between the substrate of a glacier and its flow regime (Pattyn, 1996;Truffer et al, 2001;Pimentel et al, 2011;Sergienko and Hulbe, 2011). The risk of neglecting thin-layer considerations is particularly acute where the thickness of a dilatant till approaches its nominally resolvable threshold.…”

“…Although glaciers themselves are clearly good approximations to half-spaces, subglacial tills frequently contain metre-(or sub-metre-) scale contrasts (Smith, 2007), such as the transitional boundaries between dilatant and lodged till (e.g. Clarke et al, 1984;Echelmeyer and Wang, 1987;Iverson et al, 1988Iverson et al, , 1994Truffer et al, 2001;Boulton et al, 2001;Porter and Murray, 2001;Evans et al, 2006;Iverson, 2011;Reinardy et al, 2011). It would therefore be prudent to consider how thin layering affects a seismic AVA response whenever surveys are conducted over till substrates.…”

“…Characterising the physical properties of subglacial material is a key research goal for improving the representation of basal dynamics in predictive ice-flow models (e.g. Pattyn, 1996;Payne, 1999;Truffer et al, 2001;Pimentel et al, 2010;Sergienko and Hulbe, 2011). A hard-bedded glacier is associated with different ice-flow regimes to a soft, sediment-bedded glacier, with the presence of englacial debris and/or subglacial water significantly impacting flow (Iverson et al, 2003;Cohen et al, 2005;Emerson and Rempel, 2007).…”

Mass-conserving subglacial hydrology in the Parallel Ice Sheet Model version 0.6

“…The water in till pore spaces is much less mobile than that in the linked-cavity system because of the very low hydraulic conductivity of till (Lingle and Brown, 1987;Truffer et al, 2001). Therefore, we choose an evolution equation for W til without horizontal transport for simplicity (Tulaczyk et al, 2000a), namely,…”

“…Laboratory experiments on the rheology of till (Kamb, 1991;Hooke et al, 1997;Tulaczyk et al, 2000a;Truffer et al, 2001) generally conclude that its deformation is well approximated by a Mohr-Coulomb relation (Schoof, 2006b). For this reason we adopt a compressible-Coulombplastic till model when determining the effective pressure on the till as a function of the amount of water stored in it (Tulaczyk et al, 2000a).…”