LATE-SUMMER SOLIFLUCTION AND ACTIVE LAYER DEPTHS, FOSHEIM PENINSULA, ELLESMERE ISLAND, CANADA

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1 LATE-SUMMER SOLIFLUCTION AND ACTIVE LAYER DEPTHS, FOSHEIM PENINSULA, ELLESMERE ISLAND, CANADA Antoni G. Lewkowicz, Shawne Clarke Centre for Research on Cold Environments, Department of Geography, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 Abstract Measurements of solifluction using electro-mechanical meters over a period of 5 years at Hot Weather Creek (80 N, 84 W) show that summers with deep thaw produce enhanced rates of movement throughout the active layer. Within a basal transitional layer, the soil thaws and moves in some years but remains immobile and frozen in others. Predictions from a simple model involving displacements in a shear zone at the bottom of the active layer agree with measured subsurface movements below depths of cm. This indicates that a shear zone, whose location depends on the depth of thaw, can explain the lower part of the velocity profile in this area of two-sided freezing, but that other processes, notably frost creep and isolated shearing, are important nearer the surface. Introduction Solifluction is a widely studied periglacial processes. The bulk of an extensive literature describes either solifluction landforms, primarily lobes and sheets, or rates of surface movement (c.f., Harris, 1981; French, 1996). Subsurface velocity profiles have been obtained only at scattered sites within polar and high altitude environments (e.g., Price, 1991; Smith, 1992). Still fewer studies have examined subsurface movement in the continuous permafrost zone, even though the literature suggests that solifluction differs significantly in this area from the same process in zones of seasonal freezing or discontinuous permafrost (Lewkowicz, 1988). The distinction between solifluction processes in areas of one-sided and two-sided freezing, first made by J.R. Mackay (1981) in the Mackenzie Delta, is that ice lensing near the base of the active layer during freeze-up is a precursor to the development of a convex-downslope velocity profile, due to plug-like movement in late summer. Plug-like movement was also observed on Banks Island by Egginton and French (1985). Although longterm rates were measured in both studies, annual variability in subsurface movement could not be differentiated using the techniques employed. Another longterm study of solifluction on Spitzbergen used surveying to examine the effects of climatic variability on near-surface movements ( kerman, 1996), but did not describe what happens at the base of the active layer, a crucial zone according to the theory of two-sided freezing. This paper reports solifluction rates measured at sites in the valley of Hot Weather Creek, Fosheim Peninsula, Ellesmere Island (80 N, 84 W) using electro-mechanical solifluction meters (Lewkowicz, 1992) and Rudbergtype pillars. These are believed to be the coldest and most northerly sites at which subsurface movements have been measured in North America. The solifluction meters allow continuous, non-destructive monitoring of movement at 8 or more levels within the active layer if linked to a data-logger, or intermittent, manual recording using a multimeter. The information obtained is the resultant movement, incorporating both vertical and downslope motions of pairs of wooden blocks within the active layer. In contrast, the pillars give only total deformation over the period between installation and excavation. The Fosheim Peninsula is both cold and arid, with a mean annual air temperature at the Eureka weather station of -19 C, and mean annual precipitation of 64 mm (Atmospheric Environment Service, 1984). However, summer temperatures are higher and vegetation more diverse and extensive than in most other areas at similar latitudes in the Canadian Arctic Archipelago (Edlund and Alt, 1989). In addition, the summer climatic conditions inland at Hot Weather Creek are still warmer than at Eureka (e.g., Woo et al., 1990). Solifluction landforms, such as sheets and lobes, are rare in the region. Modeling In order to improve understanding of the field data, a simple mathematical model was developed for compa- Antoni G. Lewkowicz, Shawne Clarke 641

2 the middle of August, after which freezing begins from the permafrost up (Lewkowicz, 1992), accumulated thawing degree-days to this date can be used as a reasonable predictor of active layer depth (see Lewkowicz, 1994). Figure 1. Generalized velocity profile used in modelling. Deformation takes place in a shear zone at the base of the active layer. rison. Laboratory experiments involving the thaw of layered ice-poor and ice-rich soil have shown that rapid displacements are concentrated in the ice-rich layer (Rein and Burrous, 1980). Deformation is limited within the overlying ice-poor material which is rafted on top of the soil where strain occurs. This view can be incorporated within a modelled velocity profile for an area of two-sided freezing, assuming that the basal part of the active layer is relatively ice-rich and that the remainder is ice-poor (Figure 1). The location of the shear zone is linked to the depth of maximum thaw in a given summer. This can be modelled using a modified Stefan equation (e.g., Lewkowicz, 1994): ktt u s z = 2 L [1] Based on the record of temperatures from Eureka for June 1-August 15, and assuming a mean value of 65 cm (a typical active layer depth for the area), the hind-cast values of active layer depths for range from 52 cm to 77 cm. The critical assumption in this analysis is that soil thermal characteristics remain constant from year to year. In reality, this variability may be somewhat reduced because in very warm summers, the soil layers which change from being part of the permafrost to being part of the active layer, generally possess high latent heat, and therefore may experience a slower rate of thaw. A predicted velocity profile can be generated using the distribution of thaw depths and assuming a thickness for the shear zone. In the model, a thickness of 10 cm was assumed and amounts of movement were calculated for layers at 1 cm intervals. The mean surface velocity was set to 2 cm/a, a typical value for mesic sites. Because the variable annual thaw depth shifts the shear zone up and down relative to the surface, the thickness of the zone in which differential movement occurs, expands to 33 cm (Figure 2). If the same calculations are undertaken for , a similar pattern of deformation arises but with a discontinuity due to particularly cool conditions in 1992 and 1996 (Figure 2). In addition, the rare movements below 72 cm are not present. The model can be refined to take into account annual variations in the amount of subsurface movement. where z is the thaw depth (m); k u is the unfrozen thermal conductivity (W/(m C)); T s is the average surface temperature ( C); t is time since the start of thaw (days); L is the volumetric latent heat of the soil (MJ/m 3 ). If it is assumed that soil thermal characteristics are constant on an inter-annual basis, the annual maximum depth of thaw at a particular site is dependent on T s and t, whose product can be represented by the sum of thawing degree-days. On the Fosheim Peninsula, since the maximum thaw depth is generally attained prior to Figure 2. Velocity profiles modelled using uniform rate of shear in the bottom 10 cm of the active layer and surface velocity of 2 cm/a. A: Profile gene-rated using predicted Eureka thaw depths B: Profile generated using predicted Eureka thaw depths (inclusive). 642 The 7th International Permafrost Conference

3 Variations would be expected because there is a net annual flux of water into the upper part of the permafrost due to temperature gradients and gravity (Mackay, 1983). Thus the greater number of years that a given layer is not thawed, the more ice-rich this layer is likely to be, increasing the probability of high porewater pressures being generated during thaw, and hence increasing the rate of solifluction. This aspect was introduced into the model using a logarithmic relationship with elapsed time since the previous thaw. A nonlinear relationship was used because there should be a decline in moisture fluxes as the layer becomes increasingly ice-rich. Velocities were calculated using: ( ) V = x V log10 x where V x is the velocity 10 cm above the base of the active layer for a given number of years since the previous thaw (cm/a); V 1 is the velocity 10 cm above the base of the active layer for 1 year since the previous thaw (cm/a); x is the number of years since the previous thaw at this depth. As for the 1-year case, a linear velocity gradient was assumed if the complete shear zone (i.e., basal 10 cm) had not thawed for the specified number of years. But if, for example, the bottom 5 cm had not thawed for 10 years, but the next 5 cm had thawed one year previously, a combination of the two velocity gradients was used. Inclusion of the relationship in Equation (2) into the model changes two aspects (Figure 3). First, velocities at [2] all levels increase, producing an average value at the surface of almost 2.5 cm/a. Second, there is increased development of the concavity in the velocity profile at the deepest levels, because movement there is enhanced since thaw is so infrequent. The same method applied to the period (Figure 3) also enhances velocities, producing an average surface rate of 2.2 cm/a. Field measurements Observations from the Fosheim Peninsula relate to two aspects of the model: (1) the presence of a shear zone, and (2) annual variation in position and rates of movement. BASAL SHEARING Evidence of shearing near the base of the active layer comes from two sources. First, shear planes or thin zones of shear have been observed in relation to activelayer detachment slides which can develop during periods of particularly rapid thaw (Harris and Lewkowicz, 1993). If large-scale shearing takes place on slopes that fail rapidly, it is very probable that smaller movements are taking place simultaneously on surrounding slopes which have slightly higher factors of safety. Second, 25 pits were excavated in 1996 to expose Rudberg-type dowel columns or garden hose cut into approximately 5 cm lengths. These markers had been put in place on slopes in 1990 or None of the slopes monitored exhibited solifluction lobes or sheets. At or near the base of several of the pits, horizontal displacements of up to 1 cm had occurred between the bottom of one pillar section and the top of the one beneath. This indicates that differential movement had taken place along a discrete plane or in a very narrow zone, most likely while the material beneath was frozen. INTER-ANNUAL VARIATION IN MOVEMENT Velocity profiles recorded by manually-monitored solifluction meters show inter-annual differences in rates of movement at the base and throughout the active layer. Meters 16 and 19 are on the same eastfacing slope at Hot Weather Creek, and are located within and just downslope of a concavity in the upper part of the slope. Snow immediately around the meters is consistently less than 15 cm deep at the end of winter, but snowbanks upslope provide moisture for about two weeks during the melt period. Near-surface rates of movement at these sites averaged 3.1 cm/a and 1.7 cm/a over 5 years. As noted above, these values are resultants of both downslope and vertical motions. Figure 3. Velocity profiles modelled using non-uniform rates of shear in the bottom 10 cm of the active layer that depend on number of years elapsed since previous thaw at a given depth. A: Profile generated using predicted Eureka thaw depths B: Profile generated using predicted Eureka thaw depths (inclusive). The movement profiles at meters 16 and 19 show a clear difference between the summers of 1993 and 1995 when deeper thaw occurred, and the other three sum- Antoni G. Lewkowicz, Shawne Clarke 643

4 Figure 4. Mid- and late-summer movement profiles recorded by two solifluction meters installed on the same slope at Hot Weather Creek, together with maximum thaw depths at the meter sites. Meter 16 is on a slope of 9, 20 m upslope of Meter 19 which is on a slope of 5. Both are installed in silty colluvium, with evidence of shearing in a pit between the two meters and in a second pit 4 m downslope of meter 19. Zero line represents the position of the meters on August 13, Note difference in movement scale between the upper and lower parts of the figure. Thaw depths measured on July have been increased by 2 cm to represent maximum values for the year, based on ground temperatures measured at the experimental solifluction site 50 m downslope. mers (Figure 4). Near-surface movement between mid-july and early August 1993 was 3 cm at meter 16 and 2 cm at meter 19. In 1992, 1994 and 1996, movements during the same period were 1 cm or less at both sites. Only a single measurement was made during 1995, but the great change in position between 1994 and 1996 suggests that movement must have been similar to that of Near the maximum thaw depths, there is an even greater contrast, although absolute rates are much smaller. At meter 16, there was virtually no latesummer movement in 1992, 1994 and In 1993, however, 0.5 cm of movement occurred at a depth of 50 cm. At meter 19, movements are more deep-seated as the thaw depth is greater. At 58 cm, the maximum level Figure 5. Late-summer movement and thaw depths for 1994 and 1995 at solifluction meter A, Hot Weather Creek. The meter was installed in 1992 on a slope of 8. (A) Movement in 1994 at 58 cm and 66 cm. (B) Movements in 1995 at 66 cm and 74 cm. Inset shows measured shear strengths: values are an average of 3 repetitions (1994) and 5 repetitions (1995), together with standard deviations. (C) Thaw depths are 5-day running means calculated after fitting polynomial functions to temperatures recorded using thermocouples at depths of 0, 10, 26, 42, 50, 58, 66, 74, 100, and 150 cm. for measurement, movement in 1993 was almost 1 cm compared to less than 0.2 cm in the colder summers. Again, the importance of movement in 1995 both before and after the single measurement on July 28 is indicated by the substantial change in position between the profiles of 1994 and Some of the movement recorded during years of deeper thaw is settlement. Because of the variability in depth of thaw and typically high ice contents at the base of the active layer and in the upper permafrost, it cannot be assumed that blocks return to the same elevation relative to the body of the meter (anchored in permafrost) even after a complete annual cycle. However, because the soil moves downslope while settling, the overall pattern of movements described above is still correct. Records from other meters with additional measurement levels show that at greater depths, movement takes place only during certain years. These levels constitute the Òtransitional layeró (Romanovskii, 1980), 644 The 7th International Permafrost Conference

5 which is part of the permafrost in some years and part of the active layer in others. Movements at the lowest thawed level and the first frozen level are shown for 1994 and 1995 for meter A, located 50 m downslope of meter 19 and linked to a data-logger (Figure 5). In 1994, a year with shallow thaw, there was virtually no movement at 58 cm, while the 66 cm level remained frozen (Figure 5A). Fluctuations of less than 0.5 mm at 58 cm are probably due to initial heave followed by settlement associated with the passage of the thaw front. In situ shear strengths measured using a Geonor shear vane inserted from the surface were greatest in surface layers but still averaged 60 kpa at a depth of 58 cm (Figure 5B inset). In 1995, surface shear strengths were also high due to the development of a crust through evaporation, but much lower values were recorded at depth where thawing was rapid. At the 66 cm level where shear strengths were lowest, combined forward movement and settlement began on July 30 when the thaw depth reached nearly 70 cm (Figure 5C), and almost 9 mm was recor-ded by August 10 (Figure 5B). Maximum rates excee-ded 1.5 mm/day. At 74 cm, retrograde movement of less than 0.5 mm was recorded from August 4-10 while this layer was cryotic. It is thought that this was due to heaving of the measurement blocks as water entered the permafrost (c.f., Mackay, 1983). The rapid decrease in velocity between 66 cm and the frost table just a few cm below confirms that a zone of shear or plastic deformation existed. Comparison of observed and modelled movements Hose pillar 19B, located 4 m downslope of solifluction meter 19, can be used to illustrate the degree of accordance between the modelled values and actual movement. This pillar was installed on August 12, 1991 and excavated 5 years later on August 5, The model inputs were manipulated to give zero movement at the appropriate depth and a variety of values of V 1 were used. It was impossible to obtain a good fit throughout the profile, but a very good reproduction of velocities in the lower half of the active layer could be produced using a value for V1 of 1.08 cm/a (Figure 6). The agreement in this zone supports the concept of two-sided freezing (Mackay, 1981) and indicates that the observations of Rein and Burrous (1980) in the laboratory may be transferable to the field. Based on the continuous measurements and observations at other sites, the two possible missing elements in the model that explain the discrepancies in the upper part of the profile, appear to be frost creep and shearing other than at the base. The former is indicated by heave Figure 6. Comparison of measured and modelled movements, hose column 19B, Hot Weather Creek. Hose column left in place from August 1991 to August Modelling based on thaw depths predicted from Eureka thawing degree days for with mean active layer for of 56 cm. V 1 set to 1.08 cm/a (see Equation (2)). in the fall, and limited movement in the spring that descends with the thaw front and is concentrated in the near-surface. Egginton and French (1985) suggested that plug-like flow on Banks Island resulted in the preservation of surface hummocks as they moved downslope and hypothesized that comparable movements were also taking place within discrete zones in the active layer. They also noted the occurrence of emergent shears in the same area and attributed these to ÒclassicÓ solifluction. Observations from Hot Weather Creek show similar preservation of surface forms on some slopes (slope hummocks, Lewkowicz and Gudjonsson, 1992) and the presence of emergent shear planes within the central part of the active layer. At some sites, the latter are linked to a downslope decrease in volumetric transport of material (e.g., from Solifluction Meter 16 to Meter 19, see Figure 4), which produces surface aggradation through shearing. Conclusions Solifluction movements reported here are from the most northerly and coldest monitoring sites in North America. Rates of up to 3 cm/a were measured. A model of solifluction movements, involving shearing in a 10 cm thick zone immediately above the maxi- Antoni G. Lewkowicz, Shawne Clarke 645

6 mum depth of thaw can reproduce the velocity profile for the lower half of the active layer for a site in Hot Weather Creek. The upper part of the profile differs from the model and it is clear that other processes, probably frost creep and isolated shearing, are significant to downslope movement. The use of solifluction meters allowed the first observations of inter-annual variation in subsurface rates of active-layer movement in an area of two-sided freezing. Substantial inter-annual differences in amounts of latesummer movement occurred throughout the active layer. In general, higher rates of movement developed in years with deeper thaw. Measurements confirm the importance of the Òtransitional layeró, recognised within the Russian permafrost literature, which thaws and moves in some years and is frozen and immobile in others. Acknowledgments This research was supported financially over 8 years by grants from the Natural Sciences and Engineering Research Council and by Research Agreements with Energy, Mines and Resources Canada. Essential logistical support was given by the Polar Continental Shelf Project, Natural Resources Canada, and at Hot Weather Creek through Dr. S. Edlund and the Geological Survey of Canada. Additional financial support came from Northern Training Grants from Indian Affairs and Northern Development Canada. Field assistance given by L. Bellisario, Dr. J. Desloges, K. Gudjonsson, Dr. C. Harris, J. Hartshorn, S. Kokelj and H. Taylor is gratefully acknowledged. The suggestions of two anonymous reviewers helped to improve a previous version of the manuscript. References kerman, H.J. (1996). Slow mass movements and climatic relationships, , Kapp LinnŽ, West Spitsbergen. In Anderson, M.G. and Brooks, S.M. (eds.), Advances in hillslope processes. Wiley, Chichester, pp Atmospheric Environment Service (1984). Eureka. Principal Station Data, Volume 79. Atmospheric Environment Service, Environment Canada, Downsview, Ontario. Edlund, S.A. and Alt, B.T. (1989). Regional congruence of vegetation and summer climate patterns in the Queen Elizabeth Islands, Northwest Territories, Canada. Arctic, 42, Egginton, P.A. and French, H.M. (1985). Solifluction and related processes, eastern Banks Island, N.W.T. Canadian Journal of Earth Sciences, 22, French, H.M. (1996). The Periglacial Environment. Longman, New York (341 pp.) Harris, C. (1981). Periglacial mass-wasting: A review of research. British Geomorphological Research Group, Research Monograph Series, No. 4 (204 pp.). Harris, C. and Lewkowicz, A.G. (1993). Micromorphological investigations of active-layer detachment slides, Ellesmere Island, Canadian Arctic. In Proceedings, Sixth International Conference on Permafrost, Beijing, China. South China University of Technology Press, Wushan, pp Lewkowicz, A.G. (1988). Slope processes. In Clark, M.J. (ed.), Advances in Periglacial Geomorphology. Wiley, Chichester, pp Lewkowicz, A.G. (1992). A solifluction meter for permafrost sites. Permafrost and Periglacial Processes, 3, Lewkowicz, A.G. (1994). Ice wedge rejuvenation, Fosheim Peninsula, Ellesmere Island. Permafrost and Periglacial Processes, 5, Lewkowicz, A.G. and Gudjonsson, K.A. (1992). Slope hummocks on Fosheim Peninsula, Northwest Territories. Current Research, Part B, Geological Survey of Canada Paper, 92-1B, Mackay, J.R. (1981). Active layer slope movement in a continuous permafrost environment, Garry Island, Northwest Territories, Canada. Canadian Journal of Earth Sciences, 18, Mackay, J.R. (1983). Downward water movement into frozen ground, western arctic coast, Canada. Canadian Journal of Earth Sciences, 20, Price, L.W. (1991). Subsurface movement on solifluction slopes in the Ruby Range, Yukon Territory, Canada - a 20- year study. Arctic and Alpine Research, 23, Rein, R.G.J. and Burrous, C.M. (1980). Laboratory measurements of subsurface displacements during thaw of lowangle slopes of a frost-susceptible soil. Arctic and Alpine Research, 12, Romanovskii, N.N. (1980). Principles of classification of polygonal-vein structures. In Third International Conference on Permafrost; English Translations of Twenty-Six of the Soviet Papers. Ottawa, National Research Council of Canada, pp Smith, D.J. (1992). Long-term rates of contemporary solifluction in the Canadian Rocky Mountains. In Dixon, J.C. and Abrahams, A.D. (eds.), Periglacial Geomorphology. Wiley, Chichester, pp Woo, M.-K., Young, K.L. and Edlund, S.A. (1990) observations of soil, vegetation, and microclimate, and effects on slope hydrology, Hot Weather Creek basin, Ellesmere Island, Northwest Territories. Current Research, Part D, Geological Survey of Canada Paper, 90-1D, The 7th International Permafrost Conference