Introduction

Cracking of the soil, the process at the origin of ice-wedges and sand-wedges and of tundra polygons(Leffingwell, 1919), is a temperature-controlled phe-nomenon. In theoretical work on the mechanics of ther-mal contraction cracking, Lachenbruch (1962) estimatedthat the required value of soil viscosity to allow frostcracking is reached when air temperatures fall between-20 and -30¡C and when the temperature at the top ofpermafrost is about -15 to -20¡C (Lachenbruch, 1966;P�w�, 1966). However, for cracking to take place, arapid drop of temperature is the triggering mechanism.With a too low cooling rate, the tensile stress inducedby thermal contraction can be released by permafrostcreep. ÒLess than half a day of cooling... is required fora close approach to the full stress value when coolingrate is 10¡C/day (a total cooling of about 4¡C over aperiod of about 9 hours). Lower cooling rates must bemaintained longer to be effectiveÓ (Lachenbruch, 1962,p. 20). Similarly, Grechishchev (1973, p. 231) concludesthat: Òthe dynamics of the formation of frost fissuresobviously look like the following: long period coolingleads to stress equal to the long term strength, and rup-ture is caused by secondary short-period temperaturefluctuationsÓ. Detection of several hundreds of frostcracking events and careful examination of field andclimatic conditions by Mackay (1974, 1978, 1984, 1992,1993) supports these theoretical studies. However, mea-

surements of air and ground temperatures at the timeand site of frost cracking have not yet been reported.

Over two winters, we measured the air and soil tem-peratures at which frost cracking occurred along a fewice-wedges around a polygon. This was done at a sitethat lies near the southern margin of the distributionrange for active ice-wedges.

Study area and characteristics of the experi-mental site

The test site is in the Foucault river valley(Narsajuaq), 12.5 km west of Salluit (Figure 1). Themean annual air temperature for the area is estimatedto be about -8¡C (Gray, 1983). At the study site, ourmeasurements in 1990 indicated a mean air tempera-ture of - 8.9¡C. The annual temperature range is about37¡C. Total precipitation is estimated to be 310 mm, ofwhich 53% is snow. Ground temperature at the depth ofzero 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 afluvial terrace at an elevation of 14.5 m a.s.l. It bears anetwork of random orthogonal, low centre polygonswith a mean diameter of 16 m (Figure 2). The soil con-sists of a 2-2.6 m thick sequence of organic-rich finesand layers alternating with 10-30 cm thick sandyorganic layers that overlie coarse fluvial and gravelysands. These fluvial sediments are themselves about

Abstract

The temperatures at which thermal cracking occurred along ice-wedges around a tundra polygon were mea-sured over two years near Salluit, northern Qu�bec. Electrical cables were buried in the active layer across fur-rows 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 thecourse of the two winters, several cables broke with the opening of thermal cracks. Over the two years, the firstcracks 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 topwas -20¡C in the first year and -19.7¡C in the second year. The data also allow estimates of the minimum tem-perature changes and cooling rates required to induce ice-wedge cracking. The cracks closed (or narrowed) andre-opened (or widened) in response to winter temperature fluctuations at the soil surface. The measured ther-mal conditions for cracking substantiate the previous theoretical work on this basic process at the origin of tun-dra polygons.

Michel Allard, Jennifer N. Kasper 5

TEMPERATURE CONDITIONS FOR ICE-WEDGE CRACKING:FIELD MEASUREMENTS FROM SALLUIT, NORTHERN QU�BEC

Michel Allard, Jennifer N. Kasper

Centre dÕ�tudes nordiquesUniversit� Laval, Sainte-Foy, Qu�bec, Canada G1K 7P4

e-mail: michel.allard@cen.ulaval.ca

7 m thick and they overlie post-glacial marine sandysilts. In river bluff sections and on ground probingradar profiles, the ice-wedges are seen to penetratedownward 5-6 m into the fluvial sediments (see Allardet al., 1993, p.7). According to the regional uplift curve,the site must have emerged about 4700 years ago, set-ting a potential date for the onset of frost cracking andinitiation of the polygons (Kasper, 1995).

Methods

INSTRUMENTATION

Two thermistor cables (thermistors YSI-44033) wereinstalled in drill-holes, one in the centre of the polygon,the other under a trough along a side, through an ice-wedge. The centre of the polygon is about 40 cm deeperthan its raised edges and the trough along the polygonside is about 30 cm deep ; therefore the soil surfaceover the wedge is about 10 cm above the level of thepolygon centre. The cables were set in PVC casingsfilled with silicone oil (Osterkamp, 1974). A thermistorwas also installed on top of a 3 m high mast, in a shel-ter, to measure air temperatures, and one was at theground surface (+/- 5 cm deep under the moss in poly-gon centre). Temperature readings were recorded by adatalogger (Campbell Scientific CR-10, 12 channels).Two channels were used to store hourly temperaturesin the air and at the ground surface. Every four hours,temperatures were recorded at 55, 105 and 255 cmdepths in the polygon centre and at 8, 58, 108, 158 and258 cm depths in the polygon trough.

As wedge ice was met in the drill hole in the trough ata depth of 33 cm, which is close to mean wedge depthin the region (62 measurements), a depth of 30 cm isused below as the wedge top depth for interpolations oftemperature of cracking.

The last available channel on the data logger was usedfor a cracking detection system which consisted of amain electric circuit including ten resistors of differentvalues in series. Each resistor was bypassed with an

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Figure 1. Location of the experimental site.

Figure 2. Plan of the instrument layout.

electrical cable loop. Incremental increase in resistancedue to the breaking of one loop spread across a poten-tial crack could be monitored with the datalogger. Theresistance increment could be used to identify the bro-ken loop. As the values of the resistors differed by equalincrements and as the circuit was sensed only everyfour hours (a design fault found afterwards), thereexisted the potential situation that if two cables brokebetween two readings, the total resistance shift on thecircuit equaled the one of a third different cable, leadingto an identification mistake. For the first winter thestructure of the data indicates that this did not occur.

The loops, or breaking cables, were first laid acrosscracks on 12 July, 1989 at depths of about 15 cm in thethawing active layer. Nine of them extended across thetop of known ice wedges (Figure 2). One was laid onthe inner side of a small polygon ridge where, appar-ently, new cracking was taking place. The cables weredug up one year later on 1 July, 1990, with a smallamount of damage resulting from excavating through5-10 cm of frozen ground. The broken cables were sol-dered and buried again, taking care that an unaffectedsegment of cable extended across the wedge. For thesecond winter, jumps in resistance were clear-cut in thedata, but identification of which cables broke on parti-cular events was not always possible.

DATA ANALYSIS

During excavation in 1990, 7 of the 10 cables showedevidence of strain. As the type of electrical wire used inthe loops was rather coarse (steel wires coated intranslucent rubber sheeting of 2 mm o.d.), it could beseen that tension on some of them had broken the steelwire but that the rubber coating had been onlystretched. As crack closing had taken place, some ofthem were still pinched in ice veins and in the cracks.On 21 August, 1991, re-excavation revealed that 8 cableshad been affected and that their mechanical behaviorhad 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 partiallyclosed and re-widened several times during the winter,each time re-closing sub-circuits and registering shiftsin the resistance channel, thus permitting assessment ofboth opening and closing events.

The logged temperature data were quality checked forconsistency and loaded into a spreadsheet software.Then, times and temperatures in the soil and in the airwere determined for each cracking and each closingevent. The peak or trough of the temperature oscillationpreceding the event was searched both in the data tableand on ÒT vs timeÓ graphs (Figures 3 and 4) to deter-mine the range of the triggering temperature oscilla-

tion, its duration and the rate of change (Table 1). Aminus sign applied to a rate means, in the tables and inthe text, a cooling rate.

Results

WINTER 1989-90The steel wires in five cables broke between 27

December and 15 January (#7, #3, #1, #4 and #6 insequence). The first rupture took place when the airtemperature 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 was10¡C/m from the surface through the top of the wedgeand diminished to 1.9¡C/m in the lower meter of theprofile. This first cracking occurred during the firstmajor cold spell of the winter which had begun 34hours beforehand and after a cooling of 10.6¡C, that isat a rate of -0.3¡C/hr (-7.4¡C/day). At 8 cm, the coolingwent back 140 hours at a rate ten times smaller and itwent 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 thesame day, may have been a response to tensile stress atthe soil surface given the small cooling rates at depthsgreater than 8 cm which are part of the general earlywinter cooling.

All crack openings of this winter (including re-openings after January 15) occurred at air temperaturesof -26.8¡C or colder (average of -31.5¡C), after a meandrop of 11.4¡C over a mean time of 34 hours (averagerate of -0.6¡C/hr or -14.3¡C/day). A few cracks openedafter a cooling time of only 4-10 hours. Openings tookplace when the temperature at 8 cm depth was below -17.5¡C (average of -21.7¡C) and after an average coo-ling period of 60 hours for a cooling rate of -0.07¡C/hr(-1.73¡C/day). The data also show that openings some-

times took place as the ground at 58 cm was warmingup a little. This implies that tensile stresses induced atthe soil surface and at the wedge top were provokedduring falling air temperatures, as waves from warmerspells of a few days before were still propagating in theground. We can interpolate that the average tempera-ture for cracking at the wedge top was about -20.0¡C. Atthis level, openings occurred after a cooling of roughly2.14¡C over an average time of 90 hours for an estima-ted rate of - 0.024¡C/hr (-0.57 ¡C/day).

All closings took place during warming spells, withthe exception of cable #4 on 20 January. This cable hadopened 4 to 8 hours before and contact was somehowre-established. The average warming associated withclosings 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 changewas -0.205 ¡C over 65 hours (Table 1). The near surfacenature of the mechanism is here again evident.

Michel Allard, Jennifer N. Kasper 7

The general picture for winter 1989-90 is rather sim-ple. From the last week of December to the first week ofMarch, air and near surface temperatures were con-stantly cold (mean air temperature of -28.2 ¡C forJanuary , -30.7¡C for February, and -21.5¡C for March)with 4-5 days oscillations. The remainder of March hadtwo warm spells and two cold ones. Openings and clo-sings followed these oscillations within a rheologicalysuitable temperature range (Figure 3).

WINTER 1990-91 Wedge cracking started on January 2 when the air

temperature was -34.5¡C, after a drop of 14.8¡C over108 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 worthnoting that three openings took place when the air waswarming, albeit at very low temperatures (average -27.2¡C). The crackings and the subsequent openingevents all occurred below -24.4¡C for an average airtemperature of -32.8 ¡C, and after a cooling of 13.1¡C

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Figure 3. Air and soil temperature curves (1989-90 and 1990-91). Upward arrowheads : opening events. Downward arrowheads : closing events.

over 102.2 hours (a rate of -0.15¡C/hr or -3.7¡C/day).Just below the soil surface, the general conditions forcracking were a temperature of -21.6¡C, after a coolingof 4.6¡C over 134.7 hours for a rate -0.038¡C/hr(-0.9¡C/day). At the wedge top, the average tempera-ture for crack openings is interpolated at - 19.7¡C ; thetriggering drop of temperature was in average of 3.8¡Cand lasted roughly 143 hours (a rate of -0.64 ¡C/day).

Table 1 shows that the three closing events occurred atvery cold air (-27¡C to -33¡C) and soil temperatures (-18.8¡C to - 25¡C). However they took place after muchshorter temperature changes (27-48 hours) than did theopenings.

The temperature curves for this winter depict a verydifferent 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) andMarch colder by 1.6¡C (mean of -23.1¡C). These diffe-rences are evident on Figures 3. But the main differencein thermal behavior lies in the fact that 1990-91 hadlong cold spells of 2-4 weeks duration separated bywarmer periods of shorter duration. Openings tookplace mainly in the lows of the cold spells, generallyafter many hours (roughly 4 days) of soil cooling, butthe cracks then closed and opened again with se-condary temperature oscillations in the range of 12 to 48 hours.

Michel Allard, Jennifer N. Kasper 9

See Conference CD-Rom for complete data files

Table 1. Average and threshold thermal conditions for opening and closing of cracks

DiscussionEach winter, a majority of cables (7 in 1989-91 ; 8 in

1990-91) spread across the cracks were visibly strainedalthough all did not break. Only cables 8 and 10 werenot strained in either year. At the start, these two cableswere laid across apparent lines of fractures of dubiousorigin, perhaps resulting from slope tension in the ve-getation on the inner side of a furrow (#10) or at a for-mer site of cracking that has become inactive (#8). Thesmall spatial and temporal scale of the observations donot attain the statistical significance of MackayÕs (1974,1992) frequency data spread over tens of frost cracks, atthree sites over many years. However the analysis ofthermal conditions when tension occurred on the wiresprovide numerical values that substantiate the previoustheoretical work (Lachenbruch, 1962, 1966). In bothyears cracking began in late December-early Januarywhen air temperatures dropped from about -20¡C tobelow -32¡C, after four months of gradual cooling. Firstand subsequent crack openings took place at similarsoil and atmospheric temperatures in both winters. Forinstance, average temperatures at opening times at thewedge top were -20.0¡C in 1989-90 and -19.7¡C in 1990-91.

The difference between the two winters is in theresponse time to oscillations and cooling rate beforecrack openings. While in 1989-90 cracks opened aftertemperature drops of about 34 hours duration at amean rate of -14¡C/day, they did so the following yearafter 102 hours at a rate of -3.7¡C/day. This observationapplies also to the soil; in response to temperaturechanges at 8 cm, cracks opened after drops lastingabout 60 hours at a rate of -1.7¡C/day the first winterand lasting over twice as long (134 hours) the fol-lowing year at about half the rate (-0.9¡C/day). In factthe ratio of the difference is approximately 3 in the airbetween the two winters, (i.e. roughly three timeslonger at 1/3 of the cooling rate ; at 8 cm, it is about 2.Mackay (1993) calculated that ice-wedge cracks onGarry Island usually open after four days of decreasingair temperatures (at a rate of -1.8¡C/day), a situationthat resembles our data for winter 1990-91 but differsgreatly from 1989-90. His study used daily mean airtemperatures from Tuktoyaktuk, 80 km away, whichbrought some generalization into the data . The presentstudy shows that the thermal regime at the site governsexpansion and contraction. Comparison between thetwo 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 tem-perature variations in the soil by snowcover did nothave a serious impact on frost cracking (Mackay, 1978,1993). The surface and shallow temperatures werewarmer in the polygonal trough than in the polygon

centre in early winter on both years; however theycooled down to the same values over the second half ofJanuary. Despite the absence of snowcover observa-tions, this behavior can easily be explained by the fil-ling of the trough and of the depressed polygon centerby 30-40 cm of fresh snow in early winter, which wasnot enough to prevent the soil from cooling to crackingtemperatures. Thereafter, wind prevented further accu-mulation 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 probablytrue events and not instrumental artifacts. For instance,it is worth noting that many closing events took placewhen the temperature in the soil was colder than at thetime of earlier crack openings. Therefore, expansionand contraction of the metallic wires inside their sheet-ing is not the cause of circuit closing. The authenticityof the events is also supported by the straightforwardrelationship with temperature variations in all cases inthe first winter and in the majority of cases in the sec-ond one. In both years, and despite different tempera-ture regimes governing cracking, closing eventsoccurred either after a warming of 11-13¡C over a pe-riod of 27-30 hours or during oscillations within thecourse of long very cold spells. They may be related toground movements generated elsewhere in the frozenground stress field. In general, the occurrence of clo-sings in response to temperature changes of almost uni-form amplitude and duration suggests that, oncecracked, the permafrost and frozen active layer expandand contract regularly in response to temperature varia-tions. These expansion and contraction movements lo-gically should take place in stress fields bounded byalready open cracks, i.e., within tundra polygons.

Reopening or enlarging of existing cracks with fallingtemperatures may possibly be related with the lateralexpansion of cracks along wedges. For example,Mackay (1984) reports from an experiment at GarryIsland that the same wedge cracked on three differentdates over ten days along a propagation distance of 5 m. Each new cracking may then very well provokethe enlargement of the already open crack upstreamalong the direction of propagation.

In May 1990, crack #4, which last opened on January20, closed as the temperature increased (-9¡C at 8 cm to-12¡C at 258 cm) and as the thermal gradient in theground was reversing. This late closing is likely anotherdemonstration of the effect of frozen ground expansion.It has been demonstrated elsewhere that wedge crackscan close by as much of 80 % between their maximumwidth in winter and early summer (Mackay, 1975).However, in many excavations in the area around thesite, we found several cracks in wedges below the per-mafrost table in July and August that were still wide

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enough to introduce a thin (Swiss army) knife blade.Others were completely sealed by new ice veins andsome had corks of ice and soil that seamed their uppersections.

Conclusion

The temperatures conditions that were measuredwhen thermal cracks opened are in excellent agreementwith existing theoretical work. Air temperature oscilla-tions duringthe two years of observation, certainlyrelated to synoptic weather variations, regulated therate of cooling and the time response of ice-wedgemechanical reactions. However, crack opening alwaystook place at the same well-defined ground temperature.

These field measurements allow one to estimate win-ter conditions of past climate in regions possessing ice-wedge pseudomorphs and in permafrost regions whereice-wedges have become dormant (i.e. inactive) due to achange in climate. One must keep in mind, however,that cracking temperatures can occur in locally colderenvironments within the discontinuous permafrostzone (e.g., Burn, 1990; Hamilton et al., 1983; Payette etal., 1986)

Acknowledgments

The authors express their thanks to Dr. RichardFortier who designed and built the automatic meteoro-logical station and the crack detection circuit and forreading a first draft. Maintenance and data handlingwere subsequently carried on by Dr. Janusz Frydecki.This study received the financial and logistical supportof Natural Sciences and Engineering Research Councilof Canada (Allard), Fonds pour la Formation deChercheurs et lÕAide � la Recherche du Minist�re delÕEnseignement Sup�rieur du Qu�bec (Allard), theGeological Survey of Canada, the Polar ContinentalShelf Program of Canada and the Northern StudiesTraining Program of the Department of Indian Affairsand Northern Development (Kasper). Reviewing byDr. C. R. Burn and another, anonymous, referee greatlyhelped to improve the manuscript. Special thanks areexpressed to the community of Salluit which hosted usand provided true friendship during field work.

Michel Allard, Jennifer N. Kasper 11

References

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