ACTIVE LAYER MONITORING IN NORTHERN WEST SIBERIA

ACTIVE LAYER MONITORING IN NORTHERN WEST SIBERIA A. V. Pavlov Earth Cryosphere Institute, B RAS 142452, Zeleny-village, 5-67, Noginsk district, Moscow region, Russia e-mail: emelnikov@glas.apc.org Abstract Long-term observations on the depth of seasonal thaw have been made in northern West Siberia. Results from two permafrost stations are analyzed: Marre-Sale (Yamal Peninsula) and Parisento (Gydan Peninsula). Thaw depths range from 0.3 to 0.75 m on polygonal peatlands and flat bogs, to up to 1.5 to 1.8 m on unvegetated sandy deposits. There is a strong relationship between summer thawing degree-days and depth of thaw. Inter-seasonal variations of depth of thaw can be greater than 15 to 20% of the mean. Depth of thaw poorly reflects contemporary climatic warming. Results from the past 15 to 20 years suggest a trend for an increase in thaw not exceeding 0.5 to 0.7 cm/yr. Analysis of thaw measurements from CALM grids indicate statistically similar values can be obtained from sampling on the 1000 m grid by reducing the number of probings by a factor of 2 to 2.5. Introduction More than 90% of all the Russian reserves of natural gas are located in the Arctic and Subarctic regions of Western Siberia. Permafrost exerts a strong influence on engineering works for gas production. Permafrost stations in the Russian North are used to develop information and monitoring of the geological environment (Pavlov, 1984; Melnikov et al., 1992). Seasonal thawing of soils is one of the basic parameters of permafrost monitoring. The most detailed research on seasonal thawing of soils in the Arctic regions of Western Siberia has been carried out at the Marre-Sale permafrost station from 1978 to the present. Other stations in the region are closed or are occasionally occupied (Figure 1). In this paper, data from the Parisento permafrost station (1985 to 1993, and 1995) are also used. Both stations are located in the tundra zone, characterized by low air and ground temperatures and continuous permafrost. Many works have been published in Russia, Canada, USA and other countries on the subject of seasonal thawing of soils. Aspects considered include: (1) physics of thawing and the influence of various natural factors (Kudriavstev, 1978; Pavlov, 1984); (2) modeling of seasonal thaw of soils (Aziz and Lunardini, 1993; Pavlov, 1980); (3) surficial conditions and the influence on the depth of seasonal thawing (Brown and Grave, 1979; Pavlov, 1980); (4) estimation of contemporary change of the depth of seasonal thaw and future prediction (Nelson et al., 1993; Mackay, 1994; Pavlov, 1994, 1996). Despite these and other publications, the depth of seasonal thaw has been insufficiently utilized as a major parameter of permafrost monitoring. Experimental data from Marre-Sale and Parisento stations are useful for illustrating this fact. Study area Marre-Sale station is located in the middle part of western Yamal on the Kara sea coast, near the polar meteorological station (Figure 1). Mean annual air temperature in the Marre-Sale region is -8 C and total precipitation is 301 mm/yr. The duration of the warm season varies from 90 to 137 days. Average date of winter snow cover formation is October 10, and it melts completely on flat areas by June 12. Relief is gently hilly with erosional terraces, deeply dissected by ravines and valleys of streams and small rivers. The elevations within the station area are 0 to 40 m asl. Sand and sandy loam predominate in the near-surface, with a peat cover in many places (0.1 to 0.7 m). Vegetation is mainly grass-shrub-moss-lichen tundra changing to grass-moss bogs on the bottoms of depressions. The mean annual soil temperature (at A. V. Pavlov 875

Figure 1. Location of monitoring stations in the Arctic regions of West Siberia I - stations: 1 Marre-Sale, 2 Kharasavey, 3 Turin-To, 4 Tadibyakha, 5 Sloping, 6 Parisento, 7 Yamburg, 8 Soleny. II - boundaries of natural zones (T tundra, FT forest tundra, F taiga). III - boundaries of permafrost distribution (a) continuous permafrost; (b) discontinuous permafrost; (c) sporadic permafrost. 10 m) ranges from -2 C (in depressions) up to -7 C (on divides). Parisento station is located in the western part of the Gydan peninsula (Figure 1). Mean annual air temperature at Parisento is -10.8 C and total precipitation is 326 mm/yr. The average date for formation of continuous snow cover is October 9, and its complete disappearance is June 12. The station area is situated on a coastal plain with elevations 10 to 46 m asl. A series of terraces with numerous lakes (up to 40% of the area) are present. Sands, loamy sands and loam predominate in the near-surface. The mineral soils are enriched by peat with a thickness of 2 m in some places. Vegetation is graminoid-shrublichen and sedge-shrub-lichen tundra with a thickness of plant-organic cover of 5 to 10 cm. Willow stands with heights from 1.5 to 2 m occur on the slopes of lake basins. On level terraces, willow-grass-moss bogs and cloudberry-dwarf birch-labrador tea-lichen-moss polygonal peatland are widespread. Mean annual soil temperature ranges from -0.4 to -10 C. Methods Observations of seasonal thaw and soil temperature, including temperature measurements in boreholes, have been carried out on experimental sites at these stations. These sites and boreholes are located in areas representative of the major types of the natural landscape, including unvegetated areas. Measurements of depth of seasonal thaw on experimental grids were organized according to the Circumpolar Active Layer monitoring (CALM) program of the IPA (Akerman et al., 1996). Grids are square 1x1 km and cover most of the landscape types in the region; observations are at 100 m intervals. The Parisento grid was measured in 1992, 1993 and 1995, and Marre-Sale grid in 1995 and 1996. 876 The 7th International Permafrost Conference

The depth of seasonal thaw of soils is determined with a steel probe marked at 1-cm intervals. The probe easily penetrates thawed peat, loam, sandy loam and sand to depths of 1.5 to 2 m. The accuracy of determination of the thawed layer is 1-2 cm. Depth of seasonal thaw of soil was measured at a microsite with 5 to 10 replications. Such monitoring of the active layer was carried out every 10 days during the thaw season. Additional control for the depth of seasonal thaw was carried out with the help of hand drilling of small diameter boreholes, and sampling for soil moisture of the active layer. Determination of active layer thickness is a component of thermal monitoring of the upper layer of permafrost (Melnikov et al., 1992). Field observations The formation of the active layer depends on the sum of thawing-degree days - the thaw index Ith. At both Marre-Sale and Parisento, long-term mean summer air temperatures and the duration of the thaw season average are about 5 C and 170 days, respectively. Thus, meteorological conditions during the warm season result in almost equal thaw indexes at the stations. Both stations have about equal winter precipitation (about 170 mm). The winter air temperatures at Parisento are lower than at Marre-Sale and result in lower soil temperatures for Parisento. The meteorological conditions promote slightly greater thaw depths at Marre-Sale in comparison with Parisento for similar landscape conditions. At Marre-Sale, the depth of seasonal thaw of soil (h th ) varies from 0.4 to 0.75 m (polygonal peatlands, flat bogs) to up to 1.5 to 1.8 m (unvegetated sands) (Table 1). For river valleys and thermokarst depressions, typical values of h th are 0.8 to 0.9 m. Slightly deeper thaw (up to 1.3 m) occurs for well-drained hollows with fragments of peat. At site 1, which is vegetated, the depth of seasonal thaw of soil in on average 22% less than at unvegetated site 7 (Table 1). In peat, the depth of seasonal thaw is about 3 to 3.5 times less than in sand. Interannual variations of the thaw depth in peat are significant, especially at the beginning of summer and usually exceed 25%. For mineral soils, variations in h th are much less; in particular, year-to-year change in h th for unvegetated sand (site 7) does not exceed 15%. The largest values of h th generally occurred during the abnormally warm summers of 1984, 1989, 1993 and Table 1. Depth of thaw, h th (m) on 15 th of month at the Marre-Sale and Parisento stations A. V. Pavlov 877

Figure 2. Inter-seasonal changes of statistical parameters of variability of soil thawing in northern West Siberia. (a) Parisento (1987 to 1988), (b) Marre-Sale (1994). s h is the mean quadratic deviation of h th ; C v is the coefficient of variation. 1995. There is also a correspondence between the minimum values of mean summer air temperature and the lowest values of h th in 1980, 1986 and 1992. The ratio between h th for warm and cold summers is 1.1 to 1.4 for soils with a vegetated organic layer and 1.1 to 1.2 for soils with no organic cover. The observations of active layer at both stations appear similar in many respects; e.g., the maximum depth of seasonal thaw on unvegetated sand is 1.79 m at Parisento, and 1.77 m at Marre-Sale (Table 1). On polygonal peatlands, h th is reduced to 0.5-0.7 m. Despite a short period of observation at Parisento in comparison with Marre-Sale, the interannual variations of thaw depth were similar, especially in the first half of the thaw season. Analysis of data On the basis of these field observations, the following questions regarding active layer monitoring as a component of general monitoring of permafrost are discussed: Spatial-temporal variability of depth of soil thaw; Distribution of observation points required for statistical averages; Comparison of temporal variability of depth of seasonal thaw with variation of thermal parameters in the upper part of permafrost. Figure 3. The dependence of parameters of spatial variability of seasonal thawing of soils based on number (n) of observational points in CALM grids. 1 Parisento; 2 Marre-Sale; h th av, s h and C v (see text). The estimation of spatial-temporal variations of thawing depth of soils was carried out using the mean quadratic deviation (s h ) and the coefficient of variation C v. Calculations have shown that the statistical parameters s h and C v change in opposite directions during a the summer season (Figure 2). The value of s h increases as thaw depth increases. In contrast, the coefficient C v decreases during the thaw season. The representativeness of the CALM grids as compared to the whole station territory was evaluated. The spatial variability of seasonal thaw depth (h av, s h and C v ) for the Parisento CALM grid is within the same limits as other observations from the basic landscape types. Thus, the CALM grid covers practically the 878 The 7th International Permafrost Conference

whole range of surface environments within the station territory. The Marre-Sale grid is less representative of the whole. The average seasonal thaw depth for the grid was 23 cm greater than the mean for the whole station territory. On the basis of grid observations, we evaluated the spatial variability of the active layer as a function of the number of points (n) of observations (Figure 3). For this purpose, calculations of hav,s h and C v were carried out by reducing the number of observations. For a reduction in n from 100 up to 40 to 50 points the parameters s h and C v do not change significantly (Figure 3). Thus, an effective estimation of variability of depth of seasonal thawing in limits of the grid can be carried out with a reduction of number of observation points by a factor of 2 to 2.5. Figure 4. Data from Marre-Sale station, 1978 to 1995. (a) Thawing index I th 1/2 ; (b) Depth of seasonal thaw h th for experimental sites 2, 3, 6, 8 and 9 (see Table 2); (c) h th /h th av for the mean of sites 2 and 3; h th av = mean h th values for 1978 to 1995. Results from Marre-Sale (1978 to 1995) have revealed the variability of depth of seasonal thaw over the longterm for individual observation points (Figure 4). Interseasonal variation in depth of seasonal thaw (Æh th ) on most of the terrain elements is more than 20% of the mean. Maximum variations of (Æh th /h av th correspond, as a rule, to organic soils. A more detailed statistical estimation of interannual variation of h th is given by s h and C v. For organic soils, (h = 0.12 m and C v = 0.17 (average data for sites 2 to 5; h th = 0.4 to 0.8 m) (Table 2). For mineral soil with minor organic cover (sites 6, 8 and 9; h th = 1.1 to 1.5 m) parameters s h and C v yield appreciably smaller variability: (h = 0.09 m, C v = 0.13. The absolute maximum of h th occurred in the abnormally warm summers of 1989 to 1991 and 1995. In these years the thaw index Ith was a factor of 1.1 to 1.3 above the "norm" (mean annual Ith value) (Figure 4a). Minimum depths of thaw were observed in 1978, 1980 and 1992 at I 1/2 th values 1.5 to 1.6 lower than the "norm". The ratio between h th in warm and cold summers is 1.3 to 1.4 for soils with a rather thick organic layer and 1.1 to 1.2 for soils with thin or no organic layer. An analytical formulation of the depth of thawing-freezing of soil is the Stefan's formula (Kudriavtsev, 1978). All freezing-thawing formulas include I th 1/2 as a basic parameter. Over the long-term, the conformity between changes of seasonally thaw depth h th and I 1/2 th is marked. At the maximum I 1/2 th values (1979, 1984, 1989, 1993 and 1995), there is a corresponding maximum of h th. Over the period of 1978 to 1995, a weak tendency of seasonal thaw to increase is observed. This tendency is observed better in the relative depth of seasonal thawing h th /h av th, where h av th is the average h th value for the period of observation (Figure 4c). At Marre-Sale, the increasing trend of h th does not exceed 0.5 to 0.7 cm/yr. At the present rate, the active layer will increase 15 to 20% by 2050. The parameters s h and C v, calculated for the whole territory of Marre-Sale and Parisento stations, changed in a smaller range than for separate points of observation. Inter-seasonal changes in s h and C v for these stations do not fall outside the limits of 10% and 4%, respectively. Variations of frozen soil temperatures over a year even at a depth of 10 m exceed the accuracy of our instrumental measurement (0.1 C). The general tendency of an increase of soil temperatures over the period of observation at Marre-Sale is marked. The greatest increase of temperature of frozen soils occurs in polygonal tundra; while the smallest occurs in depressions and river valleys (Pavlov, 1994, 1996). A. V. Pavlov 879

Table 2. Statistical characteristics of inter-annual variations of seasonal thaw depth, h th, at the Marre-Sale station Conclusion The analysis of long-term observations of seasonal thaw of frozen soils in Arctic regions of Western Siberia is summarized. The depth of seasonal thawing (h th ) varies with total thawing degree-days. The highest values of h th correspond to years with abnormally warm periods. Interseasonal variations of thaw depth can be greater than 15 to 20% of the mean. The maximum values of h th /h av th are observed for organic soils. A ratio of h th in warm and cold summer periods is 1.1 to 1.4. Statistical parameters of the spatial variability of depth of seasonal thawing at Marre-Sale and Parisento stations are evaluated. Positive deviations of depth h th from the mean occur less often than negative deviations, but positive deviations are characterized by larger amplitudes. depth of seasonal thaw is not very sensitive to contemporary climatic warming, as the warming is largely caused by an increase of winter air temperatures. In Arctic regions of Western Siberia the trend in the increase of h th for the last 15 to 20 years does not usually exceed 0.5 to 0.7 cm/yr. Acknowledgments The field investigations were financed by the Russian Geological Service, Russian Academy of Sciences and the Russian company "Gazprom". CALM active layer observations were supported with the assistance of a National Science Foundation grant to the State University of New York-Albany, F.E. Nelson. The author is grateful to V. Dubrovin and Nataly Moskalenko for significant contributions at Marre-Sale and Parisento stations, and L. Kharitonov, by whose efforts the necessary meteorological information was received. The author is also grateful to J.Brown for his encouragement to prepare this paper. Estimation of variations of seasonal thaw depth is carried out to forecast global climate in the future. The References Akerman, H. J., et al. (1996). A Circumpolar Active Layer Monitoring (CALM) Program (Draft 3/22/1996) in cooperation with the International Tundra Experiment (ITEX) and the International Permafrost Association (IPA). In Fundamental Research of Earth Cryosphere in Arctic and Sub- Arctic (Results and Prospects). Pushchino, pp. 65-68. Asiz, A. and Lunardini, V. J. (1993). Temperature variations in the active layer of permafrost. In Proceedings, Sixth International. Conference of Permafrost, Vol 1. Beijing, China, South China University of Technology Press, pp. 17-22. Brown, J. and Grave, N. A. (1979). Physical and thermal disturbance and protection of permafrost. USA Cold Regions Research and Engineering Laboratory, Special Report 79-5, Hanover, NH, (42 pp.). Kudriavtsev, V. A. (ed.). 1978. General Geocryology, Moscow State University, Moscow, (463 pp). (In Russian). Mackay, J. R. (1994). Active layer change (1968-1993) following the forest-tundra fire near Inuvik, N.W.T., Canada. Arctic and Alpine Research, 27, 323-336. 880 The 7th International Permafrost Conference

Melnikov, E. S., Grechichshev, S. E., and Pavlov, A, V. (eds.). (1992). Study of engineering-geocryological and hydro-geological conditions for top layer of gas-oil permafrost regions (Methodological manual) Moscow: Nedra, (288 pp.). (In Russian). Nelson, F. E., et. al. (1993). Permafrost and Changing Climate. In Proceedings, Sixth International Conference on Permafrost, Vol. 2, Beijing, China, South China University of Technology Press, pp. 987-1008. Pavlov, A.V. (1980). Calculation and Regulation of Soil Permafrost Regime. Novosibirsk: Nauka. (240 pp.). (In Russian). Pavlov, A.V. (1984). Energy Exchange in Landscape Sphere of the Earth. Novosibirsk, Nauka. (256 pp.). (In Russian). Pavlov, A.V. (1994). Current Changes of Climate and Permafrost in the Arctic and Sub-Arctic of Russia. Permafrost and Periglacial Processes, 5,101-110. Pavlov, A.V. (1996). Permafrost - Climatic monitoring of Russia: analysis of field data and forecast. Polar Geography, 20, 44-64. A. V. Pavlov 881