The paper presents research findings of the climatic factors responsible for the activation of thermal denudation processes leading to the formation of thermocirques in the central part of the Yamal Peninsula in the north of West Siberia. Based on a comparison of multi-temporal remote sensing (2010, 2013 and 2018) coupled with climate data, an analysis of the thermocirques activity in 2010–2018 was carried out. Thermocirques are specific topographic forms that arise because of the activation of cryogenic earth flows, the formation of which is caused by an increased seasonal thawing of the upper part of the ground ice on the slopes as compared to the previous years. Analysis of the monitoring data showed that in 2012 and 2013 all the types of surfaces are characterized by a significant increase in seasonal thawing compared to the previous period 1993–2011 (12 % increase on slopes), due to the summer air temperature, the amount of summer precipitation and the increased duration of the warm period. The results of the thermocirques activity analysis are shown on a map. The results of this local study were compared with those of a regional (Yamal and Gydan Peninsulas) remote sensing study of thermocirques. This showed a higher accuracy of the local study based on field monitoring and very-high-resolution satellite imagery. The analysis of the thermocirques activity showed that 1) over the period from 2010 to 2018 thermal denudation activity in the study area increased due to the anomalous climatic conditions in the spring-summer season of 2012, and then gradually decreased; 2) in addition to “classical” thermocirques, which were at different stages of activity in the time periods analyzed (2010, 2013 and 2018), a significant number of embryonic thermocirques were identified. Such thermocirques, just as the majority of small “classical” thermocirques, can only be identified on very-high-resolution satellite imagery.

Т. In recent decades, the distribution and activation of thermodenudation, which leads to the formation of specific landforms — thermocirques (also referred to as retrogressive thaw slumps, RTS), have been intensively studied. In different regions of the cryolithozone and at different time intervals, both activation and stabilization of thermocirques are observed. As a rule, studies focus on the climatic controls of the phenomena observed, the environmental controls are discussed less often. This study presents an analysis of the dynamics of thermocirques in relation to the relief features in a specific key area of Central Yamal. To achieve this aim, the spatial distribution of thermocirques at different geomorphological levels is considered based on multi-temporal remote sensing data. Satellite images obtained in 2009, 2018, and 2023, as well as a global digital elevation model (ArcticDEM), were used. We outlined five geomorphic levels and determined their parameters: area, altitude, steepness, and the aspect of the slopes. Thermocirques were identified in the images and their parameters were measured. The dynamics of the thermocirques were analyzed by their number, area, length, width, slope aspect and angle for the periods 2009–2018 and 2018–2023, and for 14 years in total, separately for each geomorphic level. It was found that thermocirques predominate on the slopes of the III alluvial-marine plain, 5–12° steep. In 14 years, the total area of thermocirques increased by 296 %, and their number — by 61 %. A larger increase in the total area and number of thermocirques occurred during the period 2009–2018 in response to climate extremes in 2012 and 2016. Thermocirques that face west cover a higher total area, partly due to the predominance of such slopes over the area of the key site. In all the years of observation, the average areas and lengths of thermocirques are maximum on south-facing slopes. Some of the results are close to those obtained in other regions of Russia and in North America. In many of the areas studied, the increase in the total area of thermocirques exceeded the increase in their number, which means that the expansion of the existing forms prevails over the inception of the new ones. The discrepancies observed in different studies in the results of assessing the effect of relief on thermocirques are due to both the regional features and differences in satellite imagery and methods of its processing.

Строение верхней части криолитозоны и рельеф северо-востока Западной Сибири обусловлены изменением природной обстановки в позднем неоплейстоцене и голоцене. В 2016–2021 гг. изучены разрезы отложений на разных элементах рельефа III озерно-аллювиальной равнины Пур-Тазовского междуречья – увалах и термокарстово-эрозионных ложбинах. В строении верхней части равнины выделены каргинско-сартанские аллювиальные, озерные, склоновые отложения и голоценовые торфяники. На основе стратиграфии и новых геохронологических данных на Пур-Тазовском междуречье выявлены последствия активизации неотектонических процессов в сартанский период и влияния климатических факторов на дифференциацию аккумулятивных и денудационных процессов в голоцене. The structure of the upper part of permafrost and the topographic features in the northeast of Western Siberia were shaped by changes in the natural environment in the Late Pleistocene and Holocene. In 2016–2021, sections of different landforms – ridges and thermokarst-erosional hollows – were studied within the third lacustrine-alluvial plain of the Pur–Taz interfluve. The upper part of the plain includes the Kargin–Sartan alluvial, lacustrine, and slope sediments and Holocene peatlands. Based on the stratigraphy and new geochronological data on the Pur–Taz interfluve, the consequences of the activation of neotectonic processes in the Sartan period and the influence of climatic factors on the differentiation of accumulative and denudation processes in the Holocene were identified.

On Yamal Peninsula massive ice is widespread cryogenic object, lying close to the surface in different sediments, often in a combination with ice wedges. This study is dedicated to tabular massive ground ice exposed in the outcrop of the thermocirque of the third sea terrace in the Central Yamal Peninsula to establish the mechanism of ice formation and the source of water based on the isotopic signature of ice. The tabular massive ground ice body was opened with visual thickness of 2.5 m and a length of 300 m. It had formed from 43 to 37 cal ka BP according to C-14 AMS-data. A white ice body was found under the visible lower boundary of the studied tabular massive ground ice. Tabular massive ground ice yielded values of delta(18)? from -19.9 to -23.1 parts per thousand and delta H-2 from -151.8 to -164.7 parts per thousand, with variations in deuterium excess from 6.5 to 20.4 parts per thousand (average d(exc) = 13 parts per thousand). The delta(18)? and delta H-2 values of white ice vary from -17.5 to -22.5 parts per thousand, and from -132.6 to -165.3 parts per thousand respectively. The delta H-2-delta(18)? relationship for white ice indicating the injection of water and its freezing in a closed system condition. The study shows that the tabular massive ground ice is an intra-ground ice formed under sub-aquatic conditions from lake water. During syngenetic accumulation of segregated massive ice unstable freezing took place, widespread feature of cold regions, which led to intrusion of water under pressure and form injection ice.

The thawing of polygonal ice wedges determines the dynamics of polygonal peatland relief. The polygonal peat plateaus in the Pur-Taz interfluve account for an average of 6,5 % of the total area. The purpose of the proposed study is to establish the short-term rates and direction of change in the plateaus’ relief under the combined action of technogenic and natural factors, using monitoring data for the period 2005–2022. Based on satellite images and orthophotoplans, elements of the peat plateaus have been outlined and their areas have been determined for different time slices. The studies were carried out on a peat plateau immediately adjacent to the highway (T1) and on a background peat plateau at a distance of about 1.5 km from the highway (T2). To identify the natural causes of changes in the relief of the peat plateaus, the influence of climatic parameters is considered. The rates of relief change in natural conditions and under the impact of the highway are also compared. It has been established that in the area to the north-west of the highway the relief of the T1 polygonal peat plateau has stabilized. In the south-east section of T1, degradation has sharply increased after the construction of the highway. Due to the degradation of the polygons, the polygonal troughs expanded. In the background peat plateau T2, the rate of relief degradation is somewhat higher than in the northwestern portion of T1. Comparison of the main climatic parameters and degradation rates of the polygonal relief did not show any clear correlations. Probably, of greater importance are the regime of atmospheric precipitation, the redistribution of surface runoff and recurrence of flooding and drainage of the polygonal troughs, determined by the rhythmic course of the relief degradation.

The upper part of the subsurface component of the Earth’s cryosphere—polygonal peatlands—are considered in this paper. The polygonal peatlands are widespread in the cryolithozone in northwestern Siberia. When the upper permafrost layer thaws under the influence of climatic fluctuations, the ice-bearing transition and intermediate layers in the polygonal peatlands serve as a buffer zone. Therefore, the identification of these layers is a crucial task. The specific features of the layers, their relationship with ice of various origins, and the properties of peat have been determined. The differences in the structure of the intermediate layer, associated with polygonal microrelief of the surface, have been revealed. Thawing of the transitional layer against the background of modern warming has been recorded on the basis of monitoring of a seasonally thawed layer. To identify the transitional and intermediate layers, the indicators—the cryogenic structure, degree of preservation, color, water content, and density of peat—have been determined.

Summary Climate changes occurring in the Arctic regions cause the transformation of permafrost conditions, including thawing of permafrost. In the last decade, cryogenic processes have been intensifying in the Arctic, including the southern tundra of Western Siberia. In the peat bogs of the khasyreys of the Pur-Taz interfluve, thermokarst processes along polygonal-wedge ice and thermal erosion are especially widespread. However, there is a significant lack of research related to the impact of transport infrastructure on polygonal peatlands. The Ground Penetrating Radar (GPR) method has shown itself to be effective in solving geocryological problems associated with areas of development of peatlands. This work is devoted to the use of GPR for assessing the warming effect of a road in a complex of geocryological works. All profiles crossed the road, the main emphasis was on the warming effect of the road and roadside depressions on the permafrost roof. The anthropogenic impact on the permafrost roof is clearly reflected in its position on the ground penetrating radar data.

Пластовые льды, вскрытые в обнажении двух соседних термоцирков на междуречье рек Сеяха и Мордыяха, стационар «Васькины Дачи» были опробованы летом 2019 г. Пластовые льды, представленные горизонтально-слоистым ледогрунтом, залегают на гл. 1 м от поверхности. В термоцирке на выс. 28 м над ур. м пластовые льды, мощностью 1 м, вмещают повторно-жильные льды, имеют выраженный изгиб вверх у контакта с ледяными жилами и дугообразное выгибание вниз между жилами, что повторяет конфигурацию подошвы СТС в полигональных ваннах. В термоцирке на выс. 17 м над ур.м пластовые льды представляют собой сегрегационную залежь протяженностью 300 м и высотой 2,5 м. Значения O и H льда жильного льда изменяются от -21,2 до -26,2‰ и от -157,5 до -203,1‰ соответственно. Значения O и H пластового льда изменяются от -19,9 до -23,1‰ и от -151,8 до -164,7‰, соответственно. Пластовый лед является сегрегационным сингенетическим и был сформирован в переувлажнённых озёрных отложениях в условиях мелкого озера.

New data on the distribution of the isotopes δ18О and δD of ice wedges in Central Yamal are presented. In the summer of 2019, the massive wedge ice was studied in the thermal circus of the third marine terrace. On the surface of the terrace, polygonal-vein relief is common with a polygon size of about 10–20 m. The largest ice wedge № 1 in the upper part was 1.5 m wide, and its visible depth in the outcrop amounted to 2.3 m. Perpendicular to this wedge, another one № 2 was opened with a width of 60 cm, and 20 m from them the wedge № 3 was opened along the strike. At the level of the head of the wedge № 1, radiocarbon AMS dating was performed based on a sample of host deposits, which showed that the ice in the upper part was about 13 thousand years old (the ice age was of 13.6 cal. BP). According to the isotopic characteristics (average values of δ18О = −24.8, and δD = −187.6 ‰), the ice corresponds to the Yamal ice-wedge ice, which was formed during the MIS‑2 isotopic stage (the end of the Late Pleistocene). The isotopic composition of oxygen indicates temperatures of the cold period of the vein formation from −22.8 to −26.8 °C (on average 7 °C colder than the present-day) and the January temperatures from −34.2 to −40.2 °C (on average 10 °C colder than the present-day). For some ice samples (11 out of 43), high values of deuterium excesses (dexc = 12÷17 ‰) were observed, which is unusual for this type of ice. A few examples of high dexc values of ice wedges are mentioned in the literature for sections of Seyakha yedoma, Cape Sabler and Bolshoi Lyakhovsky Island, and note that for the ice dated to the Late Pleistocene. The deuterium excess values may reflect the unstable climatic conditions of the Late Pleistocene associated with changes in the vapor source and the trajectories of the air masses over the continent.

В 2021 г. продолжен мониторинг глубины сезонного протаивания, температуры многолетнемерзлых пород, активности криогенных процессов, связанных с вытаиванием подземных льдов, и изменений, происходящих с полигональными торфяниками на полуострове Ямал, севере Пур-Тазовского междуречья и Гыданского полуострова. Представлены основные результаты исследований.

Построена теоретическая модель влияния климатических изменений и техногенного воздействия на прибрежную полосу озер и рельефа полигональных торфяников на севере Пур-Тазовского междуречья. Выделены 3 участка, на которых наблюдаются максимальные изменения полигональных торфяников. Выявлены статистические данные по степени трансформации торфяников и проводится мониторинг на этих участках.

Comparative study of methanogen diversity and potential activity of different methanogenic pathways in the sediments of young thermokarst and mature polygenetic Yamal lakes was carried out. The hydrogenotrophic pathway of methanogenesis played an important role in methane formation in thermokarst lakes. The acetoclastic and methylotrophic pathways were also revealed there. In a polygenetic lake with a dissolved organic matter content closest to that of the thermokarst lakes, methanogenesis proceeded more intensively, and the relative abundance of methanogens, especially acetoclastic ones, was higher than in thermokarst lakes. The activity of methyl-reducing methanogens was also assumed there. Methanogens of the genera Methanothrix and Methanoregula, as well as representatives of the family Methanomassiliicoccaceae were identified in the sediments of all lakes. Methane-oxidizing bacteria (Methylobacter, Candidatus "Methylomirabilis") and archaea (Ca. "Methanoperedens") were also detected.

Remote sensing methods make it possible to evaluate the reaction of the cryolithozone and tundra landscapes in hard-to-reach Arctic areas based on the indicators of modern climate changes. In 2016–2019, numerous organic frost boils were discovered on the peatland surfaces of drained lakes in the southern tundra of the Pur-Taz interfluve in the northern part of West Siberia and studied. It is established that the formation of frost boils caused by organic mass injections occurs in the summertime and that they indicate local deepening of seasonal thawing. Frost boils emerging at peatlands in summertime was identified after analysis of UAV survey in 2019. Local deepening of seasonal thaw layer on polygonal peatlands is evidence of permafrost reaction to global warming.

Various climatic parameters (average annual, winter, and summer air temperature, winter and summer total precipitation) and their temporal variability are analyzed to explain the activation of cryogenic processes in Central Yamal. The effect of fluctuations of these parameters on the thaw depth and ground temperature is analyzed. The regression analysis is carried out to reconstruct ground temperature during earlier periods of time in the years preceding the activation of thermal denudation and the formation of gas emission craters.