GEOPHYSICAL OBSERVATIONS OF POLAR ICE SHEETS AND ICE SHELVES

Open Access
Author:
Horgan, Huw Joseph
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 05, 2009
Committee Members:
  • Dr Sridhar Anandakrishnan, Dissertation Advisor
  • Sridhar Anandakrishnan, Committee Chair
  • Richard B Alley, Committee Member
  • Charles James Ammon, Committee Member
  • Chris Marone, Committee Member
  • Charles Holland, Committee Member
Keywords:
  • seismology
  • greenland
  • antarctica
  • basal melt
  • grounding line
  • ice sheet
  • ice shelf
  • glaciology
  • geophyscis
  • remote sensing
Abstract:
Knowledge of processes, dynamics, and the ongoing mass-balance of polar ice sheets is essential if we are to understand the response of the cryosphere to a changing climate. Here we present a series of hypotheses and associated observations and interpretations addressing the West Antarctic and Greenland ice sheets. Specific attention is paid to the grounding line, ice-shelf mass balance, and crystal orientation fabrics in streaming ice. Initially, GLAS ICESat laser altimetry data is used in an accurate and rapid method of grounding line location. The method exploits the high surface-slope at the grounding line relative to the flat ice-shelf and ice streams. Validation is performed using ground-based observations and comparisons between the modern grounding-line and past estimates indicate that the Siple Coast grounding line has been largely static for at least several decades. During this time the ice streams have been undergoing large changes in flow speed indicating that the grounding-line position is insensitive to such changes. In order to address the mass balance of the Ross Ice Shelf, a divergence method assuming steady-state is used to estimate the spatial distribution and magnitude of basal melting (Mb). An area average rate of -0.08 +/- 0.01 m/a is estimated indicating that accretion dominates the sub-ice-shelf environment with rates of Mb=-0.32  +/- 0.01 m/a estimated in the centre of the ice shelf. Our estimates of accretion are an order of magnitude higher than previous studies and we caution that this is likely due to the divergence method misinterpreting past non-steady-state behavior of the ice streams. High melt rates (1.3 +/- 0.1 m/a) are observed at the ice shelf front. The ice front is further investigated using spatial and temporal elevation changes from GLAS ICESat laser altimetry data. Melt rates are observed to increase exponentially as the front is approached, from zero at approximately 40 km from the front to an average of 2.7 +/- 0.9 m/a within the front kilometer. Melt estimates are best fit by the relationship Mb=2.1exp(x/11800) m/a. Melt at the front is modeled as a combination of tidally-induced mixing and the ascension of buoyant water from beneath the ice shelf, indicating a relationship between melt profile and calving history. In the final section of this study, active-source seismic data are reported from an upstream location on Greenland's fastest-flowing outlet glacier, Jakobshavn Isbrae. Englacial reflectivity in these data reveal the development of complex and alternating crystal orientation fabrics, which we associate with changes in impurity loading brought about by climactic changes. These fabrics likely result in strain localization and therefore have implications for predictive ice sheet modeling.