Geophysical Exploration of Glacier Basal Processes and Grounding Line Dynamics

Open Access
Author:
Christianson, Knut Andrew
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 28, 2011
Committee Members:
  • Sridhar Anandakrishnan, Dissertation Advisor
  • Richard B Alley, Committee Member
  • Peter Christopher Lafemina, Committee Member
  • Derrick J Lampkin, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • glaciers
  • ice sheets
  • geophysics
  • ice-penetrating radar
  • GPS
Abstract:
In order to accurately predict the reaction of ice sheets to climate change and their contribution to sea level, we must understand ice sheet dynamics and incorporate these dynamics into prognostic models. Here we present observations and model results that aim to advance our understanding of the ice/bed interface of ice sheets and glaciers by examining possible grounding line retreat scenarios of a large marine-based West Antarctic glacier, investigating subglacial hydrology under a stable West Antarctic ice stream, and directly instrumenting the basal interface of Engabreen (a large, maritime glacier in northern Norway). First, we use airborne laser altimetry and ice-penetrating radar data to map the surface and subglacial topography of the grounding zone of Thwaites Glacier, West Antarctica which is especially prone to the marine ice sheet instability as it is grounded up to 2.6 km below sea level on bedrock that deepens inland and is currently losing mass. Our results show that the glacier is currently grounded on a bedrock sill, that there is at least one more coherent, large amplitude bedrock sill ∼10 km inland from the current grounding line, and also that basal crevassing and high basal reflectivity persist several kilometers inland from the grounding line. This suggests that warm ocean water may penetrate beyond the grounding zone. We use a coupled 2-dimensional ice stream/ice shelf/ocean plume model to examine the sensitivity of the glacier to ocean melt by weakening the grounding zone via propagation of basal melt inland of the grounding line and reducing the strength of the glacier’s bed. The model results suggest that if basal melt occurs across a grounding zone, and not a single grounding point, the glacier is extremely sensi- tive to local topography and that a grounding zone wider than a critical bedrock bump size will allow rapid retreat of the glacier on the ∼500 year timescale. The critical size of the bedrock bump is dependent on the relative amplitude of other bedrock sills on the same flowline, but is as small as 6 km which is similar in size to the observed grounding zone on Thwaites Glacier. In the second portion of this thesis, we present the results of kinematic GPS and ice-penetrating radar surveys of Subglacial Lake Whillans, an active subglacial lake under Whillans Ice Stream, West Antarctica. The lake is manifested on the surface as a ∼15 m depression in its low-stand state. Radar imaging of the subglacial lake, although indicating wet conditions, is consistent with a water column depth of only ∼6 m. A steep ridge in basal topography that is coincident with a strong contrast in relative basal reflectivity (∼6 dB) appears to currently confine the lake. Mapped hydropotential shows that the lake cannot drain via a simple flotation model; however, an increase in water column thickness by ∼5 m is sufficient to allow drainage via a buoyant cantilever effect. Thus Subglacial Lake Whillans acts as a temporary water storage basin beneath Whillans Ice Stream. Finally, we present a series of observations from Engabreen, a mountain glacier in northern Norway, where we can directly instrument the bed of the glacier via access provided by hydropower tunnels. Passive seismic data illustrate that the glacier can transition from a mode where a considerable portion of the basal motion is accommodated via stick-slip motion to one where basal sliding dominates with only a modest change in basal water input from surface melt (< 1 m3/s change in subglacial water flux). We also present observations of a “spring speed-up” event or the first introduction of substantial meltwater to the glacier’s bed at the beginning of the melt season. This is subglacially manifested as a 400 kPa subglacial water pressure pulse accompanied initially by a long-period seismic signal followed by high frequency seismic signal indicative of basal cracking. We interpret this as the arrival of a large horizontal force, the glacier beginning to accelerate forward, followed by a large vertical signal, the arrival of the water pulse. Similar, but lower amplitude signals, that occur after the initial event suggest that the glacier basal hydrology system continues to dynamically reorganize in response to input forcing from surface meltwater or precipitation. Therefore hysteresis may need to be incorporated into glacier models as results to input forcing may depend on the prior organization of the glacier’s subglacial hydraulic system.