Numerous supraglacial and proglacial lakes have developed on debris-covered glaciers in conjunction with 20th-century retreat associated with global warming. When a glacier holds a substantial debris cover on its lower reach and/or is calving into a proglacial water body, the behaviour of its terminus can be modified to varying degrees compared to that of land-terminating or debris-free glaciers. The terminus is not just retreating from its frontal position but it is disintegrating through several processes that are linked. An improved understanding of these glacier margins is needed for the prediction and management of hazards associated with these types of lakes for hydroelectric power generation, recreational purposes and areas threatened by potential glacier outburst floods as well as for the interpretation of glacio-geological records and reconstruction of former glacial environments and palaeoclimate.
The principal research question of this study is how processes of ice loss contribute to the terminus disintegration of a debris-covered, lake-calving glacier. This is addressed by an application of a field-based strategy which includes extensive field observations of variables, processes and their controls, and subsequent analysis of the data in the light of previous models and concepts. The study attempts to combine and integrate different aspects of glaciological research that have previously been examined mostly separately. It investigates the prevalent processes at the glacier terminus and their controls over different time periods ranging from days to years at Mueller, Hooker and Tasman Glaciers in Mount Cook National Park, New Zealand. The data form the basis for models of calving and pond development and future retreat scenarios.
This study has demonstrated that this glacial environment is characterised by ice-frontal processes with complex inter-relationships that vary between glaciers and in particular between stages of terminus development. While surface ice melt in the terminus area is substantially reduced by supraglacial debris, sub-debris melt contributes the largest fraction of ice loss. Other important effects of debris are restraining thermal undercutting, reducing subaqueous melt and decreasing buoyancy. Data from supraglacial ponds and proglacial lakes show that limnological factors become increasingly important with increasing pond/lake size. Changes in water currents and temperature lead to changes in significance and rates of ice loss processes, the most important being the change from melting to predominantly calving. This study has confirmed the hypothesis that thermal undercutting is the rate-controlling process for calving. This process is controlled by the cliff geometry, debris supply, subaqueous geometry and water temperatures, currents and level variations. The results from the examination of calving processes suggest that the process of regular, progressive calving through the stages suggested previously may not be widely applicable to slow-moving, lake-calving glaciers. The several forms of subaerial calving identified in this study can present themselves as largely independent events, a combination of events or as a progression. At the central submerged part of the ice face, subaqueous ice melt is likely to be the dominant form of ice loss, leading to horizontal ice loss. Subaqueous calving is prevalent in gently-sloping lateral areas, leading to vertical ice loss. This process is controlled by buoyancy forces which are affected by sedimentation and lake and glacier geometry. The onset of subaqueous calving in the earlier stages of lake development is a crucial process for the transition to faster disintegration and ice loss, accelerating subaqueous melt. Due to the complex inter-relationships attempts to formulate general relationships between calving or retreat rates and other glaciological parameters may not be feasible.
| Identifer | oai:union.ndltd.org:ADTP/217588 |
| Date | January 2006 |
| Creators | Roehl, Katrin, n/a |
| Publisher | University of Otago. Department of Geography |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Katrin Roehl |
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