Microstructures in a glacial shear margin

Abstract:

Microstructures, including crystallographic orientation fabric, within the margin of streaming ice can exert strong control on flow dynamics. We retrieved three cores, two of which reached the glacier base, from the flank of Jarvis glacier, eastern Alaska Range, Alaska. Core lengths were 80m (core JA, ~100m from the margin), 30m (core JB, ~75m from the margin) and 18m (core JE, ~50m from the margin). The core sites lie ~1km downstream of the ice source, and abundant water was present in the extracted cores and at the base of the glacier. All cores exhibit extensive dipping layers, which we interpret as a combination of debris bands and healed fractures, Grain areas are approximately bimodal throughout JA and JB, but small, presumably recrystallized, grains are less abundant in the higher strain ice represented by core JE. Grain circularity is also lower in JE than in JA and JB. Electron backscatter diffraction-derived crystallographic orientations indicate that c-axes are more clustered and closer to horizontal near the lateral margin. Fabric in JA increases in strength with depth, whereas we identified no significant variation with depth in JE. The measured fabric is sufficiently weak to have minimal rheologic influence. However, from these data, we conclude that despite the challenging conditions of warm ice, abundant water, and a short flow distance, many aspects of the microstructure evolved in systematic ways. In particular, measurable crystallographic fabric developed on Jarvis glacier. This finding is expected but incompletely demonstrated in natural systems. Our observations support the prediction that stronger fabrics exist in the margin of streaming ice where the ice is colder, contains less water, lies farther from source, and flows over a smoother basal boundary. In many cases, we expect those fabrics to exert a rheologically significant influence on flow dynamics.