ALBERT

Description: Mean ice grain size and crystal-preferred orientation data was obtained from vertical thin sections (ca. 6.7 x 9 cm²) of the East Greenland Ice Core Project (EGRIP) ice core to analyze the microstructural evolution and internal deformation with depth. Vertical ice thin sections were prepared and measured with the Fabric Analyzer G50 (resolution: 20 μm/pixel) in the field at the EGRIP site in the summer seasons between 2017 and 2019. 55 cm long sample were cut into six thin sections and used to calculate mean grain size values for these 9 cm long sections with the program cAxes for the upper 1340 m of the ice core. The stated depth of the section represents the center of the section. At depths of 138.92 m, 276.88 m, 415.3 m, 514.48 m, 613.3 m, 757.21, 899.94 m, 1062.65 m, 1141.2 m, 1256.98 m, and 1339.75 m the orientation of each crystal in the representitive thin section was measured with the Fabric Analyzer G50 to analyze the internal deformation at those depths. Crystal orientations per sample are represented in eleven c-axes stereo plots from those depths. Stereoplots are not geographically truly orientated due to loss of orientation during ice core retrieval.

Description: 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.