Radar characterization of ice crystal orientation fabric and anisotropic viscosity within an Antarctic ice stream

We use polarimetric radar sounding to investigate ice crystal orientation fabric, and its impact on ice viscosity, within the near-surface of Rutford Ice Stream, West Antarctica. The technique retrieves lateral and depth variation in the horizontal components of ice fabric but no direct information on the vertical fabric component. In the shallowest ice (depths 40-100 m), the fabric is consistent with flow-induced development and correlates with the surface compression direction. Notably, toward the ice-stream margin the horizontal compression angle and azimuthal fabric orientation tend toward 45° relative to ice flow which is consistent with the early stages of flow-induced fabric under simple shear. The fabric orientation in deeper ice (depths 100-300 m) is, in places, significantly misaligned with shallower ice and the surface compression direction due to sharp depth-transitions in orientation. We then use a rheological model to bound effective anisotropic viscosities (directional hardness) of ice that are consistent with the radar measurements. Toward the shear margin, we show that the shallow-ice fabric does not appreciably soften the ice to lateral shear, although this may happen in deeper ice. In the center of the ice stream, we show that lateral and depth variation in the fabric alignment relative to ice flow results in corresponding changes in uniaxial ice viscosities relative to ice flow. Our results indicate that spatial variability in fabric translates to variability in viscosity that widely used isotropic ice-flow models are unable to consider.

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