Abstracts
SEARCH Open Science Meeting
October 27, 2003
Seattle, Washington, USA
Submarine Melting at Temperate Tidewater Glacier Termini: How Significant is it?
Roman J. Motyka1, Martin Truffer2
1Geophysical Institute, University of Alaska, 903 Koyukuk Drive, PO Box 757320, Fairbanks, AK, 99775-7320, USA, Phone 907 586-1994, Fax 907 586-5774, jfrjm@uas.alaska.edu
2Geophysical Institute, University of Alaska, 903 Koyukuk Drive, PO Box 757320, Fairbanks, AK, 99775-7320, USA, Phone 907 474-5359, martin.truffer@gi.alaska.edu
One of the most important unresolved questions concerning temperate tidewater glaciers is the role that submarine melting and proglacial convection play in controlling terminus stability. Little is known about ocean thermal forcing of temperate tidewater glaciers even though its seasonal and long-term variation may significantly influence calving speed, terminus dynamics and ocean convection. Relationships developed from field, experimental, and analytical studies on icebergs drifting and melting in seawater have been used to estimate submarine melting at calving termini. However, these calculations give estimates that are typically a small fraction of total calving rate.
Recently, Motyka et al. (2003) used heat and mass balance analysis based on glacier and fjord measurements at LeConte Glacier, a tidewater glacier in southeast Alaska that terminates in 250-m-deep water, to estimate submarine melting. They found that proglacial convection was substantial and that submarine melting contributed significantly to ice loss at the terminus during late summer. Melting was at least as significant as calving in controlling terminus position - if not more. In a similar study at Columbia Glacier, Walters et al. (1988) also found that melting there was seasonally significant, with melt being about half the iceberg calving flux during the summer. These field studies indicate that iceberg analogies do not accurately reflect the dynamic process of turbulent convective flow along the terminus face that is driven by discharge of buoyant subglacial and englacial water.
In our model we propose that turbulent upwelling of subglacial freshwater draws in warm ocean waters and that the mixture rises along the submarine face and melts ice. A consequence of this model is that submarine melt rates should vary as a function of ocean water temperature and subglacial discharge. We suggest that seasonal fluctuations in the terminus position of tidewater glaciers are directly related to seasonal changes in submarine melting, much as termini of land-terminating glaciers are affected by seasonal changes in surface ablation.
Submarine melting may also be involved in controlling the long-term stability of tidewater glacier termini through direct oceanic thermal forcing. Submarine melting could help explain the correlation between annual "calving speed" and water depth found for many well-grounded tidewater glaciers. This is because the percentage area of the terminus face exposed to submarine melting would increase as a function of water depth. It has also been noted that the calving speed - water depth correlation only holds when annually averaged values are used and breaks down for shorter time periods. Our model is consistent with this observation as seasonal changes in convective flow and seawater temperatures would significantly affect melt rates but annual melt rates should be approximately the same.
Buoyancy-driven submarine ablation and seawater temperatures could also help explain the order-of-magnitude disparity in "calving speeds" between tidewater and lacustrine settings. The lack of a strong density contrast and the generally cooler water temperatures encountered at lacustrine calving glaciers would inhibit convection and melting at a sublacustrine face in contrast to submarine environments.
Lastly, there may be a spectrum of submarine melting regimes, from polar ice shelves with little subglacial discharge (e.g., Pine Island, Antarctica) to those with significant subglacial discharge (e.g., Jakobshavn, Greenland) to temperate tidewater glaciers with no floating tongue and strong seasonal subglacial discharge.
Abstract Categories: Changes in the Sea
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