Description: Subgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization. Statistical data obtained from the EPICA Dronning Maud Land (EDML) deep ice core drilled in East Antarctica cannot detect a depth region of increased subgrain-boundary formation. Grain-boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths. The data do not support the classical view of a change of dominating recrystallization regimes with depth. Three major types of subgrain boundaries, reflecting high mechanical anisotropy, are specified in combination with crystal-orientation analysis.

Description: With the warming climate, ice masses on Earth are expected to increasingly contribute to a rising sea level. As for any material, the ice bodies’ temperature is a key variable to change the material’s properties, especially the rheology. In the case of ice in natural environments on Earth, temperature is always close to the material’s melting point. Therefore ice can be regarded as a ‘hot material’ (homologous temperatures T/T_m ca. 0.7 to 0.9). This means that recrystallization plays a decisive role in governing the state and thus the behaviour of the material, as it continuously resets the mechanical properties. Recrystallization as a set of control mechanisms has been recognized and interpreted in many ice cores in the last decades, and certain recrystallization regimes have been assigned to special ice-sheet depth ranges. This assignment was based on microstructure observations (mainly grain size) and estimated boundary conditions (temperature and stress/strain amounts) which change systematically with depth. To generalize the use of recrystallization regimes we decouple their occurrence from the ice-sheet depth information and connect them directly to the activators and causes: strain rate and temperature.

Description: ABSTRACT. We investigated the large-scale (10–1000m) and small-scale (mm–cm) variations in size, number and arrangement of air bubbles in the EPICA Dronning Maud Land (EDML) (Antarctica) ice core, down to the end of the bubble/hydrate transition (BHT) zone. On the large scale, the bubble number density shows a general correlation with the palaeo-temperature proxy, d18O, and the dust concentration, which means that in Holocene ice there are fewer bubbles than in glacial ice. Small-scale variations in bubble number and size were identified and compared. Above the BHT zone there exists a strong anticorrelation between bubble number density and mean bubble size. In glacial ice, layers of high number density and small bubble size are linked with layers with high impurity content, identified as cloudy bands. Therefore, we regard impurities as a controlling factor for the formation and distribution of bubbles in glacial ice. The anticorrelation inverts in the middle of the BHT zone. In the lower part of the BHT zone, bubble-free layers exist that are also associated with cloudy bands. The high contrast in bubble number density in glacial ice, induced by the impurities, indicates a much more pronounced layering in glacial firn than in modern firn.

Description: The behaviour of Earth’s ice sheets is intensely monitored via surface and remote sensing techniques to improve predictions of sea level evolution. In the 3rd dimension however, in particular concerning the ice material properties, this behaviour can only be studied via ice core drilling. Material properties control the deformation in general, and specifically the strain localization such as observed in ice streams, which supply the major discharges into the oceans. Currently, the first ice core on an active ice stream, the North-East Greenland Ice Stream (NEGIS) is being drilled. EastGRIP (East Greenland Ice-Core Project) drilling started in 2016 and will likely be ongoing until 2019. This is the first chance to study ice fabrics from a dynamically active region, with a deformation regime differing from the usual locations of previous long ice cores, which are usually situated on domes or on ice divides. We will present the results from the CPO (c-axes fabric) and the grain size measurements of the uppermost 350 m, which is the depth to which the ice core has been processed for analysis so far. 54 core pieces (bags) were selected for measurements, with a minimum depth resolution of 10 m. From these 275 thin sections were prepared in total, and measured and processed on site by means of an Automated Fabric Analyzer and a Large-Area-Scanning Macroscope (LASM). Mostly entire bags have been measured, to ensure constrains on small-scale variability with depth. The CPO patterns found in the upper 350 m at EastGRIP show (1) a more rapid evolution of c-axes anisotropy with depth compared to other ice cores and (2) partly novel characteristics in the c-axes distributions. (1) The microstructural measurements begin at a depth below the firn ice transition at 118 m. Starting with a very broad single maximum distribution, the alignment of the c-axes happens much more rapidly with depth than seen in ice cores from divides or domes. In our deepest samples available (350 m) we observe an anisotropy of a strength comparable to samples from ∼1000 m depth at for example GRIP, NEEM and EDML. (2) Between 118 m and 160 m depth the almost random to very broad single maximum is similar to shal- lowest samples in other ice cores. Classically, we interpret this distribution as a result of vertical compression caused by the weight of overlying layers. An alternative interpretation may be a snow metamorphosis influenced by the temperature gradient. This weak pattern is, however, quickly overprinted in 160 m to 200 m, where a progressive evolution to girdle distribution is observed. Such a vertical great girdle can evolve with extension along flow, and, thus, the observed distribution indicates that the ice at this depth is deforming under conditions close to pure shear, rather than being translated by rigid block movement. This early-onset of deformation seems further supported by the observation of a broad “hourglass shaped” girdle, developing into a “butterfly shaped” cross girdle, which is observed for the first time in ice.

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{\textendash}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{\textendash}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{\textendash}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 {\textquoteleft}Microdynamics of ice{\textquoteright}.

Description: Ice in polar ice sheets undergoes deformation during its flow towards the coast. Deformation and recrystallization microstructures such as subgrain boundaries can be observed and recorded using high-resolution light microscopy of sublimation-edged sample surfaces (microstructure mapping). Subgrain boundaries observed by microstructure mapping reveal characteristic shapes and arrangements. As these arrangements are related to the basal plane orientation, full crystallographic orientation measurements are needed for further characterization and interpretation of the subgrain boundary types. X-ray Laue diffraction measurements validate the sensitivity of different boundary types with sublimation used by microstructure mapping for the classification. X-ray Laue diffraction provides misorientation values of all four crystal axes. Line scans across a subgrain boundary pre-located by microstructure mapping can determine the rotation axis and angle. Together with the orientation of the subgrain boundary this yields information on the dislocation types. Tilt and twist boundaries composed of dislocations lying in the basal plane, and tilt boundaries composed of nonbasal dislocations were found. A statistical analysis shows that nonbasal dislocations play a significant role in the formation of all subgrain boundaries.

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.