Anisotropy in ice sheets - some types and causes

Neutrino-astronomy meets ice drilling and glaciology The 2017 Fall Collaboration meeting is hosted by DESY and the HU Berlin. As part of the pre-meeting schedule, it includes the one-day IceCube Polar Science workshop. This interdisciplinary workshop will include scientists and engineers from Europe and the US discussing their areas of expertise, including glaciology, drilling, sampling, logging, and remote sensing techniques in the polar environment. Current status and future plans, requirements and challenges will be reviewed

Ice physics studies using deep ice cores in the light of global warming

Our changing climate will affect human lives in manifold ways. Especially sea-level rise is driven by diminishing amounts of snow and land ice. The cryosphere as part of the Earth’s climate system provides interconnecting feedbacks between its components, leading to multifaceted changes. To understand these feedback mechanisms we have to consider the various interactions between the atmosphere, the oceans and the ice covering both polar land and seas. The state and evolution of Earth’s glaciers and ice sheets play a key role therein and thus profound ice physics knowledge with respect to ice sheet dynamics is required. Ice sheet dynamics on multiple scales, from microscopic processes to continental-sized phenomena, are in the focus of scientific attention to improve climate predictions on a global scale. The state and evolution of ice sheets and glaciers is partly recorded in deep ice cores drilled through the Antarctic and Greenlandic ice. Advanced analysis of ice core material can teach us the climate history of our planet and reveal physical mechanisms leading to ice motion. The mapping of ice micro-structures, which are reflecting the deformation and recrystallisation processes that control ice sheet flow reveal the active processes in the material. The connection between ice dynamics, microstructures, process modelling as well as phenomenological modelling of ice deformation then tests and completes our current understanding of ice sheet dynamics and result in improved future projections

Anisotropic backscatter in ice-penetrating radar data: potential mechanisms and implications

Airborne and ground-based radar have been used extensively in the past to measure ice thickness and to investigate the internal structure of ice sheets in terms of layering. The main reflection mechanisms for internal reflections are changes in density, conductivity, and crystal orientation fabric, which alter thepermittivity of the ice. Linking the different mechanisms to the individual reflection horizons enables thededuction of glaciological parameters like accumulation rates or age-depth estimates. If no sample material from snow pits or ice-cores are available, multi-frequency and multi-polarization measurements must be applied to distinguish between the different reflection mechanisms. The backscattered power of horizons caused by changes in conductivity varies with the center frequency whereas in the case of horizons originating from changing crystal orientation the backscattered power is dependent on the polarization plane of the carrier signal.In this study we examine a sample data set near the German summer station Kohnen (drill site for theEPICA-EDML ice core) on the Antarctic plateau. The data were acquired with an airplane sliding on ground, producing varying incident polarization with a circular profile and several cross profiles with different headings. We find that the backscattered power changes with varying antenna orientation (i.e. polarization). In the upper third of the ice column the backscatter has two maxima with a 180° symmetry. The maxima align with the direction of minimal surface strain. At approximately 900 m depth the anisotropy is shifted by 90° in heading azimuth, with the maxima now being parallel to the maximum in surface strain. This dataset is unique, as airborne systems (primarily designed for the sounding of ice thickness) are usually not used for ground-based applications. The observed anisotropy appears clearly and is intriguing as the reason for it is entirely unknown. As primary suspects we consider the role of changing crystal orientation and ellipsoidal shaped air bubbles. The effect is visible from 200 1400 m. It appears distributed along the entire interval, and not restricted to individual layers. It seems that the polarization dependence becomes visible by a changing background level of the acquired signal, which is otherwise largely dominated by layer-like, polarization independent reflections. Hence we apply a (semi-analytical) volume scattering model in order to understand the different reflection mechanisms better. From ice-core measurements it is known that the crystals in the upper hundred meters are only weakly aligned (if at all), and it is unclear how the crystal orientation changes overshort depth intervals (~10 m). The rotation of the anisotropy coincides with the clathrate transition in the ice core and thus we first focus on the effect of anisotropic air bubbles. In an in-coherent approach we treat the ice matrix as a random medium and use the vector radiative transfer theory to incorporate boundary conditions. In a second step we model the effect of crystal orientation to estimate both, the degree of alignment and the statistical variance in the permittivity tensor needed to generate the observed pattern in backscatter. Doing so, we eventually aim at pinning down the mechanisms for the anisotropy in the upper interval, lower interval and the interrelation of the two by a shift of 90°.Anisotropic air bubbles as well as aligned crystal orientation allow to deduce stress and strain rates and a potential change thereof along depth. So far it is largely unclear, how surface strain rates relate with strain rates within the ice. If one of the two suspected mechanisms can be excluded or confirmed, this study may serve as a case study for future polarimetric surveys with low-frequency radars, in order to supply ice-sheet modelling with adequate boundary conditions - including changes in the internal structure of ice sheets along depth

Determination of crystal orientation fabric from seismic wideangle data

It is known from ice core analyses that the crystal orientation fabric (COF) of ice sheets is anisotropic and changes over depth. A better understanding of these anisotropies as well as their remote detection is important to optimize flow models for ice. Here we show how seismic wideangle measurements can be used to determine the COF remotely. We demonstrate the principle formalism how observed seismic traveltimes can be related to COF properties by a forward model and then apply the formalism to field data. The eigenvalues that describe the ice fabric of the ice core EDML (Dronning Maud Land, Antarctca) are set into a relationship with the elasticity tensor. From the elasticity tensor the expected seismic velocities and reflection coefficients are calculated. Additionally we calculate the value eta from the Thomsen-parameters epsilon and delta. The value eta gives a measure of the anisotropy of vertical transverse isotropic (VTI)-media and is an important tool for the NMO-correction of anisotropic data. The approximation of reflection horizons as hyperbolas is not valid anymore in anisotropic media. The calculation of the moveout is therefore performed by a 4th order NMO-correction with the rms-velocity and the effective eta value as variables. This approach is applied to data from a wideangle survey shot at Halvfarryggen, Dronning Maud Land, Antarctica. From this data we derived rms-velocities and effective eta values. These values were than recalculated to interval velocities and interval eta values to give a hint on the measure of anisotropy of the different layers. The results give first insight into the anisotropies at Halvfarryggen

Location and distribution of micro-inclusions in the EDML and NEEM ice cores using optical microscopy and in situ Raman spectroscopy

Impurities control a variety of physical properties of polar ice. Their impact can be observed at all scales – from the microstructure (e.g., grain size and orientation) to the ice sheet flow behavior (e.g., borehole tilting and closure). Most impurities in ice form micrometer-sized inclusions. It has been suggested that these μ inclusions control the grain size of polycrystalline ice by pinning of grain boundaries (Zener pinning), which should be reflected in their distribution with respect to the grain boundary network. We used an optical microscope to generate high-resolution large-scale maps (3 μm pix^-1, 8 x 2 cm^2) of the distribution of micro-inclusions in four polar ice samples: two from Antarctica (EDML, MIS 5.5) and two from Greenland (NEEM, Holocene). The in situ positions of more than 5000 μ inclusions have been determined. A Raman microscope was used to confirm the extrinsic nature of a sample proportion of the mapped inclusions. A superposition of the 2-D grain boundary network and μ-inclusion distributions shows no significant correlations between grain boundaries and μ inclusions. In particular, no signs of grain boundaries harvesting μ inclusions could be found and no evidence of μ inclusions inhibiting grain boundary migration by slow-mode pinning could be detected. Consequences for our understanding of the impurity effect on ice microstructure and rheology are discussed

A Review of the Microstructural Location of Impurities in Polar Ice and Their Impacts on Deformation

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

Micro-inclusions in the EGRIP ice core identified with Raman-spectroscopy

Soluble and insoluble impurities play a crucial role regarding the deformability and thus the flow of polar ice. To better understand this interplay from a mechanistic point of view it is especially important to investigate the location and chemical composition of micro-inclusions (Stoll et al., 2021), which are among the most abundant impurities in polar ice. New results from a systematic analysis of micro-inclusions in Holocene ice from the East Greenland Ice Core Project (EGRIP), which has been drilled near the onset of the Northeast Greenland Ice Stream (NEGIS), offer unique insights into the dynamics of fast flowing ice over different scales, ranging from kilometres to micrometres. Investigating the small-scale properties of eleven samples from Holocene ice, i.e. the upper 1340 m of the EGRIP ice core, we mapped the locations of several thousand micro-inclusions inside the ice. The use of cryo-Raman spectroscopy allowed us to obtain a representative overview of the mineralogy of these inclusions in the ice without the risk of contamination. We identified a variety of Raman spectra, mainly from sulphates (dominated by gypsum) and terrestrial dust, such as quartz, mica and feldspar. The observed mineralogy changes with depth and EGRIP Holocene ice can be categorised in two different depth regimes, i.e. the upper (100-900 m) and lower (900-1340 m) regimes, depending on their mineralogy. Furthermore, micro- inclusions show certain spatial patterns, such as clustering or layering, which are partly related to their mineralogy. We thus conclude that Greenlandic Holocene ice has a broader, and more variable, mineralogy than previously reported and that chemical reactions might take place within the ice sheet, possibly altering the paleo-climate record. Our approach further demonstrates the added value of systematic, combined high-resolution impurity and microstructural studies, and the importance of considering different spatial scales and is thus another step towards a more holistic understanding of impurities in ice

Location and composition of micro-inclusions in deep ice from the EDML ice core (Antarctica) using optical microscope and cryo-Raman spectroscopy

The impurity content in meteoric ice from polar regions is relatively low compared to other natural materials. However, it controls a variety of physical properties of ice - from dielectric response to its mechanical behaviour. Links between impurity concentration, changes in ice micro-structure and deformation rate have been reported on several scales. In order to approach the responsible mechanisms, a better understanding is needed regarding the in-situ form, location, and distribution of the different species within the polycrystal. We used an optical microscope to generate high-resolution 2D-maps of micro-inclusions in deep ice from the EDML ice core (Antarctica). Superposition of the grain boundary network and micro-inclusion distributions shows no significant correlations between grain boundaries and micro-inclusions. Implications for the relevance of Zener pinning during grain boundary migration and redistribution of impurities by grain boundary drag are discussed. Raman spectra of micro-inclusions in selected regions were obtained using a confocal cryo-Raman system. Comparison with ion chromatography shows that most of the available ions in ice precipitate in form of micro-inclusions. However, indications were found that some of the residual components could coexist in form of solid solution

Microstructure, micro-inclusions, and mineralogy along the EGRIP (East Greenland Ice Core Project) ice core – Part 2: Implications for palaeo-mineralogy

Impurities in polar ice do not only allow the re- construction of past atmospheric aerosol concentrations but also influence the physical properties of the ice. However, the localisation of impurities inside the microstructure is still un- der debate and little is known about the mineralogy of solid inclusions. In particular, the general mineralogical diversity throughout an ice core and the specific distribution inside the microstructure is poorly investigated; the impact of the mineralogy on the localisation of inclusions and other pro- cesses is thus hardly known. We use dust particle concen- tration, optical microscopy, and cryo-Raman spectroscopy to systematically locate and analyse the mineralogy of micro- inclusions in situ inside 11 solid ice samples from the up- per 1340 m of the East Greenland Ice Core Project ice core. Micro-inclusions are more variable in mineralogy than pre- viously observed and are mainly composed of mineral dust (quartz, mica, and feldspar) and sulfates (mainly gypsum). Inclusions of the same composition tend to cluster, but clus- tering frequency and mineralogy changes with depth. A va- riety of sulfates dominate the upper 900 m, while gypsum is the only sulfate in deeper samples, which however contain more mineral dust, nitrates, and dolomite. The analysed part of the core can thus be divided into two depth regimes of different mineralogy, and to a lesser degree of spatial distri- bution, which could originate from different chemical reac- tions in the ice or large-scale changes in ice cover in north- east Greenland during the mid-Holocene. The complexity of impurity mineralogy on the metre scale and centimetre scale in polar ice is still underestimated, and new methodological approaches are necessary to establish a comprehensive un- derstanding of the role of impurities. Our results show that applying new methods to the mineralogy in ice cores and recognising its complexity, as well as the importance for lo- calisation studies, open new avenues for understanding the role of impurities in ice cores

The new frontier of microstructural impurity research in polar ice

Deciphering the localisation of solid and dissolved impurities on the micron-scale in glacial ice remains a challenge, but is critical to understand the integrity of ice core records and internal deformation. Here we report on the state-of-the-art in microstructural impurity research by high- lighting recent progress in bringing together cryo-Raman spectroscopy and laser ablation induct- ively coupled plasma mass spectrometry (LA-ICP-MS). We show the potential of both methods and discuss possibilities to improve inter-method approaches aiming for a more holistic under- standing of the evolution of impurity localisation throughout the ice column, including post-depositional processes. In this framework, we elaborate on future research priorities such as LA-ICP-MS imaging on firn samples and integrating a large cryo-cell with imaging capabilities