Recrystallization in uniaxial compression tests on ice from the GRIP core, Central Greenland (Abstract)

Physics of Ice Core Records, Edited by T. Hondoh, Hokkaido University Press, p. 116, Sapporo 2000 

T. Thorsteinsson
Alfred-Wegener-Institut für Polar- und Meeresforschung, Sektion Geophysik, Postfach 120161, D-27515 Bremerhaven, Germany.
D. Dahl-Jensen
Departement of Geophysics, The Niels Bohr Institute of Astronomy, Physics and Geophysics, University of Copenhagen.

ABSTRACT.
Knowledge of the mechanical properties of ice in polar ice sheets is of vital importance for the interpretation of ice core records and for improved modelling of ice sheet flow and evolution. Results from comprehensive studies on textures and fabrics in the GRIP core have shown that the Central Greenland ice sheet becomes highly anisotropic with depth, and that the lowest 500 m of the ice sheet consists of interspersed layers of widely different crystal size, fabric strength and impurity content, leading to strong rheological contrasts.
To improve understanding of the flow properties of ice from different climatic periods, uniaxial compression tests have been carried out on a number of samples from the GRIP core. The tests are conducted at a temperature of -16 ^oC, with a fixed octahedral compression stress of 3 bars. All samples are compressed until tertiary strain rate has been reached.
Crystal sizes and c-axis orientation fabrics have been measured on the samples prior to testing and at various stages during compression. Initial crystal sizes vary between 3 mm and 16 mm, but these differences are strongly reduced as the samples have reached a tertiary strain rate. By then, most samples attain a grain size of 4-5 mm, which does not change markedly after that. The initial c-axis orientations also differ greatly, varying between random and strong single maximum fabrics. After tertiary strain has been reached, however, nearly all samples display a similar kind of girdle fabric, with most c-axes lying within 20^o-45^o from the axis of compression.
It appears that initial strain-rates during the tests are strongly influenced by the initial fabrics, but much less so by the initial crystal sizes. The increase in strain-rate when tertiary creep is reached can be attributed to recrystallization forming new crystals with orientations favourable for deformation in compression. Varying impurity content appears to influence the recrystallization processes taking place and thereby the flow properties. The relevance of these results for ongoing discussions of flow disturbances in the Central Greenland ice cores will be addressed.