Oil initiation of super slippery surfaces in sediments: a driver of instability in glacial systems

McCerery, Rebecca (2021) Oil initiation of super slippery surfaces in sediments: a driver of instability in glacial systems. Doctoral thesis, Northumbria University.

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Abstract

The driving mechanisms of glacier fast flow and the cyclical instability inherent in ice streams and surging glaciers are not fully understood. Current theories of sliding and basal deformation insufficiently explain glacier dynamics. Until now, the interface physics occurring at the bed of glaciers and ice sheets has not been considered in glacier flow theory. In this thesis, the role of interface physics, particularly the existence and stability of super slipperiness, superhydrophobicity and slippery liquid-infused porous surfaces (SLIPS) in glacial till, are explored.

First, physical models were used to examine sediment-water interactions on hydrophobic and oil-impregnated models of super slippery sediments. The models were characterised using contact and sliding angles to determine the existence and extent of water repellence and water mobility. Superhydrophobicity was created on clay and clay aggregate size surfaces by coating the particles with a hydrophobic chemistry, producing droplet contact angles of ≥150° and air plastrons between particles. It was also possible to produce sediment-SLIPS with droplet sliding angles ≤5° on clay to silt size particles by impregnating the model sediments with a lubricating oil. This thesis presents the first physically modelled example of a superhydrophobic sediment and sediment-SLIPS, providing a physical basis for the behaviour that could occur in Earth systems where hydrophobic and SLIPS inducing compounds are present.

Second, petroleum geochemistry techniques were employed to detect oil compounds in a glacial environment. Biomarker and non-biomarker hydrocarbon diagnostic ratios were used to identify the key geochemical signatures of Alberta Oil Sands samples from the Aurora Mine, Amphitheatre Outcrop, and Cold Lake deposits. The biomarker analysis revealed the compounds gammacerane and 28,30-bisnorhopane were ubiquitous throughout the Alberta Oil Sands, indicative of a hypersaline depositional environment of the hydrocarbon source rock. A further 12 diagnostic ratios of source, depositional environment, maturity, and biodegradation were identified as key indicators of Alberta Oil Sands contamination based on the small value ranges between samples and good separability from the geochemical signature of North Sea Oil, an unrelated petroleum product. This analysis was then applied to surficial sediment from the Central Alberta Ice Stream in order to detect glacially mobilised SLIPS-inducing oil deposits. Evidence of Alberta Oil Sands contamination including the presence of gammacerane and 28,30-bisnorhopane was detected throughout sediments from the Central Alberta Ice Stream flow track, in particular at the terminating margins to the east of Calgary and in the Cooking Lake area to the southeast of Edmonton. These results indicate glacial erosion and long distance mobilisation of oil sands deposits from Northern Alberta through the Central Alberta Ice Stream. This suggests oil induced SLIPS may have played a significant role in fast flow of the Laurentide Ice Sheet ice streams and flow instability in Alberta.

Three scenarios of SLIPS at the ice-bed interface can be assumed from these results; (i) an oilwet macroscale SLIPS, (ii) a water-wet macroscale SLIPS, and (iii) a microscale SLIPS all of which would influence the degree of ice-bed coupling and therefore the proportion and rates of sliding and basal deformation. These findings also have important implications for understanding soil physics and mechanics, and wider Earth system processes such as slope and sediment fan instabilities. The mechanisms reported in this thesis demonstrate that SLIPS and superhydrophobicity can occur in natural sediments, providing a new mechanism for water shedding in the environment. By understanding the physics occurring at the ice-bed interface it is possible to better predict glacier flow conditions, such as the degree of sliding and basal deformation. It is therefore critical that properties affecting wettability and water shedding of sediments such as sediment geochemistry and microbiology are considered in our understanding of transient flow conditions.

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