Study of the Cryosphere of the Zeravshan and Hissar Ranges (Tien Shan)

Introduction

According to media reports, the World Meteorological Organization (WMO) has recognized the last eight years as the hottest since the beginning of meteorological observation. The global average temperature in 2022 will be about 1.15 degrees Celsius higher than in the pre-industrial period (1850–1900). This means that every year since 2016 has been record-breaking hot. WMO experts state that greenhouse gas emissions into the atmosphere have led to an increase in the level of the world's oceans and the melting of ice, as a result of which extreme weather has been recorded in various parts of the world. In Greenland, at an altitude of 3.2 thousand meters above sea level, it rained for the first time. Thus, since the main elements of the cryosphere in high-altitude areas are frozen soil, underground ice, and glaciers, their condition is most sensitive to the global and regional climatic fluctuations currently occurring. According to the calculations of Gorbunov and Ermolin ^[1981], the volume of underground ice in the Tien Shan is 320 km³. Over the past sixty years, the volume of underground ice in relation to the volume of glaciers has significantly decreased. This trend persists in all mountainous regions of Central Asia (Gorbunov, 2018). Such permafrost thawing rates, in turn, can provoke the development of dangerous permafrost geological processes, which is significant in shaping the ecological situation in mountainous areas. The increased seismic and dynamic activity of high-altitude permafrost development areas facilitates the development of dangerous and often catastrophic cryogenic geological processes. These factors must be taken into account both when drawing up projects for the economic development of these areas and carrying out measures to protect structures already built in the mountains.

Methods

According to the 1982 Glacier Catalog, there were 72 glaciers with a total area of 162.02 km² in the Zeravshan Glacier basin as of the glaciation in 1957, of which 12 had an area of less than 0.1 km². According to the Catalog of Glaciers of 1980, as of the glaciation in 1980, 63 glaciers with a total area of 141.62 km² are represented in the Zeravshan Glacier basin. In the conditions of glaciation degradation in the Zeravshan Glacier basin, the number of small glaciers with an area of less than 0.1 km² was 17 in 2021. According to the USSR's catalogs in 1957 and 1980, there were 12 glaciers.

In 2016, the Department of Geocryology of Moscow State University also organized a stationary geocryological site in the area of the Hissar Range (Anzob Pass, Ziddin Valley) [Zheltenkova et al., 2020]. 3 geocryological wells have been drilled in this area with a 3-to-5-meter depth, equipped with thermal sensors. The first two wells are located at an altitude of 3,372 m above sea level, and the third well was drilled later in 2019 in the Ziddin Valley (abs. height 2,000 m). With the help of wells, routine observations of the temperature state of the soil and geophysical and laboratory work were carried out to determine the composition, structure, and properties of frozen soil.

The materials of observation and calculations of the depth of soil freezing at the Anzob Pass (Tajikistan) given in this article are taken partly from the works of Frolov et al., 2021 and Frolov et al., 2022. The methodology given in this article for calculating the effect of snow cover on the depth of soil freezing is taken from Frolov, 2019; Frolov, 2021; Frolov, 2020; Frolov, 2020; Frolov, 2021; and Frolov, 2020. Thus, according to these thermometric observations, seasonal freezing of the soil in this area on the slopes of the northern exposure has been observed since mid-October and continues until the end of April.

At around 1.5 m, the temperature of the rocks already changes from negative to positive at the end of May, and by the beginning of June, the rocks are completely thawed. Taking into account the snow cover, rock composition, and humidity, as well as other factors affecting the freezing depth, the freezing depth was simulated according to the developed calculation scheme. Calculations of changes in the depth of soil freezing were carried out according to the proposed calculation scheme based on data on the thickness of snow cover and air temperature based on a three-layer model of the medium (thawed soil, frozen soil, snow) and assuming a linear change in temperature in the media and heat flow according to Fourier's law.

The design scheme was based on the problem of thermal conductivity of a three-layer medium (snow, frozen, and thawed soil) with a phase transition at the boundary of frozen and thawed soil. The heat balance equation included the energy of the phase transition, the inflow of heat from the thawed soil and outflow into the frozen soil and, in the presence of snow cover, into the atmosphere through it. The heat flow was calculated according to Fourier's law as the product of thermal conductivity and temperature gradient. It was assumed that the temperature in each medium varies linearly (for example, [DeGaetano, 2001]). For snow cover and frozen ground, the formula of thermal conductivity of a two-layer medium was used. The work also involved literary data ([Usupaev Sh.E. et al., 2019], [Khmelevskoy V.K. et al., 2010], [Shatravin V.I., 2007], [Yurkevich N.V. et al., 2020], [Brown J., 1994], [Marchenko S., 2001] and [Williams M.W. et al., 2006]).

Results and conclusions

In this work, based on the developed calculation scheme, the depth of soil freezing is estimated for the last few winter seasons based on data on the thickness of the snow cover and air temperature for the Anzob Pass (Tajikistan).

The Aznob Pass (Tajikistan) is located at latitude 39.07 and longitude 68.88, with an altitude of 3,373 m above sea level. The average annual temperature is -2.7 °C, but due to heavy snow accumulation, there is no long-term freezing—only seasonal is observed. Computational modeling showed the presence of seasonal frozen rocks on the slope of the northern exposure at a depth of up to 1.5 m. Thus, in winter 2018, on the slopes of the northern exposure, the depth of seasonal soil freezing was 1.5 meters (Fig. 1). In winter 2020, on the slopes of the northern exposure, the depth of seasonal soil freezing was 1.2 meters at an average annual soil temperature of 2.42 °C (Fig. 2).

Drawing. 1. Changes in air temperature, snow cover thickness, and ground freezing depth according to calculations and observations for the Anzob Pass weather station for the winter period 2017/18.Drawing.

2. Changes in air temperature, snow cover thickness, and ground freezing depth according to calculations and observations for the Anzob Pass weather station for the winter period 2019/20. The geophysical studies carried out at the Anzob pass made it possible to correct the geoelectric section, on which, according to the difference in electrical resistance, loams with a thickness of up to 3 m, a zone of coarse-grained rocks with sandy aggregate, a zone of rocks and a zone of faults are well distinguished.

As a result of annual studies, maps and schemes of the distribution of frozen rocks of the Hissar Ridge were constructed (Fig. 3 and Fig. 4). An approach was used to compile the maps, which takes into account differences in the altitude position of the MMP distribution boundaries for macrosclines with the most pronounced differences in the geothermal regime. These are the macrosclines of the northern and southern exposures. The soil samples collected at the Anzob Pass under artificial freezing in laboratory conditions are characterized by a massive cryogenic texture. The distribution of ice over the entire volume of the soil is observed in the form of cement. The formation of ice slots is observed only with an increase in humidity up to 30%.

Figure 3. Map of the cryolithozone in the upper reaches of the Zeravshan River (Matcha River). Figure 4 Map of the cryolithozone in the upper reaches of the Varzob River (Zideh River).

The applied calculation method is physically well justified.

The solution, according to the method, describes the process of changing the depth of freezing during the winter season, which is important for the successful operation of the method and is the most accurate possible setting of the initial data.

According to calculations, the ground under the snow cover remains frozen on the Anzob Pass from December to April. The power of the accumulated snow cover can reach one and a half meters or more. At the same time, the soil under the snow-covered surface freezes, according to calculations, by an average of 1.5 m. Thus, the proposed method for calculating the dynamics of the depth of soil freezing based on data on air temperature and snow cover thickness allows us to assess soil freezing as a factor of soil stability during the construction of village and avalanche protection structures. Thus, the Anzob Pass belongs to an area of seasonal freezing of rocks. Considering the gradient of the average annual temperature of the rocks, we can conclude that permafrost rocks on the Hissar Range can be expected at altitudes of more than 4,000 meters.