Dynamic recrystallization and energy balance within glacier shear margins

Abstract:

Most of the mass loss from the Antarctic Ice Sheet occurs through fast-flowing glaciers and ice streams. Shear margins separate fast-flowing ice within the ice stream from the near-stagnant ice in the ridges and therefore exhibit rapid rates of deformation. The work required to deform ice drives thermomechanical processes that govern the temperature and viscosity of ice, and subsequently the speed of further deformation. While in theory, deformational work is partitioned into both heat and grain-scale processes (such as dynamic recrystallization and crystal growth), current research is based on the assumption that all of the work is dissipated as heat, effectively neglecting other changes in internal energy that arise from deformation. Here, we derive an idealized thermomechanical model for steady-state ice temperature and grain size, including a novel parameterization for migration recrystallization in deforming glacier ice, to study the partitioning of deformational work between heat dissipation and other changes in internal energy, namely stored elastic and surface energy. Under the conditions common in ice stream shear margins, we find that approximately half of the deformational work is dissipated as heat, with the remainder stored as elastic energy (associated with dislocations). We thus estimate lower ice temperatures in ice stream shear margins than has been found previously, along with coarse (~20 mm) average grain size that increases with depth. This suggests a different picture of the ice stream energy budget within shear margins from that commonly presumed. Since deformational energy can be characterized as a combination of changes in thermal energy, surface energy, and elastic energy, changes in elastic strain energy likely play an important a role in the dynamics of glacier shear margins. Thus, dynamic recrystallization may be a key process in glacier dynamics that is poorly understood and not captured in current ice flow models.