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 KyrϪ1 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°36.63ЈN, 76°12.85ЈW). 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 Ϫ3.5°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 ϩ2.8°C and Ϫ3.7°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 Ϫ30°C and ϩ80°C with an accuracy of Ϯ0.2 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 Ϯ0.1 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 ϩ0.45°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 C) ˚ 5 -2 0 -5 -4 Temperature ( -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 -0.55 -0.4 -1.2 -0.81 C ° 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 KyrϪ1(Fig. 6). This obser- of around Ϫ3.5°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.