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PERMAFROST - Seventh International Conference (Proceedings),
Yellowknife (Canada), Collection Nordicana No 55, 1998
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TEMPERATURE CONDITIONS FOR ICE-WEDGE CRACKING:
FIELD MEASUREMENTS FROM SALLUIT, NORTHERN QUƒBEC
Michel Allard, Jennifer N. Kasper
Centre dÕŽtudes nordiques
UniversitŽ Laval, Sainte-Foy, QuŽbec, Canada G1K 7P4
e-mail: [email protected]
Abstract
The temperatures at which thermal cracking occurred along ice-wedges around a tundra polygon were measured over two years near Salluit, northern QuŽbec. Electrical cables were buried in the active layer across furrows or cracks in the soil at various places around the polygon. The time of breaking of the electrical cables,
and the air, soil surface and ground temperatures down to 2.5 m were monitored with a datalogger. In the
course of the two winters, several cables broke with the opening of thermal cracks. Over the two years, the first
cracks opened in late December-early January when the temperature at the permafrost table was about - 15¡C,
and after a drop of air temperature from about -20¡C to below -32¡C. Mean cracking temperature at wedge top
was -20¡C in the first year and -19.7¡C in the second year. The data also allow estimates of the minimum temperature changes and cooling rates required to induce ice-wedge cracking. The cracks closed (or narrowed) and
re-opened (or widened) in response to winter temperature fluctuations at the soil surface. The measured thermal conditions for cracking substantiate the previous theoretical work on this basic process at the origin of tundra polygons.
surements of air and ground temperatures at the time
and site of frost cracking have not yet been reported.
Introduction
Cracking of the soil, the process at the origin of icewedges and sand-wedges and of tundra polygons
(Leffingwell, 1919), is a temperature-controlled phenomenon. In theoretical work on the mechanics of thermal contraction cracking, Lachenbruch (1962) estimated
that the required value of soil viscosity to allow frost
cracking is reached when air temperatures fall between
-20 and -30¡C and when the temperature at the top of
permafrost is about -15 to -20¡C (Lachenbruch, 1966;
PŽwŽ, 1966). However, for cracking to take place, a
rapid drop of temperature is the triggering mechanism.
With a too low cooling rate, the tensile stress induced
by thermal contraction can be released by permafrost
creep. ÒLess than half a day of cooling... is required for
a close approach to the full stress value when cooling
rate is 10¡C/day (a total cooling of about 4¡C over a
period of about 9 hours). Lower cooling rates must be
maintained longer to be effectiveÓ (Lachenbruch, 1962,
p. 20). Similarly, Grechishchev (1973, p. 231) concludes
that: Òthe dynamics of the formation of frost fissures
obviously look like the following: long period cooling
leads to stress equal to the long term strength, and rupture is caused by secondary short-period temperature
fluctuationsÓ. Detection of several hundreds of frost
cracking events and careful examination of field and
climatic conditions by Mackay (1974, 1978, 1984, 1992,
1993) supports these theoretical studies. However, mea-
Over two winters, we measured the air and soil temperatures at which frost cracking occurred along a few
ice-wedges around a polygon. This was done at a site
that lies near the southern margin of the distribution
range for active ice-wedges.
Study area and characteristics of the experimental site
The test site is in the Foucault river valley
(Narsajuaq), 12.5 km west of Salluit (Figure 1). The
mean annual air temperature for the area is estimated
to be about -8¡C (Gray, 1983). At the study site, our
measurements in 1990 indicated a mean air temperature of - 8.9¡C. The annual temperature range is about
37¡C. Total precipitation is estimated to be 310 mm, of
which 53% is snow. Ground temperature at the depth of
zero annual amplitude (23 m) is about
-6.2¡C, as measured in drill holes near Salluit airport
(Allard et al., 1995). The experimental site itself is on a
fluvial terrace at an elevation of 14.5 m a.s.l. It bears a
network of random orthogonal, low centre polygons
with a mean diameter of 16 m (Figure 2). The soil consists of a 2-2.6 m thick sequence of organic-rich fine
sand layers alternating with 10-30 cm thick sandy
organic layers that overlie coarse fluvial and gravely
sands. These fluvial sediments are themselves about
Michel Allard, Jennifer N. Kasper
5
Methods
INSTRUMENTATION
Two thermistor cables (thermistors YSI-44033) were
installed in drill-holes, one in the centre of the polygon,
the other under a trough along a side, through an icewedge. The centre of the polygon is about 40 cm deeper
than its raised edges and the trough along the polygon
side is about 30 cm deep ; therefore the soil surface
over the wedge is about 10 cm above the level of the
polygon centre. The cables were set in PVC casings
filled with silicone oil (Osterkamp, 1974). A thermistor
was also installed on top of a 3 m high mast, in a shelter, to measure air temperatures, and one was at the
ground surface (+/- 5 cm deep under the moss in polygon centre). Temperature readings were recorded by a
datalogger (Campbell Scientific CR-10, 12 channels).
Two channels were used to store hourly temperatures
in the air and at the ground surface. Every four hours,
temperatures were recorded at 55, 105 and 255 cm
depths in the polygon centre and at 8, 58, 108, 158 and
258 cm depths in the polygon trough.
Figure 1. Location of the experimental site.
7 m thick and they overlie post-glacial marine sandy
silts. In river bluff sections and on ground probing
radar profiles, the ice-wedges are seen to penetrate
downward 5-6 m into the fluvial sediments (see Allard
et al., 1993, p.7). According to the regional uplift curve,
the site must have emerged about 4700 years ago, setting a potential date for the onset of frost cracking and
initiation of the polygons (Kasper, 1995).
As wedge ice was met in the drill hole in the trough at
a depth of 33 cm, which is close to mean wedge depth
in the region (62 measurements), a depth of 30 cm is
used below as the wedge top depth for interpolations of
temperature of cracking.
The last available channel on the data logger was used
for a cracking detection system which consisted of a
main electric circuit including ten resistors of different
values in series. Each resistor was bypassed with an
Figure 2. Plan of the instrument layout.
6
The 7th International Permafrost Conference
electrical cable loop. Incremental increase in resistance
due to the breaking of one loop spread across a potential crack could be monitored with the datalogger. The
resistance increment could be used to identify the broken loop. As the values of the resistors differed by equal
increments and as the circuit was sensed only every
four hours (a design fault found afterwards), there
existed the potential situation that if two cables broke
between two readings, the total resistance shift on the
circuit equaled the one of a third different cable, leading
to an identification mistake. For the first winter the
structure of the data indicates that this did not occur.
The loops, or breaking cables, were first laid across
cracks on 12 July, 1989 at depths of about 15 cm in the
thawing active layer. Nine of them extended across the
top of known ice wedges (Figure 2). One was laid on
the inner side of a small polygon ridge where, apparently, new cracking was taking place. The cables were
dug up one year later on 1 July, 1990, with a small
amount of damage resulting from excavating through
5-10 cm of frozen ground. The broken cables were soldered and buried again, taking care that an unaffected
segment of cable extended across the wedge. For the
second winter, jumps in resistance were clear-cut in the
data, but identification of which cables broke on particular events was not always possible.
DATA ANALYSIS
During excavation in 1990, 7 of the 10 cables showed
evidence of strain. As the type of electrical wire used in
the loops was rather coarse (steel wires coated in
translucent rubber sheeting of 2 mm o.d.), it could be
seen that tension on some of them had broken the steel
wire but that the rubber coating had been only
stretched. As crack closing had taken place, some of
them were still pinched in ice veins and in the cracks.
On 21 August, 1991, re-excavation revealed that 8 cables
had been affected and that their mechanical behavior
had been the same as in the previous winter.
Despite the coarse time interval for cracking detection
(4 hours), discrete cracking events could be discerned.
An unexpected outcome was that open cracks partially
closed and re-widened several times during the winter,
each time re-closing sub-circuits and registering shifts
in the resistance channel, thus permitting assessment of
both opening and closing events.
The logged temperature data were quality checked for
consistency and loaded into a spreadsheet software.
Then, times and temperatures in the soil and in the air
were determined for each cracking and each closing
event. The peak or trough of the temperature oscillation
preceding the event was searched both in the data table
and on ÒT vs timeÓ graphs (Figures 3 and 4) to determine the range of the triggering temperature oscilla-
tion, its duration and the rate of change (Table 1). A
minus sign applied to a rate means, in the tables and in
the text, a cooling rate.
Results
WINTER 1989-90
The steel wires in five cables broke between 27
December and 15 January (#7, #3, #1, #4 and #6 in
sequence). The first rupture took place when the air
temperature was -32.3¡C; ground temperatures were
-17.5¡C at 8 cm depth, and -15.3¡C at the wedge top
(interpolated). The vertical temperature gradient was
10¡C/m from the surface through the top of the wedge
and diminished to 1.9¡C/m in the lower meter of the
profile. This first cracking occurred during the first
major cold spell of the winter which had begun 34
hours beforehand and after a cooling of 10.6¡C, that is
at a rate of -0.3¡C/hr (-7.4¡C/day). At 8 cm, the cooling
went back 140 hours at a rate ten times smaller and it
went to 240 hours at 58 cm, at a very low cooling rate of
-0.0096¡C/hr (-0.23¡C/day) (Table 1). This first
cracking, and the second one which occurred in the
same day, may have been a response to tensile stress at
the soil surface given the small cooling rates at depths
greater than 8 cm which are part of the general early
winter cooling.
All crack openings of this winter (including reopenings after January 15) occurred at air temperatures
of -26.8¡C or colder (average of -31.5¡C), after a mean
drop of 11.4¡C over a mean time of 34 hours (average
rate of -0.6¡C/hr or -14.3¡C/day). A few cracks opened
after a cooling time of only 4-10 hours. Openings took
place when the temperature at 8 cm depth was below
-17.5¡C (average of -21.7¡C) and after an average cooling period of 60 hours for a cooling rate of -0.07¡C/hr
(-1.73¡C/day). The data also show that openings sometimes took place as the ground at 58 cm was warming
up a little. This implies that tensile stresses induced at
the soil surface and at the wedge top were provoked
during falling air temperatures, as waves from warmer
spells of a few days before were still propagating in the
ground. We can interpolate that the average temperature for cracking at the wedge top was about -20.0¡C. At
this level, openings occurred after a cooling of roughly
2.14¡C over an average time of 90 hours for an estimated rate of - 0.024¡C/hr (-0.57 ¡C/day).
All closings took place during warming spells, with
the exception of cable #4 on 20 January. This cable had
opened 4 to 8 hours before and contact was somehow
re-established. The average warming associated with
closings is 11.5¡C over an average period of 30 hours.
At 8 cm, the warming and the period were, respectively,
2.2¡C over 29.8 hours. But at 58 cm the average change
was -0.205 ¡C over 65 hours (Table 1). The near surface
nature of the mechanism is here again evident.
Michel Allard, Jennifer N. Kasper
7
Figure 3. Air and soil temperature curves (1989-90 and 1990-91). Upward arrowheads : opening events. Downward arrowheads : closing events.
The general picture for winter 1989-90 is rather simple. From the last week of December to the first week of
March, air and near surface temperatures were constantly cold (mean air temperature of -28.2 ¡C for
January , -30.7¡C for February, and -21.5¡C for March)
with 4-5 days oscillations. The remainder of March had
two warm spells and two cold ones. Openings and closings followed these oscillations within a rheologicaly
suitable temperature range (Figure 3).
8
WINTER 1990-91
Wedge cracking started on January 2 when the air
temperature was -34.5¡C, after a drop of 14.8¡C over
108 hours. Along the thermal profile in the wedge cable,
the temperature was -16.6¡C at 8 cm and
-14.1¡C at permafrost or wedge top (Table 1). It is worth
noting that three openings took place when the air was
warming, albeit at very low temperatures (average
-27.2¡C). The crackings and the subsequent opening
events all occurred below -24.4¡C for an average air
temperature of -32.8 ¡C, and after a cooling of 13.1¡C
The 7th International Permafrost Conference
Table 1. Average and threshold thermal conditions for opening and closing of cracks
See Conference CD-Rom for complete data files
over 102.2 hours (a rate of -0.15¡C/hr or -3.7¡C/day).
Just below the soil surface, the general conditions for
cracking were a temperature of -21.6¡C, after a cooling
of 4.6¡C over 134.7 hours for a rate -0.038¡C/hr
(-0.9¡C/day). At the wedge top, the average temperature for crack openings is interpolated at - 19.7¡C ; the
triggering drop of temperature was in average of 3.8¡C
and lasted roughly 143 hours (a rate of -0.64 ¡C/day).
Table 1 shows that the three closing events occurred at
very cold air (-27¡C to -33¡C) and soil temperatures
(-18.8¡C to - 25¡C). However they took place after much
shorter temperature changes (27-48 hours) than did the
openings.
The temperature curves for this winter depict a very
different history from the previous year (Figure 4).
January was colder by 3.7¡C (mean of - 31.9¡C),
February was warmer by 3.5¡C (mean of -27.2 ¡C) and
March colder by 1.6¡C (mean of -23.1¡C). These differences are evident on Figures 3. But the main difference
in thermal behavior lies in the fact that 1990-91 had
long cold spells of 2-4 weeks duration separated by
warmer periods of shorter duration. Openings took
place mainly in the lows of the cold spells, generally
after many hours (roughly 4 days) of soil cooling, but
the cracks then closed and opened again with secondary temperature oscillations in the range of 12 to
48 hours.
Michel Allard, Jennifer N. Kasper
9
Discussion
Each winter, a majority of cables (7 in 1989-91 ; 8 in
1990-91) spread across the cracks were visibly strained
although all did not break. Only cables 8 and 10 were
not strained in either year. At the start, these two cables
were laid across apparent lines of fractures of dubious
origin, perhaps resulting from slope tension in the vegetation on the inner side of a furrow (#10) or at a former site of cracking that has become inactive (#8). The
small spatial and temporal scale of the observations do
not attain the statistical significance of MackayÕs (1974,
1992) frequency data spread over tens of frost cracks, at
three sites over many years. However the analysis of
thermal conditions when tension occurred on the wires
provide numerical values that substantiate the previous
theoretical work (Lachenbruch, 1962, 1966). In both
years cracking began in late December-early January
when air temperatures dropped from about -20¡C to
below -32¡C, after four months of gradual cooling. First
and subsequent crack openings took place at similar
soil and atmospheric temperatures in both winters. For
instance, average temperatures at opening times at the
wedge top were -20.0¡C in 1989-90 and -19.7¡C
in 1990-91.
The difference between the two winters is in the
response time to oscillations and cooling rate before
crack openings. While in 1989-90 cracks opened after
temperature drops of about 34 hours duration at a
mean rate of -14¡C/day, they did so the following year
after 102 hours at a rate of -3.7¡C/day. This observation
applies also to the soil; in response to temperature
changes at 8 cm, cracks opened after drops lasting
about 60 hours at a rate of -1.7¡C/day the first winter
and lasting over twice as long (134 hours) the following year at about half the rate (-0.9¡C/day). In fact
the ratio of the difference is approximately 3 in the air
between the two winters, (i.e. roughly three times
longer at 1/3 of the cooling rate ; at 8 cm, it is about 2.
Mackay (1993) calculated that ice-wedge cracks on
Garry Island usually open after four days of decreasing
air temperatures (at a rate of -1.8¡C/day), a situation
that resembles our data for winter 1990-91 but differs
greatly from 1989-90. His study used daily mean air
temperatures from Tuktoyaktuk, 80 km away, which
brought some generalization into the data . The present
study shows that the thermal regime at the site governs
expansion and contraction. Comparison between the
two winters suggests that synoptic weather variations
(duration and interval of passages of colder air masses)
regulated the rhythm of cracking over the two years.
During the observation period, the damping of temperature variations in the soil by snowcover did not
have a serious impact on frost cracking (Mackay, 1978,
1993). The surface and shallow temperatures were
warmer in the polygonal trough than in the polygon
10
centre in early winter on both years; however they
cooled down to the same values over the second half of
January. Despite the absence of snowcover observations, this behavior can easily be explained by the filling of the trough and of the depressed polygon center
by 30-40 cm of fresh snow in early winter, which was
not enough to prevent the soil from cooling to cracking
temperatures. Thereafter, wind prevented further accumulation on the site (average wind velocity of 5.3 m/s,
recorded maximum of 24 m/s ; data not shown).
Almost all of the closing events recorded are probably
true events and not instrumental artifacts. For instance,
it is worth noting that many closing events took place
when the temperature in the soil was colder than at the
time of earlier crack openings. Therefore, expansion
and contraction of the metallic wires inside their sheeting is not the cause of circuit closing. The authenticity
of the events is also supported by the straightforward
relationship with temperature variations in all cases in
the first winter and in the majority of cases in the second one. In both years, and despite different temperature regimes governing cracking, closing events
occurred either after a warming of 11-13¡C over a period of 27-30 hours or during oscillations within the
course of long very cold spells. They may be related to
ground movements generated elsewhere in the frozen
ground stress field. In general, the occurrence of closings in response to temperature changes of almost uniform amplitude and duration suggests that, once
cracked, the permafrost and frozen active layer expand
and contract regularly in response to temperature variations. These expansion and contraction movements logically should take place in stress fields bounded by
already open cracks, i.e., within tundra polygons.
Reopening or enlarging of existing cracks with falling
temperatures may possibly be related with the lateral
expansion of cracks along wedges. For example,
Mackay (1984) reports from an experiment at Garry
Island that the same wedge cracked on three different
dates over ten days along a propagation distance of
5 m. Each new cracking may then very well provoke
the enlargement of the already open crack upstream
along the direction of propagation.
In May 1990, crack #4, which last opened on January
20, closed as the temperature increased (-9¡C at 8 cm to
-12¡C at 258 cm) and as the thermal gradient in the
ground was reversing. This late closing is likely another
demonstration of the effect of frozen ground expansion.
It has been demonstrated elsewhere that wedge cracks
can close by as much of 80 % between their maximum
width in winter and early summer (Mackay, 1975).
However, in many excavations in the area around the
site, we found several cracks in wedges below the permafrost table in July and August that were still wide
The 7th International Permafrost Conference
enough to introduce a thin (Swiss army) knife blade.
Others were completely sealed by new ice veins and
some had corks of ice and soil that seamed their upper
sections.
Conclusion
The temperatures conditions that were measured
when thermal cracks opened are in excellent agreement
with existing theoretical work. Air temperature oscillations duringthe two years of observation, certainly
related to synoptic weather variations, regulated the
rate of cooling and the time response of ice-wedge
mechanical reactions. However, crack opening always
took place at the same well-defined ground
temperature.
These field measurements allow one to estimate winter conditions of past climate in regions possessing icewedge pseudomorphs and in permafrost regions where
ice-wedges have become dormant (i.e. inactive) due to a
change in climate. One must keep in mind, however,
that cracking temperatures can occur in locally colder
environments within the discontinuous permafrost
zone (e.g., Burn, 1990; Hamilton et al., 1983; Payette et
al., 1986)
Acknowledgments
The authors express their thanks to Dr. Richard
Fortier who designed and built the automatic meteorological station and the crack detection circuit and for
reading a first draft. Maintenance and data handling
were subsequently carried on by Dr. Janusz Frydecki.
This study received the financial and logistical support
of Natural Sciences and Engineering Research Council
of Canada (Allard), Fonds pour la Formation de
Chercheurs et lÕAide ˆ la Recherche du Minist•re de
lÕEnseignement SupŽrieur du QuŽbec (Allard), the
Geological Survey of Canada, the Polar Continental
Shelf Program of Canada and the Northern Studies
Training Program of the Department of Indian Affairs
and Northern Development (Kasper). Reviewing by
Dr. C. R. Burn and another, anonymous, referee greatly
helped to improve the manuscript. Special thanks are
expressed to the community of Salluit which hosted us
and provided true friendship during field work.
References
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Payette, S., Gauthier, L. and Grenier, I. (1986). Dating icewedge growth in subarctic peatlands following deforestation. Nature, 322, 724-727.
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The 7th International Permafrost Conference