Creep deformation and buttressing capacity of damaged ice shelves: theory and application to Larsen C ice shelf

C P Borstad,J Mouginot,M P Schodlok,E Rignot

doi:10.5194/tc-7-1931-2013

C P Borstad, J Mouginot + Show 2 more

Open Access

https://doi.org/10.5194/tc-7-1931-2013

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Abstract

Abstract. Around the perimeter of Antarctica, much of the ice sheet discharges to the ocean through floating ice shelves. The buttressing provided by ice shelves is critical for modulating the flux of ice into the ocean, and the presently observed thinning of ice shelves is believed to be reducing their buttressing capacity and contributing to the acceleration and thinning of the grounded ice sheet. However, relatively little attention has been paid to the role that fractures play in the ability of ice shelves to sustain and transmit buttressing stresses. Here, we present a new framework for quantifying the role that fractures play in the creep deformation and buttressing capacity of ice shelves. We apply principles of continuum damage mechanics to derive a new analytical relation for the creep of an ice shelf that accounts for the softening influence of fractures on longitudinal deformation using a state damage variable. We use this new analytical relation, combined with a temperature calculation for the ice, to partition an inverse method solution for ice shelf rigidity into independent solutions for softening damage and stabilizing backstress. Using this new approach, field and remote sensing data can be utilized to monitor the structural integrity of ice shelves, their ability to buttress the flow of ice at the grounding line, and thus their indirect contribution to ice sheet mass balance and global sea level. We apply this technique to the Larsen C ice shelf using remote sensing and Operation IceBridge data, finding damage in areas with known crevasses and rifts. Backstress is highest near the grounding line and upstream of ice rises, in agreement with patterns observed on other ice shelves. The ice in contact with the Bawden ice rise is weakened by fractures, and additional damage or thinning in this area could diminish the backstress transmitted upstream. We model the consequences for the ice shelf if it loses contact with this small ice rise, finding that flow speeds would increase by 25% or more over an area the size of the former Larsen B ice shelf. Such a perturbation could potentially destabilize the northern part of Larsen C along pre-existing lines of weakness, highlighting the importance of the feedback between buttressing and fracturing in an ice shelf.

Highlights

The majority of the Antarctic ice sheet drains to the ocean through floating ice shelves (Barkov, 1985), most of which are contained in embayments or run aground against ice rises, ice rumples or islands
Even though Cochran and Bell (2012) measured a maximum gravity anomaly associated with Bawden ice rise, this ice rise is not represented in the bathymetry data set used by the ocean circulation model
The analytical theory for the creep deformation of a floating ice shelf was extended using continuum damage mechanics to account for the softening influence of fractures on longitudinal deformation

Summary

Introduction

The majority of the Antarctic ice sheet drains to the ocean through floating ice shelves (Barkov, 1985), most of which are contained in embayments or run aground against ice rises, ice rumples or islands. Ice shelves play a major role in modulating the mass balance and contribution to sea level rise of the Antarctic ice sheet. This influence was brought into sharp focus following the collapse of the Larsen B ice shelf in 2002, after which the tributary glaciers that fed the shelf accelerated three- to eightfold (Rignot et al, 2004; Scambos et al, 2004) with sustained dynamic thinning and retreat ongoing (Rott et al, 2011).

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