| Summary: | Support is provided to allow the PIs to understand and model, through an improved understanding of grain boundary migration processes in deforming ice, the following phenomena: 1. The survival of extensive deformation and domination of the microstructure at depth of favorably-oriented grains, 2. The growth of slipping grains in the direction of shear, and whether this is a function of the relative orientations of neighboring grains or a fundamental characteristic of slipping grains, and 3. The effects of pressure on the rate and/or balance of the processes responsible for fabric development at depth. Their working hypothesis is that slipping grains can propagate without requiring recrystallization, and that this process is closely associated with anisotropy in grain boundary mobility during deformation. To seek evidence of these mechanisms, they propose the following series of laboratory observations on granular freshwater ice: 1. Conduct confined compression experiments at low deviatoric stresses and a range of pressure and temperature to develop a preferred c-axis orientation and cause growth. 2. Use standard techniques to document c-axis orientations, grain size and shape of the deformed microstructures for several temperatures and shear stress levels. 3. Employ electron backscatter pattern (EBSP) analysis to document a- and c-axis orientations of slipping and abutting grains in the deformed material. 4. Use surface etching to identify slip line arrays in the deforming grains, and couple these observations with the grain boundary structures observed in the EBSP results. The fundamental understanding derived from this study is anticipated to foster improved understanding of paleoclimate as derived from the interpretation of ice cores, as well as improved models of glacier and ice sheet mechanics.
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