Year of Award

2014

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Joel T. Harper

Commitee Members

Jesse V. Johnson, William Woessner, Marco Maneta, Neil F. Humphrey

Keywords

Glaciology, Greenland, Hydrology, Modeling, Thermodynamics

Publisher

University of Montana

Abstract

In this dissertation I investigate the dynamics of a land-terminating reach of the west Greenland ice sheet through three projects utilizing unique field data and modeling experiments. In Chapter 1 I use in-situ water pressure data and numerical modeling to elucidate the conceptual model of subglacial hydrologic drainage beneath Greenland. Measurements in boreholes drilled to the ice sheet bed along a transect in the ablation zone reveal water pressures that question the stability of water-draining conduits. I apply numerical techniques to model transient evolution of subglacial conduits and show that seasonal growth of such features is unsupported in the ice sheet interior. Low potential gradients that drive energy availability to melt channel walls limit conduit growth. This elucidates the importance of other processes in facilitating seasonal development of the subglacial hydrologic system in the interior setting. In Chapter 2 I investigate the effect of thermal boundary conditions on the thermo-mechanical state of western Greenland. I propose new boundary fields from measurements of temperature near the surface and basal heat flux beneath the ice sheet. Comparison of these observation-based fields with model-driven datasets suggests that model-derived basal heat flux is too high, and surface temperatures too low in the study area. By applying different boundary conditions to a thermo-mechanically coupled ice sheet model I show that thermal conditions at the ice/bedrock interface critically depend on the boundary conditions at both the surface and bed. Unrealistically cold conditions are induced if basal heat flux alone is driven by observations. Warmer surface conditions consistent with observations are sufficient to reintroduce melted conditions at the bed, elucidating the importance of the surface boundary in thermo-mechanical model exploration. In Chapter 3 I address the processes responsible for inducing a region of anomalously low driving stress that is evident in west-southwest Greenland. I show that the feature corresponds to a consistent reduction in surface slope rather than a strong bedrock topographic expression. Kinematic wave experiments show that the diffusive nature of the ice sheet renders the development of such a feature infeasible from surface mass balance perturbations. Low driving stress necessitates a change in dynamics and I surmise it is this variation in basal sliding that is an important factor in inducing changes in the surface slope, and thus the driving stress.

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