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The taxonomic composition and distribution of Boreal Carnian ammonoids from northeastern Asia and Arctic Canada have been clarified, the generic assignment of some species has been revised, and their definitions have been standardized. A qualitative comparative analysis of ammonoid assemblages from northeastern Asia and the Canadian Arctic Archipelago has been conducted for the omkutchanicum, armiger, pentastichus, and combined yakutensis and bytschkovi phases. The presence of ammonoids of the genus Boreotrachyceras in the early Carnian of Arctic Canada has been substantiated. This has allowed for the first comparison of coeval ammonoid assemblages of the omkutchanicum phase from northeastern Asia and Arctic Canada, demonstrating the commonality of the ammonoid generic composition of this time throughout the Boreal Realm. It has been established that the early Carnian ammonoid fauna of northeastern Asia differs from that of Arctic Canada by the presence of Tethyan genera (Trachyceras, Striatosirenites) and families (Arpaditidae). A comparative analysis of the ammonoid fauna of the late Carnian has been conducted for the first time, separately for the formation time of the Arctosirenites canadensis beds, equivalent to the pentastichus phase, and for the formation time of the Jovites borealis beds, equivalent to the yakutensis and bytschkovi phases. As a result, for the first time, with phase accuracy, the time of penetration of Tethyan genera into the paleobasins of the Yukon and Arctic Canada (the time of formation of layers with Jovites borealis) was determined, and the belonging of Arctic Canada to the Tethyan Realm was substantiated.

Zones of hydrothermal vein-disseminated, disseminated sulfide, predominantly pyrite, mineralization in Riphean rocks of the Kyvvozhsky region are characterized by elevated contents of Au, Cu, Se, Mo, Ag, Pb, Bi, Sb, Zn. According to the results of the correlation analysis, the following groups of ore elements were identified, within which the strongest positive correlations were established: Ag-Bi-As-Со; Pb-Cu-Zn-Se; Co-Ni-As. Pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, cobaltite, monazite, xenotime, bornite, and covellite are found in association with pyrite in sulfide veinlets. Native gold is rare; particles with peripheral low-grade zones alternate with high-grade gold overgrowths are present. Inclusions of pyrite, pentlandite, monazite, and albite are observed. The gold contains Ag, and sometimes Cu and Pd, as impurities. Judging by the relationships between the minerals, pyrite was the earliest, followed by pyrrhotite, chalcopyrite, and cobaltite, followed by sphalerite, galena, and gold. The formation of sulfide mineralization is associated with hydrothermal processes that occurred in pre-Middle Devonian times, associated with the activation of tectonic-magmatic processes. Geological data, the heavy isotopic composition of pyrite sulfur with a variation in δ³⁴S values from +15.8 to +23.6 ‰, suggest that the most important role was played by regional metamorphic processes that facilitated the mobilization and migration of hydrothermal solutions along faults with the borrowing of various components from the host rocks and subsequent crystallization of sulfides and gold. In some cases, the presence of Cu and Pd impurities in gold indicates a possible influence on the mineral-forming solutions of basic rocks or their partial depth nature as derivatives of basite-hyperbasite magmatism. The Kyvvozhsky district is of interest for primary gold deposits and merits continued exploration.

Multimetric indexes (MMI) are an important biomonitoring tool that is widely used to assess the ecological status of aquatic ecosystems worldwide. The effectiveness of MMI lies in their ability to combine both the structural and functional features of communities of organisms, as well as the features of the entire ecosystem. The use of organisms such as macroinvertebrates in the development of MMI is most common in lake biomonitoring. No such work has been carried out in Russia to assess the ecological state of lakes. In this study, a multimetric index based on the composition and structure of macroinvertebrate communities was developed for low-altitude Altai lakes experiencing high recreational and agricultural stress. The studied lakes were grouped into two groups according to the degree of organic pollution: reference and disturbed. Using a step-by-step statistical analysis, five metrics were selected from 55 indicators of macrozoobenthos communities, which were included in the final multimetric index. The selection tests included sensitivity (discrimination), stability (seasonality), redundancy, and correlation with environmental variables (abiotic factors). The new index was compiled by the following metrics: the Pielou Evenness index, the number of mayflies, the occurrence of shredders, the number of caddisflies and gammarids. These indicators were evaluated on a continuous scale and divided into 6 classes of water quality in accordance with the classification adopted in Russia. The developed index demonstrated the ability to distinguish not only dirty and clean lakes, but also moderately polluted reservoirs. The new index can become an informative tool for monitoring and assessing the ecological status of lakes.

Based on the interpretation of high-resolution seismoacoustic profiles and borehole drilling data in the Barents and Kara Seas, a GIS database was developed, and a digital map of the distribution and depth of the top of submarine permafrost in the Barents and Kara Seas was constructed. In the southwestern and northeastern parts of the Kara Sea, the submarine permafrost boundary partially coincides with the morphostructure of the denudation-tectonic slope of the seafloor. Three regions with significantly different depths of the top of the submarine permafrost were identified on the shelf. On the Barents Sea shelf, the average depth of the submarine permafrost top is (13.3 ± 6.6) m; in the southwestern part of the Kara Sea, (16.7 ± 8.1) m; in the northeastern part, (29.5 ± 15.5) m. The average rate of the subsidence of the submarine permafrost top in the Holocene is 8.2 mm/yr for the Barents Sea and 5.5 mm/yr for the Kara Sea.

The application of capacitive resistivity surveying in conjunction with other methods of geotechnical monitoring of oil and gas gathering pipelines in the permafrost zone is considered to improve monitoring efficiency and informativeness. It is established that electromagnetic surveying is efficient to map frost-susceptible soils because of the dependence of electrical resistivity on the soil pore water content. The use of electrical resistivity allowed the identification of low-resistivity sections along the pipeline, where active frost jacking of the piles occurs. The proposed electrical surveying at shallow depths is effective to identify areas of frost-susceptible soils when used as part of an integrated geotechnical monitoring program.

Spatiotemporal changes in the rate of thermoabrasion of the shores of the East Siberian Sea have been studied on the basis of measurement data and formulated zero-dimensional and one-dimensional mathematical models. The results indicate that the rate of thermoabrasion of ice-rich coasts in the western part of the sea decreases from south to north. The reason for this is a decrease in the duration of thermal abrasion northward because of the negative meridional air temperature gradient during the ice-free period. The zero-dimensional model indicates that climate warming with an increase in air temperature of the ice-free period by 1 °C increases the rate of thermоabrasion of the shores by 1.84 times independently from the latitude of the studied coastal areas. According to the one-dimensional model, the influence of positive air temperatures on the development of coastal cryogenic processes decreases at higher latitudes. As a result, the response of ice-rich coasts to changes in air temperature of the ice-free period weakens by three times in the northern part of the coast compared to the southern part.

Experimental modeling has been used to analyze the influence of drilling fluid composition and temperature on the destabilization of intrapermafrost gas hydrates. The need for this research is linked to the drilling of oil and gas wells in the Arctic permafrost zone, which, in addition to ice, may contain gas hydrate formations, as well as horizons of saline rocks and cryopegs. Since drilling fluids are an integral part of the well drilling process, studying their interaction with the host ice- and hydrate-containing rocks is essential for preventing various emergency situations associated with the dissociation of pore gas hydrates and the melting of ice inclusions. Experiments have been performed on frozen, artificially hydrate-saturated sand samples exposed to drilling fluids of varying compositions, including those containing contaminated cryopegs. The experimental modeling results suggest that the composition of drilling fluids and their temperature can have a significant impact on the destabilization of intrapermafrost gas hydrate formations, especially those under self-preservation conditions. Contamination of drilling fluids during drilling operations with cryopegs leads to a significant intensification of the decomposition of pore hydrates in the frozen rock strata due to active salt transfer processes.

This paper presents an analysis of the relationship between snow cover density and snow depth across the Arctic Zone of the Russian Federation over the modern climatic period (1991-2020). The study is based on snow survey data from various regions of the Russian Arctic. Due to substantial climatic, topographic, and meteorological differences, a single statistically significant correlation between snow density and depth could not be established for the entire Arctic Zone of the Russian Federation. Instead, regional empirical relationships were developed. The results are compared with formulas commonly used in engineering practice and with foreign empirical models. The study highlights the significant influence of factors, such as wind speed, air temperature, snow metamorphism type, and snow stratigraphy on snow density formation. The obtained dependences can be used to estimate snow water equivalent, model ground thermal regimes, and calculate snow loads in regions where snow density data are scarce or unavailable.

Based on the analysis of monthly solar radiation at 5-degree latitudinal zones in the Arctic, the characteristics of changes in their solar climate in the 21st century have been determined. The tendencies of long-term changes in the annual irradiation patterns of 5-degree latitudinal zones in the entire Arctic region (65-90° N) are positive for the period from March to June and negative from July to October. In addition, in the latitudinal range of 70-80°, positive changes in irradiance are observed in February and November; and in the latitudinal range of 65-70°, also in January and December. In the interannual variability of the monthly irradiation intensity, the maximum relative values are observed in March (0.047 %) and October (0.045 %) in the latitudinal zone of 85-90°, as well as in January (0.044 %) and December (0.044 %) in the latitudinal zone of 65-70°. The meridional gradient of insolation (MGI: summer, winter, and annual) in the current century increases in all latitudinal zones of the Arctic. This tendency becomes less pronounced with increasing geographic latitude. Winter MGI exceeds summer MGI by 2.279-2.782 times, and this excess increases northward. In the 21st century, insolation seasonality (IS) of the Arctic solar climate has been weakening, especially in the high latitudes.

On December 25, 2025, at the age of 66, Sergey Vladimirovich Alexeev, Corresponding Member of the Russian Academy of Natural Sciences, Doctor of Geological and Mineralogical Sciences, chief researcher at the Institute of the Earth’s Crust of the Siberian Branch of the Russian Academy of Sciences, a recognized leader in the field of permafrost hydrogeology, passed away. Sergey Alexeev will remain in the memory of the scientific community as an outstanding researcher of the frozen zone of the lithosphere, the author of fundamental works on cryohydrogeology, a talented science manager and teacher.