Permeability Anisotropy of Foliated Glacier Ice

Abstract

Within the temperate ice of ice stream shear margins, high strain and accompanying recrystallization likely result in longitudinal foliation characterized by thin, steeply dipping ice layers with distinct variations in grain size and bubble content. The sensitivity of ice permeability to these factors, particularly grain size, implies that foliation causes shear-margin ice to be hydraulically anisotropic. In this study, the permeability of foliated ice is measured in disks cut from cores from Athabasca Glacier, allowing permeability anisotropy to be assessed. We collected cores oriented normal and parallel to foliation from beneath the weathered crust of the glacier. Permeability values range from approximately $6\times 10^{-13}\text{to}10^{-16}$ m² and correlate with the textures and orientations of foliation layers. Results indicate that the anisotropic permeability of foliated ice can be approximated using a model that incorporates an empirical grain-size/permeability relationship and a model of vein clogging by air bubbles. For water flow parallel to foliation, the arithmetic mean of the area-weighted permeability closely approximates the bulk permeability; for flow perpendicular to foliation, measurements agree with the harmonic mean permeability, weighted to the thickness of each layer. These findings imply hydraulic anisotropy spanning several orders of magnitude in temperate glacier ice, with water flux governed by the most and least permeable layers in the flow-parallel and flow-perpendicular cases, respectively.