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Dynamic implications of discontinuous recrystallization in cold basal ice: Taylor Glacier, Antarctica

D. Samyn,1 A. Svensson,2 and S. J. Fitzsimons3

1  Department des Sciences de la Terre et de l’Environment, Faculte des Sciences, Universite de Bruxelles, Brussels, Belgium.

2  Department of Geophysics, Niels Bohr Institute for Astronomy, Geophysics, and Physics, University of Copenhagen, Copenhagen, Denmark.

3  Department of Geography, University of Otago, Dunedin, New Zealand.


Abstract

Crystallographic investigations have been conducted of cold (-17°C) debris-bearing ice from the base of an Antarctic outlet glacier (Taylor Glacier). The 4-m-thick sequence studied has been retrieved from a 20-m-long tunnel dug from the glacier snout and has been analyzed with an automatic ice fabric analyzer (AIFA). The top and bottom of the sequence consists of clean meteoric ice (englacial facies), whereas alternating debris-rich and clean bubbly ice layers are found in the middle part (stratified facies). Ice from the englacial facies displays a polygonal texture and a strong c-axis clustering toward the vertical, denoting recrystallization through ‘‘subgrain rotation’’ (SGR). In contrast, clean ice from the stratified facies shows SGR fabrics which are delimited at the contact with debris-rich layers by large, interlocking grains organized in ribbons. These two distinct textures within the stratified facies are associated with looser c-axis patterns at the scale of single thin sections, which is interpreted as resulting from ‘‘migration recrystallization’’ (MR). The change from SGR to MR trends marks a clear increase in grain boundary and nucleation kinetics (hence the term ‘‘discontinuous recrystallization’’) and may be associated with strain localization at rheological interfaces during basal ice genesis. Analogies with bottom ice from deep polar ice sheets, where temperature is commonly higher than at the studied site, are highlighted. Two recrystallization scenarios are proposed, accounting for the development of both types of fabrics. It is shown that by controlling the repartition of stress and strain energy within basal ice, the rheology of debris-bearing ice layers plays a decisive role in recrystallization dynamics at structural interfaces. We also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate ice sheets, provided that strain accumulation has been high enough in the former. This challenges the common belief that migration fabrics observed in bottom ice from deep ice sheets are exclusive to warm, stagnant, annealed ice.

 
Reference

Samyn, D., Svensson, A. and Fitzsimons, S. J. 2008. Dynamic implications of discontinuous recrystallization in cold basal ice: Taylor Glacier, Antarctica. Journal of Geophysical Research. 113, F03S90, doi:10.1029/2006JF000600. [pdf 1690Kb]

 

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