Abstract
Glacier and ice-shelf shear margins are localised zones of intense deformation where flowing ice is in contact with confining valley walls, coastal headlands, or slower-moving ice. Within these zones, ice crystal alignment, grain size, temperature, liquid water content, and brittle damage are modified by and contribute to rapid deformation. Shear stresses generated along lateral margins resist the flow of ice from upstream, yet despite their mechanical importance, shear margins are some of the least sampled sites in Antarctica due to their remoteness and extensive crevassing that limits field access. Analyses of shear-margin mechanics over space and time are generally restricted to satellite-based approaches and are limited by the spatial resolution of the associated data products. Because weakening along glacier margins directly contributes to the increasing rate of mass loss from Antarctic outlet glaciers, shear-margin deformation is a timely process to investigate. This thesis investigates ice deformation in situ within the floating, true-left shear margin of Priestley Glacier, an outlet glacier in the Ross Sea region of Antarctica. Priestley Glacier has an exposed ice surface that allows direct observation of shear-margin kinematics and structure. Four related studies are undertaken to make methodological advances and to examine shear-margin deformation with high precision over timescales ranging from recoverable strains driven by tidal flexure, to longer-term viscous creep.
The first study develops a low-cost field method to improve the spatial density of Global Navigation Satellite Systems (GNSS) positioning records of ice motion. We designed and built low-cost (<$1,000 NZD) u-blox ZED-F9P GNSS stations and evaluated their positioning performance against survey-grade Trimble R10 receivers (~$20,000 NZD) under high latitude, glaciated conditions. Positioning errors are nearly identical between the two systems under stationary and on-ice kinematic conditions. Four u-blox GNSS systems deployed across Priestley Glacier's shear margin in November 2022 successfully resolved subtle tidal flexure signals with vertical amplitudes <3 cm, confirming that the low-cost stations operate with sufficient precision to investigate ice kinematics in detail.
The second study examines the capability of optical geodetic methods for high-precision measurement of ice deformation in a fast-deforming setting. A total station and prism transect were deployed across an ice-stream grounding zone to measure surface compression and extension associated with tidal flexure. The method achieves millimetre-level precision when tracking ice displacement and resolves strains of 10-4 with percent errors of 1 to 11%. These precise observations resolve the spatial distribution of tidal strain, and the different deformation processes that contribute to it, including elastic bending, fracture opening and closing, and viscous creep. Tidal strains are localised within a 500 m-wide zone downstream of the grounding-zone flexure limit, and >70% of the measured strain arises from fractures rather than surface bending. These results indicate that the ice margin response to tidal forcing may be more complex than simple elastic bending of a continuous, homogeneous material, a common approximation in ice-flexure modelling studies. Together, the geodetic field methods introduced in the first and second studies improve the quantity and quality of kinematic measurements of fast-deforming margins.
The third study uses high-precision geodetic measurements to analyse the across-flow tidal flexure of Priestley Glacier's floating shear margin. Continuous GNSS records of vertical ice motion and total-station measurements of surface compression and extension are used together to constrain a 2D dynamic ice-flexure model. Matching the model to observed ice displacement and strain associated with tidal flexure requires a spatially variable elastic modulus to account for apparently softer ice in the shear margin than in the glacier interior. The combined patterns of tidal displacement and strain are also used to infer the degree of mechanical coupling between the margin ice and sidewall boundary. Strain measurements over two length scales (200 m and 1200 m) are used to investigate the dependence of inferred ice properties on the spatial resolution of the measurement. The apparent reduced ice stiffness in the shear margin is consistent with grain-size reduction and strong c-axis alignment observed in ice-core samples from the field site. The addition of high-precision in situ strain measurements leads to improved insight into elastic deformation and properties that cannot be obtained using GNSS positioning alone.
The fourth study examines how the spatial resolution of observational data affects interpretations of shear-margin kinematics. Using in situ GNSS-derived velocity gradients from Priestley Glacier's shear margin, the study assesses the ability of four NASA MEaSUREs satellite velocity products to resolve the kinematics of rapidly deforming ice. All satellite products underestimate shear-margin velocity gradients and these biases propagate into derived strain rate fields, resulting in erroneously widened shear zones and underestimated peak shear strain rates. A higher-resolution (64 m) TerraSAR-X-derived deformation map and in situ ice structure observations show that within the shear margin itself, deformation is localised into discrete, discontinuous bands of elevated strain rates associated with the merging of ice bands from upstream tributaries. Resolving finer-scale patterns of strain localisation will yield more accurate descriptions of shear-margin rheology and weakening processes, and will reduce biases in model simulations of outlet glacier dynamics.
This thesis provides new evidence for spatial complexity in shear-margin deformation. Processes responsible for this complexity operate over a range of spatial scales, from the individual ice crystal to the tributary flow bands that together comprise the margin. Neither ice fabric nor its development within the composite margin is represented in the ice-flow models used to project future changes in Antarctic ice-sheet dynamics. High-precision field-based measurements that resolve the fine-scale structure of shear margins can bridge the gap between laboratory ice-sample analysis and whole ice-sheet simulation by identifying the spatial scales at which specific deformation mechanisms must be parameterised and represented in ice-flow models. The observational methodologies developed and applied in this thesis achieve that goal. Further progress in understanding shear margins and outlet-glacier mechanics will depend on measurements that resolve strain and strain rate patterns at spatial resolutions much finer than the ice thickness.