Modelling the Evolution, Rheology, and Buttressing of Antarctic Pinning Points

Abstract

The Antarctic Ice Sheet has the potential to substantially influence global sea level under a warming climate. Precise projections of ice loss rely on an understanding of physical mechanisms and an accurate model representations of coastal ice flow dynamics. Regulation of ice flow towards the open ocean occurs due to the drag caused by ice rises and ice rumples, areas of grounded ice which form in ice shelves and are surrounded by otherwise floating ice. Due to their complex flow regimes and small size, isle-type ice rises and ice rumples are often under-represented in ice flow models. Using the finite element model Elmer/Ice with a full Stokes setup, the evolution of ice rises and ice rumples in response to sea level variation over glacial-interglacial timescales is investigated. Findings show that the grounded area of ice rises and ice rumples responds with hysteresis to sea level variation. The hysteresis is reflected in the upstream velocity field, meaning that ice shelf buttressing has the potential to be irreversible under a changing climate having significant consequences given the large number of pinning points in coastal Antarctica. Building on work highlighting the importance of pinning points in continental grounding line evolution, the results of the quantification of pinning points buttressing forces using full Stokes simulations with varying pinning point sizes and upstream ice shelf fluxes are presented.

In a real-world application, three-dimensional simulations of the velocity and age fields of Derwael Ice Rise (DIR) are performed building on previous studies of the two-dimensional simulated isochronal stratigraphy of ice rises which were restricted to grounded ice and did not allow for through-plane flow. Results show a good match between modelled isochronal stratigraphy and observed isochronal stratigraphy obtained from airborne data in grounded ice and across the grounding zone. This work provides a tool for comparison with ice cores and radargrams, allowing co-validation and extrapolation of the age field beyond observed data points, as well as for choosing sites for ice core drilling. Building on a study which found that the often-used Glen’s flow law of is an under-estimation in ice shelves, an investigation of the influence of the choice of Glen’s flow law exponent on the age field of DIR is performed. The results show that differences between using a Glen’s flow law exponent of n=3 and n=4 result in age differences of <5% at depth. The largest differences between simulations with Glen’s flow law exponents of n=3 and n=4 are seen in the shear zones between DIR and the ice shelf, where shear strain rates are higher in the simulation, and could have consequences, for example, in fracture modelling. Finally, the results of simulations of the three-dimensional anisotropy field of DIR are presented. In these simulations, the influence of the strain-rate and deviatoric stress tensors on the crystal orientation evolution is varied, based on previous studies. Results vary across the ice rise, but most significantly where horizontal divergence of flow occurs. A framework for comparing modelled anisotropy with observations is developed and estimates are presented for the error resulting from an assumption in radar data acquisition that one crystal orientation tensor eigenvector is vertical. The results presented in this thesis advance the understanding of the role of pinning points in coastal Antarctic ice-flow dynamics and provide improved modelling frameworks for comparison with geophysical observations.