Cryostructures and stable oxygen and hydrogen isotopes in the Pleistocene sediments discovered by a deep borehole in the Churapcha village (Lena-Aldan interfluve, Russia)

Vasil'chuk Yurii Kirillovich ORCID: 0000-0001-5847-5568 Doctor of Geology and Mineralogy Professor, Lomonosov Moscow State University, Faculty of Geography, Department of Landscape Geochemistry and Soil Geography 119991, Russia, Moscow, Leninskie Gory str., 1, office 2009 vasilch_geo@mail.ru Other publications by this author     Ginzburg Alexander Pavlovich Junior Researcher, Department of Landscape Geochemistry and Soil Geography, Faculty of Geography, Lomonosov Moscow State University and Scientific Center for Arctic Studies 119991, Russia, Moscow, Leninskie Gory str., 1, office 2007 alexandrginzburg13154@yandex.ru Other publications by this author     Sisolyatin Robert Gamletovich ORCID: 0000-0003-2633-1919 PhD in Geology and Mineralogy Head of the permafrost station; North-Eastern Permafrost Research Station of the Institute of Permafrost Science SB RAS 685000, Russia, Magadan region, Magadan, Portovaya str., 16 robertseesaw@gmail.com Tokarev Igor' Vladimirovich ORCID: 0000-0003-1095-0731 PhD in Geology and Mineralogy Leading Specialist, Resource Center "X-ray Diffraction Research Methods" of the Science Park of St. Petersburg State University 199034, Russia, Saint Petersburg, Universitetskaya str., 7 tokarevigor@gmail.com Other publications by this author     Korolyova Ekaterina Sergeevna PhD in Geology and Mineralogy Senior Researcher; GAU YANAO 'Scientific Center for Arctic Studies' 629007, Russia, Yamalo-Nenets Autonomous Okrug, Salekhard, Republic str., 20, office 306 koroleva_katy@inbox.ru Palamarchuk Valentina Anatol'evna Junior Researcher, Scientific Center for Arctic Studies and Institute of the Earth's Cryosphere of the Tyumen Scientific Center SB RAS 629007, Russia, Salekhard, Republic str., 20 VAPalamarchuk@yanao.ru Other publications by this author     Budantseva Nadine Arkad'evna ORCID: 0000-0003-4292-5709 PhD in Geography Senior Researcher, Department of Landscape Geochemistry and Soil Geography, Faculty of Geography, Lomonosov Moscow State University 119991, Russia, Moscow, Leninskie Gory str., 1, office 2007 nadin.budanceva@mail.ru Other publications by this author

Vasil'chuk Alla Constantinovna ORCID: 0000-0003-1921-030X Doctor of Geography Leading Scientific Associate, Lomonosov Moscow State University, Faculty of Geography, Laboratory of Geoecology of the Northern Territories 119991, Russia, Moscow, Leninskie Gory str., 1, office 10 alla-vasilch@yandex.ru Other publications by this author

Introduction

In the study of the cryolithozone of the Siberian Platform, extensive material has been accumulated on permafrost rocks (MMP), which allows us to characterize their current thermodynamic state, find quantitative dependences of temperature and MMP power on a complex of natural factors and perform paleoreconstruction of MMP dynamics and reliable prediction of geocryological conditions during the development of the territory.^[1] This is especially important in connection with the active development of mineral deposits of the Siberian platform concentrated mainly in the MMP development zone: iron ores (Tarynnakh, Taiga, Sulumat), coal (Apsat, Elge, Ungra), copper (Udokan, Charuoda, Chin), rare metals (Katugino), titaniummagnetite ores (Chin), semi-precious stones (Murun), apatites (Ukduska, Seligdar, Kabakhanyr)^[1].

A series of structural depressions of the Aldan shield, composed of rocks of Mesozoic age and extending from the Olekma River in the west to the Idyum River in the east, is known by the common name – Mesozoic depressions. Coal deposits on the territory of the Chulman and Tokinskaya depressions form the basis of the unique South Yakut coal basin (high-quality coking coals account for more than 80% of its reserves, proven reserves of more than 4 billion tons), the Tokarikan, Guvilgrinskaya and Ytymdzhinskaya depressions are now considered as potentially suitable for development.^[2]

The geocryological study of the Mesozoic depressions is characterized by extreme heterogeneity. The geocryological conditions of the Chulman and Tokinsky depressions described in detail by numerous researchers contrast sharply in terms of study with the Tokarikan, Guvilgrinsky and Ytymdzhinsky depressions, for which only isolated data of geocryological conditions are available.^[2]

The project of organizing sites for monitoring the temperature of permafrost rocks at weather stations along the Kolyma federal highway in the territory of the Republic of Sakha (Yakutia) is called the Kolyma Permafrost transect. This is a network of seven wells with a depth of 30 meters (Churapcha, Ytyk-Kyuel, Teply Klyuch, Vostochnaya, Agayakan, Tomtor and Delankir) located near long-term monitoring sites for meteorological parameters, which is created to determine the reaction of the temperature regime of frozen rocks to changes in climatic factors.^[2]

One of the wells in the area of the village. Churapcha, drilled with core sampling, including ice ice slots for isotope analysis. The purpose of this article is to study the distribution of the isotopic composition of oxygen and hydrogen in the texture–forming ice of the Churapchinsky well to a depth of 17.7 m and to identify the main differences in the formation of thicknesses of different isotopic compositions.

Physical and geographical conditions of the research area

Geographical location

The site where the well was laid is located in the central part of the Leno-Aldan interfluve in Central Yakutia on the northwestern outskirts of the village. Churapcha (Fig. 1). The laid-down well has coordinates 62°0’26.08” s.w., 132°24’27.98” v.d.

Fig. 1. Cartographic diagram of the key research area: 1 – the location of the well; 2 – the boundaries of the site of the Churapcha weather station; 3 – lake-alasny basins; 4 – the spread of thermokarst. Compiled on the basis of a Google satellite image (Maxar Technologies), the date of shooting is 07/17/2022.

The geological structure of the territory

The northern part of the Leno-Aldan interfluve, where Churapcha is located, is composed of a complex of polygenetic quaternary deposits of various compositions. Thus, to the West and Northwest of Churapchi, in the valley of the Lena River, the foundation of the territory is represented by Jurassic sedimentary rocks overlain by the Middle and Upper Pleistocene alluvium of the Lena and Great Aldan. To the east of Churapchi, in the area of the village of Ytyk-Kuel, several outcrops of Cretaceous and Neogene sedimentary strata are observed, overlain by Lacustrine-alluvial Pliocene-Lower Pleistocene sands, silts and sandy loams, often enriched with organogenic material.^[3]. Pleistocene permafrost deposits, including the ice complex (edoma) and lake deposits, including numerous fragments of underground ice, are widespread directly in the Churapchi area.^[4] (fig. 2).

Fig. 2. Exposure of underground ice in the shore of thermokarst lake near the village of Chakyr (Churapchinsky district). Photo by V.V. Ogonerov

Relief and hydrographic network

The territory is a hilly-westerly plain with absolute heights of 300-400 m, rarely crossed by valleys of small and medium-sized rivers, as well as dotted with thermokarst lakes of various sizes.^[4-6]. Active melting of the MMP plays an important role in the redistribution of surface water runoff, contributing to the formation of new drainage basins and temporary watercourses.

Permafrost rocks and cryogenic processes

The research area is located in the area of continuous distribution of MMPs, the vertical capacity of which reaches 400-500 m here. Under the beds of large rivers (Lena, Aldan, Amga), through taliks are developed here.^[7]. In the Leno-Aldan interfluve, the average annual temperature of rocks ranges from -4.6 – -6.2 ° C, in river valleys they are increased to about -2 – -1 ° C. At the same time, there are quite strong deviations from the average latitude values.

Fig. 3. Zoning of the territory of Central Yakutia according to the average annual MMP temperatures at depths of zero seasonal fluctuations (a) ^[8] and vertical temperature distributions of MMP in the Leno-Aldan interfluve (b): III terrace (yellow) and low and high floodplains (green). From ^[7]

The MMPs in the Churapchi area are characterized by a relatively high ice content from 0.26 – 0.47 cu. The ice is located in rocks in the form of interlayers and forms a layered cryotexture. The increased iciness of the upper layer of frozen rocks contributes to the activation of thermokarst processes in the summer, and sufficiently deep thawing, on average about 1.5 – 2.0 m per year, leads to thawing of vein ice included in edom deposits. The average depth of the ice vein heads here is 1.7 m in the river valleys and 2.1 m in the interfluves.^[8,9,10] The proportion of the area affected by thermokarst in the vicinity of Churapchi by thermokarst reaches 70%^[11] as a result, most of the territory suitable for agriculture is currently being degraded.^[10] Anthropogenic decreases in MMP temperatures and anthropogenic-provoked activation of thermokarst processes were recorded within the village itself.^[12]

Climate features

The climate of Central Yakutia is sharply continental with long and very cold winters and short hot summers. The average annual air temperature in the village of Churapcha is -8 – -9 °C, since 1933 it has increased slightly, by about 1-1.5 °C. The average January temperature drops to about -40 – -45 °C, the average July temperature rises to +18 – +21 °C. Over the 90-year period of meteorological observations, these temperatures also increased by no more than 1.5 °C. During the year, the largest amount of precipitation falls in July, often their amount reaches 100 mm/month, and in winter their amount decreases by about 10 times (Fig. 4, b). Positive air temperatures in Churapcha are set in the middle to the end of April, and negative temperatures are set at the end of September (Fig. 4, a). The height of the snow cover is relatively low, not exceeding 0.5–0.7 m, as a result of which a sufficiently deep freezing of rocks occurs in winter, and large temperature amplitudes (about 80 ° C) provide strong frost cracking.

Fig. 4. Graphs of the course of the average monthly (a) and average annual temperatures (b) at the Churapcha weather station: 1 – average January; 2 – average July; 3 – average annual. 4 – the average annual precipitation. According to ^[13]

Soils and vegetation cover

The zonal vegetation in the area of the Leno-Aldan interfluve is represented by larch forests (Larix sibirica) with an admixture of spruce (Picea obovata) and pine (Pinus sibirica) cranberry-green mosses.^[3] Forests are distributed mainly in uplands that are not involved in agriculture. In the lowlands, areas of steppes and meadow steppes, as well as alasian basins, are more common.^[14] These areas are actively being plowed.^[10]

Accordingly, the zonal types of soils (according to the classification and diagnosis of soils of the USSR ^[15]) are permafrost taiga fawn (including rejuvenated and podzolized) in complexes with alasic soils.^[3] Calcic Solonetz (Calcic Solonetz), including arable, as well as malt, including glazed (Protocalcic Gleysols), are found in steppe areas.^[16]

Research methods

During the field work, the authors investigated the core of permafrost deposits, including, probably, the ice complex (edoma), opened by a deep drilling well. The well has a depth of 30 m and is equipped to monitor the geothermal state of permafrost. This well was included in the geocryological monitoring network in Yakutia. 11 samples of frozen rocks and underground ice were taken from the well, and cryolithological descriptions of frozen rock cores were carried out in the field. The isotopic composition of oxygen and hydrogen was studied in the segregated schlier ice exposed by the well.

Well drilling and sampling

A deep 30-meter drilling well was laid on the outskirts of the village. Churapcha is in the immediate vicinity of the weather station of the same name. Drilling was performed vertically 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. Purging and flushing were not used during drilling. Sampling was carried out from frozen core, the weight of each sample reached 500 g. The samples were packed in two layers of plastic bags with a zip-lock closure.

Field descriptions of cores

The lithological composition and cryotextures were described in the drilled cores in the field. The lithological description included color, structure, plasticity, soil and ice inclusions, and characteristics of cryotextures.

Laboratory tests

Pretreatment of the sample included its complete thawing at room temperature without access to sunlight and transfusion into plastic matte vials with a volume of 30 ml with an airtight lid. The lid was additionally sealed with a cellophane gasket to avoid contact of the gas in the sample with the surrounding air.

Measurements of the isotopic composition of oxygen and hydrogen of ice were performed using a Picarro L 2130-i laser infrared spectrometer at the X-ray Diffraction Research Methods Resource Center at the Scientific Park of St. Petersburg State University. The following international standards are used: V-SMOW-2, GISP, SLAP, USGS-45 and USGS-46. The error of determination was: for δ18 ^O – ±0.02 %, for δ2 H – ± 0.3 %. In total, 11 samples of textured ice were analyzed.

Results

Lithological column

A well in the area of the village. Churapcha deposits of the ice complex were uncovered, having mainly dark brown and gray coloration, especially bluish-gray shades characteristic of heavy loams and clays. The seasonally shallow layer, including the soil profile, has a sandy loam composition, and below, at depths from 1.5 to 6.0 m, the deposits are loamy (Fig. 5, A). 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. Presumably, this is due to previous epochs of soil formation or peat accumulation. The lower part of the section is represented by medium and heavy loam.

Fig. 5. Lithological column (A), inclusions and neoplasms (B) and cryolithological features (C) of the deposits of the ice complex opened by a deep well in the area of the settlement. Churapcha: 1 – sand, 2 – sandy loam, 3 – light loam, 4 – medium loam, 5 – heavy loam, 6 – light clay, 7 – medium clay, 8 – plant root system, 9 – iron-manganese nodules and primers, 10 – layers of organic matter, 11 – cryoturbation, 12 – the sole of the seasonally thin layer, 13 – ice interlayers, 14 – massive cryotexture, 15 – large–scale cryotexture, 16 – small-scale cryotexture, 17 - ice sampling points for isotope analysis.

Cryolithological structure of the section

The thickness of the seasonally shallow layer according to the results of drilling the well is 1.5 m, below in sandy loam-loamy layers of low-ice deposits there are massive cryotextures. Below, they are replaced by small-scale and then large-scale cryotextures, confined to heavier loamy and clay deposits. Visually, the iciness of loams and clays reaches 50-60%. Also, loamy and clay layers include several high–ice interlayers, as well as one layer of almost pure ice at depths of 14.87-14.95 m (Fig. 5, B, C). In sandy sediments, high ice content is observed in the interlayers of organogenic material. In the lower part of the section, the vertical layering is disrupted by numerous cryoturbations.

A well with a depth of 30 m uncovered permafrost deposits, which have mainly dark brown and gray colors, especially bluish-gray shades characteristic of heavy loams and clays. The seasonally shallow 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.

Distribution of δ18 ^O and δ2 ^{H values} over the well section

The distribution of δ18 ^O and δ2 H values was studied in textured ice up to a depth of 17.7 m. In the upper 12 m (pack 1), the isotopic composition is noticeably more negative: the values of δ18 ^O vary from -29.44 to -34.35, and the values of δ2 H range from -213.5 to -253.6. In the range of 12-17.7 m (pack ²), the values of δ18 ^O vary from -25.94 to -28, and the values of δ2 H range from -195.3 to -214.3 (Table 1, Fig. 6).

Table 1. Changes in the values of δ18 ^O, δ2 H and d _exc in textured ice from a deep well section in the area of the settlement. Churapcha

In the upper 12 m (pack 1), the average value of δ18 ^O is -30.98%, and the average value of δ2 H is -228.5%, the average value of d _exc is 19.3%. In the range of 12-17.7 m (pack ²), the average value of δ18 ^O is -26.86%, and the average value of δ2 H is -205.7%, the average value of d _exc is 9.2% (Table 2).

Table 2^. Minimum (min.), maximum (max.) and average (average) values of δ18 ^O, δ2 H and d _exc in the textural ice of two isotopically contrasting bundles of sediments along the section of a deep well in the area of the settlement. Churapcha

.

Fig. 6. Lithological column (A), cryolithological structure (B) and distribution of δ18 ^O (1) and δ2 H (²) values by section, as well as neoplasms and permafrost deposits (C) opened by a deep well in the area of the settlement. Churapcha. For the remaining symbols, see Fig. 5

Discussion

Permafrost rocks in the area of the village. Churapcha are characterized by a variety of morphological manifestations. One of the most notable is the polygonal flat mounds formed during the erosive processing of polygonal–vein structures on the territory of the village and the former airfield (Fig. 7).

In the summer of 2023, the core of the Bulgunnyakh was eroded in Central Yakutia near the village of Tolon (Central Yakutia), not far from the village. Churapcha (Fig. 8). Bulgunnih is located in the central part of the Alas and has an elongated shape in the form of an "eight" oriented to the northwest. The highest and widest part of Bulgunnyakh was destroyed with the formation of a lake with a diameter of about 35 m. The height of the bulgunnyakh is 13 m, the length in the north direction is 130 m, in the wide destroyed part – 76 m.^[17] The ice of the Bulgunnyakh core is predominantly pure. The color of the ice is whitish or bluish. Under the sole of the cover layer at a height of 6 m relative to the water level of the lake, the core ice is transparent.

Fig. 7. A flat-polygonal relief formed during the thermal erosion processing of re-core ice in the area of the village. Churapcha. Photos from the website https://theplanetpress.blogspot.com/

Fig. 8. Opening of the core of bulgunnyakh in Central Yakutia near the village of Tolon: a – the appearance of bulgunnyakh, b – a crater with a lake in the center of the hillock, c – ice on the edges of the crater, d – contact of different types of ice. Photos from the website https://fishki.net/4473732-v-jakutii-iz-za-jerozii-pochvy-na-poverhnosty-vystupila-vech.html

The ice from the cores was formed from the water of a frozen atmospheric talik, when ice formation most likely occurred in a closed system. The value of δ18 ^O in the Bulgunnyakh ice is on average -17.41%, the value of δ2 H = -149.26%, and 0.9 by d _exc = -9.97%. The sample taken at the boundary between bluish core ice and whitish ice has a lighter isotopic composition: the value of δ18 ^O is -24.07%, the value of δ2 H = -191.73%, and d _exc = 0.9%.^[17] This makes us think of a different source of water for whitish ice.

A comparison of the isotopic composition of the ice of the Bulgunnyakh core and the segregation ice opened by a well near the village of Churapcha showed that the Bulgunnyakh ice is noticeably more isotopically heavy, and is also characterized by much lower values of d _exc (even negative), which shows a difference in the conditions of freezing and ice formation, as well as in water sources: the bulgunnyakh core, most likely, it was formed in conditions of a closed system from the water of a suspicious talik. This water could be a mixture of precipitation from different seasons (snowmelt and rains) and was exposed to evaporation during the hot summer months.

The water for the formation of segregation ice from the well could be formed to a greater extent from melted winter snow and less exposed to evaporation. The contrasting values in the ice of the two studied packs may indicate different sources of water – for ice in pack 1, the water source could be to a greater extent precipitation of the winter season (surface waters formed from melted snow), the presence of a negative peak in the distribution of isotopic values vertically in pack 1 may indicate bilateral freezing (from above and below from below) and about isotopic fractionation in the freezing process.

The sediments of pack 2 are represented by heavy loams, probably lake loams, saturation of these sediments could occur with lake water, which consisted of a mixture of precipitation of both the winter and summer seasons and was characterized by higher values of isotopic composition. The freezing of these loams, most likely, judging by the configuration of the isotope diagram in its lowest part (a steady trend of decreasing ^δ18 O values from -24.8 to -28 and Δ2H values from -200.3 to -214.3 in the depth range from 17.7 ^{to 14} m), occurred from bottom to top and could begin already with shallowing lakes.

We compared our data on the isotopic composition of segregational ice from the well with the data on the isotopic composition of textural and formation ice studied in central and northern Yakutia.

V.B.Spector and co-authors ^[18] studied formation ice in a well drilled in the Leno-Amginsky interfluve, about 200 km west of the village of Churapcha. In the depth ranges of 13.5-14.2 m and 23.3-24.5 m, ice layers were opened by the well. The values of δ18 ^O ranged from -29.2 to -32.3, and the values of δ2 H ranged from -213.5 to -236.9 in the upper ice horizon. In the lower ice horizon, the values of δ18 ^O ranged from -30.3 to -31.7, and the values of δ2 H ranged from -219.9 to -234.7. These values for formation ice are close to the values of the segregation ice from pack 1 studied by us (the average value of δ18 O = -31%, the average value of δ2 H = -228.5%, see Table ²). According to the conclusions of V.B.Spector and co-authors^[18], the formation ice studied by them has a firn origin, i.e.K. in terms of isotopic composition, it is close to modern snow and in terms of the ratio δ ² H-δ ¹⁸ O, it is close to the local line of meteoric waters.

According to the authors^[19] textural ice in the edom thickness of the Zeleny Mys section (lower reaches of the riverKolyma) in the depth range from 11 to 33 m were characterized by values of δ18 ^O from -27 to -30.6%, while lower values of δ18 ^O (from -29.1 to -34.1%) were obtained for ice veins. In the textural ice in the edom thickness of the Plakhinsky Yar in the lower reaches of the Kolyma River, the values of δ18 ^O varied from -22 to -33%, which is also generally lower than in the re-vein ice of this section (the variations of δ18 ^O from -29.9 to -34.7%).^[20]

Textural ice was studied in the edom lake deposits of Central Yakutia in the Yukechi region, in the Leno-Aldan interfluve.^[4] The depth of the well was about 20 m. It is shown that the ice was characterized by rather low values of isotopic composition (values of δ18 ^O ranged from -27 to -31%) and rather high values of d _exc – on average about 15-17%. These values for textural ice are comparable to the values of the isotopic composition of the re-vein ice of the MIS-3 stage studied in other regions of central Yakutia and may indicate the great role of isotopically light winter precipitation in the source water for textural ice in the extremely cold continental climate during MIS-3.^[4]

In the Klondike region, Yukon, Canada, cryostratigraphy was studied in detail in frozen sediments uncovered during mining ^[21,22] The isotopic composition of textural ice was determined in sediments to a depth of about 8 m. The difference in values in two different facies (bundles) is shown. The ice of the lower pack was characterized by lower values of isotopic composition (δ18 ^O from -32 to -29, δ2 H from -234 to -257), which, according to the authors, characterizes typically glacial climatic conditions. The ice in the upper pack is isotopically heavier (δ18 ^O values range from -28 to ^-21, δ2 H values from -164 to -225), which, according to the authors, marks the transition to warmer and wetter conditions at the end of the ice Age before the onset of the Holocene epoch, about 11.6 thousand years ago.^[21]

An experiment to study the isotopic composition of water and ice in dispersed soils has shown that during the interaction of water with soils, its migration and ice formation during freezing, isotope fractionation occurs, depending on the composition of the soil and freezing conditions.^[23] Water extracted from dispersed soils is isotopically heavier than the source water, on average by 2-3% for values of δ18 O. This is due to the more active interaction of soil with lighter isotopes of water, while it was shown that bentonite, due to its greater adsorption capacity, interacts somewhat more actively than kaolinite and powdery loam.

In D.V.Mikhalev's dissertation^[24] it was shown that the isotopic composition of textural ice depends on two factors: the isotopic composition of soil moisture before freezing and isotopic fractionation during freezing.^[24] The slower the freezing occurs, the more the formed ice is enriched with isotopes. During bilateral freezing (from above and below), the most isotopically heavy ice is formed in the upper and lower horizons of the freezing layer, while the average (dried up) horizon is characterized by lower isotopic values of the ice formed. Such a distribution is observed in the ice of the pack 1 studied by us, which allows us to conclude that this pack is bilaterally frozen. According to D.V.Mikhalev^[24], epigenetic freezing of sediments is characterized by a gradual easing of the isotopic composition of textural ice with depth due to isotopic fractionation.

The analysis of data on the isotopic composition of textural ice in edom deposits of various regions of Yakutia showed a close range of values with pack 1 ice from the well we studied in Churapcha. The heavier isotopic composition of Pack 2 ice is probably explained by another water source and the influence of isotopic fractionation during the interaction of clay particles with water. It is also possible that the freezing of loams of pack 2, judging by the configuration of the isotope diagram, occurred from bottom to top and could begin already with the shallowing of the lake.

As for the contrast of the isotopic characteristics of the ice of pack 1 and pack 2, we note that a similar ratio, when the isotopic composition in the upper pack is on average steadily 3-5% lower than in the lower pack, is quite rare in MMP. After reviewing dozens of isotope diagrams for wells in the MMP of northern Russia, Alaska and Northern Canada, we found only isolated examples where such an isotope inversion was recorded. One illustrative example is the isotopic distribution obtained by F.By Michael and P.Fritz^[25] at well 79-8, located near the sea coast of Mackenzie Bay, where to a depth of 2.5-3.0 m the values of δ18 O are -¹⁸− -20%, and below they rise sharply and in the range of 3.5-5.5 m are constantly above -14%.^[25] In this case, the contrast of the isotope ratios of the upper and lower bundles can probably be explained by a change in the facies regime and the predominance of marine sedimentation of the lower bundle.

Drilling of a 30-meter well with core sampling in the village. Churapcha was carried out within the framework of the Kolyma Permafrost Transect project – a project for the organization of sites for monitoring the temperature of permafrost rocks at weather stations along the Kolyma federal highway in the territory of the Republic of Sakha (Yakutia). A network of seven wells with a depth of 30 meters located near the sites of long-term monitoring of meteorological parameters is being created to determine the reaction of the temperature regime of frozen rocks, taking into account the maximum possible number of climatic factors. The creation of the Kolyma permafrost transect involves drilling with core sampling of 30-meter wells at seven weather stations: Churapcha, Ytyk-Kyuel, Teply Klyuch, Vostochnaya, Agayakan, Tomtor (Oymyakon) and Delankir.^[26]

In early August 2021, a well with a depth of 15 m was drilled at the Vostochnaya weather station. The time to stand the well and restore the temperature field after drilling was five days. After that, for three weeks, a decrease in temperature was observed at depths of 3, 5 and 15 m. The results of the control measurement show the temperature distribution of rocks in the annual heat turnover layer at the beginning of September. With a negative temperature of rocks before the winter season at a depth of 3 m and a positive temperature at a depth of 1 m, it can be assumed that the thickness of the seasonal thawing layer of rocks was 1.5-1.6 m. The lowest temperature (about -6.5 °C) is characterized by rocks in the depth range from 10 to 15 m.

The exploration well at the Lazurnoye ore deposit, located 60 km to the southwest, is the nearest site for conducting deep geothermal measurements, which made it possible to determine the thickness of the permafrost in this area (210 m). The value of the intra-terrestrial heat flow set for the region varies between 75-85 MW/^m2. The thickness of the permafrost at the Vostochnaya weather station, assuming a stationary distribution of rock temperature below the annual fluctuations layer, is 310-320 m.^[26]

Further implementation of the Kolyma Permafrost Transect project will make it possible to determine the temperature and thermophysical parameters of frozen rocks in seven different regions of Yakutia and complement the previously obtained ^[27,28] materials on the geocryology of the largest Mesozoic depressions in Yakutia. And studying the isotopic characteristics of textured ice will allow us to assess the nature of freezing, identify undoubtedly syncreogenic strata and separate them from possibly epicryogenic ones.

Conclusion

A well with a depth of 30 m uncovered permafrost deposits, which are mainly dark brown and gray in color, especially bluish-gray shades are characteristic of heavy loams and clays. The seasonally shallow 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 δ18 ^O and δ2 H values was studied in textured ice up to a depth of 17.7 m. In the upper 12 m, the isotopic composition is noticeably more negative: the values of δ18 ^O range from -29.44 to -34.35, and the values of δ2 H range from -213.5 to -253.6. In the range of 12-17.7 m, the values of δ18 ^O vary from -25.94 to -28, and the values of δ2 H range from -195.3 to -214.3.

The analysis of data on the isotopic composition of textural ice in the edom deposits of Yakutia showed a close range of values with pack 1 ice from the well we studied in Churapcha. The heavier isotopic composition of Pack 2 ice is probably explained by another water source and the influence of isotopic fractionation during the interaction of clay particles with water.

As for the contrast of the isotopic characteristics of pack 1 and pack 2, we note that a similar ratio, when the isotopic composition in the upper pack is on average steadily 3-5% lower than in the lower pack, is quite rare in Late Pleistocene MMPs.

The water for the formation of segregation ice from the well could be formed to a greater extent from melted winter snow and less exposed to evaporation. The contrasting values in the ice of the two studied packs may indicate different water sources – for ice in pack 1, the water source could be to a greater extent precipitation of the winter season (surface waters formed from melted snow), the presence of a negative peak in the distribution of isotopic values vertically may indicate bilateral freezing (top and bottom) and isotopic fractionation during freezing.

The sediments of pack 2 are represented by heavy loams, probably lacustrine. It can be assumed that the saturation of these sediments occurred with lake water, which consisted of a mixture of precipitation from both the winter and summer seasons and was characterized by higher values of isotopic composition.

In any case, so far the probable explanation for the heavier isotopic composition of pack 2 is isotopic fractionation in water when interacting with clay particles and, possibly, a different water source than in pack 1. Freezing of loams of pack 2, judging by the configuration of the isotope diagram, most likely occurred from bottom to top and could begin already when the lake was shallowed.

Thanks

The authors express their gratitude to the Institute of Permafrost Science SB RAS for organizing the summer field school-seminar "Cryogenic processes of Central Yakutia", during which field work was carried out, as well as to the staff of the Institute V.V. Ogonerov, N.E. Baishev and N.I. Tananaev for assistance in collecting and transporting samples.