Radar Characterization of Ice Crystal Orientation Fabric and Anisotropic Rheology within an Antarctic Ice Stream

Abstract:

An ever-present challenge in glaciology is understanding how different rheological mechanisms (e.g. temperature, ice crystal orientation fabric, structural damage) impact on ice flow across the ice sheets. Typically, only spatial variability in temperature is incorporated in Glens’ flow-law, leaving other rheological mechanisms unaccounted for. This is especially problematic for ice fabric, which results in rheological anisotropy of glacier ice (i.e. ice that is preferentially softer for different deformation modes and directions). Direct ice fabric measurements are generally limited to slow-flow regions of ice-sheets. Consequently, to understand the impact of ice fabric on fast-flowing ice streams, geophysical measurement techniques are undergoing a period of rapid development.

Here, we use polarimetric radar sounding to investigate spatial variation in ice crystal orientation fabric within Rutford Ice Stream, West Antarctica. To assess the influence of the fabric on ice rheology and flow, we develop a framework where the radar fabric estimates (azimuthal anisotropy) are used to parameterize an anisotropic flow-law. In the shallowest ice (top 100 m) the fabric orientation is consistent with flow-induced development that correlates with the surface strain field. In deeper ice, however, the fabric orientation can be significantly misaligned from shallower ice and inconsistent with the surface strain. The rheological modeling illustrates that the fabric results in spatially variable enhancement of uniaxial and shear deformation within the ice stream. Taken together, the results support that the fabric acts as a viscous feedback mechanism, serving to enhance heterogeneity within the ice flow.