PERMAFROST AND PERIGLACIAL PROCESSES
Permafrost and Periglac. Process., 24: 39–55 (2013)
Published online 14 February 2013 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/ppp.1767
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard, Investigated by
Two- and Three-Dimensional GPR and Ground Thermal and Acceleration Regimes
Tatsuya Watanabe,1,2* Norikazu Matsuoka3 and Hanne H. Christiansen2
1
2
3
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki Japan
Department of Geology, The University Centre in Svalbard, Longyearbyen, Norway
Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki Japan
ABSTRACT
GPR is applied to image subsurface structures below non-sorted polygons in Kapp Linné, Svalbard, where ice and
active-layer soil wedges co-exist within a small area. Two-dimensional GPR images ice wedges as hyperbolic
reflections extending down from the frost table. However, some ice-wedge signals are obscured or masked by
similar hyperbolic reflections produced by stones or active-layer soil wedges. Three-dimensional GPR images ice
wedges as linear amplitude anomalies, which excludes the possibility of misinterpretation and offers more reliable
results. GPR investigations show that ice wedges are distributed sporadically in lower (younger) beach ridges, but
not in higher (older) ones.
Inter-site monitoring of ground temperature, soil moisture, slow ground deformation and cracking during 2004–09
and the determination of near-surface soil texture and stratigraphy suggest that snow cover and soil thermal properties
determine the distribution of ice wedges. Most ice wedges are considered to be inactive due to relatively high
permafrost temperatures. Shock loggers and extensometers detected shallow (soil wedge) cracking in sandy
sediments, when the ground surface temperature dropped to 12 C and the thermal gradient in the upper 20 cm
of ground reached 10 C m 1. Copyright © 2013 John Wiley & Sons, Ltd.
KEY WORDS:
GPR; permafrost; non-sorted polygon; ice wedge; soil wedge; thermal contraction; Svalbard
INTRODUCTION
In Svalbard, most thermal contraction polygons occur in
the bottom and at the lower part of slopes of inland valleys
(Sørbel et al., 2001) and on strandflats (Åkerman, 1980,
1987). Excavation and boreholes indicate that ice wedges
are predominant in Adventdalen (inland valley), while
active-layer soil wedges are more common at Kapp
Linné (strandflat), despite similar landform morphology
(Matsuoka and Hirakawa, 1993). Sandy sediments and
relatively warm permafrost in Kapp Linné (Christiansen
et al., 2010a) are apparently unfavourable for deep contraction cracking into the permafrost, which restrains icewedge formation (Matsuoka et al., 2004). On the other
hand, Åkerman (1980) mapped some wide troughs accompanied by ice wedges in the same region. This contradiction
* Correspondence to: T. Watanabe, Geological Survey of Hokkaido,
Sapporo 060-0819, Japan. E-mail: watanabe-tatsuya@hro.or.jp
Copyright © 2013 John Wiley & Sons, Ltd.
arises from the absence of criteria for surface morphology
and a threshold condition for ice-wedge growth, but also
that detailed geophysical methods and automated field
monitoring have not been applied to study the ground.
Recent advances in geophysical methods make it possible
to resolve shallow subsurface structure in periglacial
environments (Hauck and Kneisel, 2008). GPR is useful
for imaging near-surface structures because electromagnetic
waves penetrate into various materials, in both dry and
frozen states. Waves reflected from layer boundaries where
electrical properties change display subsurface structures at
high resolutions. GPR has been applied to permafrost
studies since the 1970s to measure the dielectric properties
of perennially frozen ground (Annan and Davis, 1976;
Davis et al., 1976), identify the extent of massive ground
ice (Dallimore and Davis, 1987; De Pascale et al., 2008),
estimate the spatial variation in active-layer thickness
(Doolittle et al., 1990; Moorman et al., 2003) and to
examine the internal structure of rock glaciers (Berthling
et al., 2000; Monnier et al., 2008), pingos (Ross et al.,
2005; Yoshikawa et al., 2006), palsas (Doolittle et al.,
Received 15 January 2011
Revised 29 November 2012
Accepted 4 January 2013
40
T. Watanabe et al.
1992; Horvath, 1998) and ice-wedge polygons (Hinkel
et al., 2001; Fortier and Allard, 2004; Munroe et al.,
2007; Watanabe et al., 2008). Meanwhile, automated techniques in field monitoring allow us to detect the timing
and magnitude of ground movements and to relate these
events to thermal and hydrological parameters, which have
advanced significantly since the 1990s in periglacial process
studies (Matsuoka, 2011).
This paper presents the results of GPR surveys mapping
and visualising wedge structures to identify the distribution
of ice- and soil-wedge polygons in Kapp Linné. Modern
thermal and subsurface conditions constraining the distribution of ice wedges and the frequency of thermal contraction
cracking are also investigated with ground temperature data
at three of the study sites.
STUDY AREA AND SITES
by the difference in vegetation density between the two.
Åkerman (1980) mapped most of the troughs as underlain
by active-layer soil wedges, while extraordinarily large
troughs (ELTs) measuring 100–250 cm wide, 10–20 cm
deep and > 100 m long were interpreted as underlain by
ice wedges (Figure 2a, d, f). The ELTs do not constitute
polygonal patterns, but occur as single troughs. Similar
large troughs, which Åkerman (1980) suggested are
underlain by ice wedges, sporadically occur in bogs with
peat accumulation (Figure 2e). However, they are rarely
accompanied by polygonal patterns and are obscured by
continuous vegetation.
Study Sites and their Exposure Ages
GPR surveys were conducted at six study sites on beach
ridges composed of gravel and sand (KL1–6) (Figure 1b).
Geomorphology and Meteorology
Kapp Linné lies at the southern edge of the mouth of
Isfjorden in western Spitsbergen (Figure 1). The area is
located on the wide coastal strandflat plain composed of a
sequence of raised marine terraces mantled by beach ridges,
small lakes and bogs. Geologically, the area consists of subvertical, north-trending metamorphic rocks ranging in age
from pre-Devonian to the Middle Carboniferous (Hjelle,
1993). Total uplift of the coastal plain since deglaciation
(11000-5500 yr BP) is c. 80 m (Landvik et al., 1987). The
area was deglaciated early due to its location on one side
of the Svalbard archipelago.
The Kapp Linné area is situated in a maritime, relatively
warm periglacial environment with a mean annual air
temperature (MAAT) of 4.7 C and a mean annual precipitation of 480 mm at the Isfjord radio station (1955–75: Førland
et al., 1997). Recently, a MAAT of 3.4 C for the period
2007–09 has been reported (Christiansen et al., 2010a).
Although the majority of the precipitation falls as snow in
winter, the snow cover is thin due to the strong southeasterly winds. Permafrost is nearly continuous except
in a limestone karst area, Vardeborgsletta, located on
the north side of Linnévatnet (Salvigsen and Elgersma,
1985). The active-layer thickness varies from 0.3 m in
bogs (organic soils) to 2.2 m in well-drained beach
ridges with only sparse vegetation (Åkerman, 2005).
During the 2007–09 period, active-layer thickness has
been observed to vary from 0.8 m in organic-rich beach
deposits, to 2.5 m in bedrock, with 1.8 m in beach ridge
sediments (Christiansen et al., 2010a).
Some relatively small polygons (5–15 m in diameter)
have developed on high-level raised beach ridges consisting
of gravelly sand, gravel and blocks (Figure 2). Most
polygons are flat or slightly high-centred and delineated
by narrow, shallow troughs, generally less than 10 cm deep
and 20 to 60 cm wide, that lack raised rims on both sides.
The polygon centres and troughs are well distinguished
Copyright © 2013 John Wiley & Sons, Ltd.
Figure 1 (a) Overview map of the Kapp Linné area. (b) Aerial photograph
of the Kapp Linné area showing locations of the individual study sites and
and results of radiocarbon dating at sampling points.
Permafrost and Periglac. Process., 24: 39–55 (2013)
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard
41
Figure 2 Photographs of the six study sites. ELT = Extraordinarily large trough.
Whereas distinct polygonal patterns exist at KL1–4, only
few sporadic troughs occur at KL5 and 6 (Figure 2).
KL1 is located on the highest and oldest studied ridge,
c. 38 m a.s.l., north of Linnévatnet (Figure 2a). Radiocarbon
dating by Landvik et al. (1987) indicated that this site was exposed above sea level 10000-9000 14C yr BP. Matsuoka and
Hirakawa (1993) observed only active-layer soil wedges
below all of the five excavated polygon troughs involving an
ELT at KL1.
KL2 is located on a beach ridge (c. 23 m a.s.l.) lying 1.1 km
east of Tunsjøen (Figure 2b). A whale bone was sampled from
the marine sediment very close to the KL2 study site at 24 m a.
s.l. in summer 2005, providing an age of 9300 110 14C yr
BP (T-18297). Approximately 450 m northwest of KL2, a
sample of shell fragments was collected from the surface of a
beach ridge at 14 m a.s.l. in the summer of 2005 and provided
Copyright © 2013 John Wiley & Sons, Ltd.
an age of 9080 85 14C yr BP (TUa-5853). Polygons at KL2
are delineated only by narrow troughs, all of which are
mapped as soil-wedge polygons (Åkerman, 1980).
KL3 (c. 10 m a.s.l.) is located on a beach ridge south of
Tunsjøen (Figure 2c). A series of beach ridges about 10 m
a.s.l. emerged from the sea around 7700 14C yr BP (Landvik
et al., 1987). Whereas most of the polygons are delineated
by narrow troughs, ELTs occur sporadically.
KL4 (c. 8 m a.s.l.) is located on a tongue-shaped beach
ridge north of Tunsjøen (Figure 2d). Shell fragments were
sampled from a depth of 0.2 to 1.44 m below the surface
of the homogeneous beach ridge sediment in 2004 during
the installation of temperature sensors. They provided an
age of 7375 80 14C yr BP (AAR-9452). Polygonal
patterns are mostly delineated by narrow troughs, but an
ELT crosses this beach ridge.
Permafrost and Periglac. Process., 24: 39–55 (2013)
42
T. Watanabe et al.
KL5 (c. 8 m a.s.l.) lies in a bog, located in a depression
beside a beach ridge (Figure 2e). Sporadic polygons are
mapped as ice-wedge polygons by Åkerman (1980).
KL6 lies on the lowest beach ridge (c. 5 m a.s.l.), where
ELTs (mapped as ice-wedge polygons by Åkerman, 1980)
are present sporadically (Figure 2f). At the outermost end of
a distinct coastal spit with beach ridges, located approximately 450 m east of KL6, shell fragments were sampled
on the surface of the beach ridge at 5 m a.s.l. in summer
2005. This sample has an age of 6605 50 14C yr BP
(TUa-5852).
METHODS
GPR Survey
Ice wedges produce strong subsurface reflections in twodimensional (2D) GPR profiles (Hinkel et al., 2001). The
contrast between the dielectric properties of ice wedges
and the surrounding frozen soils produces identifiable,
high-amplitude hyperbolic reflections. However, the
reflections may be obscured by other hyperbolic reflections
emanating from point structures (e.g. stones), causing
misinterpretation.
Recently, improved software has enabled the illustration
of three-dimensional (3D) GPR images, which extract the
wave amplitude anomalies by combining data from
multiple, closely-spaced radar transverses. The 3D GPR data
manipulation is known as ‘amplitude slice-map analysis’, in
which the difference in amplitude of reflected radar energy
is averaged between adjacent parallel radar traverses within
a specific time (i.e. depth) interval. 3D GPR reveals significantly higher resolution and more detailed subsurface
structure than 2D GPR. In fact, studies with 3D GPR have
demonstrated potential for imaging the configuration of
ice- and sediment-filled-wedge networks (Munroe et al.,
2007; Doolittle and Nelson, 2009). Consequently, we
applied 3D GPR to obtain more detailed subsurface images,
to complement 2D GPR results.
The GPR survey was conducted in the summers of 2007
and 2009. The GPR used in this study was a Noggin Smart
System (produced by Sensors & Software Inc., Mississauga,
Canada) equipped with a 250-MHz shield antenna. In 2007,
2D survey lines (30–90 m long) were sounded mostly at
10-m intervals at KL2–4, where a wide range of polygonal
patterns is observed. At KL5 and 6, where polygons are
sparse, survey lines across polygon troughs were established at right angles. For the 3D surveys, 25 m 25 m
survey squares were established at KL1, 3, 4 and 6 in
2009. A total of 51 parallel traverses at 50-cm intervals
were required for each square. The number of 2D transects
and 3D grids in the study sites is shown in Table 1.
Radar records were processed with EKKO_Mapper 4
software (Sensors & Software Inc.). Standard processing
included conversion of the two-way travel time to a depth
scale, removal of the direct ground wave during radar
Copyright © 2013 John Wiley & Sons, Ltd.
Table 1 Outline of GPR surveys.
Location
KL1
KL2
KL3
KL4
KL5
KL6
No. 2D lines
No. 3D squares
Estimated radar
velocity (m ns 1)
51
12
117
65
1
53
1
0
2
1
0
1
0.11
0.10
0.09
0.09
0.06
0.08
acquisition and gain adjustments. Converting to a depth
scale required determination of the radar transmission
velocity in the ground. The velocity was determined by
point-source reflection analysis, which estimated the
velocity using the shape of the hyperbolic patterns produced
by point structures. The accuracy of the estimated velocity
was confirmed by the depth of a subsurface reflector (e.g.
frost table) revealed by excavations. For generating slice
images, further processing (including dewow, migration
and envelope) was applied to remove unwanted low-frequency
signals, reduce hyperbolic diffraction and enhance amplitude
anomalies by replacing negative signal amplitudes with
positive values.
The GPR images were compared with the results of
excavation and/or from boreholes (130–270 cm deep)
carried out at several sites. Soil samples were taken from
soil wedges and host sediments at five sites (KL2–6), to
determine the grain size distribution using sieves for
particles from 0.064 to 16 mm. This analysis was necessary
because grain size affects the dielectric constant, generating
wave reflection, and accordingly, the interpretation of
GPR images.
Monitoring of Ground Thermal and Soil Moisture
Regimes, and Soil Deformation and Cracking
The ground thermal regime of thermal contraction cracking
in ice-wedge polygons has been recorded with a combination of several sensors in silty sediments (Allard and
Kasper, 1998; Matsuoka, 1999; Fortier and Allard,2005;
Matsuoka and Christiansen, 2008; Christiansen et al.,
2010b). As with these studies, an automatic monitoring
system was installed in a narrow polygon trough at the
KL2 site in late July 2007, to detect the timing and thermal
regime of thermal contraction cracking in an active-layer
soil wedge. The system consisted of thermistors, time
domain reflectometry (TDR) probes, horizontal extensometers, breaking cables and shock loggers (Figure 3).
Unfortunately, most of the system was damaged by a polar
bear in November 2008, and, as a result, only one-year
records of horizontal soil movements and air and ground
temperatures and two years of soil cracking and moisture
contents were collected.
Ground temperatures were measured hourly at 3-, 20-,
60-, 100-, 140- and 180-cm depths below the polygon
Permafrost and Periglac. Process., 24: 39–55 (2013)
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard
43
the detailed tracking of various aspects of ground motion,
including pre-failure ground contraction, the timing of
crack generation and subsequent crack propagation
(Matsuoka and Christiansen, 2008). The observed data on
ground movement and ground thermal regime were combined to define more precisely the threshold for thermal
contraction cracking.
Ground and/or air temperatures were also recorded using
Tinytag TGP-4020 miniature temperature dataloggers
(MTDs) at KL4 (2004 to the present) and KL5 (2005–07)
as a part of the IPY research project Permafrost Observatory
Project: A Contribution to the Thermal State of Permafrost
in Norway and Svalbard (TSP Norway) (2009). These records
are accessible directly from the Norwegian permafrost
database, NORPERM (www.ngu.no/norperm). MTDs measured hourly temperatures at 0-, 25-, 75- and 144-cm depths
at KL4 and at 0-, 13- and 27-cm depths at KL5. Data from
the deepest sensors at both sites approximated the TTOP.
Air temperatures were measured at KL4 every hour. In
addition, shock loggers were placed in the top of troughs
adjacent to the MTDs.
RESULTS
Figure 3 Overview of the automatic monitoring system at study site KL2.
TDR = Time domain reflectometry.
trough. The deepest thermistor represented the temperature at
the top of permafrost (TTOP). Campbell (Lugan, USA) TDR
probes measured the volumetric liquid moisture contents of
the soil at 20- and 50-cm depths every 6 h. Kyowa (Tokyo,
Japan) extensometers (resolution c. 0.04 mm), attached to
two steel frames anchored in permafrost, measured horizontal
movements across the trough every hour. Two extensometers
at different heights (0.2 and 0.4 m above the ground) can
document tilting of the frames. Miniature one-dimensional
accelerometers (Tinytag (Chichester, UK) high-sensitivity
shock dataloggers, TGP-0605) were placed in the top of thermal contraction cracks in the trough. A shock logger with a
built-in accelerometer, with a small known mass on an elastic
mounting attached to a strain gauge, measured the displacement
of the mass relative to its normal static resting place
(Christiansen et al., 2010b). The loggers record surface
movement as acceleration along an axis perpendicular to
the position of the logger’s top and bottom. Thus, loggers
placed vertically in polygon troughs record acceleration especially by thermal contraction cracking, and should be
able to register exactly when cracking occurs. Subsurface
cables made of very thin copper wires were installed across
the trough to detect the timing of thermal contraction cracking, recognised by the loss of electrical current when the
wire is cut. The cables were installed at 20- and 50-cm
depths across the trough to delimit the extent of any crack
depth. The overall combination of measurements enables
Copyright © 2013 John Wiley & Sons, Ltd.
2D GPR
Figure 4 shows representative 2D radar profiles across
polygon troughs at the six research sites. Most profiles are
characterised by strong horizontal reflections generated
from thermal transition (i.e. frost tables) and hyperbolic
reflections indicating vertical (i.e. ice or soil wedge) or
point (i.e. large stone or whale bone) structures (Moorman
et al., 2003).
Distinct hyperbolic reflections extend downwards
from the ground surface in polygon troughs, probably
representing active-layer soil wedges. However, hyperbolic
reflections extending downward from the permafrost table
and representing ice wedges are not common below the
narrow troughs. In contrast, the profiles crossing ELTs
(90–170 cm wide, 4–21 cm deep) at KL3 and 6 show
double hyperbolic reflections extending downward from
both the ground surface and the frost table (Figure 4c, f).
At KL5, although the 2D GPR survey was undertaken
across several troughs, only one profile was available
because the wet surface significantly attenuated the radar
signals. The available profile (Figure 4e) displayed a
narrow hyperbolic reflection extending downwards from
the frost table below the surveyed trough.
Excavated troughs at KL2 (Figure 5a) displayed activelayer soil wedges confined within the upper 20–50 cm of
the active layer. In contrast, active-layer soil wedges extend
below the frost tables at KL3 and 5 (Figure 5b, c). Further
drilling into the frozen layer at KL3 confirmed the presence
of an underlying ice wedge, reaching deeper than 270 cm.
Although the top width of the ice wedge was not measured
directly, the outline of the overlying soil wedge implied that
Permafrost and Periglac. Process., 24: 39–55 (2013)
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T. Watanabe et al.
Figure 4 Representative 2D GPR profiles at the study sites.
the width of the ice-wedge top is around 50 cm. Thus, the
reflection extending downwards from the frost table,
displayed by the 250-MHz impulse wave, indicates the ice
wedge. 2D radar profiles crossing ELTs at KL1 and 4 lack
hyperbolic reflections indicative of an ice wedge, although
these ELTs are mapped as ice-wedge troughs by Åkerman
(1980). A possible explanation for this absence is that ice
wedges are too small to be detected with the 250-MHz
wavelength antenna, but may be detected with a high-frequency
antenna (e.g. 400 MHz). Alternatively, the wedge structure is
confined to the active layer (i.e. the active-layer soil wedge), as
observed directly by Matsuoka and Hirakawa (1993).
Copyright © 2013 John Wiley & Sons, Ltd.
An excavation at KL5 (Figure 5c) displayed a surface peat
layer 20 cm thick underlain by gravelly marine sediments. A
peat wedge penetrated into the marine sediments below the
trough. The frost table varies from 25-cm depth in the peat
wedge to 35 cm in marine sediment. The shallower thaw
depth in the peat wedge may partly result from delayed
thawing of snow in the trough, but the large latent heat
capacity and low thermal conductivity of the saturated peat
may also delay thaw. The peat wedge extends below the frost
table, and at 40-cm depth it changes to an ice wedge 30 cm in
width. Drilling into the frozen layer confirmed that the bottom
of the ice wedge exceeded a depth of 130 cm.
Permafrost and Periglac. Process., 24: 39–55 (2013)
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard
45
Figure 5 (a) Two cross-sections of polygon troughs at the KL2 site, excavated on 22 July and 24 July 2007, respectively. (b) Cross-section of the extraordinarily large trough at the KL3 site, excavated on 25 July 2007. (c) Cross-section of a polygon trough at the KL5 site, excavated on 26 July 2007.
The near-surface wedge fillings tended to be significantly
finer-grained than the host sediments, except at KL3
(Table 2). Excavation of an ELT at KL3 displayed a soil
wedge composed of gravelly sand (just below the trough),
surrounded by pure marine sand (Figure 5b; Table 2). In
either case, the hyperbolic reflections were generated by
the differences in soil densities and dielectric properties
between the host sediment and wedge filling.
The radar propagation velocity in the active layer was
estimated by point-source reflection analysis using hyperbolic reflections generated by active-layer soil wedges.
The estimated velocity varies from 0.06 m ns 1 in wet
organic soil (KL5) to 0.11 m ns 1 in well-drained beach
sand (KL1), and decreases gradually from the highest beach
ridge (KL1) to the lowest (KL6) (Table 1). The variation in
velocity reflects the difference in moisture contents between
the sites. Finally, strong horizontal reflections at around
160 180-cm depth below the higher beach ridges (KL1
and 2), 100–150-cm depth below the lower beach ridges
(KL3, 4 and 6) and at 20-cm depth in a bog (KL5) (Figure 4)
are consistent with the depths of the frost table revealed by
excavation, drilling and/or ground temperature recording.
3D GPR
Figure 6 displays polygonal patterns in 3D survey squares at
KL1, 3, 4 and 6 and a sequence of time-slice images
Copyright © 2013 John Wiley & Sons, Ltd.
Table 2 Grain size composition of host sediments and soil
wedges.
Location
Host sediment
Silt and
clay (%)
KL2
KL3
KL4
KL5
KL6
0.4
0.4
1.4
1.8
1.8
Soil wedge
Sand Gravel Silt and
(%) (%)
clay (%)
Sand Gravel
(%) (%)
10.0
93.9
44.4
40.9
43.7
49.4
68.7
69.9
77.5
63.4
89.6
5.7
54.2
57.3
54.5
2.1
9.5
10.4
19.5
1.2
48.5
21.8
19.7
3.0
35.4
extracted from the ground surface, middle active layer and
uppermost permafrost. In the surface images, white linear
features of high-amplitude anomalies delineate some
polygon troughs, but the polygonal patterns are not well
expressed. The anomalies mainly occur under deeper,
vegetated troughs, reflecting the wetter surface condition
than the surroundings. Among the images of the middle
active layer, only that from KL6 (Figure 6d) shows
amplitude anomalies (in black) under the polygon troughs,
which imply a distinct dielectric difference between soil
wedges and host sediments in the active layer. In contrast,
the images from KL1, 3 and 4 (Figure 6a c) lack linear
amplitude anomalies, which indicates either a minimum
Permafrost and Periglac. Process., 24: 39–55 (2013)
46
T. Watanabe et al.
Figure 6 Polygon maps in 3D survey grids and a sequence of time-slice images. Numbers in the polygon map show the depth of the polygon trough (cm). The black
dashed lines in the polygon map indicate new open cracks observed in the summer of 2009. The time-slice images obtained from KL1 produced black lines parallel to the
vertical axis because two erratic 2D profiles were removed from the 3D analysis. Top row: Surface polygon patterns. Second row: Images at the surface. Third row: Images
of the middle of the active layer. Bottom row: Images of the upper part of the permafrost. This figure is available in colour online at wileyonlinelibrary.com/journal/ppp
contrast between soil wedges and host sediments or wedge
structures confined to the upper part of the active layer. In
the images of the uppermost permafrost, such anomalies
Copyright © 2013 John Wiley & Sons, Ltd.
appear on the images from KL3, 4 and 6 (Figure 6b d).
At KL6, the anomalies on the image of the uppermost
permafrost correspond to the hyperbolic reflections
Permafrost and Periglac. Process., 24: 39–55 (2013)
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard
47
Figure 6 (Continued)
extending downward from the permafrost table in the 2D
radar profile (Figures 4f and 6d). Hence, the linear anomalies reflect the presence of ice wedges. At KL4, a linear
anomaly occurs at 180-cm depth below the ELT, despite
the lack of a hyperbolic reflection indicative of an ice wedge
(Figures 4d and 6c); thus, the ice wedge at KL4 is
Copyright © 2013 John Wiley & Sons, Ltd.
overlooked in the 2D radar profile. Furthermore, 3D GPR
is capable of delineating narrow ice wedges (c. 50 cm wide)
as revealed by an excavation at KL3 (Figure 5b).
These results demonstrate that 3D GPR is more reliable
in displaying ice wedges than 2D GPR. This conclusion
implies that the absence of linear amplitude anomalies at
Permafrost and Periglac. Process., 24: 39–55 (2013)
48
T. Watanabe et al.
KL1 (Figure 6a) indicates that wedge structures are
confined to the active layer even below the ELT, supporting
direct observations by Matsuoka and Hirakawa (1993).
However, there could be smaller ice wedges which were
not resolved with the 250-MHz antenna.
Air and Ground Temperatures
Winter air temperature generally fluctuated between 25 C
and 0 C, occasionally rising above 0 C (Figure 7). A
comparison between the current and past winter air temperatures revealed a rise in the average winter air temperature,
and a decrease in extremely cold spells (Table 3). Kapp
Linné experienced cold winters in the 1960s, nearly equivalent to the present winter conditions in Adventdalen
(Figure 1a), which is close to the limit of modern ice-wedge
activity (Matsuoka and Christiansen, 2008; Christiansen
et al., 2010).
The ground surface temperatures followed the air
temperatures, and showed lower spatial and interannual
variations (Figure 7). At KL4, the mean annual ground
surface temperatures over four years (2005–08) fluctuated
between 1.3 C (2005–06) and 3.0 C (2008–09), and
with mean annual TTOPs varying from 2.2 C (2005–06)
to 2.9 C (2008–09).
Winter conditions varied from year to year. The winter of
2004–05 had a cold period including stable low air temperatures ( 13 to 25 C) for three weeks in March (Figure 8a).
The minimum air temperature ( 24.8 C) recorded 12
March was the lowest during the five-year monitoring
period. Then, the ground surface temperature at KL4 fell
below 23 C. The winter of 2005–06 was relatively mild,
but the air temperature dropped close to 20 C several
times in February and March (Figure 8b). However, ground
surface temperatures never fell below 15 C at both KL4
and 5. The winter of 2006–07 was also relatively mild, but
the ground surface cooled to 18 C at KL4 and to 13 C
at KL5 at the end of January, when the air temperature
temporarily dropped to 19 C (Figure 8c). By contrast,
the winter of 2007–08 was relatively cold, particularly in
late March, with a minimum air temperature of 21 C
(Figure 8d). Nevertheless, ground surface temperatures
never fell below 15 C at both KL2 and 4. The winter of
2008–09 had a cold period in early January (Figure 8e).
The air temperature fluctuated between 15 C and 24 C
for two weeks, and then the ground surface temperature at
KL4 reached 20 C.
In general, ground surface temperature responded to air
temperature fluctuation except in the late winters of 2004–05
and 2006–07 at KL4 (Figure 8a, c). By contrast, ground
temperature was much less sensitive to air temperature
Figure 7 Summary of automated monitoring data at KL2, 4 and 5 (2004–09).
Copyright © 2013 John Wiley & Sons, Ltd.
Permafrost and Periglac. Process., 24: 39–55 (2013)
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard
49
Table 3 Summary of winter air temperatures ( C) in Kapp Linné and Adventdalen.
Location and recorded period
Kapp Linné (1935–75: Isfjord radio) (Førland et al., 1997)
Monthly mean
Lowest monthly mean
Absolute minimum
Kapp Linné (2005–08: KL4 site)
Monthly mean
Lowest monthly mean
Absolute minimum
Adventdalen (2000–09: UNIS weather station)
Monthly mean
Lowest monthly mean
Absolute minimum
December
January
February
March
8.9
17.8 (1968)
33.5 (1968)
10.7
18.6 (1967)
32.0 (1967)
11.5
20.2 (1963)
32.2 (1963)
12.2
19.4 (1969)
32.3 (1966)
6.9
8.0 (2007)
19.1 (2008)
7.1
9.6 (2007)
19.3 (2007)
9.1
10.1 (2006)
19.0 (2006)
12.0
14.4 (2005)
24.8 (2005)
9.0
11.8 (2000)
29.2 (2008)
12.1
22.7 (2003)
37.6 (2009)
13.7
18.7 (2004)
35.9 (2004)
16.2
21.4 (2002)
38.1 (2002)
UNIS = The University Centre in Svalbard.
fluctuation at KL2 and 5 (Figure 8b–d) due to a somewhat
thicker snow cover than at the extremely exposed KL4 site.
Temperature variations were less pronounced at the top of
the permafrost than near the ground surface. At KL2 and
4, the TTOP (measured at 180-cm and 144-cm depth, respectively) reached its annual minimum in late winter
(March–April), but never fell below 10 C. By contrast,
at KL5, the TTOP (at 27-cm depth) attained its annual
minimum just after the surface temperature reached its
minimum; and the TTOP dropped below 10 C briefly in
late January 2007, when the ground surface cooled to
13 C (Figure 8c).
The thermal gradient displayed a considerable spatial
variation between the three sites (Figure 8). The largest
negative thermal gradients (> 15 C m 1) favourable for
thermal contraction cracking (Mackay, 1986) occurred in
the uppermost soil (0 25-cm depth) at KL4, where the
ground surface cooled most rapidly. However, the thermal
gradients diminished to < 15 C m 1 below 25-cm depth
and further to < 7 C m 1 below 75-cm depth. The KL2
and 5 sites exhibited smaller (negative) thermal gradients
than the KL4 site, although they were still large enough
(> 10 C m 1) to produce cracking (Mackay, 1986) in
the topmost 20 cm of ground at KL2 and in the topmost
27 cm at KL5.
The cooling rates in the ground also displayed a distinct
difference between the three sites. KL4 and 5 often experienced rapid cooling (> 0.4 C h 1) at the ground surface,
but KL2 did not. At KL5, the top permafrost was also often
cooled at a rate of 0.4 to 0.3 C h 1 because of the thin
active layer (c. 27 cm). By contrast, the cooling rate at
KL2 never fell below 0.3 C h 1 even at 20-cm depth.
The volumetric liquid water content in the active layer
varied seasonally in response to annual freeze/thaw cycles
(Figure 7c). In early summer, the thawed soil experienced
a rapid, temporary wetting by infiltrating meltwater. The
water contents at both 20- and 50-cm depths never exceeded
20 per cent throughout the year, reflecting good drainage
due to coarse sediments. Rapid percolation of meltwater
would have accelerated thawing and resulted in relatively
Copyright © 2013 John Wiley & Sons, Ltd.
thicker active layers. In mid-summer, the water content in
the active layer fluctuated around 10 per cent, and then
quickly decreased as seasonal freezing started, which
implies that the voids in the seasonally frozen soil were
mainly filled with air. In winter, the water content in frozen
soils apparently showed low and stable values (< 5%), but
the unfrozen water content in frozen soil has not yet been
calibrated.
Soil Deformation and Cracking
The horizontal extensometer at KL2 showed seasonal
movement of a trough expanding in early winter and
shrinking in early summer (Figure 7). In addition, minor
trough expansion (c. 3 mm) was observed at the beginning
of February 2008 (Figure 8d).
The miniature shock loggers located in troughs detected
frequent acceleration events (Figure 7). The events
associated with intensive ground cooling in winter most
likely indicate thermal contraction cracking; others possibly
reflect animal or human disturbance, however, mainly in
summer, when the ground was not frozen. At KL4, large
acceleration events were recorded in the winters of 2006–07
and 2008–09, but not in 2005–06 and 2007–08
(Figure 8).
In the winter of 2007–08, two large acceleration events
occurred at KL2 (Figure 8d). The first occurred on 2
February 2008 immediately after the extensometer recorded
minor trough expansion. The occurrence of thermal contraction cracking in the trough is inferred from this coincidence.
The breaking cables at 20- and 50-cm depths, however,
remained connected, which implies that crack penetration
was confined to the uppermost 20 cm. When acceleration
occurred, the ground surface temperature dropped to 12 C
and the thermal gradient reached 11 C m 1 in the uppermost soil (3–20-cm depth) (Figure 8d). The second event
occurred on 19 February 2008, but was not accompanied with
horizontal trough expansion. Since the ground surface temperature fluctuated around 6 C at that time, the event may have
been triggered by residual stress or animal disturbance.
Permafrost and Periglac. Process., 24: 39–55 (2013)
50
T. Watanabe et al.
Although no large acceleration events were registered in
the monitored trough at KL4 in the winter of 2007–08,
thermal contraction cracking might have occurred in other
troughs at KL4. In other winters, large winter acceleration
events at KL4 were generally associated with ground
surface temperatures below 12 C and negative thermal
gradients of 10 C m 1 or below in the uppermost active
layer (0–25 cm). Thus, these values may represent the
Figure 8 Air and ground temperatures, rates of temperature change, thermal gradients and soil deformation through the winters of 2004–05 to 2008–09 from 11
December to 10 April. Air temperature data are representative values measured at KL4. TTOP = Temperature at the top of permafrost.
Copyright © 2013 John Wiley & Sons, Ltd.
Permafrost and Periglac. Process., 24: 39–55 (2013)
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard
51
Figure 8 (Continued)
thresholds for thermal contraction cracking in sandy
sediments.
DISCUSSION
Which Structures Can 2D and 3D GPR Detect?
In general, GPR surveys in permafrost environments are
best conducted when the active layer is frozen, because
the radar signals can penetrate well (Hinkel et al., 2001).
Although summer is generally unfavourable for GPR
surveys, as the radar signal tends to attenuate in the thawed
active layer, dry coarse sediments with a relatively low
dielectric constant attenuate the signal less than wet, fine
sediments, enabling the structure of the uppermost permafrost and ice wedges to be imaged.
Both ice and soil wedges show strong hyperbolic reflections on 2D GPR profiles. The reflection of ice wedges
results from the difference in dielectric constants between
ice and frozen soils (Hinkel et al., 2001). Likewise, the
Copyright © 2013 John Wiley & Sons, Ltd.
reflection of soil wedges is probably affected by the difference in dielectric properties between the soil wedge and
the host sediment. Ice and active-layer soil wedges are
distinguished by the depth of hyperbolic reflections
despite the similar reflection patterns. Accordingly, 2D
GPR is a useful tool for distinguishing ice wedges from
soil wedges. Furthermore, 2D GPR permits the rapid
examination of a large number of troughs. These
advantages allow mapping of the subsurface distribution
of ice and soil wedges. Nevertheless, a large shortcoming
of 2D GPR is that both linear (e.g. ice wedge) and point
(e.g. stone) structures provide similar hyperbolic reflections, which might cause misinterpretation. Moreover,
large hyperbolic reflections produced by soil wedges or
large clasts may mask the reflection of ice wedges, as at site
KL4 (Figure 4d).
3D GPR displays ice wedges as linear, high-amplitude
anomalies in time-slice images, which simplifies the
distinction of ice wedges from clasts and minimises the
possibility of misinterpretation. Furthermore, 3D GPR
can identify an ice wedge hindered by the reflection of a
Permafrost and Periglac. Process., 24: 39–55 (2013)
52
T. Watanabe et al.
soil wedge in 2D GPR profiles. Overall, 3D GPR provides
more reliable images of subsurface wedge structures
underlying non-sorted polygons compared to 2D GPR
profiles. Data acquisition is time-consuming with a large
number of survey lines, but is significantly faster and
non-destructive compared with direct excavation to the
permafrost.
One advantage of 3D GPR is that it can detect the depth
of subsurface structures. However, estimating ice-wedge
dimensions is difficult due to the lack of prominent anomalies in sequentially deeper slices. Narrow ice wedges (30–50
cm wide) in the study site may also complicate the imaging
of deeper structures.
Factors Controlling Ice-Wedge Distribution
GPR surveys revealed that ice wedges only underlie ELTs
on the lower (younger) beach ridges (KL3, 4 and 6) and
primary troughs in the bog (KL5) (Figure 9). This indicates
that ice wedges have developed during the last 7 ka following the emergence of the 10-m a.s.l. beach ridge (KL3). In
contrast to the lower beach ridges, no ice wedges were
detected – even below the ELTs – on the higher (older)
beach ridges (KL1 and 2) that emerged above sea level
9–10 ka ago. If the substrates are similar in all beach
ridges, then more mature ice wedges would be expected
in higher beach ridges, but this is not the case. This
contradiction implies that near-surface environments
constrain the ice-wedge distribution at the six study sites.
According to the viscoelastic theory of Lachenbruch
(1962), ice-wedge cracking requires a rapid and pronounced
fall in ground temperature. A comparison of winter
ground thermal conditions between the KL2, 4 and 5
sites (Figures 7 and 8) indicates that KL4 tends to experience the most rapid cooling, probably reflecting the
thin snow cover in winter. Strong southeasterly winds
remove the snow cover from exposed beach ridges such
as KL4, which permits large fluctuations in ground
temperature and rapid cooling throughout the winter. In
contrast, a high mountain ridge (Figures 1 and 9) blocks
the southeasterly winds, leading to moderate snow cover
at KL2.
Depressions such as KL5 preferentially trap snow,
reducing rapid ground cooling. However, the presence of
ice wedges at KL5 implies that the thermal properties of
the peat contribute to ice-wedge development despite the
snow conditions (Williams and Smith, 1989, p. 90). In
summer, the peaty active layer is significantly shallower
at KL5 than the inorganic active layer at all the other
study sites. This difference results from the low thermal
conductivity of unfrozen peat compared to that of
coarse-grained mineral sediments in summer. In contrast,
wet, frozen peat has higher thermal conductivity than unfrozen peat (Kujala et al., 2008), which favours rapid
ground cooling in winter. In fact, the TTOP at KL5
quickly responded to fluctuations in the surface ground
temperature and often experienced rapid cooling ( 0.4
to 0.3 C h 1) in winter (Figure 8b, c). Accordingly,
ice wedges at KL5 are considered to have developed due
to the seasonal contrast with insulating unfrozen peat
preventing ice veins from melting in summer, while
conductive frozen peat accelerates rapid ground cooling
Figure 9 Schematic transect across uplifted marine terraces, representing spatial variations in height above sea level, surface morphology and subsurface
structure conditions for the Kapp Linné study site, Svalbard.
Copyright © 2013 John Wiley & Sons, Ltd.
Permafrost and Periglac. Process., 24: 39–55 (2013)
Ice- and Soil-Wedge Dynamics in the Kapp Linné Area, Svalbard
53
in autumn and winter. In contrast to KL5, intensive
ground cooling rarely occurred at KL2 even when a
larger (negative) thermal gradient (< 10 C m 1) developed in the upper 20 cm (Figure 8d). The low thermal
conductivity of dry, frozen mineral soil probably
prevents rapid ground cooling that produces effective
thermal stress.
Furthermore, spatial variations in grain size and ice
content influence the linear coefficient of thermal contraction for frozen soils, which may also affect the magnitude
of thermal contraction cracking (Lachenbruch, 1962). In
general, the coefficient increases from gravel (<
1 10 5 C 1) to sand (1 10 5 to 1 10 4 C 1) and
further to ice-rich peat (2 10 4 to 4 10 3 C 1)
(Gamaleia and Brushkov, 1987; Yershov, 1998; Mackay,
2000). The lower coefficient of gravel layers is less favourable for generating thermal stress large enough for cracking
to reach the permafrost. In fact, the excavations at KL1 and
2 revealed that wedge structures were stopped by a gravel
layer (Figure 5a; Matsuoka and Hirakawa, 1993, Figure 2c).
The well-drained condition of the gravel, indicated by the
low soil moisture winter values at KL2 (Figure 7c), also
restricts the penetration of thermal contraction cracking
into the permafrost, because the contraction coefficient
decreases with desiccation (Romanovskii, 1973). In
contrast, sandy or finer sediments would permit deep
penetration of thermal contraction cracks on the lower
beach ridges (KL3–6), where ice wedges are sporadically
developed. At KL5, peat accumulation seems to favour
ice-wedge formation, because peat has a higher linear
coefficient than coarse-grained mineral soils. The peatfilled soil wedge implies that peat accumulation activates
thermal contraction cracking. Indeed, some field studies
have emphasised that ice-rich frozen peat, with a high
linear coefficient of thermal contraction, promotes the growth
of ice wedges (Mackay, 2000; Fortier and Allard, 2004).
In summary, snow cover and soil thermal properties in
combination are likely to be the main controls on ice-wedge
distribution in the Kapp Linné strandflat study area,
although other factors (e.g. vegetation and creep of frozen
ground) may also affect cracking (Romanovskii, 1973;
Mackay, 1993).
Christiansen, 2008; Matsuoka, 2011). However, the critical
temperatures have not yet been defined for cracking in
coarse sediments. In addition, the above thermal thresholds
are not unequivocal even within a small area, because the
creep and strength properties of frozen soils vary with local
factors such as ice content and grain size (Williams and
Smith, 1989, p. 171). In Kapp Linné, air temperature never
fell below 25 C in recent winters (2004–09), which
prevented cooling of the ground surface below 20 C, even
at the exposed beach ridge (KL4). Nevertheless, monitoring
revealed the occurrence of shallow (soil wedge) cracking,
when the ground surface temperature fell below 12 C and
the uppermost soil was subjected to large negative thermal
gradients (> 10 C m 1). This relatively high critical
temperature limit suggests that non-viscous coarse sediments
activate thermal contraction cracking at higher temperatures
compared to viscous fine sediments, although the crack
penetration is probably confined to a relatively shallow depth.
Our monitoring recorded the timing of thermal contraction
cracking, but the cracks are unlikely to have reached the permafrost and contributed to ice-wedge growth. This is because: (1)
the TTOP never fell below 10 C during the monitoring period at both KL4 and 5 (Figure 7a, b), where ice wedges were
displayed by GPR; (2) the thermal gradient through the active
layer at KL4 (0–144-cm depth) never fell below 10 C m 1;
(3) the organic crack filling observed at KL3 (Figure 5b) implied that recent cracking was confined within the active layer;
and (4) new cracks were rarely observed in the ELTs underlain
by ice wedges. With regard to the fourth point, preferential
snow accumulation in deep troughs prevents intensive cooling
of these locations.
The climate statistics from the middle of the 20th century
(1935–75: Førland et al., 1997) indicate that the Kapp Linné
area often experienced relatively low MAATs ( 8 to 6 C)
and cold winters during the 1960s, following a relatively
mild climate between the 1930s and 1950s. Ice-wedge
cracking may activate temporarily during cold periods.
However, the latest intensive activity of ice-wedge cracking
in Kapp Linné may date back to the early 20th century when
the MAAT was well below 6 C (Hanssen-Bauer et al.,
1990) or before then, during the Little Ice Age.
Activity of Ice Wedges and Soil Wedges
CONCLUSIONS
In the summers of 2007 and 2009, new open cracks were
widely observed in narrow polygon troughs on the beach
ridges (see the polygon map in Figure 6), indicating active
thermal contraction cracking. In contrast, new open cracks
were rare in the summer of 2008. These observations
correspond well with the annual variation of winter ground
acceleration events (Figure 8).
Previous studies in Arctic regions suggest that ice-wedge
cracking requires lowering of ground surface temperatures
below 20 C, and the TTOP below 10 C in fine-grained
sediments (Mackay, 1993; Allard and Kasper, 1998;
Christiansen, 2005; Fortier and Allard, 2005; Matsuoka and
The combination of 2D and 3D GPR surveys with monitoring of ground thermal conditions and accelerations significantly improves understanding of the controls of
ice- and soil-wedge growth in the present climate in
the Kapp Linné region.
GPR surveys are useful for mapping the extent of ice and
soil wedges below polygon troughs. 2D GPR permits rapid
imaging of numerous troughs, although interpretation is
often obscured by reflections generated from structures such
as stones. 3D GPR provides more precise images which
greatly reduce misinterpretation and obscuration. In the
Kapp Linné area, ice wedges underlie ELTs on lower,
Copyright © 2013 John Wiley & Sons, Ltd.
Permafrost and Periglac. Process., 24: 39–55 (2013)
54
T. Watanabe et al.
younger beach ridges and sporadic polygon troughs in bogs
developing in depressions between or within beach ridges.
This suggests that ice wedges have been active in the last
7 ka. In contrast, no ice wedges are detected in higher
(older) beach ridges, even below ELTs.
Ice-wedge distribution in the Kapp Linné region is
controlled primarily by surface and near-surface conditions,
including snow cover and soil thermal properties (thermal
conductivity and the coefficient of thermal contraction).
New, open cracks observed in the field indicate that thermal
contraction cracking is still active under the present
relatively warm climate, contributing to the growth of some
active-layer soil wedges. Cracking was recorded when the
ground surface temperature dropped to 12 C and the
thermal gradient reached 10 C m 1 in the uppermost soil.
However, most ice wedges are considered to be inactive
because the TTOP is too high and the active-layer thermal
gradient is too small for cracking to reach the permafrost.
Ice wedges may have been active until the early 20th
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ACKNOWLEDGEMENTS
This study was financially supported by Grants-in-Aid
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