A first glimpse into the EGRIP ice core: An analysis of the influence of deformation and recrystallisation on fabric and microstructures of the Northeast Greenland Ice Stream


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Ilka.Weikusat [ at ] awi.de

Abstract

Global sea level has been rising over the last century, and one of the contributors and the main source of projection uncertainties is ice sheet mass loss by solid ice discharge. Projections currently lack sufficient confidence, partly due to the difficulty in simulating ice flow behaviour, which is highly influenced by deformation modes and the physical properties of ice, such as grain microstructure and c-axis orientation anisotropy. This thesis aims to deliver an overview about the deformation regimes and microstructural properties, as well as crystal-preferred orientation (CPO) anisotropy, of the Northeast Greenland Ice Stream (NEGIS) by examining an ice core from the East Greenland Ice Core Project (EGRIP). Ice streams are major features to conduct the discharge from inland ice towards the coasts and NEGIS is the largest and most dominant one in Greenland. Therefore, microstructure and fabric data from almost 800 thin sections were analysed by an automated Fabric Analyser and a Large Area Scanning Macroscope. The result is an almost continuous record of the physical properties of the upper 1714m of the ice core. The major findings regarding crystal-preferred orientations are (1) a much more rapid evolution of c-axes anisotropy in shallow depths compared to lower dynamics sites and (2) partly novel characteristics in the CPO patterns. These findings are accompanied by highly irregular grain shapes, the regular occurrence of protruding grains and further indicators for an early onset of dynamic recrystallisation. Grain size values are similar to results from other ice cores and show an increase in grain size, followed by a strong decrease in the Glacial. Until a depth of 196 m, a broad single maximum CPO was observed, indicating vertical compression from overlaying layers. A crossed girdle of Type I and Type II, observed in natural ice for the very first time, dominates until 294 m, probably caused by a fluctuation between non-coaxial and coaxial deformation, accompanied by simple shear and the activation of multiple slip-systems. Between 294 and 500m a transition into a vertical girdle CPO occurs. Extensional deformation along flow leads to a distinct vertical girdle between 500 and 1150 m. This CPO pattern develops into a horizontal maxima CPO, also observed as a novel feature in polar ice, which is probably caused by additional simple shear. This new microstructure and fabric information improves our understanding of ice dynamics, and should be considered in future ice flow law parameterisations to improve ice-sheet models.



Item Type
Thesis (Master)
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Primary Division
Programs
Primary Topic
Research Networks
Peer revision
Not peer-reviewed
Publication Status
Published
Eprint ID
49287
Cite as
Stoll, N. (2019): A first glimpse into the EGRIP ice core: An analysis of the influence of deformation and recrystallisation on fabric and microstructures of the Northeast Greenland Ice Stream , Master thesis, University Bremen FB5.


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