Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Numerical simulation of the temperature field of a palsa reveals strong influence of convective heat transport by groundwater

G. Delisle Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany M. Allard Centre d’études nordiques, Université Laval, Ste-Foy, Québec, Canada

ABSTRACT: Slow changes of the internal temperature field in six boreholes within a cryogenic mound (mineral palsa or lithalsa) located east of Umiujaq, Eastern Hudson Bay, Canada have been monitored since July 2000. The permafrost depth and the internal temperature field are controlled by the temperature contrast of groundwater flow along the base of the permafrost at a depth of about 10 m and the negative mean annual surface temperatures that existed in the past. One pressure transducer, placed into the freezing front at the base of the mound, records strong underpressure, presumably in response to cryosuction. We conclude from this observation the presence of a strong hydraulic gradient and slow fluid flow into the palsa. This observation and the recorded steady warming at the palsa base with rates of 0.04–0.05 Kyr1 might be considered as the first evidence that the mound has entered the early stage of decay.

1 INTRODUCTION

Palsas appear to undergo a life cycle in the sense that they grow with time to a fully developed cryogenic mound, only to eventually decay to a ring structure by melt-out of the central ice-rich core (see for example Pissart, 2000). The objective of our ongoing program is to study the dynamics of the underlying physical processes in order to develop a better understanding of the processes that control the life cycle of a palsa. This paper presents a progress report based on the data of the first 11 months of monitoring of a palsa.

2 THE SITE

In June 2000 we initiated an investigation of a palsa Figure 1. Location of palsa site under investigation. located at an elevation of about 200 m near the eastern shore of the Hudson Bay (Fig. 1) east of Umiujaq show damage to heights of approximately 1.5 m. This (56°36.63N, 76°12.85W). The intent is to monitor is generally interpreted as an indication of the thickness temperature changes in the permafrost core of the palsa of the annual snow cover. The palsa surface, covered and thereafter to numerically simulate the processes with frost boils, consist of marine clay. The frost boils that regulate them. This paper discusses first results. and low tundra vegetation are taken as an indication of The mound is circular with a diameter of 50 m. It minimal snow cover in winter as a consequence of has a rather flat top and steep slopes. It is between 2.1 strong winds sweeping across these elevated palsa and 2.7 m high above the surrounding wetland on the surfaces. A mean annual air temperature of 3.5°C northern side and about 3.4 m high on its southern for a site near Umiujaq was reported by Fortier side. Four small ponds are found around the periphery et al. (1992); additional data for mean annual soil sur- of the palsa; the water level of the two ponds on the face temperatures at sites in the region, ranging northeast and northwest side is about 1 m higher than between 2.8°C and 3.7°C, were given by Ménard the two ponds on the southeast side (Fig. 2). et al. (1998), snow cover being the principal factor of Many other circular palsas with similar characteris- variation. tics exist in the immediate vicinity. The surrounding Six boreholes (P1–P6) have been drilled through peat surface is covered by shrubs and trees, which the palsa, four of them along a cross-sectional line.

181 roughly with the lower freezing front of the perma- frost body. The boreholes serve three purposes: – provide a continuous record of the ice lenses for subsequent isotopic analyses (not to be discussed in this paper) – installation of strings with small dataloggers to mon- itor the two dimensional temperature field of the palsa – installation of a thermistor cable in the central bore- hole together with a pressure transducer at the lower end of the string to monitor pressure changes within the ground near the basal freezing front. In addition, annual temperature changes in the top soil of the adjacent peat plain and in a pond were recorded to assess the surrounding temperature field.

3 THE INSTRUMENTATION

Two approaches were chosen to monitor the annual course of the soil temperatures within the palsa. Five of the boreholes were fitted with PVC-tubing with closed off bottom sections. The tubes were filled with silicon oil. Handylogs™ (a recording device with inte- grated temperature sensor, memory and battery), attached to a steel cable were emplaced in these bore- holes at predefined positions. One borehole (P3) was fitted with a conventional temperature sensor string and one pressure transducer at the lower cable end and refilled with gravel surrounding the pressure trans- ducer and sandy clay over the transducer position. Soil temperatures and pore pressure are measured twice daily. The handylogs register within a temperature range of 30°C and 80°C with an accuracy of 0.2 K and a resolution of 0.003 K. Each handylog is capable to store 64000 readings. Readings of the thermistors (temperature sensor string) are immediately digitised and downloaded in a memory unit via the so-called “intelligent sensor modules” (ISM) technology, which allows us to meas- ure with an accuracy of ϳ0.002°C. The pressure transducer in use measures absolute pressures within the range of 0 to 16 bars with an accuracy of 0.1 bar and a resolution of 5 m bar. Figure 2. Plan and cross section through the investigated mound with location of boreholes shown in Fig. 4. 4 RESULTS All boreholes penetrated an active layer about 1.5 m deep (maximum) followed by a chaotic sequence of The recorded soil temperatures and pressures were ice lenses, typically 10 cm thick and massive, separated downloaded in June 2001 for the first time. Figure 3 by highly distorted clay layers. The holes were all ter- presents the soil temperature measured at P3 at a depth minated when they reached the crystalline bedrock at a of 10 cm below soil surface. Surprisingly, the measured depth of about 10 m. Subsequent temperature record- mean annual soil temperature of 0.45°C, approxi- ings (see below) show that this boundary coincides mated on the basis of 11 months of measured data in P3

182 15 Temperature (˚C) 1. Oct. 2000 -10 -5 0 5 10 10 0 15 Dec. 2000 5 -2 0

-5 -4 Temperature ( ˚ C) -10 -6 200 -15 Depth (m) 180 280 380 480 580 300 Days -8 400 Figure 3. Soil temperatures recorded at a depth of 10 cm 500 in borehole P3. Measurements started on 16 July 2000 (day -10 198). Note deviation of ground temperatures from October to Mid-December from a sinusoidal curve, corresponding to the zero curtain effect. -12 Figure 5. Recorded soil temperatures from July 2000 (200 days) to Mid-June 2001 (500 days) in borehole P3. S N

P1 P3 0.2 P4 P2 7.5 P1 -1.2 6.6 5.24 C -0.55 -0.4 -1.2 -0.81 0 P2 -0.84 -0.56 -0.98 -0.33 Ground water level P6 -0.59 -0.87 -0.56 P5 -0.44 -0.48 -0.69 -0.2 P3 Marine clay -0.63 -0.44 under peat -0.37 -0.23 0.12 0.0ºC -0.33 -0.04 -0.16 Not to scale Temperature ° -0.42 0.08 0.23 -0.4 Crystalline bedrock Pressure transducer P4 -0.6 190 290 390 490 Days Figure 4. Recorded soil temperatures in the palsa in Mid- June 2001. Note the apparent thermal effect of ground water Figure 6. Temperatures recorded by sensors at the base of intrusion into the frozen core on the northern side of the palsa. three boreholes (P3–P5). Note the steady increase during in the first eleven months of recording. is positive. The difference between mean annual air and soil temperatures is surprisingly large, if one considers that the point of measurement is located in an elevated, Soil temperatures below a depth of about 3.5 m show vegetation-free position with little snow cover in winter nearly no evidence for the penetration at depth of an because of strong wind and snow drift. annual temperature wave (Fig. 5). Likewise, the frozen core of the palsa (Fig. 4) is sur- Basal temperature sensors in the three boreholes prisingly warm when considered in relation to the (P3–P5) indicate continuous steady warming of per- recorded mean annual air temperatures for the region mafrost at a rate of 0.04–0.05 Kyr1(Fig. 6). This obser- of around 3.5°C. The lowest soil temperatures exist at vation and the existence of a non-linear thermal gradient the southern side, which coincides with the highest from the palsa base to the active layer clearly indicate elevation of the mound above the surrounding terrain. that the palsa is actually in a transient ther-mal state. A The freezing front lies within bedrock on the southern very surprising observation was the recorded pressure side of the palsa and passes above the contact between at the base, near the frozen fringe in borehole P3. sediment and bedrock at its northern side. The pressure transducer had initially measured the The temperature field of the palsa shows on the combined pressure of the air and water column northern side lateral heat flow into the frozen core at (1.65 bar) just after drilling and installation. It was two levels and at the base of the palsa (Fig. 4). The expected that the pressure transducer would record a presence of the ponds and of saturated unfrozen pressure increase, as the borehole started to refreeze. ground all around the palsa strongly suggest that this The transducer however, yielded a record of a system- heat is conveyed by groundwater flow. atic reduction of pore water pressure in the following

183 2 (0.4°C to 0.6°C) in 2000–2001 was significantly 1.8 warmer than the regional mean annual air temperature 1.6 1.4 ( 3.5°C), but also colder than the mean annual near 1.2 surface soil temperature (0.45°C). The soil value 1 probably results from a particularly thick snow cover in 0.8 the winter and warm spring temperatures. In fact, air 0.6 Pressure (bar) 0.4 temperature and soil surface temperature measurements 0.2 on another palsa (Sheldrake site) only 10 km away show 0 190 240 290 340 390 440 490 540 that the region has been warming since 1992. For the 12 Days month period spanning July 2000–June 2001 the mean annual surface temperature on this other palsa was also Figure 7. Measured pore water pressures by the pressure well above 0°C (0.14°C). In fact, the gradual reduction transducer in borehole P3 at a depth of 10.55 m. Pressures dropped rapidly to sub-atmospheric pressures. of the temperature gradient towards the surface (Fig. 5) suggests such a warming event. Approximately linear temperature gradients are only observed in the bottom 0 section of the mound, which allow an estimate of the vertical heat flow. Heat flow values vary, depending on -0.4 the season, between 100 and 140 m Wm2, these values -0.8 being about two- to threefold higher than the regional terrestrial heat flow. The lower boundary of the frozen -1.2 core of the palsa is apparently defined by the relatively Temperature ( º C) warm groundwater flow at the sediment bedrock inter- -1.6 470 490 510 530 face. The high basal temperature gradient is therefore Days controlled by the temperature contrast between the surface temperature and the temperature of intruding Figure 8. Recorded sudden temperature increase (influx ground water at the interface between sediment and of water?), followed by thermal relaxation at 1.14 m depth in borehole P6 on 17 May 2001. bedrock. Another very surprising observation lies in the fact that at depths of more then 3.5 m only minimal soil temperature fluctuations were observed, an indica- 11 months. In June 2001, the recorded pore pressure tion that in effect neither the winter nor summer wave was 0.61 Bar (Fig. 7). penetrated down to this depth. Several thermal sensors, located near the base of Impact of ground water flow on the palsa: Figure 4 the active layer have recorded sudden changes in tem- shows, from the northern side of the palsa, three levels perature of the order of between 0.01°C and 0.5°C, an where unfrozen layers penetrate into the frozen core. effect which cannot be explained by pure heat con- In particular, the strongly developed unfrozen zone at duction (see also Fig. 8). the sediment bedrock interface clearly suggests that The monitoring of soil temperatures 10 cm below the the base of the frozen core is defined by the position surface at a position about 7 m away from the northern of ground water flow paths. palsa rim did yield a mean annual soil temperature Frost heaving evidence: Fracturing of the frozen of 2.8°C. The recorded temperatures suggest freez- core, apparently in response to expansion of freezing ing of the palsa margins only in February 2001, which ice and heaving of the whole structure, occurs under is undoubtedly related to the insulating effect of a the active layer and near the base. Sudden inflow of deep snow cover. The datalogger emplaced to record water into the frozen fringes of the mound was observed water temperatures at the bottom of a lake next to the by sudden temperature increases, followed by thermal palsa failed. Temperatures measured at the bottom of relaxation to ambient temperatures within days in some a lake in discontinuous permafrost outside of the village cases (see Fig. 8). of Umiujaq yield a mean annual lake bottom temper- Internal pressure field: The strong underpressure ature of 5.8°C however. observed, is presumably the result of cryosuction processes. We conclude from this observation the pres- ence of a strong hydraulic gradient and episodic, per- 5 DISCUSSION haps even permanent inflow of water into the frozen part of the mound. This assumption is independently We have made the following key observations during supported by the observation of finger-shaped unfrozen the first eleven months of the monitoring programme: portions pointing laterally into the frozen mound. Temperature field and heat flow in the palsa: The Ground temperatures adjacent to the palsa: The at mean temperature of the frozen core of the palsa least 1.5 m high snow cover prevents the penetration of 184 the cold winter wave into the ground. The result is a Temperature (ºC) significantly higher mean annual soil temperature -10 -5 0 5 10 (2.8°C) next to the palsa in comparison to the palsa 0 top. Temperatures at the bottom of lakes are even higher than in adjacent soils. The very effective heat 2 storage in lakes and rivers during summer and the development of a thick winter lake ice promotes the 4 preferential storage of the summer heat in the water body and serves as a permanent source of “warm” 6 ground water. The difference in elevation between the Dept (m) ponds from the northern to the southern side of the 8 palsa also suggests that a groundwater gradient exists 91 days around and possibly underneath, the permafrost body. 10 182 days

273 days 6 INTERPRETATION 12 365 days We have made extensive efforts to numerically simu- late the observed temperature field. Several key prob- Figure 9. Calculated annual variation of soil temperatures lems were encountered however. The surprisingly in palsa. Close match with measured distribution (Fig. 5) is shallow permafrost depth at the site cannot be achieved only by the assumption of ground water flow at depths below 10 m. explained as a reflection of the regional heat flow den- sity (estimated at 45 mW m2 for Canadian Shield ter- rain). This heat flow value would be equivalent to a of 0.7°C was derived by the linear extrapolation of mean annual soil surface temperature of 0.2°C the measured and by the annual T-wave least perturbed under the assumption of an average thermal conduc- T-gradients at borehole depths between 5–8 m. This 1 1 tivity of 2.1 W m K for the ice rich palsa core. The value for the soil surface is thought to represent the frozen core of the palsa is colder however, which average thermal condition in the past decades. Close fit points to a former time period of clearly colder sur- between calculated and measured soil temperatures was face conditions. Secondly, the minimal annual soil achieved only by assuming a deviation from the sinu- temperature variations at depths of more than 3.5 m soidal T-curve for the top soil in the time span between indicate a process that in effect dampens the down- days 182 and 254 in agreement with the measured sur- ward penetration of the cool winter wave. face soil temperatures (Fig. 3). Ground temperatures Our numerical simulation of the temperature field were kept at 0°C in this time period. within the palsa follow the mathematical approach As ground water flow appears to control the depth of discussed in Delisle (1998). The governing equations permafrost, a basal heat flow value of 145 m W m2 dT/dt l/rcd2T/dz2 (1) was chosen. This value reflects exactly the tempera- ture contrast between ground water and the assumed mean annual surface temperature of 0.7°C (with l(dT1/dz – dT2/dz) LredX/dt (2) l 2.1 W m1 K1). with To ensure minimal changes in calculated soil tem- l thermal conductivity (W m1 K1) peratures at depths below 3.5 m, a freezing point of r density (kg m3) the marine clay within the palsa of 0.4°C had to be c heat capacity (Ws kg1 K1) assumed. The choice of the freezing point in effect 1 dT1/dz T-gradient above freezing front (K m ) keeps the soil temperatures “in place”. Any warming is 1 dT2/dz T-gradient below freezing front (K m ) impeded by uptake of latent heat, cooling only occurs L latent heat (Ws kg1) moderately down to a depth of 3.5 m because of the 3 re water content in sediment (kg m ) damping effect due to the modification of the otherwise dX/dt rate of movement of phase change boundary sinusoidal surface temperature curve. The temperature in (m s1) distribution calculated under these assumptions is z depth (m) shown in Fig. 9. were solved numerically with the following boundary The pressure transducer has recorded a strong under- conditions: pressure at the basal centre of the palsa. Since hydro- A sinusoidal course of the annual surface temperature static conditions exist in the permafrost free area next with amplitude of 15°C, in association with an annual to the palsa, we can calculate a hydraulic gradient of mean temperature of 0.7°C, was adopted. The value about 0.0005 bar cm1 (Pressure difference of 1.9 bar

185 Table 1. Estimated flow rates into the palsa as function of with respect to its thermal state. It appears that the permeability of the frozen palsa core. palsa is currently in a thermally transient state. The thermal record from the base points to slow warming. Surface of Permability lateral face Flow rate Annual The frozen core is clearly colder than the recorded (Darcy) of palsa (m2) (m s1) flow (m3) mean annual surface temperature for the season 2000/2001. It appears that the temperature field and 1 mD 1335 5.0 109 210 the vertical extent of permafrost in the palsa is con- 0.1 mD 1335 5.0 10 10 21 trolled by the thermal influence of the ground water 11 0.01 mD 1335 5.0 10 2.1 flow at the interface between bedrock and sediment, 12 0.001 mD 1335 5.0 10 0.21 not by the regional heat flow density. Despite strong underpressure in the palsa, water appears to enter the frozen palsa core by diffusion in only low amounts. to 0.61 bar over a lateral distance of 25 m). The flow Water apparently enters additionally through opening rate is then dependent only on the internal permeabil- cracks as evidenced by recorded sudden temperature ity of the palsa. Shown in Table 1 are the estimated changes near the freezing front. The palsa appears to flow rates through the side of the palsa (the depth be in an early stage of thermal decay, induced by lateral level considered is 10 m below palsa surface) for dif- inflow of ground water into the frozen core along two ferent assumed permeability values of the frozen core. zones of weakness on the northern palsa side. It is apparent that significant amounts of water can migrate over time into the palsa, even if very low per- meability values are assumed. Does the palsa draw water into its structure and is REFERENCES new ice developing within the frozen core today? If this is the case, then the latent heat liberated by the Delisle, G. 1998. Numerical simulation of permafrost freezing process would have to be conducted away. growth and decay. Journal of Quaternary Science The heat can only flow away to the cool palsa top 13(4): 325–333. along the existing temperature gradient. The observed Fortier, R., Sheriff, F., Seguin, M.-K. & Allard, M. 1992. amount of heat flowing to the surface limits the amount Détermination des contenus en eau et en glace d’échan- tillons gelés à l’aide de la méthode calorimétrique de of ice that might form annually in the interior of the 3 terrain. Le Climat 10(1): 57–68. palsa to at most a few m in total. It therefore appears Ménard, E., Allard, M. & Michaud, Y. 1998. Monitoring of that little ice segregation is taking place. The frozen ground surface temperatures in various biophysical core of the palsa appears to possess a very low perme- micro-environments near Umiujaq, Eastern Hudson ability with the exception of places where cracking Bay, Canada. In: Proceedings, 7th International Con- near the freezing front occurs (see above). ference on Permafrost, Yellowknife, Canada, Centre d’études nordiques, Université Laval: 723–729. Pissart, A. 2000. Remnants of lithalsas of the Hautes Fagnes, Belgium: a summary of present-day knowl- 7 CONCLUSIONS edge, Permafrost and Periglacial Processes 11(4): 327–355. The 11 month data record available from our investi- gated palsa only allows us to draw tentative conclusions

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