Microstructural Analysis of Intensely Sheared, Coarse-Grained Marginal Ice on Storglaciären, Sweden, Using Cryo-Electron Backscatter Diffraction (EBSD)

Abstract:

Anisotropy in the form of a crystallographic preferred orientation (CPO) develops during plastic deformation and modifies the rheological properties of ice, influencing dynamic feedbacks and large-scale flow rates that impact glacial discharge. Warm (T>-10° C), coarse-grained (>20mm) ice, common deep in ice sheets, valley glaciers and polar outlet glaciers, is particularly important because basal ice in ice sheets likely accommodates a significant amount of deformation and is responsible for much of the flow. This ice is often characterized by a multi-maxima (typically 3-4 pronounced clusters) c-axis CPO pattern. Its rheological properties are not well known. Deformation is accommodated primarily by glide on the basal plane, and a-axes as well as c-axes are needed to fully define the CPO and characterize deformation, recovery and recrystallization processes, vital for understanding kinematics and rheology. Previous microstructural studies on coarse-grained ice were limited by techniques that provide only c-axis orientations. We developed a new sample-preparation method, in which we construct composite sections to determine full crystallographic orientations (c- and a-axes) of coarse-grained ice using cryo-electron backscatter diffraction (EBSD). CPOs from intensely sheared marginal ice of Storglaciären, a polythermal valley glacier, yield a c-axis maximum normal to the shear plane that is elongate or split in a plane normal to the shear direction. They also include a c-axis sub-maximum ~50° antithetic to the shear direction, and an a-axis girdle parallel to the shear plane with a concentration of a-axes perpendicular to the shear direction, and parallel with the inferred vorticity axis. This CPO compares well with experimentally sheared ice at high homologous temperatures and strains <1.5. However, shear strains at the margin of Storglaciären are >2. Grains characteristically interlock with ameboidal grain boundaries, show no apparent shape preferred orientation, lack significant internal distortion, and show evidence of grain-boundary drag around bubbles. Taken together, these factors indicate that dynamic recrystallization involving grain-boundary migration might explain the similarity, and likely play a role in resetting the CPO to local conditions with limited strain history.