Jökull - 01.01.2016, Side 95

Changes in the flow pattern of Breiðamerkurjökull reflected by
bending of the Esjufjallarönd medial moraine
Snævarr Guðmundsson1 and Helgi Björnsson2
1South East Iceland Nature Research Center, Litlubrú 2, Höfn í Hornafirði, Iceland
2Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík, Iceland
Correspondence to: snaevarr@nattsa.is
Abstract — The Esjufjallarönd medial moraine separates two branches of the south flowing Breiðamerkurjök-
ull outlet of Vatnajökull ice cap, Southeast-Iceland. The more rapidly flowing easternmost branch (Norðlinga-
lægðarjökull) descends along a trench, reaching 200–300 m beneath sea level, and calves into the Jökulsárlón
lagoon. Recently the rate of calving has increased and a depression has formed in the glacier surface. Hence,
inflow of ice toward the eastern branch has led to a lateral shift of the Esjufjallarönd medial moraine bending
it eastward by up to 900 m, during the period 2006 to 2016. Thus, the moraine has been shifted by ice flow into
the lagoon.
SITE DESCRIPTION
Breiðamerkurjökull is the fourth largest outlet of
Vatnajökull ice cap, SE-Iceland. It contains three
major ice branches (Mávabyggðarjökull, Esjufjalla-
jökull and Norðlingalægðarjökull) that are separated
by two medial moraines named Mávabyggðarönd
and Esjufjallarönd (Figure 1), originating from the
nunataks Mávabyggðir and the Esjufjöll, respectively.
Norðlingalægðarjökull flows down a 25 km long
and 300 m deep subglacial trench (Björnsson et al.,
1992; Björnsson, 1996) which may have been exca-
vated by Breiðamerkurjökull during its Little Ice Age
advance (until late 19th century), including several
surge events (see Pálsson, 1945; Henderson, 1815;
Watts, 1962; Thoroddsen, 1931; Thorarinsson, 1943;
Sigbjarnarson, 1970; Björnsson, 1998; Björnsson et
al., 2003; Björnsson, 2009, 2016). Norðlingalægðar-
jökull is the fastest flowing branch of Breiðamerkur-
jökull (Björnsson et al., 2001, Björnsson, 2016, fig-
ure 3.14, p. 118). Since the 1890s the withdrawal
of Breiðamerkurjökull’s margin has been from 33 to
59 m yr−1 (Guðmundsson, 2014).
Jökulsárlón lagoon occupies the southernmost
part of the subglacial trench. The lake emerged during
the rapid retreat of Breiðamerkurjökull in the 1930s
and has gradually become larger as the recession has
proceeded (Björnsson, 2009). Since the formation of
the lagoon the medial moraine has terminated at the
west margin of Jökulsárlón. The retreat slowed down
from the 1980s until 1995, but has since then been in-
creasing. From 2010–2016 the front retreated by 0.8–
1.4 km. The calving front has retreated on the aver-
age by 200–280 m yr−1 although the downward speed
of the ice stream is about 550 m yr−1 (Björnsson et
al., 2001; Eyjólfur Magnússon, personal communica-
tion). A horseshoe-like ice front has been formed in
the lake (Figure 2) and a depression in the ice surface
upstream from the lagoon has become more and more
evident in the 21st century.
The first map showing the Esjufjallarönd me-
dial moraine was surveyed by the Danish General
Staff (DGS) in 1904 (Herforingjaráðið 1905). The
moraine, however, may have been paid attention to by
Gunnlaugsson (1844) when mapping Iceland, as he
drew up regional district limits for >13 km upglacier
from the Jökulsá river. The medial moraine is located
above the western slopes of the trench of Norðlinga-
lægðarjökull, where it slopes into the trench from 20
JÖKULL No. 66, 2016 95