Inferring ice rheology in Antarctic ice shelves using remotely-sensed surface velocity and ice thickness observations

Abstract:

Glaciers and ice sheets flow as a consequence of ice rheology. At the temperatures and pressures found on Earth, several creep mechanisms allow glacier ice to flow as a non-Newtonian (shear-thinning) viscous fluid. A semi-empirical constitutive relation known as Glen’s Flow Law is often used to describe ice flow and to provide a simple expression for an effective viscosity that decreases with increasing stress and deformation rate. Glen’s Flow Law is a power-law relation between effective strain rate and deviatoric stress, with two parameters defining the rheology of ice: a rate factor, A, and stress exponent, n. The rate factor depends on features such as temperature and grain size, while the stress exponent is primarily representative of the creep mechanism. Neither A nor n are well constrained in natural ice, and the stress exponent is typically assumed to be n=3 everywhere. Here, we take advantage of recent improvements in remotely-sensed observations of surface velocity fields to infer the values of A and n in Antarctic ice shelves. We focus on areas of ice shelves that flow in a purely extensional regime, where extensional stresses are proportional to observed ice thickness, drag at the base of the ice is negligible, and extensional strain-rates are calculated from the gradients of observed surface velocity fields. In this manner, we use independent observational datasets to derive spatially dependent constraints on A and n. The robust spatial variability provides insights into the temperatures, grain sizes, and creep mechanisms of ice, using Glen’s Flow Law to interpret the flow of ice shelves. Notably, our analysis indicates that n > 3, and is often closer to 4, in the regions applicable to our framework. Our results suggest that dislocation creep is the dominant creep mechanism in most areas of Antarctic ice shelves. This analysis unites theoretical work and synoptic-scale observations of ice flow, providing insights into the rheology and stress-states of ice shelves in Antarctica.