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Thermal structure and drainage system of a small valley glacier (Tellbreen, Svalbard), investigated by ground penetrating radar

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Proglacial icings accumulate in front of many High Arctic glaciers during the winter months, as water escapes from englacial or subglacial storage. Such icings have been interpreted as evidence for warm-based subglacial conditions, 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 Spitsbergen, where a large proglacial icing develops each winter, to determine the location and geometry of storage elements. Digital elevation models (DEMs) of the glacier surface and bed were constructed using maps, differential GPS and ground penetrating radar (GPR). Rates of surface lowering indicate that the glacier has a long-term mass balance of −0.6 ± 0.2 m/year. Englacial and subglacial drainage channels were mapped using GPR, showing that Tellbreen has a diverse drainage system that is capable of storing, transporting 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 terminus, these channels reach the bed and contain stored water 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 subglacial drainage system, which allows significant storage and delayed discharge.
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The Cryosphere, 5, 139–149, 2011
www.the-cryosphere.net/5/139/2011/
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 74and 80N and 10and 35E. The climate is
relatively mild and dry compared to other locations at similar
latitudes, with an average annual air temperature of 6C
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 78.13N, 16.5E 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 20C, 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 (σ0) it follows that the magnetic permittiv-
ity µµ0when 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,1εr,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.
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144 K. Bælum and D. I. Benn: Thermal structure and drainage system of a small valley glacier
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.
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525000 526000 527000 528000 529000 530000
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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).
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 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)
Ground (bwb) Trace
R
(
b
wb
)
TRT (wbw)
TR3T (wbw)
TTRTT (bwb)
TTR3TT (bwb)
Trace
2 6 8 4 10 0
0
50
100
150
R (wbw)
TRT (bwb)
TRRRT (bwb)
TTRTT
(
wbw
)
TTRRRTT (wbw)
0
200
600
800
C:\Documents and Set tings \Karoli ne\Skr ivebord\Al t tel l\Lengt h corre
c
Position
C
o
m
m
e
n
t
3
C
o
m
m
e
n
t
3
C
o
m
m
e
n
t
3
(m)
20 30 40
20 10
Trace
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±0.2myr1. 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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www.the-cryosphere.net/5/139/2011/ The Cryosphere, 5, 139–149, 2011
... We identified many hyperbolic reflections across the glacier (Figure 4). We presume that the majority of them are linked to englacial conduits as has been noted in many studies [14,[67][68][69]. Nonetheless, some could also be linked to crevasses or englacial debris bands or large boulders [70], especially close to glacier margins. ...
... The area of the intense scattering zone that is located in the upper part of the glacie (accumulation area) occupies 0.46 km 2 . According to numerous studies where direct tem perature measurements in boreholes were correlated with GPR survey results in attempt to explain such intense scattering [5,6,14,28,65,68,71,72], we interpret it as indication o temperate ice. The volume of temperate ice in Irenebreen equals to 16,950,676 m 3 (0.017 km 3 ). ...
... The area of the intense scattering zone that is located in the upper part of the glacier (accumulation area) occupies 0.46 km 2 . According to numerous studies where direct temperature measurements in boreholes were correlated with GPR survey results in attempts to explain such intense scattering [5,6,14,28,65,68,71,72], we interpret it as indication of temperate ice. The volume of temperate ice in Irenebreen equals to 16,950,676 m 3 (0.017 km 3 ). ...
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Although measurements of thickness and internal structure of glaciers are substantial for the understanding of their evolution and response to climate change, detailed data about polythermal glaciers, are scarce. Here, we present the first ground-penetrating radar (GPR) measurement data of Irenebreen, and high-resolution DEM and orthomosaic, obtained from unmanned aerial vehicle (UAV) photogrammetry. A combination of GPR and UAV data allowed for the reconstruction of the glacier geometry including thermal structure. We compare different methods of GPR signal propagation speed determination and argue that a common midpoint method (CMP) should be used if possible. Our observations reveal that Irenebreen is a polythermal glacier with a basal temperate ice layer, the volume of which volume reaches only 12% of the total glacier volume. We also observe the intense GPR signal scattering in two small zones in the ablation area and suggest that intense water percolation occurs in these places creating local areas of temperate ice. This finding emphasizes the possible formation of localised temperate ice zones in polythermal glaciers due to the coincidence of several factors. Our study demonstrates that a combination of UAV photogrammetry and GPR can be successfully applied and should be used for the high-resolution reconstruction of 3D geometries of small glaciers.
... Since the early studies on temperate valley glaciers, it has been known that surface meltwater can reach the glacier bed, while cold ice was presumed to be practically impermeable for water (Gulley and others, 2009a). Despite this assumption, complex englacial and subglacial drainage networks evidently develop in cold and polythermal glaciers of the Arctic (Gulley and others, 2009b;Baelum and Benn, 2011;Irvine-Fynn and others, 2011;Naegeli and others, 2014;Teminghoff and others, 2018), and their evolution requires further investigation. ...
... Björnsson and others, 1996;Sevestre and others, 2015). Despite scientific interest for a long time period, making Svalbard among the best-studied sectors of the Arctic, data about the ice thickness, volume and thermal structure obtained with in situ methods are available for only a limited number of glaciers (Martín-Español and others, 2013;Procházková and others, 2019), and there have been relatively few ground-penetrating radar (GPR) investigations of drainage systems (Stuart and others, 2003;Baelum and Benn, 2011;Teminghoff and others, 2018;Hansen and others, 2020). One of the best specimens, for example, was described by Baelum and Benn (2011) who mapped the englacial and subglacial drainage channels using GPR at Tellbreen and showed that even a cold High Arctic glacier may have a diverse drainage system capable of storing, transporting and releasing water year-round. ...
... Despite scientific interest for a long time period, making Svalbard among the best-studied sectors of the Arctic, data about the ice thickness, volume and thermal structure obtained with in situ methods are available for only a limited number of glaciers (Martín-Español and others, 2013;Procházková and others, 2019), and there have been relatively few ground-penetrating radar (GPR) investigations of drainage systems (Stuart and others, 2003;Baelum and Benn, 2011;Teminghoff and others, 2018;Hansen and others, 2020). One of the best specimens, for example, was described by Baelum and Benn (2011) who mapped the englacial and subglacial drainage channels using GPR at Tellbreen and showed that even a cold High Arctic glacier may have a diverse drainage system capable of storing, transporting and releasing water year-round. Nowadays, a growing number of investigations on glacier thermal structure have been conducted outside Svalbard, further demonstrating the widespread existence and complexity of polythermal glacier thermal structures (e.g. ...
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Understanding glacier drainage system behaviour and its response to increased meltwater production faces several challenges in the High Arctic because many glaciers are transitioning from polythermal to almost entirely cold thermal structures. We, therefore, used ground-penetrating radar data to investigate the thermal structure and drainage system of Waldemarbreen in Svalbard: a small High Arctic glacier believed to be undergoing thermal change. We found that Waldemarbreen retains up to 80 m of temperate ice in its upper reaches, but this thickness most likely is a relict from the Little Ice Age when greater ice volumes were insulated from winter cooling and caused greater driving stresses. Since then, negative mass balance and firn loss have prevented latent heat release and allowed near-surface ice temperatures to cool in winter, thus reducing the thickness of the temperate ice. Numerous reflectors that can be traced up-glacier are interpreted as englacial channels formed by hydrofracturing in the crevassed upper region of the glacier. The alternative cut and closure mechanism of conduit initiation only forms conduits in parts of the lower ablation area. Consequently, Waldemarbreen provides evidence that hydrofracturing at higher elevations can play a major role in englacial water drainage through cold ice.
... In glaciological studies it is often related to the presence of different quantities of liquid water due to the temperature of the ice, referred as warm ice in polythermal glaciers (e.g. Baelum and Benn, 2011). In fact, in polythermal glaciers, there is typically a clear transition from cold to warm ice due to the abrupt change in water content, in turn related to different thermal regimes (Pettersson et al., 2003). ...
... In glaciology, the reflection amplitude analysis takes advantage due to the properties of the involved materials (i.e. ice, free water, rocks, debris and sediments, air) which can produce reflection coefficients spanning from about − 0.8 up to about +0.8 (Baelum and Benn, 2011) in turn determining very highamplitude reflections. Water has a relative dielectric permittivity close to 80 (in the typical frequency interval used for GPR in glaciology), which is one order of magnitude higher than most of geological materials being such a physical parameter close to 3 for pure ice, equal to 1 for air, and in the range between 5 and 10 for most of dry coarse sediments and rocks (e.g. ...
Article
In GPR profiles, ice is usually imaged as a mostly electromagnetic transparent facies. However, diffraction events, as well as internal layering, can be also observed. In some cases, the bedrock below glaciers is masked by dense diffractions usually interpreted as the effect of liquid water pockets inside the so-called warm ice. However, the actual physical meaning of such GPR facies is not always obvious, because it can be related also to mixed debris and ice deposits. We adopted a strategy well known in medical sciences and referred as “differential diagnosis” in order to infer which is the actual meaning of a high scattering facies imaged within the Eastern Gran Zebrù glacier (Central Italian Alps) and, more generally, of all the internal glacier features. In fact, in many cases, there is no direct information to limit the subjectivity of geophysical interpretation; therefore, we provide all the discriminative hypotheses based on both independent and integrated criteria including GPR attribute analysis, imaging effects, reflection analysis, GPR frequency evaluations combined with geomorphological and remote sensing data obtained by two photogrammetric UAV and thermal infrared surveys. On the basis of the differential diagnosis, we concluded that the high scattering zone embedded within the studied glacier is most likely related to a mixture of ice and debris probably formed during a past shrinking phase. Beside this case study, this approach could be helpful in other GPR glaciological surveys, in which the target is related not only to the bedrock detection, but also to a detailed analysis of the internal facies of a glacier.
... With respect to identifying the location and geometry of enand subglacial channels using GPR, virtually all previous attempts have been based upon two-dimensional (2-D) profiles, or at the very most quasi-three-dimensional (3-D) surveys involving multiple parallel 2-D acquisitions with a large spacing between the survey lines (e.g. Zirizzotti and others, 2010;Baelum and Benn, 2011;Church andothers, 2019, 2020;Temminghoff and others, 2019). Although these types of surveys can provide highly useful information, they are limited in the sense that the corresponding data cannot be properly imaged and visualized in 3-D because of the strong sampling bias in the along-line direction and the high degree of spatial aliasing in the cross-line direction. ...
... Indeed, past research has largely involved channel detection along 2-D GPR survey lines and subsequent interpolation of flow pathways when multiple line data were available (e.g. Baelum and Benn, 2011;Temminghoff and others, 2019). Our study, in contrast, focuses on analysis of the amplitude characteristics of the glacier bed reflection in order to create a detailed map of the suspected channel pathways. ...
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Studying subglacial drainage networks is important for understanding the potential relationship between channel dynamics and rapid glacier recession as well as the role of subglacial channels in subglacial sediment evacuation. In order to delineate the planform geometry of snout marginal subglacial channels, densely spaced ground-penetrating radar (GPR) measurements at a frequency of ~70 MHz were carried out over the snout marginal zones of two temperate glaciers in the southwestern Swiss Alps, the Haut Glacier d'Arolla and the Glacier d'Otemma. Three-dimensional (3-D) data processing and amplitude analysis of the GPR reflection along the glacier bed was used to map the channels. At the Haut Glacier d'Arolla, two relatively straight channels of several meters in width were identified. The positions of these channels correspond well with the locations of channel outlets at the glacier terminus, as well as with fractures appearing on the glacier surface one month after the GPR data acquisition. The latter are believed to represent the beginning of ice collapse above the subglacial channels. At the Glacier d'Otemma, a major subglacial conduit was detected with similar dimensions to those identified at the Haut Glacier d'Arolla, but greater sinuosity. The position of this channel was confirmed by drone-based imagery acquired after glacier margin collapse. Our results confirm that high-density 3-D GPR surveys can be used to map subglacial channels near temperate alpine glacier margins.
... There exist only a small number of studies that investigate seasonal changes within the englacial hydrological network, and all of these have been undertaken on coldice glaciers. Across several years, GPR measurements were performed by Baelum and Benn (2011) over a small coldice valley glacier to investigate the glacier's thermal regime. Pettersson et al. (2003) used time-lapse GPR imaging, separated by 12 years, to detect changes to the cold-temperate ice transition surface, and Irvine-Fynn et al. (2006) used repeated GPR measurements to investigate hydrological seasonal changes on a polythermal glacier. ...
... However the filling material would remain unknown. Such an approach was adopted in Baelum and Benn (2011) (plotting the reflection-normalised amplitude of the glacier's bed). The workflow could be extended to specular glacier basement reflectors in order to detect subglacial con-duit networks. ...
Article
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Englacial conduits act as water pathways to feed surface meltwater into the subglacial drainage system. A change of meltwater into the subglacial drainage system can alter the glacier's dynamics. Between 2012 and 2019, repeated 25 MHz ground-penetrating radar (GPR) surveys were carried out over an active englacial conduit network within the ablation area of the temperate Rhonegletscher, Switzerland. In 2012, 2016, and 2017 GPR measurements were carried out only once a year, and an englacial conduit was detected in 2017. In 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations in the detected englacial conduit. The resulting GPR data were processed using an impedance inversion workflow to compute GPR reflection coefficients and layer impedances, which are indicative of the conduit's infill material. The spatial and temporal evolution of the reflection coefficients also provided insights into the morphology of the Rhonegletscher's englacial conduit network. During the summer melt seasons, we observed an active, water-filled, sediment-transporting englacial conduit network that yielded large negative GPR reflection coefficients (
... The latter include predominantly active (Peters et al., 2008;Zechmann et al., 2018;Church et al., 2019) and passive (Podolskiy and Walter, 2016;Lindner et al., 2020;Nanni et al., 2020) seismic measurements or ground-penetrating radar (GPR) measurements (Moorman and Michel, 2000;Stuart et al., 2003;Irvine-Fynn et al., 2006;Harper et al., Published by Copernicus Publications on behalf of the European Geosciences Union. 3976 G. Church et al.: Ground-penetrating radar imaging reveals glacier's drainage network in 3D 2010; Baelum and Benn, 2011;Hansen et al., 2020). Most glaciological GPR studies published so far relied on twodimensional (2D) data, where GPR measurements were acquired along profiles. ...
Article
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Hydrological systems of glaciers have a direct impact on the glacier dynamics. Since the 1950s, geophysical studies have provided insights into these hydrological systems. Unfortunately, such studies were predominantly conducted using 2D acquisitions along a few profiles, thus failing to provide spatially unaliased 3D images of englacial and subglacial water pathways. The latter has likely resulted in flawed constraints for the hydrological modelling of glacier drainage networks. Here, we present 3D ground-penetrating radar (GPR) results that provide high-resolution 3D images of an alpine glacier's drainage network. Our results confirm a long-standing englacial hydrology theory stating that englacial conduits flow around glacial overdeepenings rather than directly over the overdeepening. Furthermore, these results also show exciting new opportunities for high-resolution 3D GPR studies of glaciers.
... Direct observations have been made from boreholes (Fountain et al., 2005), tracer testing (Nienow et al., 1996), speleology (Gulley, 2009;Gulley et al., 2009;Temminghoff et al., 2019) and geophysical measurements. The latter include predominantly active (Peters et al., 2008;Zechmann et al., 2018;Church et al., 2019) and passive (Podolskiy and Walter, 2016;Lindner et al., 2019;Nanni et al., 2020) seismic 30 measurements or ground-penetrating radar (GPR) measurements (Moorman and Michel, 2000;Stuart et al., 2003;Irvine-Fynn et al., 2006;Harper et al., 2010;Baelum and Benn, 2011;Hansen et al., 2020). Most GPR studies, published so far, relied on two-dimensional (2D) data, where GPR measurements were acquired along profiles. ...
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Hydrological systems of glaciers have a direct impact on the glacier dynamics. Since the 1950’s, geophysical studies have provided insights into these hydrological systems. Unfortunately, such studies were predominantly conducted using 2D acquisitions along a few profiles, thus failing to provide spatially unaliased 3D images of englacial and subglacial water pathways. The latter has likely resulted in flawed constraints for the hydrological modelling of glacier drainage networks. Here, we present for the first time 3D ground-penetrating radar (GPR) results that provide unprecedented high-resolution 3D images of an alpine glacier’s drainage network. Our results confirms a long-standing englacial hydrology theory stating that englacial conduits flow around glacial overdeepenings rather than directly over the overdeepening. Furthermore, these results also show exciting new opportunities for high-resolution 3D GPR studies of glaciers.
... In glacierized watersheds, icings can form from the different water sources that are active during the winter. Both polythermal (Bukowska-Jania and Szafraniec, 2005;Hambrey, 1984;Sobota, 2016;Stachnik et al., 2016;Wadham et al., 2000;Yde et al., 2012) and cold-based glacier meltwater (Baelum and Benn, 2011;Hodgkins et al., 2004Hodgkins et al., , 1998Naegeli et al., 2014) can be responsible for icing formation. At some locations, features such as lakes (Moorman, 2003;Veillette and Thomas, 1979;Wainstein et al., 2014;Yde and Knudsen, 2005) and buried ice formations within proglacial fields (Gokhman, 1987) can contribute to icing growth. ...
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The ongoing warming of cold regions is affecting hydrological processes, causing deep changes, such as a ubiquitous increase in river winter discharges. The drivers of this increase are not yet fully identified mainly due to the lack of observations and field measurements in cold and remote environments. In order to provide new insights into the sources generating winter runoff, the present study explores the possibility of extracting information from icings that form over the winter and are often still present early in the summer. Primary sources detection was performed using time-lapse camera images of icings found in both proglacial fields and upper alpine meadows in June 2016 in two subarctic glacierized catchments in the upper part of the Duke watershed in the St. Elias Mountains, Yukon. As images alone are not sufficient to entirely cover a large and hydrologically complex area, we explore the possibility of compensating for that limit by using four supplementary methods based on natural tracers: (a) stable water isotopes, (b) water ionic content, (c) dissolved organic carbon, and (d) cryogenic precipitates. The interpretation of the combined results shows a complex hydrological system where multiple sources contribute to icing growth over the studied winter. Glaciers of all sizes, directly or through the aquifer, represent the major parent water source for icing formation in the studied proglacial areas. Groundwater-fed hillslope tributaries, possibly connected to suprapermafrost layers, make up the other detectable sources in icing remnants. If similar results are confirmed in other cold regions, they would together support a multi-causal hypothesis for a general increase in winter discharge in glacierized catchments. More generally, this study shows the potential of using icing formations as a new, barely explored source of information on cold region winter hydrological processes that can contribute to overcoming the paucity of observations in these regions.
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The polythermal Aldegondabreen is one of the most widely studied glaciers of the Nordenskjöld Land (Svalbard). However, the structure of its internal drainage network remains poorly understood. In order to determine the position and hydro-chemical characteristics of the surface and internal drainage channels of the glacier complex studies were carried out including ground penetrating radar (GPR) measurements and hydrological surveys. The GPR profiling performed in 2018–2020 identified four channels of internal drainage network, two of which are found along the northern side of the glacier in the area of cold ice and are subglacial. The other two are located in the area of temperate ice along the southern side of the glacier and are englacial, stretching at the cold-temperate surface. At the outlet grotto, the subglacial waters have a bicarbonate-calcium composition and low salinity (electrical conductivity 30–40 μS/cm), inherited from the surface meltwater streams that enter the moulins in the upper part of the glacier. No noticeable increase in mineralization occurs during the movement of the flow along the glacier bed. The englacial channels’ waters at the outlet grotto have the same bicarbonate-calcium composition but a higher salinity (electrical conductivity 100 μS/cm), which we attribute to the filtration through the rocks of the riegel near the Aldegonda terminus, or, alternatively, to the influx of the groundwater at the same spot. Measuring the hydrochemistry of the Aldegonda river tributaries both on the glacier’s surface, at the grottos and on the moraine in the valley made it possible to identify the zone of enrichment of the main volume of the low-mineralization glacial meltwater of bicarbonate-calcium composition by the high-mineralization (electrical conductivity up to 760 μS/cm) groundwater of sulphate-calcium composition coming from the springs on the riegel in front of the glacier’s terminus in the central part of the Aldegonda Valley. Presumably, the springs are fed by the deep filtration of melted glacial waters along the Aldegonda subglacial talik.
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Cold glacier beds, i.e., where the ice is frozen to its base, are widespread in polar regions. Common theories state that stable permafrost should exist under glacier beds on shorter timescales, varying from years to decades. Presently, only a few direct measurements of both sub-glacial permafrost and the processes influencing its thermal regime exist. Here, we present subglacial permafrost and active layer measurements obtained from within the basal drainage systems of two cold-based glaciers on Svalbard during the summer melt season. Temperature observations were obtained from subglacial sediment that was accessed through the drainage systems of the two glaciers in the previous winters. The temperature records cover the periods from spring to autumn in 2016 and 2019 at the glaciers Lars-breen and Tellbreen in central Svalbard. The ground temperature below Larsbreen indicates colder ground conditions, whereas the temperatures of the Tellbreen drainage system show considerably warmer conditions, close to the freezing point. We suggest the latter is due to the presence of liquid water all year round inside the Tellbreen drainage system. Both drainage systems investigated show an increase in subglacial sediment temperatures after the disappearance of snow bridges and the subsequent connection to surface melt-water supply at the start of the summer melt season. Temperature records show influence of sudden summer water supply events, when heavy melt and rain left their signatures on the thermal regime and the erosion of the glacier bed. Observed vertical erosion can reach up to 0.9 m d −1 at the base of basal drainage channels during summer. We also show that the thermal regime under the subglacial drainage systems is not stable during summer but experiences several freeze-thaw cycles driven by weather events. Our results show the direct importance of heavy melt events and rain on the thermal regime of subglacial permafrost and the erosion of the glacier bed in the vicinity of subglacial drainage channels. Increased precipitation and surface melt, as expected for future climate, will therefore likely lead to increased degradation of subglacial permafrost, as well as higher subglacial erosion of available sediment around the preferential hydrological paths. This in turn might have significant impacts on proglacial and fjord ecosystems due to increased sediment and nutrient input.
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Radio-echo soundings in four different frequency bands ranging from 30 to 1000 MHz were compared with temperature measurements in boreholes in the accumulation area and ablation area of Finsterwalderbreen (77°26′ N, 15°15′ E), southern Spitsbergen. Finsterwalderbreen is a polythermal surge-type glacier in the quiescent phase after its last surge around AD 1900. The objective of the study was to investigate the relation between internal echos and the glacier ice temperature to map the overall thermal structure of the glacier. The thermal structure is important for ice flow velocities and hydrology of glaciers, and it also affects their ability to surge. At the borehole site in the accumulation area (three boreholes within a range of 60 m), a change in the relative amplitude of the reflected signal is detected in the 320–370 and 600–650 MHz bands at 52–55 m depth. The high-resolution temperature measurements with 2 m intervals show that the transition zone between cold and temperate ice corresponds to the change in the relative amplitude on the 320–370 and 600–650 MHz bandwidth data. The overall thermal structure of the glacier was mapped based on the radar sounding. The radar results show (a) that the glacier is at the pressure-melting point over most of its bed except within 500–700 m of the terminus, and (b) that there is an upper cold ice layer of variable thickness (25–170 m) underlain by temperate ice. This thermal structure is confirmed by the thermistor-instrumented access holes to the bed in both the accumulation and ablation zones of the glacier. The variations in the thermal structure in lower parts of the accumulation area are explained by superimposed ice and ice layers that cause variations in the downward heat transfer by refreezing of meltwater.
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Digital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method. Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.
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Englacial temperature measurements in Arctic valley glaciers suggest in the ablation zone the existence of a basal layer of temperate ice lying below the bulk of cold ice. For such a polythermal glacier, a mathematical model is presented that calculates the temperature in the cold part and the position of the cold-temperate transition surface (CTS). The model is based on the continuum hypothesis for ice and the ice-water mixture, and on the conservation laws for moisture and energy. Temperate ice is treated as a binary mixture of ice and water at the melting point of pure ice. Boundary and transition conditions are formulated for the free surface, the base and the intraglacial cold-temperate transition surface. The model is reduced to two dimensions (plane flow) and the shallow-ice approximation is invoked. The limit of very small moisture diffusivity is analysed by using a stationary model with further reduction to one dimension (parallel-sided slab), thus providing the means of a consistent formulation of the transition conditions for moisture and heat flux through the CTS at the limit of negligibly small moisture diffusion. The application of the model to the steady-state Laika Glacier, using present-day conditions, results in a wholly cold glacier with a cold sole, in sharp contrast to observations. The present polythermal state of this glacier is suspected to be a remnant of the varying climatic conditions and glacier geometry during the past few centuries. Steady-state solutions representing a polythermal structure can indeed be found within a range of prescribed conditions which are judged to be realistic for Laika Glacier at the last maximum extent of the glacier.
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A network of passages situated along three-grain intersections enables water to percolate through temperate glacier ice. The deformability of the ice allows the passages to expand and contract in response to changes in pressure, and melting of the passage walls by heat generated by viscous dissipation and carried by above-freezing water causes the larger passages gradually to increase in size at the expense of the smaller ones. Thus, the behavior of the passages is primarily the result of three basic characteristics: (1) the capacity of the system continually adjusts, though not instantly, to fluctuations in the supply of melt water; (2) the direction of movement of the water is determined mainly by the ambient pressure in the ice, which in turn is governed primarily by the slope of the ice surface and secondarily by the local topography of the glacier bed; and, most important, (3) the network of passages tends in time to become arborescent, with a superglacial part much like an ordinary river system in a karst region, an englacial part comprised of tree-like systems of passages penetrating the ice from bed to surface, and a subglacial part consisting of tunnels in the ice carrying water and sediment along the glacier bed. These characteristics indicate that a sheet-like basal water layer under a glacier would normally be unstable, the stable form being tunnels; and they explain, among other things, why ice-marginal melt-water streams and lakes are so common, why eskers, which are generally considered to have formed in subglacial passages, trend in the general direction of ice flow with a tendency to follow valley floors and to cross divides at their lowest points, why they are typically discontinuous where they cross ridge crests, why they sometimes contain fragments from bedrock outcrops near the esker but not actually crossed by it, and why they seem to be formed mostly during the later stages of glaciation.
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Antarctica, the sixth continent, was discovered more than 160 years ago. Since then this large, mysterious continent of ice and penguins has attracted world interest. Scientific expeditions from various countries have begun to study the geographical and natural conditions of the icy continent. Systematic and comprehensive inves­ tigations in the Antarctic started in the middle of our century. In 1956 the First Soviet Antarctic Expedition headed to the coast of Antarctica. Their program included studies of the atmosphere, hydrosphere and cryosphere. Thirty years have since passed. Scientists have unveiled many secrets of Antarctica: significant geophysical processes have been investigated, and a large body of new information on the Antarctic weather, Southern Ocean hydrology and Antarctic glaciers has been obtained. We can now claim that the horizons of polar geo­ physics, oceanology, and particularly glaciology, have expanded. Scientific inves­ tigators have obtained new information about all Antarctic regions and thus have created the opportunity to use the Antarctic in the interests of mankind.