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Sea-level projections depend sensitively on the parameterization used for basal slip in glacier flow models. During slip over rock-beds, ice-bed separation increases with slip velocity and basal water pressure. We present a method for using these variables and measured bed topography to estimate the average bed slope in contact with ice, ${\bar m}$. Three-dimensional numerical modeling of slip over small areas of former beds has shown that changes in ${\bar m}$ with increasing slip velocity and water pressure mimic changes in basal drag. Computed values of ${\bar m}$ can thus provide the form of the slip law that relates drag to velocity and water pressure, avoiding computationally expensive numerical modeling. The method is applied to 618 sections from four former glacier beds. Results generally show an increase in ${\bar m}$ , and hence inferred basal drag, with slip velocity up to a limiting value, consistent with a regularized Coulomb slip law. [ABSTRACT FROM AUTHOR]

Reconstructing the size and glacial style of past ice-sheet advances requires interpreting complex glacial sedimentary facies. We use diatoms, a major component of Antarctic continental shelf deposits, to infer the physical conditions under which these deposits were emplaced. The degree of diatom fragmentation and the presence of diatoms of varying stratigraphic age in glacial sediments provide means to qualitatively gauge glacial mixing and transport. Here we report an experimentally calibrated index of diatom fragmentation that provides a simple but objective method of assessing the degree of subshearing imparted on marine glacial sedimentary deposits. By using a ring-shear device to subject diatomaceous sediment to stresses comparable to those beneath the Ross ice streams, we quantitatively assess patterns of diatom comminution resulting from compaction and from progressive shear stress. Elongate pennate diatoms are found to break disproportionately to discoid centric diatoms when subjected to shear stress; thus, a simple ratio of unbroken centric to pennate diatoms provides a reliable gauge of past shearing. Comparison of ring-shear results with a suite of previously analyzed sediments that represent a variety of glacial, glacial-marine, and hemipelagic settings of the Ross Sea and subglacial Ross Embayment demonstrates that this index can be employed for estimating relative subglacial stresses in this setting. Keywords: diatom, fragmentation, ring shear, Antarctic, Ross Sea, ice stream, till, diamicton, pennate, centric.

Glacier movement is resisted partially by debris, either within glaciers or under glaciers in water-saturated layers. In experiments beneath a thick, sliding glacier, ice containing 2 to 11% debris exerted [...]

Slow, stable shear of granular materials in landslides, beneath glaciers, and along fault surfaces is common, despite little or no intrinsic strengthening of such materials with increasing deformation rate. Compacted, water-saturated sediments, subjected to constant stresses in a ring-shear device, sheared slowly without unstable acceleration in repeated episodes that included pore dilation during shear, attendant pore-water-pressure decline, and consequent strengthening, followed by gradual pore-pressure recovery and weakening. Time-averaged shear velocities (2-800 mm/d) depended inversely on the magnitude of pore dilation with shear and were significantly lower for fine-grained sediment than for coarse-grained sediment, owing to different rates of pore-pressure diffusion in the two materials. When sediment had dilated to its critical-state (steady) porosity and therefore could not dilate further, shear accelerated catastrophically. These data indicate that pore-pressure decreases and consequent strengthening caused by shear-induced dilation may not only suppress rapid shear of landslide debris, subglacial till, and fault gouge, but may also result in slow episodic shear at rates that depend on both material porosity and hydraulic diffusivity. Keywords: shear, porosity, pore-water pressure, landslides, till, gouge.

Results from laboratory experiments show that ice at its melting temperature regelated through dense, three-dimensional arrays of clasts abrading a rock bed. Regelation theory for an isolated, stationary array of clasts successfully approximates periods when ice separated from the bed, thermal gradients, and clast-bed contact forces, despite the relative motion of clasts and incoming heat from the bed. The process may entrain sediment at glacier beds and restrict deposition to zones where sediment in ice is concentrated, effective pressure is low, and basal melting is rapid.

Theory and experiments indicate that ice–bed separation during glacier slip over 2-D hard beds causes basal shear stress to reach a maximum at a particular slip velocity and decrease at higher velocities. We use the sliding theory of Lliboutry (1968) to explore how friction between debris particles in sliding ice and a rock bed affects this relationship between shear stress and slip velocity. Particle–bed contact forces and associated debris friction increase with increasing slip velocity, owing to increased rates of ice convergence with up-glacier facing surfaces. However, debris friction on diminished areas of the bed counteracts this effect as cavities grow. Thus, friction from debris alone increases only slightly with slip velocity, and for sediment particles larger than ~60 mm in diameter, debris friction peaks and decreases with increasing slip velocity. The effect on the sliding relationship is to steepen its rising limb and shift its shear stress peak to a slightly higher velocity. These results, which exclude the effect of debris friction on cavity size and debris concentrations above ~15%, indicate that the effect of debris in ice is to increase basal shear stress but not significantly change the form of the sliding relationship. [ABSTRACT FROM AUTHOR]

Elongate clasts in subglacial till and in fault gouge align during shearing, but the relation between clast-fabric strength and cumulative shear strain for such materials is effectively unknown. This relation was explored in experiments with a large ring-shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains. As expected, rotation of clasts in the putty is closely approximated by the theory of G.B. Jeffery, who derived the orbits of rigid ellipsoids in a slowly shearing fluid. Clast rotation in the till, however, is strikingly different. Rather than orbiting through the shear plane as predicted by Jeffery, most clasts rotate into the shear plane and remain there, resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts. This divergent behavior is likely due to slip of the till matrix along the surfaces of clasts, which is a natural expectation in a granular material but violates the no-slip condition of Jeffery's model. These results do not support the widespread belief that subglacial till deformation results in weak clast fabrics. Thus, many tills with weak fabrics thought to have been sheared subglacially to high strains, like many basal tills of the Laurentide Ice Sheet, may have been sheared only slightly with little effect on either ice-sheet dynamics or sediment transport. In addition, these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of A. March, who treated inclusions as passive markers. Keywords: deformation, fabric, gouge, granular material, till.

Subhourly measurements of bed deformation, bed shear strength, subglacial water pressure, and surface speed at Storglaciaren, a glacier in northern Sweden, showed that the shear-strain rates of the bed decrease during periods of high water pressure and surface speed. High water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing. The instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation, as has been commonly thought., Many glaciers are underlain, at least in part, by unlithified sediment that may deform readily if basal water pressures are near the ice overburden pressure. Such deformation has been invoked [...]