Softening of Temperate Ice by Interstitial Water

Abstract

Ice at depth in ice-stream shear margins is thought to commonly be temperate, with interstitial meltwater that softens ice. Models that include this softening extrapolate results of a single experimental study in which ice effective viscosity decreased by a factor of ∼3 over water contents of ∼0.01–0.8%. Modeling indicates this softening by water localizes strain in shear margins and through shear heating increases meltwater at the bed, enhancing basal slip. To extend data to higher water contents, we shear lab-made ice in confined compression with a large ring-shear device. Ice rings with initial mean grain sizes of 2–4 mm are kept at the pressure-melting temperature and sheared at controlled rates with peak stresses of ∼0.06–0.20 MPa, spanning most of the estimated shear-stress range in West Antarctic shear margins. Final mean grain sizes are 8–13 mm. Water content is measured by inducing a freezing front at the ice-ring edges, tracking its movement inward with thermistors, and fitting the data with solutions of the relevant Stefan problem. Results indicate two creep regimes, below and above a water content of ∼0.6%. Comparison of effective viscosity values in secondary creep with those of tertiary creep from the earlier experimental study indicate that for water contents of 0.2–0.6%, viscosity in secondary creep is about twice as sensitive to water content than for ice sheared to tertiary creep. Above water contents of 0.6%, viscosity values in secondary creep are within 25% of those of tertiary creep, suggesting a stress-limiting mechanism at water contents greater than 0.6% that is insensitive to ice fabric development in tertiary creep. At water contents of ∼0.6–1.7%, effective viscosity is independent of water content, and ice is nearly linear-viscous. Minimization of intercrystalline stress heterogeneity by grain-scale melting and refreezing at rates that approach an upper bound as grain-boundary water films thicken might account for the two regimes.

Transient evolution of basal drag during glacier slip

“…S6 for other choices of p ). Findings by Adams and others (2021) show the exponent on the ice flow law for temperate ice with sufficient intercrystalline water is equal to 1.1, which may in part indicate why p = 1 was found to best approximate the cavity evolution response in the experiments. The RSF model describes the timescale and magnitude of the drag change well but does not precisely replicate its form.…”

A permeameter for temperate ice: first results on permeability sensitivity to grain size

“…Our most pressing priority is to study the dependence of ice permeability on water content by varying it systematically and measuring it using the calorimetric method described herein and used previously (Duval, 1977; Adams and others, 2021). This dependence for both ice and other rocks with a melt phase is viewed as more uncertain than the dependence of permeability on ice grain size (e.g.…”

“…At the high end of this range, effectively all meltwater produced by shear heating drained away, leading to a negligibly small meltwater content in most of the temperate part of the shear zone. In contrast, at the lowest permeability value thought to be possible, drainage was inhibited sufficiently to result in a water content of ~8%, well above the range over which the effect of water on ice softening is known (Duval, 1977; Adams and others, 2021) and above values observed at the grain scale in glacier ice (<3%). Previous models of ice-stream shear margins that routed meltwater to the bed (but did not include the softening effect of water from shear heating) would have also benefited from reliable permeability values for temperate ice (Suckale and others, 2014; Meyer and others, 2018).…”

“…To measure the liquid water content of ice, the device is configured to use the calorimetric method of our ice-deformation experiments (Adams and others, 2021, see also Duval, 1976). The lateral edges of the ice disk can be abruptly chilled to below the PMT, and the speed of the resultant freezing front that moves radially inward can be tracked with thermistors in the ice.…”

“…In addition, shear heating in ice-stream margins dissipates sufficient heat to overcome the effects of cold-ice advection from the glacier surface and from adjacent slow-moving ice to cause many margins to be temperate at depth (Meyer and Minchew, 2018). Interstitial water in the temperate ice reduces its effective viscosity (Duval, 1977; Adams and others, 2021), affecting the shearing resistance, velocity distribution and strain heating of shear margins (Haseloff and others, 2019). Water, through its substantial latent heat, also influences the energy balance that dictates ice temperature (Schoof and Hewitt, 2016) and its feedback on ice effective viscosity (e.g.…”