Thermal controls on ice stream shear margins

Pierce Hunter,Marianne Haseloff,Colin Meyer,Alan Rempel,Brent Minchew

doi:10.1017/jog.2020.118

Abstract

AbstractIce stream discharge responds to a balance between gravity, basal friction and lateral drag. Appreciable viscous heating occurs in shear margins between ice streams and adjacent slow-moving ice ridges, altering the temperature-dependent viscosity distribution that connects lateral drag to marginal strain rates and ice stream velocity. Warmer ice deforms more easily and accommodates faster flow, whereas cold ice supplied from ice ridges drives advective cooling that counteracts viscous heating. Here, we present a two-dimensional (three velocity component), steady-state model designed to explore the thermal controls on ice stream shear margins. We validate our treatment through comparison with observed velocities for Bindschadler Ice Stream and verify that calculated temperatures are consistent with results from previous studies. Sweeping through a parameter range that encompasses conditions representative of ice streams in Antarctica, we show that modeled steady-state velocity has a modest response to different choices in forcing up until temperate zones develop in the shear margins. When temperate zones are present, velocity is much more sensitive to changes in forcing. We identify key scalings for the emergence of temperate conditions in our idealized treatment that can be used to identify where thermo-mechanical feedbacks influence the evolution of the ice sheet.

Highlights

Outlet glaciers are responsible for most of the ice discharge from Antarctica into the ocean (Rignot and others, 2011)
Vertical strain rates in ice streams are relatively small, so ice stream flow speed is well-approximated by the integrated lateral strain rate across the stream half-width, which highlights the importance of the viscosity structure in shear margins (Raymond, 1996; Schoof, 2010; Haseloff and others, 2015)
We present a case study of Bindschadler Ice Stream (BIS) – a ridge-controlled glacier with two distinct flow regimes – and find our model conforms well to surface data and results from previous studies (e.g. Alley and others, 2018; Meyer and others, 2018b)

Summary

Introduction

Outlet glaciers are responsible for most of the ice discharge from Antarctica into the ocean (Rignot and others, 2011). Ice flows from tributaries into ice streams that are separated by nearly stagnant ridges, and ice is advected from the ridges into the ice streams. The transition between these fast and slow flowing regions is called a shear margin due to the concentration of high shear strain rates (Raymond, 1996; Raymond and others, 2001; Schoof, 2004). Lateral advection of cold ice from the ridge to the stream reduces temperatures within the margin, resulting in higher ice viscosities that reduce strain rates and rates of meltwater generation (Haseloff and others, 2019). Vertical strain rates in ice streams are relatively small, so ice stream flow speed is well-approximated by the integrated lateral strain rate across the stream half-width, which highlights the importance of the viscosity structure in shear margins (Raymond, 1996; Schoof, 2010; Haseloff and others, 2015)

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Conclusion