title: Physical analysis of an Antarctic ice core-towards an integration of micro- and macrodynamics of polar ice.
abstract: Microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail. Ice core parameters (crystal lattice-preferred orientation (LPO), grain size, grain shape), mesostructures (visual stratigraphy) as well as borehole deformation were measured in a deep ice core drilled at Kohnen Station, Dronning Maud Land (DML), Antarctica. These observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the EDML ice core drilling site (European Project for Ice Coring in Antarctica in DML). The results suggest a division of the core into five distinct sections, interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock-parallel shear. Region 1 (uppermost approx. 450 m depth) with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization. Region 2 (approx. 450-1700 m depth) shows a girdle-type LPO with the girdle plane being perpendicular to grain elongations, which indicates triaxial deformation with dominating horizontal extension. In this region (approx. 1000 m depth), the first subtle traces of shear deformation are observed in the shape-preferred orientation (SPO) by inclination of the grain elongation. Region 3 (approx. 1700-2030 m depth) represents a transitional regime between triaxial deformation and dominance of shear, which becomes apparent in the progression of the girdle to a single maximum LPO and increasing obliqueness of grain elongations. The fully developed single maximum LPO in region 4 (approx. 2030-2385 m depth) is an indicator of shear dominance. Region 5 (below approx. 2385 m depth) is marked by signs of strong shear, such as strong SPO values of grain elongation and strong kink folding of visual layers. The details of structural observations are compared with results from a numerical ice sheet model (PISM, isotropic) for comparison of strain rate trends predicted from the large-scale geometry of the ice sheet and borehole logging data. This comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet.This article is part of the themed issue 'Microdynamics of ice'.
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany ilka.weikusat@awi.de.|Department of Geosciences, Eberhard Karls University, Tübingen, Germany.
authors.forename: Ilka
authors.identifier: http://orcid.org/0000-0002-3023-6036
authors.initials: I
authors.lastname: Weikusat
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany.
authors.forename: Daniela
authors.initials: D
authors.lastname: Jansen
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Binder
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Eichler
authors.affiliation: BC3-Basque Centre for Climate Change, Ikerbasque, Bilbao, Spain.|NUT-Nagaoka University of Technology Nagaoka, Niigata, Japan.
authors.forename: Sérgio H
authors.initials: SH
authors.lastname: Faria
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany.|Georg-August-Universität Göttingen, Göttingen, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Wilhelms
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany.
authors.forename: Sepp
authors.initials: S
authors.lastname: Kipfstuhl
authors.affiliation: CIC, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Simon
authors.initials: S
authors.lastname: Sheldon
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany.
authors.forename: Heinrich
authors.initials: H
authors.lastname: Miller
authors.affiliation: CIC, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Dorthe
authors.initials: D
authors.lastname: Dahl-Jensen
authors.affiliation: AWI-Glaciology, Alfred-Wegener-Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Kleiner
keywords: borehole deformation; fabric; ice flow modelling; microstructure; polar ice core; texture