Crystal-orientation fabric variations on the cm-scale in cold Alpine ice: Interaction with paleo-climate proxies under deformation and implications for the interpretation of seismic velocities


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johanna.kerch [ at ] awi.de

Abstract

The macroscopic flow of a glacier is substantially influenced by the plastic anisotropy of individual ice crystals on the microscale. A preferred crystal orientation develops with depth in a glacier and is subjected to the influence of the temperature, deformation and recrystallisation regime as well as the climate-dependent impurity load in the ice. Detailed knowledge about the crystal anisotropy in a glacier is thus required to better constrain the response of ice sheets in a changing climate. While the gradual change in anisotropy on a large scale of tens to hundreds of metres can mostly be explained, this is not the case for changes in anisotropic fabric on a shorter scale of centimetres to decimetres. This work aims to improve the understanding of how and why the anisotropic crystal-orientation fabric (COF) changes on a short scale in a glacier. Fabric data from an ice core, drilled at the high-altitude Alpine site Colle Gnifetti, were measured in continuously sampled sections, covering 10 % of the entire core length. The distribution of crystal axes was analysed in high-resolution together with impurity data from meltwater analysis. It is found that the fabric anisotropy exhibits a strong variability on the short scale in all depths of the ice core with extreme eigenvalue differences within one metre of ∼ 0.2, often associated with small- or large-grained layers. The clear connection between the grain size variation and the impurity content leads to the conclusion that the influence of impurities on short-scale fabric variations is partially conveyed by the impurity-controlled grain size in combination with the local deformation regime. To further connect ice-core fabric data and COF measurements using seismics, a framework for the exact calculation of seismic phase velocities based on the asymmetric fabric distributions obtained from ice cores is developed and evaluated in two case studies.



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Thesis (PhD)
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Not peer-reviewed
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Published
Eprint ID
42799
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Kerch, J. (2016): Crystal-orientation fabric variations on the cm-scale in cold Alpine ice: Interaction with paleo-climate proxies under deformation and implications for the interpretation of seismic velocities PhD thesis, Universität Heidelberg.


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