New data on chemical and isotopic properties of waters from 35 mud volcanoes of Azerbaijan show that they are represented by two contrasting types characterized by different mineralization: Cl-Na (M = ∼30–80 g/L) and HCO₃-Cl-Na (M = ∼8–15 g/L). Waters of the last type are usually enriched with alkaline metals, B, and Br. Waters of the Cl-Na type are similar in their Cl/Br coefficient with seawater. According to Mg-Li and Na-Li geothermometers, waters were formed at temperatures varying from ∼20 to ∼140°C. Taking into consideration these estimates, “roots” of the volcanoes under consideration should be located at depths of 6.0–7.5 km. The fluid generation temperatures exhibit a tendency for their growth toward the Greater Caucasus orogen. The δD, δ¹⁸O, and δ¹³C (TDIC) values in dissolved inorganic carbon vary from −32 to −12 and −0.6 to +10.4‰ (V-SMOW) or −12.9 to +37.3‰ (V-PDB), respectively. The highest δD, δ¹⁸O, and δ¹³C values are typical of HCO₃-Na waters. It is shown that the growth of formation (Mg-Li) temperatures is accompanied by a significant increase of the δ¹⁸O(H₂O) and δ¹³C(HCO ₃ ⁻ ) values along with HCO ₃ ⁻ concentrations. The degree of water enrichment with Br, B, and alkaline metals also increases. This allows the formation of soda waters from mud volcanoes with elevated ¹⁸O and ¹³C contents to be attribute to relatively high-temperature transformations of mineral and organic matter during the catagenic alteration sedimentary rocks.

Chukotka is an arctic region located in the continuous permafrost zone, but thermal springs are abundant there. In this study, for the first time, the microbial communities of the Chukotka hot springs (CHS) biofilms and sediments with temperatures 54–94 °C were investigated and analyzed by NGS sequencing of 16S rRNA gene amplicons. In microbial mats (54–75 °C), phototrophic bacteria of genus Chloroflexus dominated (up to 89% of all prokaryotes), while Aquificae were the most numerous at higher temperatures in Fe-rich sediments and filamentous “streamers” (up to 92%). The electron donors typical for Aquificae, such as H₂S and H₂, are absent or present only in trace amounts, and the prevalence of Aquificae might be connected with their ability to oxidize the ferrous iron present in CHS sediments. Armatimonadetes, Proteobacteria, Deinococcus-Thermus, Dictyoglomi, and Thermotogae, as well as uncultured bacteria (candidate divisions Oct-Spa1-106, GAL15, and OPB56), were numerous, and Cyanobacteria were present in low numbers. Archaea (less than 8% of the total community of each tested spring) belonged to Bathyarchaeota, Aigarchaeota, and Thaumarchaeota. The geographical location and the predominantly autotrophic microbial community, built on mechanisms other than the sulfur cycle-based ones, make CHS a special and unique terrestrial geothermal ecosystem.

The chemical and isotopic characteristics (δ¹⁸O and δ²H) of water from 42 mud volcanoes of the Kerch–Taman region (Crimean–Caucasus district) were analyzed. The formation temperatures of mud volcanic waters were estimated using the Mg-Li hydrochemical geothermometer (t(Mg-Li) = 40–134°C). It was revealed that dehydration waters released during the smectite–to-illite transformation play significant role in the total fluid balance of the mud volcanic systems (up to 80% of the total fluid volume). These waters are characterized by low Cl ion contents, δ¹⁸O from +13 to +17‰, and δ²H from –25 to –20‰ relative to VPDB, and high concentrations of \({\text{HCO}}_{3}^{ - }\), B, Li, Ba and other trace elements. Correlations between the concentration of HCO₃, B, δ¹⁸O, and t(Mg-Li) values were revealed.

This study presents the first regional hydrogeochemical portrait of the mineral waters and associated gases of the Central Asia region, shaped by the Tien Shan and the Pamir. A geochemical survey of more than 50 fluid discharges from the Northern Tien Shan to the Pamir was carried out between 2018 and 2019. Isotopic (δD, δ¹⁸O, d¹⁵N₂, d¹³C_CO2, d¹³C_CH4) and chemical data allow elucidating fluid genesis and general evolution in the continental collision zone. Geothermometric estimations as well as the content of the chemical components in waters (Cl, Li, B, Br) and gases (N₂, CO₂) suggest that the studied waters are not related to the presence of any active hydrothermal systems at shallow depth. Silica and cation geothermometers along with thermodynamic equilibrium calculations indicate that the temperature of unmixed deep fluids does not exceed 110 to 150 °C. The determination of d¹⁵N₂ and d¹³C_CO2 has revealed that the mantle genesis of gas flux matches with the areas of CO₂-rich waters manifestations. The dislocation of mineral and thermal waters of Central Asia along the major regional tectonic structures is provided by topographically driven and well-developed long circulation of waters at the depth of 1 to 4 km.

Mineralogical and isotope-chemical characteristics of carbonate travertines of the Greater Caucasus are investigated. It is shown that concentrations of many chemical elements, which predominantly precipitate together with iron hydroxides, decrease along the strike of the travertine dome. At the same time, δ¹³C and ¹⁸O values in carbonates systematically increase due to kinetic effects of isotope fractionation. This leads to the formation of isotopically heavy calcite (δ¹³C up to 16.3‰) near the travertine dome base. Concentrations of other elements (Mg, Sr, Ba, Na, S, and Li), which form a isomorphic part of calcite crystal lattice, almost do not change along the strike of the dome. Comparison of concentrations of these elements in travertines and initial water solutions makes it possible to get ideas on trends of their redistribution in the carbonate-water system. Correlations found in this work define not only TDS content and concentration of some ions, but also REE spectra and formation temperatures of ancient fluid systems.