| Summary: | Glacier and ice sheet mass balance is sensitive to climate change. The geological record has revealed that the polar ice sheets in the past responded rapidly to periods of warming, most likely caused by dynamic changes in ice flow patterns. The rapid ice-sheet dynamical changes observed in the past may cause mass loss in the near future to exceed current best estimates. Ice flow in larger ice sheets focuses in fast-moving streams due to mechanical non-linearity of ice. These ice streams often move at velocities several magnitudes larger than surrounding ice and consequentially constitute a majority of the ice-sheet mass flux. Understanding their physical behavior and sensitivity to changes is of greatest importance for describing ice sheet configuration in the past, present and future. In-situ measurements and interpretations from the Pleistocene sedimentary record have revealed that many glaciers move by deforming their sedimentary beds. Several modern ice streams, in particular, move as plug flows due to basal sediment deformation. An intense and long-winded discussion about the appropriate description for subglacial sediment mechanics followed this discovery, with good reason. The mechanical behavior is likely very important for the evolution of ice-sheet flow in a changing climate, and secondly directly influences the genesis of subglacial landforms seen in previously glaciated areas. Previous studies of subglacial sediment mechanics have relied on field and laboratory experiments. The approach in this PhD project has been to understand fundamental granular and fluid deformation, and apply the insights to improve the understanding the processes governing mechanical stability of subglacial granular materials. For this purpose a numerical formulation for granular and fluid mechanics has been implemented and applied. The computational approach allowed for analysis in unsurpassed detail during progressive deformation. The computational experiments show that granular deformation at glacial velocities conforms to the rate-independent Mohr-Coulomb plasticity. In select cases, however, viscous effects from meltwater deformation can provide additional rate-dependent strengthening. The strengthening may act to stabilize patches of the deforming bed, triggering differential advection and hydrological exchange between the bed and the ice-bed interface. We also show that granular advection during shear deformation is dependent on effective pressure, potentially causing unstable growth of bumps at the ice-bed interface. The process creates wavy subglacial bumps similar to common geomorphological features in past glaciated areas, but the proposed instability mechanism was until now incompatible with commonly accepted till rheology models. Variation in pore-water pressure proves to cause reorganization in the internal stress network and leads to slow creeping deformation. The rate of creep is non-linearly dependent on the applied stresses. Granular creep can explain slow glacial velocities previously associated with elastic or viscous ice deformation. If a glacier dominated by subglacial creep experiences prolonged events of strong surface melt or increased driving stresses, the plastic strength limit can cause rapid acceleration downslope due to imbalance of stresses.
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