Sensitivity of the effective viscosity of temperate ice to its water content

Abstract

Flow speeds of West Antarctic ice streams depend on the shearing resistance of ice in their lateral margins. Ice at depth in these regions is theorized to be at the pressure-melting temperature and thus to contain interstitial meltwater. This water’s influence on the rheology of temperate ice is a major source of uncertainty in efforts to model ice-stream motion. These models rely on scant experimental data indicating effective viscosity decreases by a factor of ~3 as ice water content increases from ~0.01 to 0.8%. Recent modeling incorporating this dependence indicates that the sensitivity of viscosity to water content localizes strain in shear margins, causing strain heating that increases meltwater available for enhancing slip at ice-stream beds.To expand the database on softening of ice by interstitial water, we conducted a series of shearing experiments on temperate ice under compression with a large viscometer (modified ring-shear device). Ice rings had inner and outer diameters of 0.5 and 0.9 m, respectively, and were approximately ~0.2 m thick. Ice with crystal sizes of 2-4 mm was made by mixing deionized water with sieved snow and maintained at the pressure-melting temperature by a temperature-controlled bath (0.01°C precision). Water content was varied from ~0.2 to 1.7% by imposing various combinations of confining pressure (700-1500 kPa) and shear strain rate (7.1 x 10-9 - 1.2 x 10-7 s-1). Ice was sheared only until a peak shear stress was attained to avoid the complicating influence of fabric development in tertiary creep. Water content was measured by inducing a freezing front at the ice-ring walls, recording its speed as it moved toward the ice-ring center, and solving the relevant Stefan problem. Results indicate that effective viscosity is independent of water content above values of ~0.6%, In contrast, at water contents increasing from 0.2 to 0.6%, effective ice viscosity decreased by a factor of 4.4, about 1.8 times more than in previous experiments conducted to tertiary creep. A stress exponent of n = 1.1 was determined for ice water contents above ~0.6% and indicates Newtonian flow, probably resulting from internal melting and refreezing facilitated by wetted grain boundaries. This hypothesis is supported by effective ice viscosities at these high water contents that are the same in secondary and tertiary creep, indicating a deformation mechanism that is independent of ice-fabric development in tertiary creep. The insensitivity of ice effective viscosity, when n ≈ 1 at low water contents (< 1.7%), is