Jökull - 01.01.2016, Blaðsíða 22

Eyjólfur Magnússon et al.
served displacement has a significant cross track com-
ponent relative to the modelled flow direction, partic-
ularly in 1946–1985 and again in 2011–2014. This
may indicate that these features do not only move
with the glacier surface but also migrate in the sur-
face. The direction of the migration may be associ-
ated with the alignment of the crevasses opening in the
surges, traversing these depressions approximately in
the cross track direction of the flow, and existing un-
derneath the snow cover for years after the surges.
Wind scouring and snow drift may also cause dif-
ference in snow accumulation between the north and
south end of the depressions enhancing migration of
these features in a northward direction.
CONCLUSIONS
The dense RES-survey of Drangajökull ice cap has
produced the most detailed bedrock DEM of an Ice-
landic ice cap, available at present. The bedrock to-
pography of Drangajökull is similar to its vicinity
(i.e. the Vestfirðir peninsula). The ice cap volume is
15.4±0.4 km3 with an average thickness 107±3 m,
relative to the glacier surface in July 2011. The corre-
sponding values relative to the surface of March 2014
are 16.1±0.4 km3 for volume and 112±3 m for av-
erage thickness. The largest proportion of the ice
is stored in the ice catchment of Kaldalónsjökull or
4.6±0.1 km3 (∼30% of the total volume) and it also
includes the thickest glacier measured, 284±14 m
(relative to July 2011). In 2011 about 71% of the
ice cap volume was stored in the ice catchments on
the west side of the Drangajökull (Leirufjarðarjökull,
Kaldalónsjökull and SW-Drangajökull).
The east part of Drangajökull, with both less vol-
ume and significantly thinner ice, has also been losing
volume at ∼3 times faster rate on average than the
west part of the ice cap. The glaciers on the west side,
will therefore exist for much longer than the glaciers
on the east side if the observed pattern of retreat con-
tinues. Thus glacial melt water in the rivers draining
towards east will disappear long before it disappears
in rivers draining west, if similar retreat continues.
By integrating the glacier thickness, obtained
from the difference between the bed DEM and surface
DEMs at various times, a record of the ice cap volume
has been obtained. This record shows that Dranga-
jökull’s volume was reduced from ∼18 km3 in the
autumn of 1946 to ∼16 km3 in the autumn 1975, fol-
lowed by ∼0.5 km3 volume gain until the fall of 1994.
In the autumn of 2014 the volume of Drangajökull
corresponded to ∼15 km3, corresponding to ∼17%
volume reduction since 1946. Around half of this re-
duction occurred in the period 1994–2011. The sea-
sonal fluctuation in the volume of Drangajökull cor-
responds to around a third of the net volume change
since 1946.
The detailed RES-data reveals several interesting
features of Drangajökull ice cap. These include:
Significant horizontal shift between the surface
crest and the topographic ridge in the glacier bed at
the ice divides between the east and west part of the
ice cap. This is particularly pronounced south of the
summit, Jökulbunga, where the surface crest is typi-
cally shifted 300–600 m in SW direction relative to
the bedrock ridge. The direction of the shift fits well
with the governing snow drift direction from NE. Due
to this mismatch, it is likely that water divides be-
tween the eastern and western drainage catchments
of Drangajökull will propagate eastwards if thinning
of the glacier continues, enlarging the drainage catch-
ments of Mórilla and Selá at the expense of Reykjar-
fjarðarós and Bjarnarfjarðará.
Typical ice thickness of 40–60 m underneath de-
bris patches of Langahraun near the margin of SW-
Drangajökull. Prior to this work it was not known
that Langahraun is debris covered glacier ice. A faint
internal layer that can be traced from the surface at the
upper edge of Langahraun down to the bed over a dis-
tance of ∼300 m implies that Langahraun is partially
accumulated debris, which has been scraped from the
glacier bed and transported to the surface by a main
glacier sliding over a smaller ice unit.
Ridges and troughs beneath S-Drangajökull with
dominant N-S strike, of which some can be traced
across the ice cap. The same alignment in landforms
is very common south and east of Drangajökull and is
most likely joints and fractures in the bedrock.
The new bed DEM opens opportunity for further
studies of Drangajökull ice cap, e.g. numerical ice
flow modelling. An example shown here is a study
22 JÖKULL No. 66, 2016