Abstract: The subject of the study is the cryostructures of Pleistocene sediments uncovered by a 30-m borehole in the Churapcha village and the distribution of stable oxygen and hydrogen isotopes in the schlieren ice and thin ice lenses. The research was located in the area of continuous permafrost distribution, the vertical thickness of which reaches 400–500 m here. The average annual temperature of rocks ranges from -4.6 to -6.2 °C, in river valleys the temperatures are increased to about –2 to –1°C. Permafrost sediments in the Churapcha area are characterized by relatively high iciness, from 26 to 47%. Ice is found in deposits as interlayers that form a layered (as well as lenticular and braided) cryostructure. During the fieldwork, frozen cores opened by a drilling hole at a depth of 30 meters were studied. Drilling was carried out using a large-sized drilling rig (PBU-2) based on a Kamaz truck. A core drilling pipe with a diameter of 146 mm was used. Sampling was carried out from the frozen core. The well-uncovered permafrost deposits, which have mainly dark brown and gray colors, especially bluish-gray shades characteristic of heavy loams and clays. The seasonally active layer has a sandy loam composition, and below, at depths from 1.5 to 6.0 m, the deposits are loamy. At depths of 6.0–20.0 m, alternating layers of loams and clays of different thickness are observed, and below there is a sharp change in the lithological composition to medium and fine sand (20.75–24.0 m), including a large number of layers of organic matter. The distribution of δ18O and δ2H values was examined in structured ice up to a depth of 17.7 meters. In the upper 12 m, the isotopic composition is significantly more negative: δ18O values vary from –29.44 to –34.35‰, while δ2H values range from –213.5 to –253.6‰. In the 12-17.7 m range, δ18O oscillates between –25.94 and –28‰, whereas δ2H ranges from –195.3 to –214.3‰. Analysis of data on the isotopic composition of structure ice in the yedoma deposits of Yakutia showed a close range of values with the ice of unit 1 from the borehole in Churapcha that we studied. The heavier isotopic composition of ice of unit 2 is probably explained by another source of water and the influence of isotopic fractionation during the interaction of clay particles with water. As for the contrast of the isotopic characteristics of unit 1 and unit 2, we note that such a ratio, when the isotopic composition in the upper unit is on average consistently 3-5‰ lower than in the lower unit, is quite rare in permafrost.
The water for the formation of segregation ice from the borehole could have been formed to a greater extent from melted winter snow and was less subject to evaporation. Contrasting values in the ice of the two studied units may indicate different water sources; for the ice in unit 1, the water source could have been, to a greater extent, winter precipitation (surface water formed from melted snow); the presence of a negative peak in the vertical distribution of isotopic values may indicate two-sided freezing (from above and below) and isotopic fractionation during freezing.
The deposits of unit 2 are represented by heavy loams, probably lacustrine. It can be assumed that the saturation of these deposits under these conditions occurred with lake water, which consisted of a mixture of precipitation from both winter and summer seasons and was characterized by higher values of the isotopic composition. Freezing of these loams could have begun already when the lake became shallow. In any case, for now the probable explanation for the heavier isotopic composition of unit 2 is isotopic fractionation in water during interaction with clay particles and a different water source than in unit 1.

Abstract: The study's focus is on the distribution of oxygen and hydrogen isotopes in the ice cores of frost blisters in the Chara River valley, Transbaikalia. The Kodar and Udokan mountain ranges surround the Chara Depression. Cryogenic (frosty) heaving is common in high humidity areas with loamy, clay, and peaty sediments. In summer 2023, the authors described two seasonal frost blisters in swampy, forested depressions on the periphery of the Charsky Sands (in the so-called thermosuffusion funnels). Oxygen and hydrogen isotopic compositions of oxygen  (δ18O) and hydrogen (δ2H) in blister ice were analyzed. It has been established that seasonal frost blisters are formed mainly due to downward freezing of the water-saturated active layer sediments, but in some cases, ice was formed as well due to upward freezing from the lower boundary of the active layer. The vertical distribution of the isotope values, as well as the δ2H-δ18O ratio in ice, indicates a relatively rapid freezing of water in closed system conditions during one cycle of freezing. In the ice layers sampled horizontally, very similar isotopic composition values were obtained (for example, in blister ice 1, δ18О values ranged from –17.51 to –17.32‰), which indicates consistent horizontal freezing of ice layers. The slopes of the δ2H-δ18О ratio lines for blister ice are 5.82 (blister ice 1) and 5.95 (blister ice 2). A decrease of the δ2H and δ18О values of blister ice on frost mounds with depth and a slope of the δ2H-δ18O ratio line less than 8–7.3 indicates water freezing under closed system conditions during one cycle without moisture inflow. The distribution of isotopic values in the blister ice of seasonal frost mound 2 may indicate simultaneous bilateral freezing of the water volume.