Impact of fabric on viscosity of Rutford Ice Stream, Antarctica

The viscous deformation of glacier ice is governed by its temperature and the bulk ice crystal orientation fabric. Due to the mechanical anisotropy of ice crystals, the fabric’s influence on viscosity is directional: depending on the deformation direction, the ice becomes softer or harder. Representing the mechanical anisotropy in numerical ice sheet models is crucial for accurately predicting the future contributions of the Greenland and Antarctic ice sheets to global sea-level rise. However, the fabric strength, orientation, and its impact on viscosity are largely unexplored in fast-flowing ice streams and glaciers. Consequently, the fabric’s influence on ice dynamics is currently inadequately accounted for in ice sheet models. Advances in ground-based radar technologies and improved analysis methods enable the determination of depth profiles of the crystal orientation fabric. In this study, we investigate the fabric and its influence on the viscosity of the Rutford Ice Stream, Antarctica. We analyzed polarimetric measurements performed with an Autonomous phase-sensitive Radio Echo Sounder (ApRES) using a novel approach that allows the determination of fabric-depth profiles to significantly greater depths than previously possible. The results demonstrate a rapid increase in fabric strength within the upper 200 to 300 m depth, followed by a relatively stable fabric strength over depth. In the center of Rutford Ice Stream, our analysis revealed an average fabric strength ranging between 0.4 and 0.5 within the upper 1200 m and fabric rotation by 45° to the ice flow direction. Closer to the shear margin, the fabric strength increased up to 0.8, where the orientation is aligned with the ice flow direction. The findings indicate a substantial influence of the fabric on the effective viscosity, particularly near the shear margin where the ice is softened by a factor of three for horizontal-shear deformation. These findings contribute to a more comprehensive understanding of the distribution of fabric and its influence on the viscosity within ice streams and serve as validation for fabric evolution models.