Air bubbles in glacial ice may be useful indicators of ice dynamics. To address this issue, I developed two methods to image air bubbles. Micro-computed tomography (Micro-CT) imaging enables bubble populations to be quantified in three dimensions. Optical imaging of thin (~5mm) slices enable bubble populations to be quantified in two dimensions. The optical method is faster and cheaper and therefore can be applied to more samples.

These two approaches are applied to samples from the lateral shear margin of the Priestley Glacier, Terra Nova Bay, Antarctica. These imaging techniques showed that bubble shape varies from near spherical to highly elongate within the same sample area. There are two populations of bubbles seen in the ice samples, those whose orientations are consistent with the bulk strain of the ice and those aligned sub parallel to the c-axis maximum. Complexities in bubble orientation could be explained by fold structures and other strain heterogeneities in the ice. This research has showed a redistribution of bubble volume within the glacier depth in response to ablation and temperature gradients with bubbles moving up the glacier depth over time redistributing air volume as this occurs. The bubbles are less stretched than we would expect for the amount of shear constrained from field data. This poses the question of what controls the kinetics of bubble shape evolution. How do deformation conditions (e.g., temperature, strain rate) affect the balance between bubble deformation (stretching the bubbles) and restoration to a sphere, driven by surface energy.

I use a novel a deformation apparatus that allows measuring bubble evolution iteratively during ice deformation. A thin (5mm) slice, with two larger side pieces of ice are deformed together. Teflon sheets separate the thin slice from the side pieces so that it can be removed to take photos of the bubbles and then return to the assembly for more deformation after this. The photos taken at regular time intervals during deformation allow us to measure bubble deformation kinetics and understand better the processes important during deformation. Deformation experiments were conducted at temperatures of ~11°C at stresses between 0.40 and ~1.2MPa with experiment periods ranging from 1 to 32 days. Strain rates for these experiments ranged from 8.72x10^-8s^-1 to 4.54x10^-6s^-1. All three of the experiments ran show change in bubble shape over the experiment period as a result of strain. The experiments resulted in elongation of air bubbles reflecting strain and an alignment of bubble orientation. The mechanics of these experiments are the same as more conventional deformation experiments conducted on glacial ice.

I use warming experiments to constrain the kinematics of air bubble restoration and to understand better the process. The sample that was deformed in the laboratory in experiment 2 was placed into a warming chamber at -3.5°C for a total of 255 days being removed on day 127 for imaging and data collection. Under these conditions a change in shape occurred in some of the bubbles. Using the optical images taken, changes in the bubble data could be used to calculate the rates at which bubbles of varying sizes underwent restoration. It could be determined that the rate of bubble restoration is correlated to bubble size with bubbles of smaller volume undergoing restoration at a faster rate than larger bubbles.

Results from these experiments could be scaled to natural conditions. In scaling these results it is understood that in the Priestley Glacier environment all bubbles deform at a consistent rate however only smaller bubbles with a restoration rate faster than the deformation rate (<0.2mm bubble diameter) undergo the most significant restoration being restored to a more spherical shape.