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The Cryosphere, 5, 139–149, 2011
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doi:10.5194/tc-5-139-2011
© Author(s) 2011. CC Attribution 3.0 License.
The Cryosphere
Thermal structure and drainage system of a small valley glacier
(Tellbreen, Svalbard), investigated by ground penetrating radar
K. Bælum1and D. I. Benn1,2
1Department of Geology, University Centre in Svalbard (UNIS), Longyearbyen, Norway
2School of Geography and Geosciences, University of St Andrews, UK
Received: 13 October 2010 – Published in The Cryosphere Discuss.: 21 October 2010
Revised: 17 February 2011 – Accepted: 18 February 2011 – Published: 4 March 2011
Abstract. Proglacial icings accumulate in front of many
High Arctic glaciers during the winter months, as water es-
capes from englacial or subglacial storage. Such icings have
been interpreted as evidence for warm-based subglacial con-
ditions, but several are now known to occur in front of cold-
based glaciers. In this study, we investigate the drainage
system of Tellbreen, a 3.5 km long glacier in central Spits-
bergen, where a large proglacial icing develops each win-
ter, to determine the location and geometry of storage ele-
ments. Digital elevation models (DEMs) of the glacier sur-
face and bed were constructed using maps, differential GPS
and ground penetrating radar (GPR). Rates of surface lower-
ing indicate that the glacier has a long-term mass balance of
−0.6±0.2m/year. Englacial and subglacial drainage chan-
nels were mapped using GPR, showing that Tellbreen has
a diverse drainage system that is capable of storing, trans-
porting and releasing water year round. In the upper part
of the glacier, drainage is mainly via supraglacial channels.
These transition downglacier into shallow englacial “cut and
closure” channels, formed by the incision and roof closure
of supraglacial channels. Below thin ice near the termi-
nus, these channels reach the bed and contain stored wa-
ter throughout the winter months. Even though no signs of
temperate ice were detected and the bed is below pressure-
melting point, Tellbreen has a surface-fed, channelized sub-
glacial drainage system, which allows significant storage and
delayed discharge.
Correspondence to: K. Bælum
(karolineb@unis.no)
1 Introduction
Large icings (also known as naled ice) accumulate each win-
ter in front of many glaciers in Svalbard. Although snow-
melt can occur during brief periods of positive air tempera-
ture in winter (Humlum et al., 2003), this source of water is
insufficient to explain either the volume or quasi-continuous
accumulation of proglacial icings, and the out-flowing water
most likely indicates the release of water from en- or sub-
glacial storage. Traditionally the presence of large icings
has been interpreted as evidence for warm-based polyther-
mal conditions (Hagen et al., 2003a), under which water can
be produced subglacially throughout the year. Some studies
have shown that icings also occur in front of glaciers that are
predominantly or entirely cold-based (Hodgkins, 1997), al-
though in such cases the location and distribution of stored
water are not well understood.
There have been comparatively few investigations of
drainage systems on predominantly cold based arctic glaciers
with evident outflow of water in the winter season (Hodgkins,
1997; Temminghoff, 2009; Van Hoof, 2008), and consid-
erable uncertainty exists about the source, flowpath, and
residence time of such water. In this study, we use GPR
data to investigate the thermal characteristics, structure, and
drainage system of Tellbreen, a small valley glacier in central
Spitsbergen. Specifically, the aims of this paper are: (1) to
determine the thermal regime of the glacier, particularly
whether there are any areas of temperate ice, (2) to establish
the location of stored water within and beneath the glacier,
and its relationship with englacial and subglacial drainage
channels, and (3) to determine changes in the area, volume
and thickness of the glacier since the Little Ice Age, to pro-
vide a context for other data.
Published by Copernicus Publications on behalf of the European Geosciences Union.
140 K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier
Map outline
2005 outline
GPR lines 2005
GPR lines 2009
GPR lines 2010
Fig. 1. Location of Tellbreen (inset) and 1995 aerial photo with map
outlines, ca. 1938 (thick grey), 2005 outline (thick black) and GPR
lines from 2004–05 (dark grey), 2009 (grey) and 2010 (lower third
of the glacier, light grey).
2 Setting
The Archipelago of Svalbard consists of all islands located
between 74◦and 80◦N and 10◦and 35◦E. The climate is
relatively mild and dry compared to other locations at similar
latitudes, with an average annual air temperature of −6◦C
and a mean annual precipitation of 400 mm in the central part
of Spitsbergen, the main island (Engeset and Ødeg˚
ard, 1999).
Sixty percent of the land area is covered by glaciers (Hagen
et al., 1993).
Tellbreen is a 3.5km long, land terminating valley glacier
situated at 78.13◦N, 16.5◦E in the centre of Spitsbergen
(Fig. 1), 20km northeast of the main settlement, Longyear-
byen. It has not previously been the objective of scientific
investigations and the only data available for the glacier are
the GPR and GPS measurements described in this paper, a
handful of snow pits and the maps and aerial photographs
described in Sect. 3.2. The glacier’s basic properties (esti-
mated from aerial photos from 1977) are listed in the Glacier
Atlas of Svalbard and Jan Mayen (Hagen et al., 1993) (Ta-
ble 1). The median elevation is 580 m.a.s.l. which is very
close to the mid point of the glacier (570m.a.s.l) indicating
a simple hypsometry with an area to height ratio of close to
one. Hagen et al. (1993) estimated the volume of the glacier
from the ice covered area (A) observed on the 1977 aerial
photos and the mean depth (D) which is estimated by
D=33×lnA+25 (1)
Tellbreen has a well defined catchment area, a single firn area
and has experienced substantial retreat of the terminus since
the end of the Little Ice Age (LIA). The elevation of the equi-
librium line is not known but it is evident that the accumula-
tion area is very limited as the glacier is close to snow-free on
the 1995 aerial photograph from mid August. A substantial
percentage of the accumulation is believed to be in the form
of superimposed ice and avalanches from the steep moun-
tainsides surrounding the glacier. A large icing and areas of
open water occur every winter in front of the glacier termi-
nus. We have observed open water in front of the glacier
even at times when temperatures were below −20◦C, indi-
cating release of stored englacial or subglacial water.
The geology of the area consists of sub-horizontal, poorly
cemented and readily erodible fluvial and shallow marine
sand- and siltstones of Cretaceous to Eocene age (Hjelle,
1993). There are no known faults directly under Tellbreen,
but the major Billefjorden fault zone is found 10km to
the east. Furthermore there are several faults in nearby
De Geerdalen and under Archowskifjellet (Dallmann et al.,
1999). The significance of the geological setting will be dis-
cussed later.
Traditionally, it has been assumed that drainage on cold
arctic glaciers is entirely supraglacial, because melt water is
unable to penetrate cold ice (Hodgkins, 1997). However, re-
cent work has shown that two mechanisms can enable sur-
face melt water to reach the interior, and sometimes the bed,
of predominantly cold polythermal glaciers (Gulley et al.,
2009b). Firstly, incision of perennial surface meltwater chan-
nels can form meandering englacial passages if their upper,
abandoned levels become blocked by snow or ice, or closed
by ice creep (Gulley et al., 2009a). Such “cut-and-closure”
conduits commonly form in lateral positions due to the con-
vex shape of the ablation area (e.g. Longyearbreen: (Gulley
et al., 2009a; Hansen, 2001; Humlum et al., 2005), although
they can also occur close to the glacier centreline (e.g. Aus-
tre Brøggerbreen: (Vatne, 2001). Cut-and-closure conduits
can incise down to glacier beds and form subglacial chan-
nels in cold ice (e.g. (Gulley et al., 2009a; Humlum et al.,
2005), although channels below depths of ∼20m of ice are
very susceptible to blockage, and water tends to be re-routed
to shallower flow paths.
The second mechanism for routing surface meltwater
through cold ice is overdeepening of water-filled crevasses,
or hydrofracturing (Alley et al., 2005; Benn et al., 2009;
Boon and Sharp, 2003; van der Veen, 2007). Hydrofractur-
ing can occur where ice under tensile deviatoric stress co-
incides with a sufficient water supply. This process allows
surface-to-bed drainages (moulins) to develop on a seasonal
basis, and can enable the water to access deep, warm areas
of glacier beds through great thicknesses of cold ice. On
Tellbreen, the ice surface is mostly smooth with no major
crevasse areas. Although some isolated extensional crevasses
occur on the upper part of the north-west side of the glacier,
and bergschrunds occur near its upper limit, we have found
no evidence for surface-to-bed drainage by hydrofractur-
ing on Tellbreen. Surface melt water follows well-defined
supraglacial channels in the upper part of the glacier, with a
shift to an arborescent system of partly englacial, 10–20 m
deep, incised lateral channels in the lower part.
The Cryosphere, 5, 139–149, 2011 www.the-cryosphere.net/5/139/2011/
K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier 141
Table 1. Volume and areas from 3-D models and calculated by Eq. (1). The values are plotted in Fig. 4.
Year Calculated from Hagen 1993 3-D model
Area in Mean ice Volume in Mean ice Volume in
km2thickness in m km3thickness in m km3
1910aLIA maximum 6.4 86 0.55 90 0.48
1938bMap vis. ice 5.8 83 0.48 72 0.42
1977 Glacier atlas 5.4 81 0.44 – –
1995 vis. icec3.0 61 0.18 – –
1995 dice=0d4.0 71 0.28 – –
2005 vis. icee2.6 57 0.15 50 0.13
2005 dice=0 3.0 61 0.18 50 0.15
2009 vis. icee2.5 55 0.14 52 0.13
2009 dice=0 2.8 59 0.17 50 0.14
aEstimated from the height and distribution of LIA moraines above the 1995 surface.
bModelled in Petrel from NP 1:100.000 series map (based on aerial photos from 1936–38).
cExtent of glacier ice on aerial photographs.
dThe limits of the ice inferred from the radar lines.
eIce with no detectable debris cover in radar lines. Average ice thickness dice = 33lnA + 25. Volume V= A×d (Hagen et al., 1993).
3 Methods
3.1 GPR
During the spring field seasons of 2004, 2005 and 2009 a to-
tal of 98 km of GPR lines were recorded on Tellbreen with
about 50% being repetitive to detect changes in the glacier
geometry. In 2010, a total of 26 km were recorded in a grid
covering the lower third of the glacier (Fig. 1). The equip-
ment used in 2004–2005 was a PulseEKKO 100 radar trig-
gered by an odometer wheel. The antennae were mounted
on a wooden sledge perpendicular to the direction of move-
ment with equidistant spacing of one antenna length between
them. This was done to minimize line interferences from
objects either side of the lines and to optimize the amount
of energy transmitted directly into the ground (Annan and
Cosway, 1992) as the antenna radiation pattern is that of
a half wavelength dipole (Daniels, 1996). A snow scooter
was used to tow the sledge and carry the rest of the equip-
ment. We used 50 and 100 MHz resistively damped dipolar
antennae, with bandwidths equal to their centre frequency
(Daniels, 1996).
In 2009 and 2010, a M˚
al˚
a radar system with fixed, un-
shielded 100MHz antennae was used. The antennae were
inline with an equidistant spacing of two antenna lengths be-
tween them. Since the antennae array is fixed in a Kevlar
tube, this system does not allow for making a common mid
point (CMP) survey. Petterson et al. (2003) state the theoreti-
cal values for clean dry ice as 0.168 ±0.003 m/ns. Since Tell-
breen is assumed to be cold throughout a value of 0.17m/ns
was used for depth conversion of the GPR data from Tell-
breen.
For all the GPR lines a Garmin Etrex legend GPS was
used for positioning. Kinematic Differential GPS (DGPS)
measurements covering most of the glacier surface were col-
lected in April 2009. These measurements were used to
model the 2009 surface. The accuracy given by the GPS was
in most cases 5–10m in the horizontal plane. Points with
lesser accuracy were not used. The vertical accuracy was
3–8m in 2009 when comparing the GPS to the DGPS mea-
surements taken at the same location. A vertical error of 10 m
corresponds to 2–3% of the total heights in m.a.s.l. The to-
pographic maps of the area have contour interval of 50m, so
this error is negligible with respect to the topographic models
presented.
3.2 Aerial photographs and maps
To provide context for interpretation of the radar data, long
term geometric changes of Tellbreen were reconstructed us-
ing four 3-D models of the glacier surface, based on aerial
photographs, map and GPS data. The most recent aerial
photographs of Tellbreen are the Norwegian Polar Institute
1:15000 series from 1990 and 1995. For this investigation
the pictures used were the S95 series picture 1226 and 1227
and the S90 series pictures 4611, 4612, 5175 and 5176. From
these the outline and position of surface features could be
mapped in UTM coordinates. The new edition of the Norwe-
gian Polar Institute (NPI) 1:100.000 map series of the area
was produced from this series, whereas the previous edition
was based on the 1936–38 series. Unless otherwise stated,
the map outlines refer to the 1936–38 series map whereas
the 1995 outline is taken from the 1995 aerial photographs.
All coordinates on the maps produced in this paper, except
Fig. 1, are in UTM ED50.
The volumes and ice thicknesses were calculated in two
ways; from the relationship between area and average ice
thickness stated by Hagen et al. (1993) (Eq. 1) and from the
www.the-cryosphere.net/5/139/2011/ The Cryosphere, 5, 139–149, 2011
142 K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier
3-D models described in Sect. 3. For the 2005 and 2009 mod-
els an estimate of both the area of ice visible on aerial photos
(interpreted as ice with no discernable debris cover in the
radar data) and the total ice covered area (where ice thick-
ness was zero) was modelled. The LIA maximum extent of
the glacier was approximated by raising the NPI (1936–39)
map surface to the maximum level of the prominent ice-cored
moraines that flank the glacier.
3.3 Processing
The raw GPR data were processed in ReflexW (2004 and
2005) and Rad Explorer (2009 and 2010). The processing
mainly consisted of applying gain and filters (Background
and DC removal, spherical divergence correction and AGC
(Automatic gain control)). The 2010 lines used for amplitude
mapping were not subjected to any gain function in order not
to bias the amplitude information.
In the 2004–2005 data the bottom reflector was picked and
the two-way travel time (TWT), depth, amplitude and phase
were exported into Petrel. In 2009 and 2010, the lines were
depth-converted using a velocity of 0.17m/ns and converted
into the standard SEG-Y format before being imported di-
rectly into Petrel. A digital elevation model (DEM) of the
area was created by digitizing the NPI 1:100.000 map. The
ice thickness maps and DEMs are not a direct representation
of the individual lines but rather an overall smoothed picture
of the glacier. Not all of the glacier was accessible by snow
scooter and ca. 3km2are covered by the GPR lines. The
models for 2005–2009 have a grid size of 30×30m which
is much larger than the step size (0.5–1.5m) and 10 times
the theoretical coverage as the data points are not evenly dis-
tributed on the glacier surface. A grid of 20×20 m was used
for the areas covered by the 2010 data. A comparison be-
tween the radar lines from 2005 and 2009 and the two models
created from the lines did not show significant differences.
When comparing the GPR data to the DEM models it was
clear that due to the smoothing effect of the modelling there
is a local slight underestimation of the greatest ice thick-
nesses by 3–5m. This will also influence the bottom topog-
raphy maps and the average depth and volume estimates but
to a very limited degree. The accuracy of the models based
on maps and aerial photos are difficult to quantify since no
GPR data exist from before 2004.
The models were made with a minimum of smoothing
while still obtaining reasonable ice thickness contour lines.
The bottom reflector can be picked within ±5 ns correspond-
ing to 0.4m. A potentially more significant error is the com-
putation of the depth from the TWT. The velocity of pure,
cold ice is 0.172±0.005m/ns (Bogorodsky et al., 1985).
This amounts to an error of 42.5ns or 5.8m of ice assum-
ing an ice thickness of 100m.
The underestimation of the ice thickness due to the omis-
sion of the effect of surface snow is very limited and is
therefore not taken into account. The velocity in dry firn is
Water, -0.80
Ice, -0.28
Water -0.67
Granite -0.11
Air 0.28
Shale -0.28
Wet sand -0.47
Shale 0.48
Granite 0.60
Ice 0.67
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
0,7 1,8 2,9
Air Ice Water
Dry
Wet
Wet
Air Ice Water
R
Fig. 2. Examples of reflection coefficients (R) for various inter-
faces found in a glacial environment. The top caption is the mate-
rial above the interface; the data labels state the material below the
interface and the reflection coefficient. It is clear that the presence
of water is connected to a strong reflection. Values from Davis and
Annan (1989) and Murray et al. (1997).
0.19m/ns (Pettersson et al., 2003; Bogorodsky et al., 1985)
and 0.22 m/ns for hard packed snow. This translates to a time
difference in TWT of 5ns if a 2 m snow layer replaces ice,
equivalent to 0.4m of ice.
The magnitude and polarity of a reflector can give indica-
tions of the nature of the interface. Analysis of the polarity
and strength of the bottom reflector is a widely used approach
to determine areas of water at the interface (e.g. (Gades et
al., 2000; Copland and Sharp, 2001; Pattyn et al., 2009). The
reflection coefficient (R) is a measure of the amount of en-
ergy reflected at an interface. Assuming that the conductivity
is negligible (σ≈0) it follows that the magnetic permittiv-
ity µ≈µ0when operating at very high frequencies (1MHz-
2 GHz). The relative dielectric permeability of a material (εr)
is the permittivity of the material εin relation to the permit-
tivity in free space ε0.The reflection coefficient for a planar
surface can then be expressed as:
R=√εr,1−√εr,2√εr,1+√εr,2(2)
Where the index number 1 refers to the material above the
interface and 2 the material below (Reppert et al., 2000). A
negative reflection coefficient relates to a shift in polarity of
the wave. The reflection coefficients for the most common
interfaces found in a glacial environment can generally be
divided into two major groups: dry and wet (Fig. 2). Dry
interfaces (e.g. ice to shale, air to ice) have reflection coeffi-
cients between −0.3 and +0.3 indicating that less than 30%
of the incoming wave is reflected back towards the receiving
antenna. Wet interfaces (e.g. air to water, ice to wet sand)
The Cryosphere, 5, 139–149, 2011 www.the-cryosphere.net/5/139/2011/
K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier 143
reflect as much as 80% of the incoming wave and are most
often associated with a shift in polarity. An exception to this
rule are interfaces where water is the top layer, but as the field
work was done in winter this type of interface will be intra- or
subglacial and therefore overlain by an interface with R < 0.
In order to map the properties of the glacier bed the relative
magnitude, scaled down to [−1;1], and the polarity of the
wave at the ice-bed reflection, were extracted from the data.
This was done using the Surface Attribute function in Petrel.
The algorithm used was a sum of amplitudes function. For
each trace the amplitude was calculated for a 5ns window
centred on the bed reflector (the estimated accuracy to which
the reflector could be picked). The attenuation of radar waves
in cold ice is low, in most cases below 30 dB/km (Bentley et
al., 1998; MacGregor et al., 2007; Winebrenner et al., 2003).
The ice thickness in the area covered by the 2010 radar lines
(Fig. 1) is less than 40m (in 80% of the area it is less than
25m). The attenuation is small compared to the strength of
the signal and has not been incorporated in the calculation
of the bed echo. Future work on Tellbreen, e.g. collecting
data for and producing a detailed bed reflection map of the
whole glacier, could include a more detailed analysis of the
attenuation pattern.
From these values a contour map of the relative bed re-
flection amplitude was produced using a minimum curvature
algorithm with low grade smoothing (Fig. 3). This gives an
indication of the areas where there are strong indications of a
wet interface. When combining (i) the maps of ice thickness
and (ii) the locations where structures consistent with a sub-
glacial channel are observed in the radar line, this method
presents a useful tool to resolve the origin and transport of
the water feeding the icing in front of the glacier.
4 Results and interpretations
4.1 Temporal changes in geometry, area and volume
The long-term geometric changes of Tellbreen from the last
100 years are presented in Table 1 and Figs. 4, 5 and 6.
At the LIA maximum (approximately 90 years ago; (Hagen
et al., 2003a; Hodgkins, 1997; Navarro et al., 2005)), the
glacier had a volume of 0.47km3, compared with 0.42 km3
in 1936–38, and less than 0.2 km3in 2009. Thus, in a pe-
riod of less than 100 years since the end of the LIA Tell-
breen has lost some 60–70% of its total volume and the
glaciated area has been reduced by more than 50%. The
long-term mass balance of the glacier (in metres water equiv-
alent per year) was calculated using the measured surface
lowering and an average of the glaciated area, and was
found to be −0.6±0.2m/yr. This figure is close to the
−0.55m/yr (Dowdeswell et al., 1997) for Svalbard glaciers
as a whole, and glaciers of comparable size and geometry,
such as Austre Brøggerbreen (5km2,−0.45±0.33m/yr),
Bertilbreen (5km2,−0.72±0.29m/yr), Longyearbreen
Icing
Possible en- and subglacial channels
Supraglacial channels
Radar lines 2010
Channels observed in aeial pictures
0.2
-0
-0.2
-0.4
-0.6
-0.8
-1
<-42
Fig. 3. Relative amplitude of the ice-bed reflector near the termi-
nus of Tellbreen (2010). The dark grey areas have values below
−0.5, significantly lower than would be expected from a dry in-
terface, and are most likely linked to the presence of water at the
ice-rock/sediment interface. The insert of the whole glacier with
map outline is from the 2009 survey but the areas with anomalies
are coinciding.
(4km2,−0.55±0.45m/yr) and Aldegondabreen (7.6km2,
−0.7m/yr) (Hagen et al., 2003b; Navarro et al., 2005).
4.2 Drainage channels
Numerous isolated englacial reflectors (Fig. 7) were ob-
served in the radar data. In the unmigrated data, the struc-
tures consist of strong hyperbolae located in otherwise trans-
parent ice or just above the bed. These display a reversal
of polarity and a reflection pattern consistent with models
of air- and partially water-filled englacial channels (Stuart et
al., 2003; Vatne, 2001). An englacial channel with a circular
or oval cross section will present as a single symmetric hy-
perbola, whereas a canyon-like morphology (typical of many
cut-and-closure type conduits) appears as a series of slightly
offset, stacked hyperbolae (cf. Van Hoof, 2009). More com-
plex reflection patterns can be expected for channels close
to the bed, or within debris-rich ice. The polarity and am-
plitude of the reflected wave is dependent on the reflection
coefficients at the interface (Fig. 2).
An example of a structure interpreted to be an englacial
channel is shown in Fig. 7b. The first arrival from the channel
is the reflected R-wave (white-black-white) from the top of
the channel. The next arrival is the reversed polarity (black-
white-black) TRT-wave that is transmitted (T) through the
ice- air interface at the top of the channel, reflected from the
air water interface (R) and transmitted (T) through the air-ice
interface. As the reflection coefficient is −0.80 this results
in a high amplitude, reversed polarity signal. The TRRRT-
wave is a peg-leg from within the air filled part of the cavity.
www.the-cryosphere.net/5/139/2011/ The Cryosphere, 5, 139–149, 2011
144 K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier
y = -0,0027x
2
+ 10,276x - 9747,6
R
2
= 0,9746
y = -2E-05x
2
+ 0,0904x - 84,962
R
2
= 0,9905
0
10
20
30
40
50
60
70
80
90
100
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
Ice thickness in m
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Volume in km3
Calculated ice thickness, visible
ice area
Calculated ice thickness, Z=0
area
Calculated LIA ice thickness,
maximum area
Ice thickness from model, visible
ice area
Ice thickness from model, Z=0
area
Ice thickness from model, LIA
maximum area
Calculated volume, visible area
Calculated volume, Z=0 area
Calculated volume, LIA
maximum area
Volume from model, visible ice
area
Volume from model, Z=0 area
Volume from model, LIA
maximum area
ll l d i
Fig. 4. Plot of mean ice thickness (upper curve) and volumes (lower curve) of Tellbreen as a function of time. The calculated volumes are
derived from Eq. (1). The values can be found in Table 1.
The TTRTT wave originates from the bottom of the channel
(the water-ice interface). The TTRRRTT-wave is again an
echo, this time from a peg-leg within the water. Because the
propagation velocities of electromagnetic waves in ice, wa-
ter and air are well established (Annan and Cosway, 1992),
the dimensions of the channel can be estimated. Assuming
the channel has a close to circular cross section, the height is
ca. 2.3 m, with a water level around 1.5m. For air-filled chan-
nels, the reflection pattern will consequently be simpler. The
TRT wave originates from the bottom of the air-ice interface
at the bottom of the channel and the amplitude is compara-
tively smaller (R= −0.28). The TRRRT-wave can still be
observed but not the TTRRRTT-return.
For a snow-covered supraglacial channel (Fig. 7a) the gen-
eral pattern will be the same but the overlying ice will in this
case be replaced by a snow bridge with a higher velocity. As-
suming a simple U-shaped cross section and a uniform snow-
bridge the depth of the channel can be estimated to ca. 2m.
A subglacial channel can be challenging to detect if no wa-
ter is present, due to the low reflection coefficient of ice over-
lying dry materials. The reflection pattern of a circular, partly
or completely water filled subglacial channel is similar to
those of an englacial channel (Fig. 7c). However, the reflec-
tion from the bed of the channel (TRT or TTRTT depending
on the water level) will originate from a water-rock/moraine
interface with a lower reflection coefficient than a water-ice
interface. Subglacial channels show a large variance in size,
shape and geometry, often resulting in a complicated reflec-
tion pattern complicating the estimation of geometry and wa-
ter content.
40
40
60
60
80
60
60
80
80
80
100
100
100
40
40
20
120
120
20
20
120
140
160
180
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(A) End of LIA
(B) 2005
(C) 2009
Fig. 5. Ice thickness maps and outlines of the glaciated area. NP
1:100.000 series map (light grey), 2005 measurements (dark grey)
and 2009 measurements (black).
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K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier 145
(A) Map-2005
(B) Map-2009
(C) 2005-2009
Fig. 6. The change in surface elevation and outline of Tellbreen.
Norwegian Polar Institute A7 map, 1:100.000 series outline (light
grey) and 2005 area covered by GPS measurements (dark grey).
4.3 Basal conditions
A composite of the reflection strength from the 2004, 2005
and 2009 lines shows two areas with high bed reflection
power and a distinctly negative reflection coefficient (insert
on Fig. 3, dark grey areas). One of these areas is near the
thickest part of the glacier, and is limited in extent. It is
likely that this marks a small, isolated region of wet-based ice
as there are no indications of temperate ice in the radar lines
crossing this area. The second area occurs beneath the glacier
terminus, where the ice is typically less than 20 m thick. The
terminus area was investigated in detail in 2010 (Fig. 3), and
the areas with the strongest reflectors and most prominent
shift in polarity are highly localized and coincide with struc-
tures at or near the bed consistent with a water-bearing chan-
nels. There are no indications of widespread wet basal con-
ditions beneath the terminus. Except for the areas described
above in the inset on Fig. 3 there is no evidence for water
5 10 15
A Supraglacial channel
R (wbw)
TRT (bwb)
TRRRT (bwb)
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(
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C:\Documents and Set tings \Karoli ne\Skr ivebord\Al t tel l\Lengt h corre
c
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m
m
e
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20 30 40
20 10
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400
B Englacial channel
C Subglacial channel
30
Fig. 7. Wavelet traces and GPR profiles from drainage channels.
Vertical scale is in ns TWT, horizontal in m. (A) ca. 2 m deep,
lateral melt water channel. The curvature of the hyperbola indicates
a velocity of around 0.22m/ns, a reasonable value for a snow filled
channel. (B) Englacial channel. Detail from a cross glacier line.
The radar signature can be interpreted as a 2/3 water filled englacial
channel ca. 2.3m in diameter. (C) Partially water filled subglacial
channel near the terminus. Aand Bare from the 2009 data set, Cis
from 2010.
at the bed at distances greater than 1km from the terminus,
where the ice is more than 30 m thick. However, englacial re-
flectors, consistent with partially water-filled englacial chan-
nels (Fig. 7b), are found further up on the glacier. Figure 8 is
a cross section through the glacier tongue with all channels
marked on Fig. 3 plotted as a function of depth and distance
to the profile (over or under 50m). At distances of 600–
700m from the terminus, the channels are mostly englacial
but approaching subglacial. Along the centreline closer to
the terminus the majority of the channels are near or at the
bed while they are mainly supraglacial near the margins and
the terminus where the ice thickness is less than 15m. This
drainage pattern is not consistent with the traditional view of
the evolution of drainage systems of cold-based glaciers but
it is congruent with recent findings on the distribution and
characteristics of cut-and-closure conduits (e.g. Gulley et al.,
2009b; Van Hoof, 2009).
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146 K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier
1
Length along profile [m]
275 300 325 350
275300325350
m.a.s.l.
Glacier bed
2010 surface Channels <50 from profile
Channels >50 from profile
528000 528250 528500 528750 529000
8686750 8687000 8687250
0200400 600 800
Fig. 8. Cross section of terminus with all channels identified in
the 2010 lines plotted in map view (small inset) and as a function
of depth and length along the profile. The channels are coloured
according to distance from profile line. As the surface and bed of
the glaciers changes laterally some of the channels are located above
the surface and below the bed of the profile.
5 Discussion
5.1 Thermal regime
The bed reflector below Tellbreen is generally very clear and
distinct, and the overlying ice transparent with few struc-
tures. In warm-based polythermal glaciers, there is typi-
cally a well defined transition zone from cold ice to warm
ice (Irvine-Fynn et al., 2006; Sund and Eiken, 2004; P¨
alli,
2003; King et al., 2008), due to the change in water content.
In GPR data, this is indicated by a shift from a clear layer
with few reflections to a more opaque and noisy layer with
numerous small reflections. This effect due to the backscat-
tering of the GPR waves by small scale water bodies within
the temperate ice. As no water is present in the cold ice this
appears transparent. Several of these interfaces have been
confirmed by boreholes (Hodgkins et al., 1997; Jacobel et
al., 2002; Pettersson et al., 2003; Ødeg˚
ard and Hagen, 1997).
No such change in radar signature was observed anywhere
on Tellbreen. There is no evidence in the radar data of a tem-
perate surface layer in the upper part of the glacier, such as
has been observed on other Svalbard glaciers (Type A poly-
thermal regime; Blatter and Hutter, 1991). This is in line
with the very limited accumulation area and the absence of
firn in all snow pits dug on the glacier. On several Svalbard
glaciers, cold-temperate transition surfaces (CTS) have been
reported at depths approaching or equal to the ice thickness
of Tellbreen (Melvold et al., 2003; Riger-Kusk, 2006; Sund
and Eiken, 2004; Ødeg˚
ard and Hagen, 1997). However, ex-
cept for the small, isolated area near the thickest part of the
glacier (Fig. 3), there is no evidence for warm-based ice be-
low Tellbreen. Based on the data available, Tellbreen appears
to be almost entirely cold.
In a few areas in the upper part of the glacier, complex
reflectors occur in the basal zone, interpreted as a 5–15 m
thick layer of debris-rich ice. These areas are not coincident
Fig. 9. A thrust fault near the terminus of the glacier. Detail from
line running from the terminus up the centre of the glacier. Top and
right hand scales are in m, left hand is TWT in ns. Notice the 600 m
of ice that is less than 30m thick. Gain and dewow are applied.
Also noticeable is the completely transparent ice with no evidence
of backscattering.
with the areas of a wet interface indicated in Fig. 3. There
were also several examples of narrow, inclined structures in
the lower part of the glacier which rise downglacier at angles
of 5 to 15◦. These are interpreted as thrust faults (Fig. 9).
The possible presence of both debris-rich ice and thrust faults
suggests that the glacier may have been partly warm-based at
its LIA maximum when parts of the glacier could have been
as much as 200 m thick (Fig. 5). Greater ice thicknesses and,
possibly, greater ice fluxes would have encouraged warm-
ing of basal ice by geothermal and strain heating. At that
time the glacier would have had a significant accumulation
area and could have been a Type A or Type C polythermal
glacier in the Blatter and Hutter (1991) classification. How-
ever, despite the underlying geology being readily erodible
(see Sect. 2) the DEMs of the valley beneath the glacier show
a distinct V-shape and low degree of modification, as would
be expected below a cold based glacier (Etzelm¨
uller et al.,
2000). This suggests that, although parts of Tellbreen may
have experienced warm-based conditions at times, they were
neither widespread nor prolonged.
There are no indications that Tellbreen has experienced the
retreat-advance cycles or subsequent shape changes that are
linked to a surge type glacier. The glacier is not currently
building up mass in the upper part or undergoing signifi-
cant increases of surface gradient (Fig. 6). Also there are
no indications that the glacier terminus has advanced further
into the valley than the LIA extent marked on Figs. 5 and 6.
The undisturbed and patterned ground in front of the glacier
shows no signs of having been glaciated in the last several
thousand years, which is the timescale of formation of large-
scale patterned ground and ice wedge formation (Mackay,
1990). Taken together with the absence of historical records
of the glacier surging, it is concluded that Tellbreen is not of
surge type.
The Cryosphere, 5, 139–149, 2011 www.the-cryosphere.net/5/139/2011/
K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier 147
5.2 Water in the glacier
Below the glacier terminus, subglacial water appears to be
restricted to discrete channels, and there is no evidence for a
widespread water film at the bed. In this context, it is relevant
that other areas of thin ice around the margins of the glacier
are apparently everywhere frozen to the bed (Fig. 9), as is
typically the case for Svalbard glaciers. The inferred basal
channels do not have any apparent subglacial catchment ar-
eas. Instead, it appears likely that they are fed by englacial
channels located farther up glacier, which in turn are fed
at their upper ends by supraglacial channels. The drainage
system of the glacier, therefore, appears to reflect currently
active processes (incision and roof closure of supraglacial
channels), and has not been inherited from a different thermal
regime, as has been suggested for englacial channels else-
where in Svalbard (e.g. Stuart et al., 2003). Cut-and closure
conduits appear to be very widespread on uncrevassed parts
of Svalbard glaciers (Gulley et al., 2009a, b), and have been
observed in subglacial positions below thin ice near glacier
termini. For example, cut-and-closure channels existed at the
bed of the cold-based Longyearbreen between 2001 and 2003
(Humlum et al., 2005), although this channel subsequently
became blocked and water was re-routed at a higher level
within the glacier (Gulley et al., 2009a). Other examples
have been observed near the termini of Scott Turnerbreen
(Temminghoff, 2009) and Rieperbreen (Gulley, unpublished
data). Cut-and-closure channels develop where channel inci-
sion rates exceed ice surface ablation rates, a process which
will tend to bring them into contact with the glacier bed.
Channel blockage, however, becomes more likely at greater
depths where ice-creep closure rates are higher. This re-
duces the likelihood that such channels can persist below ice
thicker than a few tens of metres. In contrast, cut-and-closure
channels could persist below the thin ice near glacier termini,
where water could be stored during the winter months.
The results of this study have wider implications for under-
standing the hydrology of High Arctic glaciers. Firstly, wa-
ter flow from beneath glaciers during winter cannot be used
to infer warm-based conditions, because subglacial storage
does not imply subglacial water production. Secondly, the
drainage systems of cold Arctic glaciers can be considerably
more complex than envisaged in traditional conceptions of
glacial hydrology. According to the widely-used model of
Shreve (1972), englacial water flow is guided by potential
gradients within the ice, and can only occur under temperate
conditions. It is now widely recognized that hydrofracturing
can allow surface water to penetrate cold ice (e.g. Alley et
al., 2005; Krawczynski et al., 2009), but it is now clear that
the cut-and-closure mechanism is also capable of creating in-
tegrated supraglacial-englacial-subglacial drainage networks
in entirely cold glaciers. Finally, the routing of oxygen-
rich surface waters to cold glacier beds implies that such
drainage systems can accomplish localized subglacial chem-
ical weathering. Cold high Arctic glaciers, therefore, are not
necessarily passive components of the landscape, but can ex-
hibit complex linkages with the atmosphere and lithosphere.
6 Conclusions
Since the end of the LIA, Tellbreen has lost ∼60–70%
of its volume and the glaciated area has been reduced by
more than 50%. The long-term mass balance of the glacier
(in metres water equivalent per year) was calculated to be
−0.6±0.2myr−1. The GPR data indicate that Tellbreen is
currently entirely cold-based, with the possible exception of
a small, isolated region near the thickest part of the glacier. It
is possible that the glacier had areas of warm ice during the
LIA maximum, although the low degree of modification and
3-D models of the underlying valley suggests that the glacier
has never had high erosional potential. There is no evidence
of surge behaviour.
Despite cold-based conditions, water exits beneath part of
the glacier all year round, and in the winter months feeds a
large icing in front of the glacier. Our results indicate that wa-
ter is stored during the winter months in two or more narrow
channels beneath the lowermost 400–500m of the glacier
tongue, from where it is gradually released. These channels
are apparently the downglacier continuations of englacial
channels, which in turn are fed by supraglacial channels
higher up the glacier. When seen in connection with the out-
flow of water from the glacier in winter and the absence of ice
at the pressure melting point in the upper part of the glacier,
this presents a strong argument for the hypothesis that, de-
spite being an entirely cold arctic glacier, Tellbreen, contrary
to the traditional view of cold based glaciers, has a diverse,
subglacial drainage system that is capable of storing, trans-
porting and releasing water year round. Results also indicate
that Tellbreen has been cold based for a significant amount
of time. The present drainage system of Tellbreen is there-
fore possibly not a relic from a previous polythermal regime
but has developed in response to processes currently active
on the glacier.
Acknowledgements. Fieldwork was funded by a Norwegian Polar
Institute Arctic Field Grand and UNIS. We thank M. Riger-Kusk,
R. Behlke, Q. Nuna, H. Rasmussen, L. J. Baddeley , C. Eide,
M. Waage and U. Silver for their indispensable help during
fieldwork.
Edited by: I. M. Howat
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148 K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier
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