Basal and englacial melting of temperate glaciers

Alexander, David (2013). Basal and englacial melting of temperate glaciers PhD Thesis, School of Geography, Planning & Env Management, The University of Queensland.

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Author Alexander, David
Thesis Title Basal and englacial melting of temperate glaciers
School, Centre or Institute School of Geography, Planning & Env Management
Institution The University of Queensland
Publication date 2013
Thesis type PhD Thesis
Supervisor James Shulmeister
Tim Davies
Nik Callow
Total pages 163
Total colour pages 19
Total black and white pages 144
Language eng
Subjects 0499 Other Earth Sciences
Formatted abstract
Omission of basal and englacial melt processes from temperate glacier mass balance models is an oversight that needs addressing. Mass balance models are one of the most widely used tools for exploring climatological-glaciological processes including the response of glaciers to climatic change. Since surface ablation is the dominant contributor to glacier melt and is relatively easy to measure, it is usually the only source of ablation used in models. Basal and englacial melting processes are traditionally considered insignificant and are more challenging to measure; consequently they are always ignored. This thesis provides new insights into the processes of basal and englacial melting and their importance in correctly quantifying temperate glacier melting.

A review of the literature reveals that while basal melt has been modelled beneath ice sheets, ice streams and ice shelves in polar settings for many years, there are few basal and englacial melting measurements or model estimates for temperate glacier settings. This is addressed in the thesis by using two temperate glaciers to investigate basal and englacial melting processes. The high-precipitation Franz Josef Glacier in New Zealand is used to investigate melting from five sources: geothermal heat flux, friction, strain heating, viscous dissipation in flowing water and heat advection. Initial order-of-magnitude estimates indicate that heat advection as a result of runoff warmer than 0°C from rainfall and snowmelt entering the glacier is one of the major basal melting processes affecting high-precipitation temperate glaciers. This process is estimated to result in mean melting across the ablation zone of up to 1.7 m a–1. Further investigations using a thermo-hydraulic melt model calibrated with field data infer that glacier-averaged melting due to heat advection is approximately twice the melting caused by the rainfall heat flux. This is the equivalent of 7% to the total glacier melt, which is a considerable component of glacier mass balance.

Another significant glacier melting process is a result of friction at the ice-bed interface. This process is investigated at a fast-flowing temperate tidewater glacier, the Columbia Glacier in Alaska using a two-dimensional basal melt model. Prior to the retreat of Columbia Glacier (pre-1980s), the mean basal melt due to friction is estimated to be 61 mm a–1 and during retreat (1980s onwards) this increased to 129 mm a–1. These basal melt rates are the equivalent of 3% and 5% of the total melting for the pre- and syn-retreat glacier profiles respectively. Interestingly, modelling indicates that even if there is a reduction in basal resistance due to high water levels within temperate tidewater glaciers, significant basal melt can still occur if ice velocities are sufficiently fast.

The effect of substantial rock avalanche debris increasing the proportion of basal and englacial melt to surface melt is incorporated into a one-dimensional ice flow model used to investigate the evolution of the Waiho Loop, the terminal moraine of the late glacial Franz Josef Glacier. Assuming that the rock avalanche occurred while the glacier was in a period of retreat, modelled basal and englacial melt accounts for 25% of the total melt discharge between the time rock avalanche debris covers the glacier and the time at which all glacier debris advects past the transient climatic terminus (30 years). Of most significance is that the model, supported by newly acquired basal topography from seismic studies in the vicinity of the Loop, demonstrates that the glacier terminus could not advance during its formation. Thus, the formation of the Loop does not reflect a climate-driven glacier advance; it actually represents no significant advance whatsoever.

At a time when glaciological models are increasingly being used to model climatological-glaciological processes including the response of glaciers to climatic change, greater attention should be given to incorporate basal and englacial melting within models to obtain more accurate model outputs. MATLAB code and MS Excel spread sheets for a one-dimensional glacier model are provided as supplementary data to assist in this process. These models facilitate a straightforward and efficient way to assess whether englacial and basal melting may be significant in any temperate glacier setting. This is an important output of the thesis. Exclusion of basal and englacial melt components when considering glacier mass balance represents a systematic underestimation of melt used for mass balance calculations and models. Modelling indicates that, regardless of the type of temperate glacier setting, basal and englacial melt generally contributes 5–10% to the total glacier melt. This may affect long-term interpretations of mass balance.
Keyword Franz Josef Glacier
Mass balance
Debris cover
Energy balance
Columbia Glacier
Waiho Loop

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Created: Fri, 21 Mar 2014, 11:26:25 EST by David Alexander on behalf of Scholarly Communication and Digitisation Service