The effect of uncertainties in creep activation energies on modeling ice flow and deformation

Abstract

Ice deformation is commonly represented by a power-law constitutive relation, Glen's Flow Law, where deformation (strain) rate equals stress raised to the power n and multiplied by a flow-rate parameter A. Glen's Law represents bulk ice rheology as a single power-law even though multiple mechanisms, each with their own power-law relation and parametric values, act together during viscous deformation (creep) of ice. The relative importance of different creep mechanisms in naturally- deforming ice sheets controls the parameters n and A in Glen's Flow Law. We couple a composite flow law that explicitly represents individual deformation mechanisms with models for ice temperature and grain size to estimate the dominant deformation mechanism in the Antarctic Ice Sheet. We demonstrate that uncertainties in creep activation energies produce significant uncertainties in the dominant deformation mechanism, and thus values of A and n. Minor variations in the activation energy values (<10% or <5 kJ/mol) can change the dominant creep mechanism, causing n to vary between 1.8<n<4. We propose a way of using observational inferences of the stress exponent n to recalibrate activation energy values in ice sheet models. This enables an improved understanding of the fundamental mechanisms of ice deformation and the controls on ice flow.