Jökull


Jökull - 01.01.2016, Page 36

Jökull - 01.01.2016, Page 36
Jonathan L. Carrivick et al. only the Upptyppingar site receives meltwater directly from Kverkfjöll so the pair of sites together provide a useful assessment of whether meltwater runoff from Kverkfjöll catchment is unusual. RESULTS AND INTERPRETATION The hill-shaded DEMs revealed considerable struc- ture, mainly crevasses, on the ice surface, because snow cover on the glacier is either absent or relatively thin, at least below ∼1500 m a.s.l. (Figure 2A) due to the late summer data acquisition. Additionally, the hill-shaded DEMs indicated roughening of the ice sur- face due to increased crevassing, notwithstanding the possibility of more snow on the surface in 2011 than in 2007, and expansion laterally of the lower parts of the glacier against lateral moraines and cliffs (Figure 2B). The DoD (Figure 2C) showed a pattern of sur- face elevation changes between 2007 and 2011 that was negative at higher elevation and positive at lower elevation, which is remarkable because it is the oppo- site of what could be expected due to an overall neg- ative mass balance regime of a normal glacier and ice dynamics. The magnitude of these surface elevation changes; tens of metres, and most importantly the spa- tial pattern of elevation changes along the glacier, can- not be explained by snow cover being thicker in one year or the other. Based on these variations in surface topography we infer that Kverkjökull surged during the 2007 to 2011 period. The boundary between the reservoir and receiving zones, which can be identified by a zero elevation change isoline, was complex and ranged from 1400 to 1528 m a.s.l. and outlines at least three lobate-shaped areas of the glacier (Figure 2C). Comparison of our dGPS measurements of spot elevations in 2007 with the ALS-derived DEM of 2007 have excellent agreement, and comparison of our 2008 dGPS elevations with the 2007 ALS data show little change (Figure 3), together giving us con- fidence that the surge had not yet started, or at least not reached the terminus, in August 2008. Further- more, comparison between our dGPS measured el- evation and those in the 2008 and the 2011 DEM demonstrate that by 2011 the north-eastern and south- western portions of the Kverkjökull terminus were behaving differently. Specifically, the north-eastern part of the glacier showed surface lowering, i.e. thin- ning, as would be expected due to a negative mass balance, or during the quiescence period of a surge- type glacier, whereas the south-western portion of the terminus had thickened, with the greatest thickening towards the terminus, as indicative of the passage of a surge front (Figure 2C). Figure 4. Ice thickness estimated via a 1D steady state ‘perfect-plasticity’ model. – Ísþykkt metin með ein- víðu, æstæðu, plastísku líkani. The asymmetric impact of the surge on ice sur- face elevations in the terminus area was conspicu- ous and warranted further investigation. Although our overall pattern of estimated ice thickness, based on a one-dimensional centreline flow model, looks rea- sonable (Figure 4) we have no direct measurements of the detailed form of the bed geometry. How- ever, the increased crevassing and amplified hum- mocky ice-surface undulations in the 2011 DEM (Fig- ure 2A,B) that extend from the centre of the termi- nus in a south-easterly direction do not correspond 36 JÖKULL No. 66, 2016
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