THICKNESS OF MINERAL COVERS ON THE ICE-MORAINE RIDGES AND AN ACTIVE LAYER OF PERMAFROST ON SPITSBERGEN

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1 Marek GRZEŚ Department of Hydrology and Waler Economy Institute of Geography, Mikołaj Kopernik University Fredry 6/ Toruń, POLAND tel/fax: (056) Wyprawy Geograficzne na Spitsbergen UMCS, Lublin, 1995 THICKNESS OF MINERAL COVERS ON THE ICE-MORAINE RIDGES AND AN ACTIVE LAYER OF PERMAFROST ON SPITSBERGEN Many phenomena in nature are directly or indirectly connected with seasonal thawing of the soil in areas with permafrost and with seasonal freezing of the soil in temperate climates. Seasonal freezing of soil is a cryologic phenomenon most commonly occurring on Earth. The difference between seasonal freezing and seasonal thawing is clear and obvious. However, there are a few common features which should be discussed. Their simple illustration is presented in Figures 1 and 2. Freezing and thawing of soils is connected with passing of this medium through 0 C whether it contains water or not and how much of it. The number and importance of natural and technical problems increases with the increase of ice amount in the seasonally thawing soils and of water in the seasonally freezing forms. Ice-moraine ridges, which have been a topic of discussion for many years, are a special case (Ostrem, 1959; Kozarski & Szupryczyński, 1973; Healy, 1975; Kozarski, 1982; Grześ, 1990a; Kłysz, 1995). Most studies on the thermoinsulation role of moraine material are of a general character. However, its thickness can be considered as the resultant - guiding value for the glacio-climatic conditions of the deglaciation period (Grześ, 1990b). The results of studies on seasonal thawing of different kinds of soils, whose aim is the determination of an active layer, and on thickness of moraine covers on Spitsbergen, indicate some dependences between these phenomena (Grześ, 1990, 1990a). During the 9th Toruń Polar Expedition to Spitsbergen in the summer of 1995, the studies on the verification of the empirical summer model of thawing of various types of soil were carried out (Grześ, 1990). The 1995 summer season in Kaffiöyra Plain was not representative for the conditions in this region due to small precipitation. Precipitation measured in July and August was 20 mm which is 30% of the average sum of precipitation in these months in the previous years (Ny Älesund). The measured thicknesses of the soil were greater than those 141

2 estimated from diagrams and formulas. The greatest differences, reaching even 0.5 m, were found in sandy forms and gravels. Taking into account the data from which the curves of soil summer thawing were approximated (Grześ, 1990a, Fig. 2, p. 74), the divergence was not great. The same conclusion can be reached analyzing the diagram proposed by R. J. Mackay (1970). The seasonal thawing of mineral covers on the ice-moraine ridges is interesting. A thesis has been brought forward that the seasonal thawing of the moraine cover cannot be deeper than the active layer for the forms of similar structure. It can be assumed that the degradation of ice-core lasts till the mineral covers reach the thickness close to the active layer for the warmest summer season. In such a situation, there must be enough moraine material in the ice-core which, on melting, causes the moraine thickness to increase. According to G. Ostrem (1959, 1964), degradation of ice-cores is very slow and depends on many years' changes of climate. It is possible only for the forms which have achieved some "maturity". After the separation of ice-cores from the glacier front and the formation of ice-moraine ridge, two phases can be distinguished in its existence. Thefirst phase is the accelerated degradation of ice-core. The predominant factor influencing this process are mass movements such as creeping down and disclosing ice. Selective melting of ice causes great differentation of relief forms. The second phase which can be described as "mature" is characterized by the disappearance of mass movements - creeping down of moraine material is very rare. Mass movements are most frequently caused by local erosion washing away and by greater moistening of the ground under the melting snow patches. The characteristic feature of this phase is the formation of thermokarst depressions in the side parts of ice-moraine ridges. This process can be seen in an idealized diagram of ridge cross-section evolution (Fig. 3). Measurements of depth of melting material which is a mineral cover on the ice-moraine ridges are very difficult. G. Ostrem (1965) used pneumatic hammers driven by air-compressors produced by the firm "Atlas" in his studies. He used manual soundings, temperature distribution measurements, natural disclosures and excavations to estimate the thickness of the thawed ground layer. The measurement results made after 15 August (5 summer seasons) were used for the analysis. In the summer of 1955 a series of measurements and observations in the forefields of a few glaciers were made in the north-west of Spitsbergen. Detailed studies were carried out on the ice-moraine ridges of the glaciers Aavatsmark and Waldemar. The results of the previous studies in the south-west of Spitsbergen were confirmed (Grześ, 1990a,b). It was found that ice-core melting under the moraine layer of thickness from 1.3 to 1.8 m in current climatic conditions is hardly noticeable. With the increase of the amount of "fine" material amount in the mineral cover, the thickness of a thawing layer decreases. A great 142

3 differentiation of mineral cover inner structure causes a differentiation of thawing active layer thickness. That is why the divergence in the estimates of the "preserving" layer thickness is 0.5 m. The interpretation of degradation of ice-moraine ridges is made somewhat difficult by the horizontal development of these forms due to the incorporation of snow patches and naledi (Kozarski, 1982; Klysz, 1995). This is not a common phenomenon and has a local significance. The analysis of ice-core degradation should take into account the fact that a potential depth of freezing is several times larger than that of seasonal thawing. Therefore the temperature of the ice-core is negative. This phenomenon is illustrated in Fig. 2. The measurements planned to be made in 1996 will show the older of these values. A general regularity is observed in the structure of mineral cover ice-moraine ridges. The thicker material (over ЗО^Ю cm diameter) is concentrated in the layer just under the surface. This results from the process of ablative moraine formation (Grześ, 1990b). The intensive mass movements destroy this original structure. An attempt has been made to verify the results of direct observation of the thermoinsulation role of the mineral cover from repeated measurements of ward heights on the ice-moraine ridges. After about 30 years, the maximum decrease of ward position was 7 m on the ice-moraine ridges of the Werenskiold Glacier. At the same time they were not observed in many places. The sites with the smallest changes were in the lower parts of the slopes. The maximum decrease was observed in the side parts (Grześ, 1990). This regularity was confirmed by the measurements made on the glaciers in the area of Kaffiöyra Plain. The wards piled up in July 1975 were measured again in August The maximum decrease of their height was found to be about 5 m. Both cases give the annual average decrease by about 0.25 m. Wards are piled up in the highest sites of the forms. In these places the thickness of moraine covers is the smallest. These data refer to the ice-moraine ridges from the decline of the 19th century. It should be stated that these changes were not observed in many places, mainly in the lower parts of the slopes. Indirect evidence for the depression of the ice-moraine ridges is the "dilapidating" of wards. Degradation of ice-core forms of contemporary origin is much faster. In the case of the Waldemar Glacier they reach 15 m during 10 years (p.i. Lankauf). The smallest changes were found in the places where the thickness of mineral cover reached and exceeded the value corresponding to that of the thawing active layer. The measurements and observation made it possible to prepare a diagram of ice-moraine ridges degradation (Fig. 3). Four characteristic shapes - phases were distinguished in the evolution of transverse profile of these forms. As mentioned before, the degradation of the ice-core is the fastest in the ridge area. At first, single thermokarst depressions are formed (2). The evidence for the advanced degradation are depressions filled with water along the ridge line (3). On 143

4 combining, they form vast thermokarst depressions (4a). The final stage of ice-core degradation is its division into two parts (4b) and covering with a moraine, whose thickness is larger than the seasonal thawing of the ground. Degradation of ice-core and change of relief give the effect of "glacier front oscillation". This refers particulary to phases 3 and 4a. Some simplification was applied in constructing the diagram which enabled the presentation of an idealized course of the process. The questions arise: How long can the ice-moraine ridges exist? What geomorphological effects occur after the ice-core melting? It is well known that the outer series of ice-moraine ridges were formed at the turn of the 19th century. These are "mature" forms which, however, contain ice inside. A complete explanation of the problems presented in this paper can contribute to the interpretation of many palaeogeographical problems. This paper constitutes only an attempt made to relate the problems of permafrost active layer and ice-moraine ridges existence. Some possibilities for greater knowledge in this matter can be provided by applying computational systems to project so called "ice stores" (Wojtkowski, 1954). REFERENCES Grześ M-, 1990a: Experimental studies of moraine covers over selected glaciers, south-western Spitsbergen. Quaestiones Geogr., 13/14. Grześ M-, 1990b: The active layer of permafrost on the western coast of Spitsbergen. Quaestiones Geogr., 11/12, Healy T. R., 1975: Thermokarst - a mechanism of de-icing ice-cored moraines. Boreas, 4. Kiysz P., 1995: Problem powstawania wałów lodowo-morenowych na przykładzie lodowców w rejonie fiordu Hornsund (Spitsbergen). Przegl. Geogr., LXVII, 1-2. Kozarski S., 1982: The genetic variety of ice cores in the marginal forms of some Spitsbergen glaciers, Hornsund region. Acta Univ. Wratisl., 525, Spitsbergen Expeditions, VI. Kozarski S., Szupryczyński J., 1973: Glacial forms and deposits in the Sidujokull deglaciaüon area. Geogr. Pol., 26. Mackay J. R., 1970: Disturbances to the tundra and forest tundra environment of the western Arctic. Canadian, Geotechnical Journal, 7. Ostrem G., 1959: Ice - melting under a thin layer ofmoraine and the existence of ice cores in moraine ridges. Geogr. Ann., 41 (4). Ostrem G., 1964: Ice-cored moraines in Scandinavia. Geogr. Ann., 46. Wojtkowski K. F., 1954: Rascet sooruzenij iz Ida i snega, Izdat. AN SSSR. 144

5 Fig. 1. Scheme of transition of seasonal freezing ground layer into a zone of permafrost with a layer of ground seasonal thawing. 1 - layer of seasonal freezing, 2 - layer of seasonal thawing, 3 - layer of seasonal amplitude of positive temperature, 4 - layer of seasonal amplitude of negative temperature of ground. After W. A. Kudriacev, 1978 A B C Fig. 2. Idea scheme of seasonal thawing and freezing of ground. 9:3, 6:6, 3:9 - periods with the negative and positive temperatures of the ground Fig. 3. Scheme of ice-moraine levee degradation. 1 - initial height of the levee (Hj), 2-formation of initial thermokarts depression with a lakelet, 3 - formation of the thermokarst depression system, 4 - formation of thermokarst depression levees parallel to the ridge line, 4a - final stage of ice-moraine levee degradation, division of ice stem into two parts and the height H 2 corresponding to them, Wj and W 2 - initial and final width of the form in the base