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Saturday, October 13, 2012

Mauri Pelto: Thwaites Glacier Tongue Major Calving Event, Antarctica

by Mauri Pelto, From A Glacier's Perspective, October 8, 2012

During the Austral winter between April and September 2012, the Thwaites Glacier (a large outlet glacier of the West Antarctic ice Sheet: WAIS) experienced a major calving event, as was pointed out by a reader on this blog. The Thwaites, along with Pine Island Glacier, is often considered the weak underbelly of the ice sheet. MODIS imagery from February, March, September and October is used here to illustrate the event. The red, magenta and green arrows point to the same location in each image. The blue dots mark the ice front. The orange arrow points to a portion of the Thwaites Glacier Ice Tongue that had previously calved in 2002/03. In the latter two images the yellow arrow indicates the new ice tongue piece that has broken off.

The Thwaites, along with the Pine Island Glacier, is an important glacier because they both tap into the heart of the WAIS but lack big buttressing ice shelves. This is why in 1985 I was busy preparing a bedrock map in front of the glacier from two shipboard seismic transects for Tom Kellogg and Terry Hughes at U Maine. Of course that map did not look nearly as good as the recent ones in Tinto and Bell (2011) from the extensive NASA Operation Icebridge. What they indicate in the two following images from their paper is that before it broke off the new ice tongue was ending on top of a bedrock high (red arrow first image below) that was acting as pinning for the ice tongue. The thicker the ice at that point, the better the pinning point. This bedrock high is noted in their Figure 3D with a white arrow. Kirsty Tinto and Robin Bell (2011) of Columbia University also predicted a faster retreat for the Thwaites Glacier. Rignot et al. (2002) noted that the Thwaites Glacier's grounding line was in retreat, the mass loss was increasing, and the glacier was thinning, although not nearly as fast as the Pine Island Glacier. In the last image below from Rignot (2008), the velocities of the Pine Island and Thwaites Glaciers are shown; the high velocity region of Thwaites is over a wider area, and this has been expanding ((Rignot et al. 2002). The behavior of the Thwaites Glacier with regard to ice frontal changes and thinning is like that of the Smith Glacier also to its west. Unlike the Petermann Glacier, where there is no past history to suggest its terminus will readvance significantly towards its former position, Thwaites Glacier has experienced large calving events in the past, such as in 2002/03. The difference on this occasion is that the ice tongue was quite small and hence more stable than in previous calving events. The continued thinning makes the readvance and subsequent pinning on the bedrock ridge less likely. The rifting is also quite extensive as MacGregor et al. (2012) have noted, I look forward to the observations that will emerge from NASA’s Operation Ice Bridge in Antarctica that begins in a week.

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