Description: Ice shelves surrounding the Antarctic perimeter moderate ice discharge towards the ocean through buttressing. Ice-shelf evolution and integrity depend on the local surface accumulation, basal melting and on the spatially variable ice-shelf viscosity. These components of ice-shelf mass balance are often poorly constrained by observations and introduce uncertainties in ice-sheet projections. Isochronal radar stratigraphy is an observational archive for the atmospheric, oceanographic and ice-flow history of ice shelves. Here, we predict the stratigraphy of locally accumulated ice on ice shelves with a kinematic forward model for a given atmospheric and oceanographic scenario. This delineates the boundary between local meteoric ice (LMI) and continental meteoric ice (CMI). A large LMI to CMI ratio hereby marks ice shelves whose buttressing strength is more sensitive to changes in atmospheric precipitation patterns. A mismatch between the steady-state predictions of the kinematic forward model and observations from radar can highlight inconsistencies in the atmospheric and oceanographic input data or be an indicator for a transient ice-shelf history not accounted for in the model. We discuss pitfalls in numerical diffusion when calculating the age field and validate the kinematic model with the full Stokes ice-flow model Elmer/Ice. The Roi Baudouin Ice Shelf (East Antarctica) serves as a test case for this approach. There, we find a significant east–west gradient in the LMI / CMI ratio. The steady-state predictions concur with observations on larger spatial scales (〉10 km), but deviations on smaller scales are significant, e.g., because local surface accumulation patterns near the grounding zone are underestimated in Antarctic-wide estimates. Future studies can use these mismatches to optimize the input data or to pinpoint transient signatures in the ice-shelf history using the ever growing archive of radar observations of internal ice stratigraphy.

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: We present initial estimates of the physical properties of meteoric and marine ice in Larsen C ice shelf, Antarctic Peninsula, as derived from quality factor (Q) and amplitude-versus-angle (AVA) analysis of reflection-seismic datasets. The data were acquired during the 2008-09 austral summer in the south-eastern sector of the ice shelf, using explosive sources deployed in shallow shot holes, and 48 vertical-component 100 Hz geophones. 24 of these phones were installed horizontally and transverse to the acquisition line, such that compressional (P), verticallypolarised shear (SV) and horizontally-polarised shear (SH) could be recorded. The recorded data are rich in reflection events, with different phases identifiable as primary and multiple P-waves, SV- and SH-waves, and also P to SV mode conversions. The AVA character of these reflections is applied in a joint inversion, with a Bayesian statistical analysis used to obtain best-fit densities and wavelet velocities for the meteoric and marine ice, which allows estimates of the ices’ Young’s moduli and Poisson’s ratios. We further use prestack Q inversion (PSQI) to determine P- and S-wave quality factors for the two ice types, and consider these in terms of ice temperature and permeability. Our estimates of the physical properties of the meteoric and marine ices will ultimately be used to inform predictive models of the flow and fracture mechanics of Larsen C Ice Shelf.

Description: Crystal Orientation Fabric (COF) of c-axes in ice cores reveals information about deformation within ice sheets. While this is a well established analysis technique for deep ice cores from ice divides, information about COF in ice streams is just now becoming available: the EastGRIP ice core is situated inside the largest ice stream in Greenland, the North East Greenland Ice Stream (NEGIS). With the ongoing analysis of samples from the EastGRIP ice core, COF is now available down to 1714 m, revealing an extremely more rapid evolution of COF anisotropy with depth compared to all other ice cores. This enables us to study the ability of polarimetric radar measurements to infer an overall pattern of COF from measurements conducted at the surface. Depending on whether the COF is isotropic or anisotropic, a radar signal is reflected differently in terms of angle dependence and polarization. We conducted these polarimetric measurements around the EastGRIP drill site and we compare them to COF data obtained from 778 thin sections, prepared and measured at EastGRIP drill site. We investigate the hypothesis that the same pattern of COF can be retrieved from the polarimetric measurements as is available from the ice core. If confirmed, this would provide an addition constraint on the (an)isotropy at locations where no ice core is available. This would potentially provide quasi spatial coverage and greatly improve our understanding of the evolution of anisotropy over from ice divides to outlet glaciers.

Description: We performed numerical simulations on the micro-dynamics of ice with air inclusions as a second phase. This provides first results of a numerical approach to model dynamic recrystallisation in polyphase crystalline aggregates. Our aim was to investigate the rheological effects of air inclusions and explain the onset of dynamic recrystallisation in the permeable firn. The simulations employ a full field theory crystal plasticity code coupled to codes simulating dynamic recrystallisation processes and predict time-resolved microstructure evolution in terms of lattice orientations, strain distribution, grain sizes and grain boundary network. Results show heterogeneous deformation throughout the simulations and indicate the importance of strain localisation controlled by air inclusions. This strain localisation gives rise to locally increased energies that drive dynamic recrystallisation and induce heterogeneous microstructures that are coherent with natural firn microstructures from EPICA Dronning Maud Land ice coring site in Antarctica. We conclude that although overall strains and stresses in firn are low, strain localisation associated with locally increased strain energies can explain the occurrence of dynamic recrystallisation.

Description: Understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large-scale ice dynamics, and thus our ability to predict future changes of ice sheets. Polar ice behaves anisotropically during flow, which can lead to strain localisation. In order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets, we present a series of numerical simulations of ice polycrystals deformed under simple-shear conditions. The models explicitly simulate the evolution of microstructures using a full-field approach, based on the coupling of a viscoplastic deformation code (VPFFT) with dynamic recrystallisation codes. The simulations provide new insights into the distribution of stress, strain rate and lattice orientation fields with progressive strain, up to a shear strain of three. Our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure (grain size and shape), while the development of lattice preferred orientations (LPO) appears to be less affected. Activation of non-basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour. Simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration. Therefore, the finite-strain history is non-directly reflected by the final microstructure. Masked strain localisation can be recognised in ice cores, such as the EDML, from the presence of stepped boundaries, microshear and grains with zig-zag geometries.

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: Impurities in polar ice cores have been studied so far mainly for the purpose of reconstructions of past atmospheric aerosol concentrations. However, impurities also critically influence physical properties of the ice matrix itself. To improve the data basis regarding the in-situ form of incorporation and spatial distribution of impurities in ice we used micro-cryo-Raman spectroscopy to identify the location, phase and composition of micrometer-sized inclusions in natural ice samples around the transition from marine isotope stage (MIS) 6 into 5e in the EDML ice core. The combination of Raman results with ice-microsctructure measurements and complementary impurity data provided by the standard analytical methods (IC, CFA, and DEP) allows for a more interdisciplinary approach interconnecting ice core chemistry and ice core physics. While the interglacial samples were dominated by sulfate salts - mainly gypsum, sodium sulfate (possibly thenardite) and iron-potassium sulfate (likely jarosite) - the glacial ice contained high numbers of mineral dust particles - in particular quartz, mica, feldspar, anatase, hematite and carbonaceous particles (black carbon). We cannot confirm cumulation of impurities in the grain boundary network as reported by other studies, neither micro-particles being dragged by migrating grain boundaries nor in form of liquid veins in triple junctions. We argue that mixing of impurities on millimeter scale and chemical reactions are facilitated by the deforming ice matrix. We review possible effects of impurities on physical properties of ice, however the ultimate identification of the deformation agent and the mechanism behind remains challenging.

Description: Over the last years AWI implemented a new ultra-wideband radar, developed by the Center for Remote Sensing of Ice Sheets (CReSIS), on AWI's polar airplane Polar 6, a Basler BT-67. After terminating the overall system tests and calibration/validation survey in 2015, the system has been in operational use in Greenland and Antarctica for several field seasons. We will present an overview of the results and experiences obtained over the last two years to illustrate the system performance in terms of achievable specifications for imaging and sounding ice sheets, and will discuss the requirements and opportunities for logistic deployment in Greenland as well as Antarctica.