Jökull - 01.01.2016, Side 28

Jonathan L. Carrivick et al.
through the analysis of surface velocity and geometry
changes (e.g. Quincey et al., 2011, 2015; Sevestre et
al., 2015) and improved understanding of surges may
shed light on velocity variations of the large ice sheets
of Greenland and Antarctica where large variations of
ice flow velocity have attracted increasing attention in
recent years (Rignot and Kanagaratnam, 2006; Rignot
et al., 2011).
Identification of glacier surges tends to focus on
rapid terminus position advances (e.g. Meier and
Post, 1969; Hewitt, 1998; Björnsson et al., 2003)
and on surface morphology that is indicative of a
surge such as looped medial moraines, looped de-
bris bands, contorted longitudinal foliation, and in-
tense and chaotic crevassing (e.g. Barrand and Mur-
ray, 2006; Hewitt, 2007; Copland et al., 2011; King
et al., 2015; Paul, 2015; Ingólfsson et al., 2016).
Characterisation of glacier surges and interpretation
of surge mechanisms usually comes from analyses of
surface velocity where dramatic speed-ups and slow-
downs and zones of high velocity propagating both
up-glacier and down-glacier have been quantified (e.g.
Raymond and Malone, 1986; Kamb and Engelhardt,
1986; Pritchard et al., 2003; Fischer et al., 2003;
Kotlyakov et al., 2008; Quincey et al., 2011; 2015;
Copland et al., 2009; Heid and Kääb, 2012; Burgess
et al., 2012; Rankl et al., 2014; Dunse et al., 2015).
In contrast, measurements of surface elevation
changes during glacier surges have been more lim-
ited, which is primarily due to poor data availabil-
ity. This is unfortunate because the surface elevation
evolution of surging glaciers is important for under-
standing surge mechanisms (Meier and Post, 1969;
Sund et al., 2009). To date, measurements of 3D
geometry changes of surges have either been (i) re-
stricted to a few along-track profiles from altimetry
data (e.g. Muskett et al., 2009; Burgess et al., 2012;
Herzfeld et al., 2013), which has good vertical accu-
racy (typically ∼1 m) or (ii) of medium (>10 m but
usually 25 m or 30 m) spatial resolution (e.g. Mus-
kett et al., 2008, Shugar et al., 2010; Kristensen and
Benn, 2012; Gardelle et al., 2013; Rankl et al., 2014;
Pitte et al., 2016) and consequently with poorer verti-
cal accuracy (typically ∼10 m). Thus, these medium
spatial resolution analyses have focussed on longitu-
dinal changes in glacier elevation profiles with some
notable exceptions from studies in Iceland where
spatially-distributed elevation changes have been re-
ported including the country-wide studies of Björns-
son et al. (2003), on the northern margin of Vatna-
jökull at Dyngjujökull, Aðalgeirsdóttir et al. (2005),
around Vatnajökull by Magnússon et al. (2005), and at
Drangajökull by Magnússon et al. (2016) and Brynj-
ólfsson et al. (2016). Globally, measurements of
3D geometry changes of surging glaciers have been
focussed on tidewater glaciers rather than on land-
terminating glaciers.
Studies that have obtained high-resolution sur-
face elevation measurements of surging glaciers
have done so typically with ∼10 m grid cell size
photogrametrically-derived digital elevation models
(DEMs) (e.g. Murray et al., 2012; King et al., 2015)
or else with ∼2 m resolution Airborne Laser Scan-
ning (ALS) data but only for a single time frame
only (e.g. Murray et al., 2012). These studies have
demonstrated the utility of these high-resolution data
for (i) quantifying spatially-distributed (longitudinal
and lateral) variability in ice surface elevation, struc-
ture and morphology during surges, (ii) computing
volume and mass displacements, and (iii) thereby as-
sessing hypotheses of surge mechanisms. The works
of Pitte et al. (2016) and Brynjólfsson et al. (2016)
are notable because they are the only studies to date
that have explicitly quantified and fully analysed the
3D geometry changes of a surging land-terminating
glacier (Table 1). In contrast, Sund et al. (2009) ex-
amined elevation changes of many surging glaciers in
Svalbard, 72% of which were land-terminating. They
identified up to +40 m elevation changes due to the
surges and, having examined multiple glaciers, iden-
tified three stages in the surge development.
In Alaska, 8% of the land-terminating glaciers
studied by Arendt et al. (2002) for their elevation
changes were identified as surging, but the eleva-
tion values for individual glaciers were not reported
(Table 1). In the Karakorum, there are 101 docu-
mented surge-type glaciers (Rankl et al., 2014), which
constitute ∼13% of all Karakorum glaciers (Barrand
and Murray, 2006). Only Gardelle et al. (2013) have
quantified elevation changes of those land-terminating
28 JÖKULL No. 66, 2016