The microstructure of polar ice. Part I: Highlights from ice core research

Faria, Sérgio H.; Weikusat, Ilka; Azuma, Nobuhiko

doi:10.1016/j.jsg.2013.09.010

Cited by 98 publications

(130 citation statements)

References 140 publications

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“…A particular rotation axis is not expected for these boundaries, although the occurrence of a strong CPO will tend to produce preferred rotation axes (Mainprice et al, 1993). At deeper depth levels in polar ice, where a characteristically strong CPO development with ice sheet depth is generally observed (Weikusat et al, 2017b;Jansen et al, 2016;, Montagnat et al, 2014;Fitzpatrick et al, 2014;Faria et al, 2014a, and references therein), we suggest that type III SGBs might occur. SGBs that develop from GBs may be recognizable as such because they are part of the grain boundary network.…”

Section: Subgrain Boundaries Without Connection To Host Grain Crystalmentioning

confidence: 65%

“…In order to evaluate this, other methods, such as the statistical evaluation of grain boundary networks (Binder et al, 2013b), are needed as larger grain populations have to be taken into account. However, in the sample studied here and for most other deep ice microstructures (Faria et al, 2014a), the vast majority of SGBs are microstructures that occur inside grains and are not low-angle SGBs that form part of a GB network. Type IV SGBs originating from grain coalescence might also be created by the mechanism of grain dissection .…”

Section: Subgrain Boundaries Without Connection To Host Grain Crystalmentioning

confidence: 91%

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EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

et al. 2017

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Abstract. Ice has a very high plastic anisotropy with easy dislocation glide on basal planes, while glide on non-basal planes is much harder. Basal glide involves dislocations with the Burgers vector b = a , while glide on non-basal planes can involve dislocations with b = a , b = [c], and b = c + a . During the natural ductile flow of polar ice sheets, most of the deformation is expected to occur by basal slip accommodated by other processes, including non-basal slip and grain boundary processes. However, the importance of different accommodating processes is controversial. The recent application of micro-diffraction analysis methods to ice, such as X-ray Laue diffraction and electron backscattered diffraction (EBSD), has demonstrated that subgrain boundaries indicative of non-basal slip are present in naturally deformed ice, although so far the available data sets are limited. In this study we present an analysis of a large number of subgrain boundaries in ice core samples from one depth level from two deep ice cores from Antarctica (EPICA-DML deep ice core at 656 m of depth) and Greenland (NEEM deep ice core at 719 m of depth).EBSD provides information for the characterization of subgrain boundary types and on the dislocations that are likely to be present along the boundary. EBSD analyses, in combination with light microscopy measurements, are presented and interpreted in terms of the dislocation slip systems. The most common subgrain boundaries are indicative of basal a slip with an almost equal occurrence of subgrain boundaries indicative of prism [c] or c + a slip on prism and/or pyramidal planes. A few subgrain boundaries are indicative of prism a slip or slip of a screw dislocations on the basal plane. In addition to these classical polygonization processes that involve the recovery of dislocations into boundaries, alternative mechanisms are discussed for the formation of subgrain boundaries that are not related to the crystallography of the host grain.The finding that subgrain boundaries indicative of nonbasal slip are as frequent as those indicating basal slip is surprising. Our evidence of frequent non-basal slip in naturally deformed polar ice core samples has important implications for discussions on ice about plasticity descriptions, rate-controlling processes which accommodate basal glide, and anisotropic ice flow descriptions of large ice masses with the wider perspective of sea level evolution.

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Section: Subgrain Boundaries Without Connection To Host Grain Crystalmentioning

confidence: 65%

Section: Subgrain Boundaries Without Connection To Host Grain Crystalmentioning

confidence: 91%

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

et al. 2017

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“…Although this is known to happen on the large scale (NEEM, 2013), it is difficult to assess this on the small scale. Indications for small-scale disturbances are provided by so-called "cloudy bands" using dark-field microscopy (Svensson et al, 2005;S.H. Faria et al, 2014), which from a depth below about 1700 m often show folds and disturbances (Jansen et al, 2016).…”

Section: Grain Morphology and Strain Distributionmentioning

confidence: 99%

Full-field predictions of ice dynamic recrystallisation under simple shear conditions

Llorens

Griera

Bons

et al. 2016

Earth and Planetary Science Letters

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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.

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“…The presence of the basal layer turns out to be widespread, especially in Antarctica (CReSIS, P. Gogineni, personal communication, 2014). As the basal ice near the bed is subject to higher stresses and elevated temperatures than the ice above, it is the region where ice physical properties on the microscale change most rapidly (Faria et al, 2014b). These include changes in crystal orientation fabric (COF) properties and distribution.…”

Section: Introductionmentioning

confidence: 99%

Seismic wave propagation in anisotropic ice – Part 1: Elasticity tensor and derived quantities from ice-core properties

Diez

Eisen

2015

The Cryosphere

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Abstract.A preferred orientation of the anisotropic ice crystals influences the viscosity of the ice bulk and the dynamic behaviour of glaciers and ice sheets. Knowledge about the distribution of crystal anisotropy is mainly provided by crystal orientation fabric (COF) data from ice cores. However, the developed anisotropic fabric influences not only the flow behaviour of ice but also the propagation of seismic waves. Two effects are important: (i) sudden changes in COF lead to englacial reflections, and (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, recorded travel times. A framework is presented here to connect COF data from ice cores with the elasticity tensor to determine seismic velocities and reflection coefficients for cone and girdle fabrics. We connect the microscopic anisotropy of the crystals with the macroscopic anisotropy of the ice mass, observable with seismic methods. Elasticity tensors for different fabrics are calculated and used to investigate the influence of the anisotropic ice fabric on seismic velocities and reflection coefficients, englacially as well as for the ice-bed contact. Hence, it is possible to remotely determine the bulk ice anisotropy.

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The microstructure of polar ice. Part I: Highlights from ice core research

Abstract: .

Cited by 98 publications

References 140 publications

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

Full-field predictions of ice dynamic recrystallisation under simple shear conditions

Seismic wave propagation in anisotropic ice – Part 1: Elasticity tensor and derived quantities from ice-core properties

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