New data on permafrost and cryogenic landforms of the Darkhad depression

The Darkhad depression is one of the largest intermountain depressions, like Baikal, in northern Mongolia covering an area of about 3320 km. The Darkhad depression climate is extreme continental, with lasting cold dry winters and warm dry summers. The average annual temperature is negative, -6 C, the mean annual air temperature in January is negative, -32,0 C, the lowest temperature is observed in January and reaches -50.4 C. Cooling in the Darkhad depression starts from the 10th of August and lasts until mid-June, so duration of the cold and cool period of the year is 290-310 days. Annual precipitation on average is 260-270 mm. Harsh climatic conditions contribute to the preservation of permafrost. The Darkhad depression is located in the developmental zone of continuous permafrost rocks. The average thickness of permafrost in sediments in the Darkhad depression is between 83.7-97.8 m and the maximum reaches 183.7 m. But soil temperature regime and cryogenic processes have been poorly studied so far. The aim of the work is the characteristics of temperature regime of the permafrost top part and cryogenic landforms. Extensive data on morphology, mor-phometry and geologic structure of frost swelling mound was collected by the author when participating in the group of the Institute of Geography named after Sh. Tsegmid of Mongolian Academy of Sciences. Cryolithozone temperature conditions were studied. In the laboratory period the collected field data processing and interpretation of space photographs were carried out as well as data from previous studies were involved. The conducted geometrical studies revealed significant fluctuations in temperature of rocks on the surface and in the depth of 1 to 15 m. Especially, large variations in temperature of rocks are typical for the surface, where the maximum temperature of the rocks vary from +35.74 C to +40.89 C. The average annual temperature of the surface layer of rocks ranges from -1.79 C to 4.04 C, the lowest of which is typical for the northern parts of the territory. In order to study the dynamics of thermokarst lakes in 1991-2010 within the Darkhad depression the author made a comparative analysis of space photographs and statistical data processing to modify areas of thermokarst lakes derived from the satellite measurements. As a result, during the study period the surface area of small (5-10 ha) thermokarst lakes varied and amounted to 663.1 ha, increased by 100.4 ha, lakes area (1050 ha) also experienced fluctuations, but as a whole, in 2010 decreased by 229 ha and the total surface of lakes with the area over 100 ha, increased by 360.8 ha. The smallest increment of lake surface is observed for the lake with the area of 50-100 ha - 10.4 ha. Thus, a decrease in the temperature of the surface layer of the lithosphere and development of thermokarst lakes in natural landscapes currently characterizes the Darkhad depression. Economic development will contribute to the development of thermokarst, particularly in the areas of continuous development of permafrost rocks.

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