The diversity and structure of microbial communities were investigated using 16S rRNA gene barcoding in the sediments of the Krasny Yar methane seep, in the zone of near-surface occurrence of gas hydrates and the presence of oxidized and restored channels. The diversity of both bacteria and archaea along the core depth was similar to the diversity found in sediments from other areas: methylotrophic methanogens and microorganisms involved in different stages of the organic matter fermentation were detected in the sediment strata of all depths investigated. Migration flows of oxygen-rich and aerobic bacteria-rich near-bottom water influenced greatly the diversity of microbial communities in oxidized channels. Fluids migrating from the deep zone to the bottom surface provided transport from anoxygenic sediments of anaerobic archaea involved in the AOM process. The data obtained are consistent with geochemical and geothermal indicators defining the zone of active migration of near-bottom waters.

An integrated study of mud volcanoes in the World Ocean is important for making assessment of potential geological-ecological disasters caused by rapid large-volume gas discharge into the water column and mud volcano eruptions at the bottom. The study of mud-volcanic activity in the past and determination of its periodicity are pioneering for the Baikal. The mud volcanoes and other hydrate-bearing structures are largely concentrated in the Middle Baikal basin along the tectonic faults. The most representative example of these phenomena is the "Gydratny" fault, four of six structures along which are mud volcanoes. An integrated geological-geophysical study (seismoacoustic and hydroacoustic sounding and geological sampling) of the "Novosibirsk" mud volcano, the largest and well-pronounced feature of the lake bottom relief, confirmed its structural identity with classical submarine mud volcanoes. The "Novosibirsk" mud volcano possesses all major elements of other single hydrate-bearing mud volcanoes of the lake which include volcanic cone in the bottom relief, vertical acoustically not transparent feeding channel, mud-volcanic breccia, gas saturation, and gas hydrates. This makes it one of the reference hydrate-bearing mud volcanic-type structures of Lake Baikal. The analysis of the bottom hydroacoustic profiling yielded evidence of the Late Pleistocene mud-volcanic eruptions shaped as two layers-flows at sub-bottom depths of 15 and 26 m (30 and 50 kyr ago, respectively). The presence of mud-volcanic breccia beneath the thin Holocene diatomic silt deposits testifies to the Holocene mud volcano activation due to the warm fluid rising from the depths to the volcano roots along the active segment of the tectonic fault in accordance with the model of the "Baikal-type" mud volcanism. Using the "Novosibirsk" mud volcano and the "Gydratny" fault as an example, it can be shown that the past tectonic activity of the Baikal basin may be determined based on the knowledge of the structure and evolution of the mud volcanoes of the lake.

The paper presents geochemical study of bottom sediments from the MSU structure located on the large Gydratny Fault in the Central Basin of Lake Baikal at a depth of 1380 m. The first detailed data on the spatial variations in the qualitative and quantitative composition of the pore waters are presented. Pioneering data were obtained on Li, B, and Sr contents in the pore water of the sediments. It has been established that fluids are actively discharged within the MSU structure, and the main pathways of their near-surface migration are confined to the tops of hills of this structure on the downthrown fault block. The fluids are highly mineralized (up to 2900 mg/L), showing the highest mineralization ever found in Lake Baikal sediments. The waters are significantly enriched in Mg, Li, B, and Sr but depleted in K. The waters are thought to be generated by the processes of authigenic formation and illitization of smectite at depths of 1 to 2.5 km in the sedimentary sequence. The maximum values of concentration gradients are recorded in the pore waters of the sediments of the western hill, which may indicate a gradual westward shift of the center of the fluid seepage activity along the fault.

The results of subaqueous landslide studies at Krasnoyarskiy are given in this paper. The landslide were located at a subaqueous part of the delta of the Selenga River. A multidisciplinary approach was applied to the study of the landslide, including seismo-acoustic, lithological, gas-geochemical and geotechnical studies. Data from landslide and surrounding soils allowed us to propose the hypothesis that a key factor in the landslide origin was high gas saturation of soils and that the reason of landslide was an earthquake. The proposed approach to the study of bottom soils in lakes and seas can be applied both in assessing their stability and in searching for focused fluid discharge zones at the bottom in areas associated with hydrocarbon deposits in the sedimentary section. As well, the approach can be applied to mapping of permafrost melting in the Arctic region offshore, where gases often accumulate below it.

Outcrops of the coal-bearing Tankhoi Formation (Oligocene–Pliocene), traced along the southern shore of Lake Baikal, submerge under its Southern Basin, where several hydrate-bearing zones of the focused hydrocarbon fluid discharge have been found. To test the hypothesis that coals of the Tankhoi Formation can be the sources of hydrocarbon gases in these zones, we collected coal samples from the Shakhterskaya Gorka outcrop. Experiment on gas generation from the selected samples was carried out in a special autoclave at a temperature of 90°C for eight months. This paper presents the obtained results, which confirm an important role of the process of gas generation from coals in the formation of fluids in the Kedr mud volcano. The further migration of gases was accompanied by the biodegradation and formation of secondary microbial methane due to CO2 reduction. This was one of the reasons for the carbon isotopic pattern observed in methane (heavier than –50‰ VPDB) and carbon dioxide (positive values) taken from the near-surface sediments and hydrates of the Kedr mud volcano, as well as for the significant enrichment of authigenic siderites in the heavy 13C isotope.

A new oil show, Zelenseep, was discovered on the eastern shore of Middle Baikal. It is characterized by natural gas discharge (seep) from the lake floor and by accumulation of oil (high- molecular-weight bitumen fraction) and gas hydrates in the top layer of the sediments. The gas in the sediments and gas hydrates is composed of 99% thermogenic methane. The carbon isotope composition (δ 13 (С–С 1 )) ranges from –48.1 to –48.8‰ for methane in the sediment and from –49.4 to –50.2‰ for that in the hydrates. Oil is not discharged into the water column. N-alkanes and isoprenoids are not found in the oil; polycyclic aromatic hydrocarbons are detected at the 1800– 2200 ppm level, including 50–90 ppm of rethene and 120–140 ppm of perylene. The perylene concentration evidences major transformation undergone by the oil on its migration path from the source. The presence of rethene in the oil is indicative of its continental origin, and that of biomarkers, hopanes, of its identity with the Baikal naphthenic-aromatic oils. The sediments of earlier Cenozoic age in Baikal Basin, occurring in its central deep-water part, were assigned to organic matter source.

The paper presents results of gas-geochemical studies of bottom sediments and petroleum potential assessment of Baikal Rift Basin. During the expeditions of the Class@Baikal project in 2014–2019, gases from the Lake Baikal bottom sediments were analyzed. The results showed a clear difference in chemical and isotopic composition of the seeping gases collected in the northwestern and southeastern parts of the lake. The seepage released from northwest part were relatively enriched by methane and had a low concentration of C2+ compounds. The seepage gases had relatively lighter carbon isotopes composition of CH4 (from -72,7 to -50,1 ‰ VPDB) and the high variability of δ13C in C2H6 (from -65 to -22 ‰ VPDB). The gases released from southeastern part of the lake had an increase in C2+ compounds and had relatively lighter carbon isotopes composition of methane (from –57,2 to –41,0 ‰ VPDB). The carbon isotopes composition of ethane varies from -32 to -25 ‰ VPDB. Asymmetric structure of the Baikal rift basin and various processes of gas migration within it might cause the variations. Diffusive process led to the lighter carbon isotopes composition of the seepage gases from the northwestern part of lake and the gas molecular composition enrichment by methane. Such molecular and isotopic fractionations caused by geochemical processes helps to understand the migration of gas from source rocks to the earth’s surface. Similar geochemical indicators of fractionation should be taken into consideration when assessing oil and gas source rocks and basin potential from gas geochemical studies data.

This paper presents a schematic summary of comprehensive analysis of seismic, reflection profiling, and hydroacoustic data on faults which caused sediment deformation in the central segment of the Central Baikal basin. According to the tectonophysical analysis results, the fault pattern within sediment fill has been recognized as zone-block, i.e., it represents a network of high-density fracture zones limiting weakly deformed blocks. The structure of large NE-trending fault zones (Olkhon, Beregovoy, Gydratny, and Svyatoy Nos) is controlled by main fault planes (or their segments) bounded by subsidiary faults. Geomorphic expression of NW cross faults in the sedimentary cover as broad zones of smaller-scale fractures accounts for early stages of the evolution of basement faults. In a longitudinal direction, they divide the basin into large fragments. The zone-block structure of the sedimentary strata was developed in different stress regimes: strike-slip and extension at the early and late orogenic rifting stages, respectively. At the modern stage of tectogenesis, the established network of fault zones controls the gaseous (including hydrate formation) and seismic activity expression in the subsurface. Hydrate-bearing mud volcanoes and seeps are confined to major faults, while earthquake epicenters are confined to fault zones and form clusters at junctions of large NE-trending faults with NW-oriented extension zones and E-W left-lateral strike-slip faults.

The purpose of the study is to describe the first finds of coal-bearing clays and coals in the bottom sediments of the southern basin of Lake Baikal and compare them with terrestrial coal-bearing deposits of the Tankhoy field. Comparative analysis of the lithological composition and colour of bottom sediments and terrestrial sections, as well as the concentration of organic carbon and conducted palynological analysis allowed their correlation. At the lake’s depth of 900 m the authors discovered a coal-bearing strata in situ (st 56), which later was stratigraphically correlated with the terrestrial coalbearing part of the Tankhoy suite. The fragments of coal found in bottom sediments basically along the entire Tankhoy field, especially bedrock coals on the underwater slope in South Baikal up to 1300 m deep prove the distribution of the coal-bearing part of the Tankhoy suite in the sublacustrine part of the lake throughout the entire slope (from 5 to 10 km offshore) and confirm the distribution area of the Tankhoy paleolake over a significant area of the contour of modern southern basin of Lake Baikal. The finds of coal-bearing strata on these and other various sub-bottom depths, i.e. under various pressure and temperature conditions, suggest that coals themselves and coal-bearing mudstones may be a generation facility of secondary microbial methane. This should be taken into account when searching for gas hydrocarbon and gas hydrate accumulations as well as assessing methane cycles in Lake Baikal.

The Gydratny Fault extends in the SW–NE direction for over 60 km in the central basin of Lake Baikal. During the Class@Baikal-2019 expedition we conducted a multidisciplinary study coupling seismic and gas sampling obtained from bottom sediment profiles intersecting the fault zone. Seismic profiles revealed that the central and northeastern sector of the fault have a pronounced footwall on the lake floor with stronger acoustic anomalies related to gas saturation in the sediments. The southwestern sector of the fault system is instead less pronounced on the lake floor with increasingly thicker hemipelagic deposits further to the west. All sampling station profiles indicate that the pore gas in the sediments is methane-dominated with greater gas concentrations at the localities above the fault. Accordingly, gas molecular and isotope composition revealed that the highest concentrations are present in the central and northeastern segments of the fault zone and in particular associated with the MSU hydrate-bearing structure and the Novosibirsk and Ukhan mud volcanoes. These structures reveal the highest concentrations of methane and C2+ homologues, as well as the highest methane carbon isotope compositions (δ13CCH4 = –57‰ VPDB) and near-surface gas hydrate accumulations. The southwestern segment of the fault is characterized by the lowest gas concentrations in the sediments and the lowest δ13CCH4 (–76‰ VPDB). By combining the geophysical and geochemical data obtained from the profiles, we propose the dominant gas migration mechanisms at various segments of the Gydratny Fault. Where the fault is well-expressed on the bottom relief (the central and northeastern sectors) focused migration is more pronounced and the thermogenic component of the methane is transported by advection from deeper units. Where the fault underlays relatively thick modern sediments (i.e., in the southwestern segment), deeper rising thermogenic gases and a significant portion of microbial methane are transported by combined diffusion and advection mechanisms, respectively. Our results reveal that this laterally extensive tectonic structure is an efficient pathway for fluid migration hosting numerous mud volcanoes and gas hydrate bearing structures. This study provides useful insights for the interpretation of offshore oil and gas geochemical prospects.

Structure, morphology, and composition of the gas for natural gas hydrates sampled in the Kedr-1 mud volcano (Lake Baikal) are studied. It is shown that all these hydrates have cubic structure II, and the hydrate‐bound gas contains about 14% of ethane (the rest is methane). One of the samples is a porous monolithic hydrate layer sandwiched between the layers of hydrate granules. The hydrate has similar structures and compositions in the layer and in the granules. As far as we know, no such morphology features of natural hydrates have been reported so far. Possible mechanisms underlying the formation of such objects are discussed.

Изучена структура, морфология и состав газа для природных газовых гидратов, отобранных на грязевом вулкане Кедр-1 (оз. Байкал). Показано, что все эти гидраты имеют кубическую структуру II, а связанный газ содержит около 14 % этана (остальное метан). Один из образцов представлял собой пористый монолитный слой гидрата с прилегающими к нему сверху и снизу слоями гранул гидрата. И в слое, и в гранулах гидрат имел одну и ту же структуру и состав. Насколько нам известно, ранее такие особенности морфологии природных гидратов не описаны. Обсуждаются возможные пути формирования подобных объектов.

We studied the composition and isotopic characteristics (δ13C and δD) of hydrocarbon gases from the Kotelnikovsky, Zmeiny, and Goryachnisky hot springs located in the coastal zone of Lake Baikal. Of greatest interest is the Zmeiny Spring located on the eastern coast, where the methane has abnormally high values of δ13C (up to +11‰) and δD (up to +267‰). These values are related to the fractionation of carbon and hydrogen isotopes during the oxidation of microbial methane. The δ13C-C1 value (‒40.2‰) in the Goryachinsky Spring can formally indicate the thermogenic origin of methane. However, the modification of carbon isotope composition in this spring due to methane oxidation is more likely, because gas from the Goryachinsky Spring contains the microbial methane as admixture. Isotopic characteristics typical of microbial gases undistorted by secondary oxidation (δ13C-C1 = ‒61.5‰, δD-C1 = ‒230.2‰, δ13C-C2 = ‒56.6‰) are recorded in the Kotelnikovsky Spring northwest of Lake Baikal.

Water purity was assessed at the site of a deep oil seepage near Cape Gorevoy Utes (Central Baikal). Polycyclic aromatic hydrocarbons (PAHs) and n-alkanes were determined in different types of oil-containing samples collected at this section of Lake Baikal. The set of studied samples included: (i) samples of water from the surface water layer; (ii) samples of water from different depths; (iii) oil on the water surface; (iv) oil from a sediment core. In the surface water layer and the water column, the total concentration of n-alkanes ranged from 0.2 to 5.3 µg/L and did not exceed the 0.1 maximum permissible concentration (MPCfish = 50 µg/L) established for hydrocarbons in water bodies of fishery importance. PAHs with carcinogenic properties were less than 0.1 ng/L. The total concentration of PAHs found in the water column did not exceed 110 ng/L; toxic equivalent (TEQ) values ranged from 0.001 to 0.110 ng/L. The distribution of petroleum hydrocarbons from the seepage site was limited, and water pollution was localised. The low level of water pollution is associated with natural mechanisms in the Baikal ecosystem. Microbiological community and phytoplankton make a decisive contribution to the purity of Baikal water, and oil fractionation during deep discharge contributes to the bioavailability of petroleum hydrocarbons.

We present the results of morphological analysis of the bottom topography in the Southern and Middle basins of Lake Baikal based on the high-precision bathymetric survey. There is a dominant influence of the fault tectonics at the origin of underwater canyons. Turbidity currents play a great role in the further evolution of the underwater valley. They intensify during abrupt seismic shocks or storm injections into the canyon heads. In cases when canyon heads are close to the river estuaries, terrestrial rivers have a significant impact on the speed of the formation of the canyon valleys. The topography of the original surface often determines the morphological features of the valleys. Climate change in the Pleistocene and fluctuations in the water level of the lake also could initiate the formation of some canyons in Lake Baikal. The studies have indicated the high morphological and genetic similarity of Baikal canyons with marine ones.

We present data, collected in 2016, on the concentration of n-alkanes and polycyclic aromatic hydrocarbons in water and bottom sediments as well as the abundance and composition of the cultured microbial community in the area of the oil seepage near Cape Gorevoi Utes. Since its discovery in 2005, the development dynamics of the oil seepage has demonstrated a decrease in the total concentration of normal hydrocarbons and polycyclic aromatic hydrocarbons in oil slicks and bottom sediments, partial degradation of oil entering the water surface, and an increase and subsequent reduction in the number of microorganisms in water and bottom sediments with the maintained structure of the cultured microbial community. From 2006 to 2016, there was a low total concentration and a narrow range of detected concentrations of n-alkanes and polycyclic aromatic hydrocarbons in the water column, which indicates the preservation of water purity in the lake near the oil seepage.

The Central basin of Lake Baikal is intersected by the North-East – South-West-oriented escarpment named the «Gydratny Fault zone». This laterally extensive structure runs subparallel to the North-Western shore of the lake. The Gydratny Fault zone has been investigated using geophysical techniques during 6 years of research in the framework of international expeditions of the Class@Baikal project. The acquired seismic data provided details of the structure of the upper part of the sedimentary section revealing a system of previously unknown faults. A new tectonic scheme of the South-Western deep-water part of the Central basin is presented. The Gydratny fault is accompanied by a system of numerous synthetic and antithetic normal faults that form a wide and extended faulted zone. These structures are unevenly distributed, and include modern and active faults as well as features buried under undeformed sedimentary units with different thickness. This parameter is used to constrain the patterns observed in several zones of the study area. The difference in the characteristics of faults and their manifestations on seismic data can be explained by complex and uneven distribution of active tectonic and sedimentary processes.