ALBERT

Description: Mass loss of the Greenland ice sheet is accelerating, which is attributed to increased ice stream discharge and changes in surface mass balance including increased runoff. Ice stream discharge is caused by both ice deformation and basal sliding. For a better projection of future mass loss, it is important to understand deformation mechanisms of polycrystalline ice in ice sheet. Deformation properties of polycrystalline material are related to its microstructure (e.g. crystal grain orientation and size). As recrystallization and recovery are occurring together in ice sheet, microstructural analysis of ice is essential. Electron backscatter diffraction (EBSD) is a method for measuring crystal lattice orientation with high angular and spatial resolutions. Both c- and a-axes of ice can be measured. We analyzed Greenland NEEM ice core and the preliminary result shows that most subgrain boundaries (SGBs) observed by optical microscopy have lattice misorientations 〈 4°. This result is in accordance with analyses of Antarctic EDML ice core by X-ray diffractometry while it differs from threshold angle of SGB/GB estimated with a dislocation theory. The observation results from ice sheet ice could contribute to better estimations of strain rate by models based on microstructural processes.

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: Here we present the ice microstructure and CPO (c-axes fabric) data from the upper 2121 m of the EastGRIP ice core, an on-going deep drilling project on the North East Greenland Ice Stream. Understanding ice flow behaviour of fast flowing ice streams is crucial for accurate projections of future global sea level rise, but is still poorly understood due to e.g. missing observational fabric data from ice streams. The presented CPO patterns found at EastGRIP show (1) a rapid evolution of c-axes anisotropy compared to deep ice cores from less dynamic sites, (2) a CPO evolution towards a strong vertical girdle and (3) CPO patterns that have not previously been directly observed in ice. Furthermore, data regarding grain properties (e.g. grain size) and indications of dynamic recrystallization, already at shallow depths, are presented. The ice CPO shows a clear evolution with depth. In the first measurements at 111 m depth a broad single maximum distribution is observed, which transforms into a crossed girdle CPO (196-294 m). With increasing depth, an evolution towards a vertical girdle c-axes distribution occurs. Below 1150 m the CPO evolves into a vertical girdle with a higher density of c-axes oriented horizontally, a novel CPO in ice. These CPO patterns indicate a depth-related change in deformation modes, from vertical compression to extensional deformation along flow. Grain size values are similar to results from other Greenlandic deep ice cores. Grain size evolution is characterized by an increase until 500 m depth, a decrease until 1360 m depth and mainly constant values in the Glacial. These findings are accompanied by indications of an early onset of dynamic recrystallisation e.g. irregular grain shapes, protruding grains and island grains. The presented high-resolution data enable, for the very first time, a detailed and data- based look into a fast-flowing ice stream and are an important step towards a better understanding of the rheology of ice and its flow behaviour.

Description: Mass loss of the Greenland ice sheet is accelerating, which is attributed to increased ice stream discharge and changes in surface mass balance including increased runoff. Ice stream discharge is caused by both ice deformation and basal sliding. For better projection of future mass loss, it is important to understand deformation mechanisms of polycrystalline ice in ice sheet. Deformation properties of polycrystalline material are related to its microstructure (e.g. crystal grain orientation and size). As recrystallization and recovery are occurring together in ice sheet, analysis of microstructure of ice is essential. Electron backscatter diffraction (EBSD) is a method for measuring crystal lattice orientation with high angular and spatial resolutions. Both c- and a-axes of ice can be measured. We analyzed Greenland NEEM ice core and the preliminary result shows that most subgrain boundaries (SGBs) observed by optical microscopy have lattice misorientations 〈 4°. This result is in accordance with analyses of Antarctic EDML ice core by X-ray diffractometry while it differs from threshold angle of SGB/GB estimated with a dislocation theory. The observation results from ice sheet ice could contribute to better estimations of strain rate by models based on microstructural processes.

Description: Behavior of Earth’s ice sheets is intensely monitored via surface and remote sensing techniques to improve predictions of sea level evolution. Radio echo sounding along with drilling ice cores are currently used to monitor the Earth’s ice sheets behaviour not only from the surface but in the 3rd dimension. Particularly the ice material properties can only be accessed via ice drilling. These properties control the deformation in general, and particularly 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, http://eastgrip.org) drilling started in 2016 and will be ongoing until 2019. This is the first chance to study ice microstructure from a dynamically active region (www.awi.de/en/focus/eisschilde/eis-ist-ein-heisses-material.html), with a deformation regime differing from the usual locations of previous long ice cores. Those were usually placed on domes or on ice divides due to straightforward kinematics and deformation rates which is advantageous for paleo-climate reconstruction from ice core records. We present CPO (c-axes fabric) and the grain size measurements of the uppermost 350m, the depth to which the ice core has been processed for analysis so far (275 thin sections discontinuous with 10m depth resolution). The CPO patterns found in the upper 350m 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 caxes distributions. (Remark: This is an invited poster.)

Description: New and more detailed investigations from the EGRIP physical properties dataset down to 1650m of the ice core will be presented. EGRIP is the first deep ice core through one of our Earth’s ice sheets partly motivated by ice dynamics’ research. It is drilled just downstream of the onset of the largest ice stream in Greenland (North East Greenland Ice Stream). Data processing of the collected ice core physical properties data was done at the Alfred Wegener Institute Helmholtz Centre for Marine and Polar Research. The two main findings regarding CPO (c-axes fabric) pattern, 1) a rapid evolution of c-axes anisotropy and 2) partly novel characteristics, were further, and in more detail, investigated. To gain a better understanding of the dominating deformation mechanisms of NEGIS, different approaches considering different length scales were chosen (1650m versus 0.55m and 0.09m scale), including several case studies. A large-scale statistical analysis of the entire dataset results in new information about the depth-dependent evolution of parameters as for example the strength of c-axes anisotropy and grain-size in the polycrystal. In general, mean grain-size decreases with depth as we drill through the Holocene ice and approach the Glacial material. The grain size variability with fine and coarse grain layers is extreme in the Holocene ice but decreases in the Glacial ice. Microstructure properties were examined, with the aim to investigate the relationship between the remarkable rapid evolution of CPO-pattern and grain properties evolution. Furthermore, the evolution of a grain-size dependent anisotropy, found in the first 350m of the ice core, is investigated and examined also in deeper sections of the core. The large-scale evolution of density distributions of c-axes orientations differ significantly from observations in deep ice cores made so far: A novel "hourglass shaped" girdle was observed, characterized by a high density of horizontally oriented c-axes within the vertical girdle. In some parts of the core, this shape develops into a "butterfly shaped" cross girdle, varying in intensity and strength. It is the first time that this cross girdle was observed in polar ice and by combining approaches considering different length scales, we aimed to verify one of our three hypotheses for its origin: a) activation of multiple dislocation slip systems (in analogy to quartz), b) a memory effect or reminiscent features from older deformation modes, further upstream or even outside of NEGIS or c) horizontal uniaxial extension causing an early onset of dynamic migration recrystallization. Small-scale high-resolution studies were carried out on several selected bags (0.55m long) from different depth regimes and were chosen as case studies to better understand the formation mechanisms of the novel CPO patterns found in the EGRIP ice core. One approach is the examination of thin sections (9 x 7 x 0.03cm) regarding the occurrence of patches of grains with similar orientations, which was observed in several samples from different depths. Small grains with similar orientations seem to cluster around large grains with a different orientation. High-resolution images (5 to 20µm/pix), derived with a Large Area Scan Macroscope (LASM), enable detailed investigations of grain shape, grain boundaries and sub-grain boundaries and therefore the possibility to find distinct features from deformation and recrystallisation in the microstructure. Grains are rarely horizontally aligned and usually show irregular, circular or rectangular shapes rather than elongated shapes. Characteristic for our case studies are also amoeboid grain shapes and sutured grain boundaries, typical features of grain boundary migration. Furthermore, layering, "sandwiched grains" and strong gradients in grain-size over distances of only a few centimetres were observed in several samples. Although still under progress, at the current state of investigation, combining fabric data and microstructure analysis, the novel CPO patterns found in the EGRIP ice core are strongly influenced by dynamic migration recrystallisation.

Description: We will present the EGRIP CPO (c-axes fabric) dataset and give preliminary interpretations concerning the processes leading to its evolution. 120 bags were selected, with a minimum depth resolution of 15m. Bags were mostly measured continuously, and in total 778 thin sections were prepared, measured and pre-processed on site. Thus, c-axes distribution CPO data are already available, while other parameters on grain stereology are still to be processed at this stage. The CPO patterns found in the upper 1650m at EGRIP show (1) a rapid evolution of c-axes anisotropy compared to lower dynamics sites and (2) partly novel characteristics in the CPO patterns. (1) Starting the measurements at 118m of depth we find a very broad single maximum distribution. The c-axes align with depth in the upper 400m much more rapidly than seen in ice cores from divides or domes. Down to only 140m depth the almost random CPO develops into a very broad single maximum which is similar to those CPOs found in the shallowest samples of other ice cores. Possible interpretations of these distributions are deformation by vertical compression from overlying layers, or alternatively a temperature-gradient snow metamorphosis. This weak CPO pattern is, however, quickly overprinted in the depth zone below 140m where a progressive evolution towards a vertical girdle distribution is observed. As vertical girdles are produced by extension along flow, the observed distribution indicates that the ice at this depth is deforming rather than just being translated by rigid block movement. From approximately 600m of depth downward we observe crystal orientation anisotropy of a strength comparable to samples from ~1400m of depth at divides (NEEM and EDML). This strong girdle CPO remains rather stable down to approximately 1300m depth, where we reach the ice deposited during the last glacial period. A novel pattern, not observed before in natural ice, is a higher densities of c-axes horizontally oriented within the vertical girdle. (2) The early onset of deformation seems further supported by the observation of a broad “hourglass shaped” girdle, which seems to develop in some depths into a “butterfly shaped” cross girdle. Another characteristic deserves attention: the distribution density within the girdle. In contrast to observations in deep ice cores so far, the highest density seems to deviate from the vertical direction being (sub-)parallel to the horizontal. The origin of this may lay in the main deformation modes, e.g. a combination of along flow extension with additional deformation modes. Especially interesting is the cross girdle, which has not yet been observed in polar ice cores so far. We suggest three possible interpretations for its origin: a) In other materials, such as quartz, cross girdles can be interpreted as activation of multiple dislocation slip systems. b) Alternatively, the CPO pattern may reflect reminiscent features from previous deformation modes, which the ice experienced upstream or possibly even outside of the ice stream. This memory effect would point to a relevance of strain dependence of the CPO. c) The cross- /double-girdle might be caused by the early onset of dynamic migration recrystallization under horizontal uniaxial extension.