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.
Highlights
Over the last few decades, flow of marine-terminating ice streams has accounted for most of the mass lost from the Antarctic Ice Sheet (Rignot et al, 2019)
Motion resulting from slip near the platen surfaces was not included in strain-rate determinations and varied roughly with confining pressure: an experiment at abnormally low confining pressure (0.33 MPa) yielded slip that accounted for 62% of the platen motion, whereas the average motion by slip was much lower, 27%
Despite slip at the platens, no discontinuity at the tips of platen teeth was visible in any experiment, unlike in Kamb’s (1972) torsion experiments in which slip was recorded at slightly subfreezing temperatures and discontinuities were visible after experiments
Summary
Over the last few decades, flow of marine-terminating ice streams has accounted for most of the mass lost from the Antarctic Ice Sheet (Rignot et al, 2019). Resultant high rates of shear strain at ice-stream margins dissipate sufficient heat to overcome 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; Hunter et al, 2021). Resultant water that resides at grain boundaries reduces the effective viscosity of the ice (Duval, 1977). Exponential thickening of pre-melted water films at grain boundaries with increasing temperature is thought to be responsible for enhanced strain rates (Hooke, 2020)
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