Microstructure and Crystallographic Preferred Orientations of an Azimuthally Oriented Ice Core from a Lateral Shear Margin: Priestley Glacier, Antarctica

Abstract

A 58 m long azimuthally oriented ice core has been collected from the floating lateral sinistral shear margin of the lower Priestley Glacier, Terra Nova Bay, Antarctica. The crystallographic preferred orientations (CPO) and microstructures are described in order to correlate the geometry of anisotropy with constrained large-scale kinematics. Cryogenic Electron Backscatter Diffraction analysis shows a very strong fabric (c-axis primary eigenvalue ∼0.9) with c-axes aligned horizontally sub-perpendicular to flow, rotating nearly 40° clockwise (looking down) to the pole to shear throughout the core. The c-axis maximum is sub-perpendicular to vertical layers, with the pole to layering always clockwise of the c-axes. Priestley ice microstructures are defined by largely sub-polygonal grains and constant mean grain sizes with depth. Grain long axis shape preferred orientations (SPO) are almost always 1–20° clockwise of the c-axis maximum. A minor proportion of “oddly” oriented grains that are distinct from the main c-axis maximum, are present in some samples. These have horizontal c-axes rotated clockwise from the primary c-axis maximum and may define a weaker secondary maximum up to 30° clockwise of the primary maximum. Intragranular misorientations are measured along the core, and although the statistics are weak, this could suggest recrystallization by subgrain rotation to occur. These microstructures suggest subgrain rotation (SGR) and recrystallization by grain boundary migration recrystallization (GBM) are active in the Priestley Glacier shear margin. Vorticity analysis based on intragranular distortion indicates a vertical axis of rotation in the shear margin. The variability in c-axis maximum orientation with depth indicates the structural heterogeneity of the Priestley Glacier shear margin occurs at the meter to tens of meters scale. We suggest that CPO rotations could relate to rigid rotation of blocks of ice within the glacial shear margin. Rotation either post-dates CPO and SPO development or is occurring faster than CPO evolution can respond to a change in kinematics.

Ultrasonic and seismic constraints on crystallographic preferred orientations of the Priestley Glacier shear margin, Antarctica

“…A fiducial line that was made in the field to provide geographic reference of the core orientation served as the 0 • reference on individual samples. The fiducial line was made perpendicular to the glacier flow direction on the surface (Thomas et al, 2021). New fiducial lines were started wherever a core break could not be fitted together and the relative orientations of the lines reconstructed using the core CPO (Thomas et al, 2021).…”

“…The fiducial line was made perpendicular to the glacier flow direction on the surface (Thomas et al, 2021). New fiducial lines were started wherever a core break could not be fitted together and the relative orientations of the lines reconstructed using the core CPO (Thomas et al, 2021). Measurements are on core sections 003 from a depth of ∼ 2.5 m (diameter d = 100.6 ± 0.3 mm), 007 from a depth of ∼ 6.0 m (diameter d = 99.9 ± 0.3 mm) and 010 from a depth of ∼ 8.5 m (diameter d = 100.3 ± 0.3 mm).…”

“…At the site the glacier is ∼ 1000 m thick and 8 km wide (Frezzotti et al, 2000). Ice flow velocities are measured to increase from ∼ 45 m/year close to the glacier margin to ∼ 130 m/year towards the center (Mouginot et al, 2019;Thomas et al, 2021), resulting in strong shear kinematics. Core samples were analysed for CPO using electron backscatter diffraction (EBSD) measurements (Thomas et al, 2021) and a strong horizontal clustering of c-axes was observed throughout the entire length of the core.…”

“…Ice flow velocities are measured to increase from ∼ 45 m/year close to the glacier margin to ∼ 130 m/year towards the center (Mouginot et al, 2019;Thomas et al, 2021), resulting in strong shear kinematics. Core samples were analysed for CPO using electron backscatter diffraction (EBSD) measurements (Thomas et al, 2021) and a strong horizontal clustering of c-axes was observed throughout the entire length of the core. The core orientation was carefully preserved during drilling which enabled the placement of CPO orientation relative to the macroscopic ice kinematics.…”

“…A continuous ice core of 58 m length was drilled and recovered in December 2019 and January 2020 (Thomas et al, 2021) in a lateral shear margin of the Priestley Glacier, located in Victoria Land, Antarctica, see Figure 1(a). At the site the glacier is ∼ 1000 m thick and 8 km wide (Frezzotti et al, 2000).…”