ALBERT

Description: Insoluble and soluble impurities, enclosed in polar ice sheets, have a major impact on the deformation behaviour of the ice. Macro- and Micro-scale deformation observed in ice sheets and ice cores has been retraced to chemical loads in the ice, even though the absolute concentration is negligible. And therefore the exact location of the impurities matters: Allocating impurities to specific locations inside the ice microstructure inherently determines the physical explanation of the observed interaction between chemical load and the deformational behaviour. Both, soluble and non-soluble impurities were located in grain boundaries, triple junctions or in the grain interior, using different methods, samples and theoretical approaches. While each of the observations is adding to the growing understanding of the effect of impurities in polar ice, the growing number of ambiguous results calls for a dedicated and holistic approach in assessing the findings. Thus, we here aim to give a state of the art overview of the development in microstructural impurity research over the last 20 years. We evaluate the used methods, discuss proposed deformation mechanisms and identify two main reasons for the observed ambiguity: 1) limitations and biases of measurement techniques and 2) the physical state of the analysed impurity. To overcome these obstacles we suggest possible approaches, such as the continuous analysis of impurities in deep ice cores with complementary methods, the implementation of these analyses into established in-situ ice core processing routines, a more holistic analysis of the microstructural location of impurities, and an enhanced knowledge-transfer via an open access data base.

Description: Ever since the first deep ice cores were drilled, it has been a challenge to determine their original, in-situ orientation. In general, the orientation of an ice core is lost as the drill is free to rotate during transport to the surface. For shallow ice cores, it is usually possible to match the adjacent core breaks, which preserves the orientation of the ice column. However, this method fails for deep ice cores, such as the EastGRIP ice core in Northeast Greenland. We provide a method to reconstruct ice core orientation using visual stratigraphy and borehole geometry. As the EastGRIP ice core is drilled through the Northeast Greenland Ice Stream, we use information about the directional structures to perform a full geographical re-orientation. We compared the core orientation with logging data from core break matching and the pattern of the stereographic projections of the crystals’ c-axis orientations. Both comparisons agree very well with the proposed orientation method. The method works well for 441 out of 451 samples from a depth of 1375–2120 m in the EastGRIP ice core. It can also be applied to other ice cores, providing a better foundation for interpreting physical properties and understanding the flow of ice.

Description: Insoluble and soluble impurities, enclosed in polar ice sheets, have a major impact on the deformation behaviour of the ice. Large-, Macro and Micro-scale deformation observed in ice cores has been retraced to impurities in the ice, and therefore the exact location of the impurities is important. Over the years different techniques have been developed to analyse the location, and chemistry, of impurities and the following figures are examples of general methodological concepts. 1_cloudy_bands_details.jpg displays a sample from a depth of 1917 m from the East Greenland Ice Core Project (EGRIP) deep ice core, NE-Greenland; the drilling has started in 2016 and is still on-going. A) shows a Visual Stratigraphy image, which was taken in June 2019 by Nicolas Stoll at the EGRIP drill site. It shows cloudy bands and the stratigraphy of the ice sample, covering a few years of age, and is used to count annual layers, and to derive information about the deformation of ice. B) and C) are microstructure maps of the same sample, created by Nicolas Stoll in July 2020 at the Alfred Wegener Institute, Bremerhaven following the technique described by Kipfstuhl et al. (2006). These show grain sizes- and shapes, grain boundaries and sub-grain boundaries in detail to derive information about the deformation of ice. 4_microstructure.jpg are microstructure maps of an EGRIP ice core sample from a depth of 757.4 m, created by Nicolas Stoll in July 2020 at the Alfred Wegener Institute, Bremerhaven following the technique described by Kipfstuhl et al. (2006). These show grain sizes- and shapes, grain boundaries, sub-grain boundaries, and micro-inclusions in detail to derive information about the location of inclusion in ice. 5_SEM_EDML.jpg are Scanning Electron Microscope images taken by Ilka Weikusat at the University of Utrecht in 2011. The sample from a depth of 2035.9 m is from the European Project for Ice Coring in Antarctica in Dronning Maud Land (EDML) deep ice core, drilled between 2001 and 2006 at Kohnen Station, Antarctica. The SEM-images were created in preparation for Electron backscatter diffraction measurements and show ice grain boundaries, frost and a filament at a grain boundary. M. Drury and G. Pennock provided the infrastructure for Cryo-SEM work, which was conducted at EM square, the electron microscopy facility of Utrecht University Weikusat et al. (2017). 6_Ramam_multiplot.jpg were created by Nicolas Stoll in March 2020 at the Alfred Wegener Institute, Bremerhaven using Cryo-Raman spectroscopy to identify the chemistry of micro-inclusions in an ice sample from a depth of 757.4 m from the EGRIP ice core, NE-Greenland. A) and B) show the location of micro-inclusions and C) is the derived Raman spectra of the micro-inclusion in B), which was identified as Quartz.

Description: Fabric Analyser data was obtained from 744 vertical thin sections (ca. 6.7 x 9 cm) 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 of 2017 and 2018. 55 cm long samples were cut into six thin sections and measurements were later background corrected and processed with the program cAxes. The stated depth (in m) of the section describes the sections' top, center, and bottom. The derived data is the number of grains, mean grain size (in pixel and μm^2), sum vector norm, regelungsgrad, regelungsgrad weighted, concentration parameter, concentration parameter weighted, spherical aperture, spherical aperture weighted, three eigenvalues (e1, e2, e3, and weighted e1, e2, e3), woodcock parameter and woodcock parameter weighted.