Description: Static (or ‘normal’) grain growth, i.e. grain boundary migration driven solely by grain boundary energy, is considered to be an important process in polar ice. Many ice-core studies report a continual increase in average grain size with depth in the upper hundreds of metres of ice sheets, while at deeper levels grain size appears to reach a steady state as a consequence of a balance between grain growth and grain-size reduction by dynamic recrystallization. The growth factor k in the normal grain growth law is important for any process where grain growth plays a role, and it is normally assumed to be a temperature-dependent material property. Here we show, using numerical simulations with the program Elle, that the factor k also incorporates the effect of the microstructure on grain growth. For example, a change in grain-size distribution from normal to log-normal in a thin section is found to correspond to an increase in k by a factor of 3.5.

Description: An important feature of natural ice, in addition to the obvious relevance of glaciers and ice sheets for climate-related issues, is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point, without losing its polycrystalline character. This fact, together with its strong mechanical anisotropy and other notable properties, makes natural ice an interesting model material for studying the high-temperature creep and recrystallization of rocks in Earth’s interior. After having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in Part I of this work (Faria et al., 2014), here in Part II we present an up-todate view of the modern understanding of natural ice microstructures and the deformation processes that may produce them. In particular, we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure (also known as the “three-stage model,” cf. Part I). These results prove that grain growth in ice sheets is dynamic, in the sense that it occurs during deformation and is markedly affected by the stored strain energy, as well as by air inclusions and other impurities. The strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal, which demand large amounts of strain accommodation. From the microstructural analyses of ice cores, we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice. Additionally, in fine-grained, high-impurity ice layers (e.g. cloudy bands), strain may sometimes be accommodated by diffusional flow (at low temperatures and stresses) or microscopic grain boundary sliding via microshear (in anisotropic ice sheared at high temperatures). Grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets. Evidence of nucleation of new grains is also observed at various depths, provided that the local concentration of strain energy is high enough (which is not seldom the case). As a substitute for the tripartite paradigm, we propose a novel dynamic recrystallization diagram in the three-dimensional state space of strain rate, temperature, and mean grain size, which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure. Dedicated to Sepp Kipfstuhl on occasion of his 60th birthday.

Description: Ice is the most frequent mineral on the Earth’s surface, however experiences conditions comparable to silicate minerals at high metamorphic grades. In all natural conditions ice is a hot material with homologous temperatures between 0.9 and 0.7 at least. Under such circumstances recrystallization plays a decisive role in governing the state and thus the behaviour of the material. This has been recognized and interpreted in many ice cores in the last decades (Faria et al. in press a) assigning recrystallization regimes to ice sheet depth ranges. This assignment made use of 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 (Faria et al. in press b). References: Faria, S. H.; Weikusat, I. & Azuma, N. The Microstructure of Polar Ice. Part I: Highlights from ice core research. Journal of Structural Geology , in press a, DOI: 10.1016/j.jsg.2013.09.010 Faria, S. H.; Weikusat, I. & Azuma, N. The Microstructure of Polar Ice. Part II: State of the Art. Journal of Structural Geology , in press b, DOI: 10.1016/j.jsg.2013.11.003

Description: Polar ice sheets play a fundamental role in Earth's climate system, by interacting actively and passively with the environment. Active interactions include the creeping flow of ice and its effects on polar geomorphology, global sea level, ocean and atmospheric circulation, and so on. Passive interactions are mainly established by the formation of climate records within the ice, in form of air bubbles, dust particles, salt microinclusions and other derivatives of airborne impurities buried by recurrent snowfalls. For a half-century scientists have been drilling deep ice cores in Antarctica and Greenland for studying such records, which can go back to around a million years. Experience shows, however, that the ice-sheet flow generally disrupts the stratigraphy of the bottom part of deep ice cores, destroying the integrity of the oldest records. For all these reasons glaciologists have been studying the microstructure of polar ice cores for decades, in order to understand the genesis and fate of ice-core climate records, as well as to learn more about the physical properties of polar ice, aiming at better climate-record interpretations and ever more precise models of ice-sheet dynamics. In this Part I we review the main difficulties and advances in deep ice core drilling in Antarctica and Greenland, together with the major contributions of deep ice coring to the research on natural ice microstructures. In particular, we discuss in detail the microstructural findings from Camp Century, Byrd, Dye 3, GRIP, GISP2, NorthGRIP, Vostok, Dome C, EDML, and Dome Fuji, besides commenting also on the earlier results of some pioneering ventures, like the Jungfraujoch Expedition and the Norwegian–British–Swedish Antarctic Expedition, among others. In the companion Part II of this work ( Faria et al., 2014), the review proceeds with a survey of the state-of-the-art understanding of natural ice microstructures and some exciting prospects in this field of research.

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.