Inferred basal friction and mass flux affected by crystal-orientation fabrics

Nicholas M Rathmann,David A Lilien

doi:10.1017/jog.2021.88

Abstract

We investigate the errors caused by neglecting the crystal-orientation fabric when inferring the basal friction coefficient field, and whether such errors can be alleviated by inferring an isotropic enhancement factor field to compensate for missing fabric information. We calculate the steady states that arise from ice flowing over a sticky spot and a bedrock bump using a vertical-slab numerical ice-flow model, consisting of a Weertman sliding law and the anisotropic Johnson flow law, coupled to a spectral fabric model of lattice rotation and dynamic recrystallisation. Given the steady or transient states as input for a canonical adjoint-based inversion, we find that Glen's isotropic flow law cannot necessarily be used to infer the true basal drag or friction coefficient field, which are obscured by the orientation fabric, thus potentially affecting vertically integrated mass fluxes. By inverting for an equivalent isotropic enhancement factor, a more accurate mass flux can be recovered, suggesting that joint inversions for basal friction and the isotropic flow-rate factor may be able to compensate for mechanical anisotropies caused by the fabric. Thus, in addition to other sources of rheological uncertainty, fabric might complicate attempts to relate subglacial conditions to basal properties inferred from an inversion relying on Glen's law.

Highlights

Basal drag provides an important resistive component in the force budget of glaciers and ice sheets, with implications for the accuracy of mass-loss projections and the dynamics of ice streams (Echelmeyer and others, 1994; Gillet-Chaulet and others, 2012; Larour and others, 2012; Morlighem and others, 2013; Schoof and Mantelli, 2021)
We investigate the effect of approximating ice as isotropic when inferring the basal friction coefficient field over a subglacial sticky spot and bedrock bump using the canonical method of an adjoint-based inversion
We investigated the isolated effect of neglecting the crystalorientation fabric when inferring the basal friction coefficient of a Weertman sliding law

Summary

Introduction

Basal drag provides an important resistive component in the force budget of glaciers and ice sheets, with implications for the accuracy of mass-loss projections and the dynamics of ice streams (Echelmeyer and others, 1994; Gillet-Chaulet and others, 2012; Larour and others, 2012; Morlighem and others, 2013; Schoof and Mantelli, 2021). Basal conditions near the onset of the largest ice stream in Greenland, the Northeast Greenland Ice Stream (NEGIS), have been extensively studied (Fahnestock and others, 2001; Christianson and others, 2014; Keisling and others, 2014; Beyer and others, 2018; Franke and others, 2021). In the upstream part of NEGIS, active-source seismics and radio-echo sounding indicate the ice rests on water-saturated till, with a strong connection to the subglacial water system of its onset, and might be controlled by variations in basal friction further downstream (Christianson and others, 2014; Keisling and others, 2014; Franke and others, 2021). The relatively smooth bed and direct evidence of subglacial sediment suggest that the Ross ice streams are likely areas where friction, rather than bedrock topography, is the primary cause of sticky spots (localised increased drag). Early studies suggested that bedrock highs are the most likely cause of sticky spots beneath these ice streams (e.g. Alley, 1993), more recent work has focused on the connection to till strength and water availability (Anandakrishnan and Alley, 1997; Tulaczyk and others, 2000)

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