Modelling of Large-scale Folds and Ice Streams in Polar Ice Sheets

Abstract

Satellite and airborne sensors have provided detailed data on ice surface flow velocities, englacial structures of ice sheets and bedrock elevations. These data give insight into the flow behaviour of ice sheets and glaciers. One significant structural phenomenon observed is large-scale folds (over 100 m in amplitude) in the englacial stratigraphy in polar ice sheets. A large population of folds is located at ice streams, where the flow is distinctly faster than in the surroundings and bound by the marginal shear zones (known as shear margins), such as the over 500 km long Northeast Greenland Ice Stream (NEGIS). Fast-flowing ice streams drain most of the inland ice from the Antarctic and Greenland ice sheets. However, there is no consensus yet on how these folds and ice streams form. Ice in ice sheets is a ductile material, i.e., it can flow as a thick viscous fluid with a power-law rheology. Furthermore, ice is significantly anisotropic in its flow properties due to its crystallographic preferred orientation. By incorporating an anisotropic, non-linear viscosity and evolving c-axis orientations of ice crystals, this thesis mainly uses the particle-in-cell full-Stokes code Underworld2 to simulate ice flow in three-dimensional large-scale ice-sheet models. The simulated folds with anisotropic ice show complex patterns on a bumpy bedrock, and are classified into three types: large-scale folds (fold amplitudes >100 m), small-scale folds (fold amplitudes <<100 m, wavelength <<km) and recumbent basal-shear folds. The results indicate that ice anisotropy amplifies the perturbations in ice layers (mainly due to bedrock topography) into large-scale folds during flow. Density differences between the warm deep ice and cold ice above may enhance fold amplification. The ice stream models show that ice streams and their shear margins can form solely due to the anisotropic rheology of ice and evolving crystallographic orientations. A fully developed, fast-flowing ice stream can form in only 1000–2000 years by internal ice anisotropy, even without external forcing such as basal melting. As the ice stream evolves, new shear margins establish and subdivide the flow into tributaries. Shear margins continue to migrate along with the ice flow, forming new margins as the system develops further. Dominant tributaries, such as NEGIS, can extend inland, nearly reaching the ice divide, within another 1000–2000 years. Hence, this thesis stresses the importance of evolving ice anisotropy in large-scale ice-sheet models to more accurately predict future ice-sheet evolution and sea-level rise during global climate change.