Using electron backscatter diffraction to measure full crystallographic orientation in Antarctic land-fast sea ice

PAT WONGPAN,KATHERINE LILLY,INGA J. SMITH,PATRICIA J. LANGHORNE,DAVID J. PRIOR

doi:10.1017/jog.2018.67

Abstract

ABSTRACTWe have mapped the full crystallographic orientation of sea ice using electron backscatter diffraction (EBSD). This is the first time EBSD has been used to study sea ice. Platelet ice is a feature of sea ice near ice shelves. Ice crystals accumulate as an unconsolidated sub-ice platelet layer beneath the columnar ice (CI), where they are subsumed by the advancing sea–ice interface to form incorporated platelet ice (PI). As is well known, in CI the crystal preferred orientation comprises dominantly horizontalc-axes, while PI hasc-axes varying between horizontal and vertical. For the first time, this study shows thea-axes of CI and PI are not random. Misorientation analysis has been used to illuminate the possible drivers of these alignments. In CI the misorientation angle distribution from random pairs and neighbour pairs of grains are indistinguishable, indicating the distributions are a consequence of crystal preferred orientation. Geometric selection during growth will develop thea-axis alignment in CI if ice growth in water is fastest parallel to thea-axis, as has previously been hypothesised. In contrast, in PI random-pair and neighbour-pair misorientation distributions are significantly different, suggesting mechanical rotation of crystals at grain boundaries as the most likely explanation.

Highlights

Ice crystals in nature have hexagonal symmetry, with three a-axes lying in the basal plane and a c-axis pointing perpendicular to this plane (e.g., Weeks, 2010; Petrich and Eicken, 2017)
Three clusters of poles to m-planes lie in the plane perpendicular to the c-axis cluster, each lying half way between two a-axis clusters. These patterns are shown in all three sample orientations (CI-h, columnar ice (CI)-v2 and CIv3) and show that the crystallographic preferred orientation (CPO) approximates a single crystal with a sub-horizontal c-axis and a vertical a-axis
The dominance of vertical a-axes in thick columnar sea ice is a new result and is consistent with Weeks (2010)’s hypothesis that growth is fastest in the a-axis direction

Summary

Introduction

Ice crystals in nature have hexagonal symmetry (ice Ih; Pauling, 1935), with three a-axes (one a linear combination of the other two) lying in the basal plane and a c-axis pointing perpendicular to this plane (e.g., Weeks, 2010; Petrich and Eicken, 2017). The initial skim of sea ice grows by vertical heat conduction from the ocean to the atmosphere. This initial layer of sea ice is composed of crystals with random orientations, usually because of turbulence in the water (Weeks, 2010). Crystals with c-axis close to the horizontal grow faster so that other orientations are eliminated. This geometric selection (Kolmogorov, 1976) results in a column-like structure called columnar ice (CI) with vertically elongated grains, each with a horizontal c-axis. Sea ice that grows by heat conducted to the atmosphere is called congelation ice. The most common form of congelation ice has a columnar structure

Results

Discussion

Conclusion