Abstract
Abstract. Basal crevasses extend upwards from the base of ice bodies and can penetrate more than halfway through the ice column under conditions found commonly on ice shelves. As a result, they may locally modify the exchange of mass and energy between ice shelf and ocean, and by altering the shelf's mechanical properties could play a fundamental role in ice shelf stability. Although early studies revealed that such features may be abundant on Antarctic ice shelves, their geometrical properties and spatial distribution has gained little attention. We investigate basal crevasses in Larsen C Ice Shelf using field radar survey, remote sensing and numerical modelling. We demonstrate that a group of features visible in MODIS imagery are the surface expressions of basal crevasses in the form of surface troughs, and find that basal crevasses can be generated as a result of stresses well downstream of the grounding line. We show that linear elastic fracture mechanics modelling is a good predictor of basal crevasse penetration height where stresses are predominantly tensile, and that measured surface trough depth does not always reflect this height, probably because of snow accumulation in the trough, marine ice accretion in the crevasse, or stress bridging from the surrounding ice. We conclude that all features visible in MODIS imagery of ice shelves and previously labelled simply as "crevasses", where they are not full thickness rifts, must be basal crevasse troughs, highlighting a fundamental structural property of many ice shelves that may have been previously overlooked.
Highlights
Basal crevasses are fractures that extend upwards from the bottom of ice bodies (Jezek, 1984; van der Veen, 1998a)
Using geophysical survey and remote sensing on the Larsen C Ice Shelf, we have demonstrated that a group of features visible in MODIS images, and previously described as “crevasses”, are completely explained as the surface expressions of basal crevasses in the form of surface troughs
We have shown that such features cannot be surface crevasses because they would not be resolved at the spatial resolution of MODIS
Summary
A better understanding of where basal crevasses may form, and how far they are likely to penetrate is desirable for future prediction of ice shelf dynamics and stability. The approach adopted by Nick et al (2010) assumes that the crevasses are closely-spaced and uses the zero stress criterion in which the balance of tensile stress, water pressure and lithostatic stress is considered. We found this method to significantly under-predict the penetration heights of both series of crevasses investigated in this study, possibly because they are more than 1 km apart (over three times the ice thickness) and cannot be considered to be “close”. The crevasses will have been advected downflow since their initiation, we choose to compare measured to modelled penetration heights using the tensile stress calculated at their present locations. The modelling approach we have taken considers only mode I stresses and does not take into account such shear processes, which may act to further propagate basal crevasses in regions with a non-simple stress regime
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