ALBERT

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.

Description: The EastGRIP (East GReenland Ice core Project) Ice Core is drilled in a highly dynamic area, the North East Greenland Ice Stream (NEGIS), with a surface velocity of 55 m/yr, representing high flow rates in this area. All previous deep ice cores in Greenland were mainly drilled to find undisturbed ice for climate reconstruction. In contrast, the main purpose of this ice core is to increase our understanding of ice flow and linking it to deformation features shown in the physical properties. Some of these deformation features can be made visible using the line scanner device. It scans a polished ice core slab illuminated by an indirect light source (similar to dark field microscopy) and thus makes internal features (e.g. impurities, bubbles, hydrates and partly grain boundaries) visible, creating a 10x165 cm image of the core. Light traveling though the core is reflected and scattered at these features thus causing the camera to detect a bright section where the impurity content is high (“cloudy bands”), whereas ice with a low impurity concentration will not reflect light and contribute a dark layer in the image (“clear bands”). This is used to make layering, i.e. the stratigraphy, visible. Ice from the last glacial period has a well layered stratigraphy resulting from fairly regular annual dust storms in spring to summer. As deformation increases and deformation modes change towards the bottom of the core, these layers will show disturbances and folding. A strong relationship between the δ18O of the water isotopes in the ice core and the impurity concentration, derived from the visual intensity of different layers, can be observed. The correlation of these two makes way for a very precise correlation of the visual stratigraphy and their δ18O age, to analyze differences in deformation modes of ice from different climatic periods. Main deformation in the upper part of the ice sheet is pure shear (stretching along the horizontal and thinning in the vertical) and simple shear in the bottom parts. The gradual change from pure to simple shear is seen in the development of small scale disturbances in the layers, such as wavy patterns. The evolution of these features into z- and s-folds is expected in greater depth. Hereby the layer will deform into a z- or s-shape, overturning a section of the layer. Wavy features, such as the ones seen here, have not been observed in other cores and could be associated to the highly dynamic drill site in NEGIS.

Description: Disturbances on the centimeter scale in the layering of the NEEM ice core (North Greenland) can be mapped by means of visual stratigraphy as long as the ice has a visual layering, such as, for example, cloudy bands. Different focal depths of the visual stratigraphy method allow, to a certain extent, a three-dimensional view of the structures. In this study we present a structural analysis of the visible folds, discuss characteristics and frequency and present examples of typical fold structures. The structures evolve from gentle waves at about 1500 m to overturned z-folds with increasing depth. Occasionally, the folding causes significant thickening of layers. Their shape indicates that they are passive features and are probably not initiated by rheology differences between layers. Layering is heavily disturbed and tracing of single layers is no longer possible below a depth of 2160 m. Lattice orientation distributions for the corresponding core sections were analyzed where available in addition to visual stratigraphy. The data show axial-plane parallel strings of grains with c-axis orientations that deviate from that of the matrix, which shows a well developed single-maximum fabric at the depth where the folding occurs. We conclude from these data that folding is a consequence of deformation along localized shear planes and kink bands. The findings are compared with results from other deep ice cores. The observations presented are supported by microstructural modeling. We are using a crystal plasticity code that reproduces deformation, applying a fast Fourier transform method (FFT), coupled with the microstructural platform ELLE to include dynamic recrystallization processes. The model results reproduce the development of bands of grains with a tilted orientation relative to the single maximum fabric of the matrix and also the associated local deformation.

Description: Visual stratigraphy of ice cores from Greenland as well as Antarctica revealed folding on a cm scale, with fold amplitudes varying from less than 1 cm to a few decimetres. Stratigraphy bands are visualized by an indirect light source scattering on surfaces inside the ice, mainly particles and air bubbles / hydrates. Due to their potential influence on the integrity of the climatic record, folds have been subject to modelling studies, however, the initial formation of the disturbances is not fully understood. In this study we present a detailed analysis of the visible folds from the NEEM ice core from Greenland and the EDML ice core from Antarctica, discuss their characteristics and frequency and present examples of typical fold structures. We also analyse the structures with regard to the deformation boundary conditions under which they formed. In case of the NEEM core the structures evolve from gentle waves at about 1500 m to overturned z-folds with increasing depth. Occasionally, the folding causes significant thickening of layers. Their similar-fold shape indicates that they are passive features and are probably not initiated by rheology differences between alternating layers. Layering is heavily disturbed and tracing of single layers is no longer possible below a depth of 2160 m. C-axes orientation distributions for the corresponding core sections were analysed where available in addition to visual stratigraphy. The data show axial-plane parallel strings of grains with c-axis orientations that deviate from that of the matrix, which shows a single-maximum fabric at the depth where the folding occurs. In case of the EDML ice cores the folding starts at a depth of about 1700 m and show very similar characteristics as found in the NEEM core. Numerical modelling of crystal viscoplasticity deformation and dynamic recrystallisation was used to improve the understanding of the formation of the observed structures during deformation. The modelling reproduces the development of bands of grains with a tilted orientation relative to the single maximum fabric of the matrix, and also the associated local deformation. We conclude from these results that the observed folding is a consequence of localized deformation at the boundaries of kink bands.

Description: Reliable knowledge of ice discharge dynamics for the Greenland Ice Sheet via its ice streams is essential if we are to understand its stability under future climate scenarios. Little however is known about the paleo ice-sheet configuration in areas still covered by ice. Here we use radio-echo sounding data to decipher the regional deformation history of the north-eastern Greenland Ice Sheet from its internal stratigraphy. We map folds deep below the surface that we attribute to the activity of a now-extinct ice stream, which shows strong similarities to the Northeast Greenland Ice Stream. We propose that locally this ancient ice flow regime reached much further inland than today’s and was ceased in the Holocene. The new insight that major ice streams may abruptly disappear will affect future approaches to understanding and modelling the response of Earth’s ice sheets to global warming.

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: We present a record of melt events obtained from the East Greenland Ice Core Project (EastGRIP) ice core in central northeastern Greenland, covering the largest part of the Holocene. The data were acquired visually using an optical dark-field line scanner. We detect and describe melt layers and lenses, seen as bubble-free layers and lenses, throughout the ice above the bubble–clathrate transition. This transition is located at 1150 m depth in the EastGRIP ice core, corresponding to an age of 9720 years b2k. We define the brittle zone in the EastGRIP ice core as that from 650 to 950 m depth, where we count on average more than three core breaks per meter. We analyze melt layer thicknesses, correct for ice thinning, and account for missing layers due to core breaks. Our record of melt events shows a large, distinct peak around 1014 years b2k (986 CE) and a broad peak around 7000 years b2k, corresponding to the Holocene Climatic Optimum. In total, we can identify approximately 831 mm of melt (corrected for thinning) over the past 10 000 years. We find that the melt event from 986 CE is most likely a large rain event similar to that from 2012 CE, and that these two events are unprecedented throughout the Holocene. We also compare the most recent 2500 years to a tree ring composite and find an overlap between melt events and tree ring anomalies indicating warm summers. Considering the ice dynamics of the EastGRIP site resulting from the flow of the Northeast Greenland Ice Stream (NEGIS), we find that summer temperatures must have been at least 3 ± 0.6 ∘C warmer during the Early Holocene compared to today.

Description: Impurities in polar ice play an essential role in ice flow and deformation and the integrity of the ice core record. In particular, cloudy bands, visible layers with high impurity concentrations, are prominent features in deep ice cores. However, these layers' physical and chemical properties are poorly understood, highlighting the need to analyse them in more detail. For this purpose, we combine measurements on different scales using a variety of methods, such as the visual stratigraphy line scanner, fabric analyser, microstructure mapping, and Raman spectroscopy. Here we report an extensive record of the microstructural spatial distribution and mineralogy of solid micro-inclusions in glacial ice from the Northeast Greenland Ice Core Project (EastGRIP) ice core ranging back to 50 ka before today. We determine major minerals related to terrestrial dust, such as quartz, feldspar, mica and hematite, carbonaceous particles, dolomite, and gypsum. Furthermore, we have identified rare minerals, such as rutile, anatase, epidote, titanite, and grossular. We thus show that inclusions in cloudy bands are of diverse mineralogy and that the respective mineralogy of micro-inclusions within and outside of cloudy bands can differ. We discuss the possible impact of these inclusions on deformation processes, and we systematically investigate and classify cloudy band properties, such as thickness, intensity, and sequences of cloudy bands, to distinguish between different types occurring in EastGRIP glacial ice. This study is thus an interdisciplinary approach to better understand the nature of cloudy bands - a regular feature defining large parts of deep polar ice cores.

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.