Full crystallographic orientation (<i>c</i> and <i>a</i> axes) of warm, coarse-grained ice in a shear-dominated setting: a case study, Storglaciären, Sweden

Abstract

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice deforms dominantly by dislocation glide on the basal plane, modified and often intensified by dynamic recrystallization. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in “warm” (T>-10∘C), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We present the first study of a axes as well as c axes in such ice by application of cryo-electron backscatter diffraction (EBSD) and do so in a shear-dominated setting. We have done this by developing a new sample preparation technique of constructing composite sections, to allow us to use EBSD to obtain a representative, bulk CPO on coarse-grained ice. We draw attention to the well-known issue of interlocking grains of complex shape and suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin sections may result in the typical multimaxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction as well as a concomitant girdle of a axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by subsampling datasets from ice sheared in laboratory experiments at high homologous temperatures up to strains of ∼1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO represents a small, final fraction of the shear history. A key result of this study is that multimaxima CPOs in coarse-grained ice reported in previous work may be due to limited sample sizes and a sampling bias related to the presence of island grains of a single host that appear several times in a thin section.

Microstructures in a shear margin: Jarvis Glacier, Alaska

“…The study area, where warm, wet ice has moved a short distance along a wet bed presents extremely challenging conditions for fabric development. Yet, the presence of a discernible fabric in this setting indicates that crystallographic alignment should become prominent in larger glaciers ice streams and/or colder settings, consistent with results from, for example, (Hudleston, 1977), (Jackson and Kamb, 1997) and (Monz and others, 2021). Given the potential rheological impact of anisotropic ice (e.g.…”

“…Although we cut most sections from a single layer of ice, for segments with very large grains (multi-cm), we cut strips from two or three segments spaced farther apart than the grain size in order to incorporate a greater number of grains in a single analysis (cf. Monz and others, 2021). We froze the sample to a similarly sized copper plate.…”

“…To our knowledge, our study is the first to report fabric from surface to bed within the margin of a temperate glacier, and only the third from any lateral margin (cf. Jackson, 1999;Monz and others, 2021). The fabric is relatively weak, but the fact that any fabric develops in the short flow distance of relatively slow-moving ice in a water-rich environment indicates that substantial fabric development is probable in larger glaciers and ice streams.…”

Structures and Deformation in Glaciers and Ice Sheets

“…In comparison with laboratory studies, shear strains in the marginal ice of Storglaciären are much higher, yet the ice is subjected to much lower natural strain rates. It is therefore likely that in natural settings, dynamic recrystallization is more evident, with the crystal orientation representing a smaller component of the overall shear history (Monz et al, 2021). Furthermore, Monz et al (2021) concluded that previous studies investigating crystal fabric in coarse-grained ice may have incorrectly reported multimaxima fabrics as a result of limited samples sizes and sampling bias.…”

Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

“…6(a)). Multiple-maxima ice c-axis fabrics arise when the number of grains measured from a single sample plane is not sufficient for a fully representative CPO (Monz et al, 2021). Cone and cluster CPOs are commonly observed in ice samples deformed under uniaxial compression conditions, and are thought to arise from varying contributions of strain-induced grain boundary migration (GBM) and grain rotation due to dislocation glide (Kamb, 1972;Vaughan et al, 2017).…”