This paper performs a detailed study of a wide set of organic-geochemical proxies in 15 sediment cores collected from the main basins of Lake Baikal (the northern, the central and the southern) where processes of focused fluid discharge were detected. A variety of studied zones includes sites with gas and hydrothermal seepage, mud volcanoes with or without gas-oil fluid discharge, gas hydrates and authigenic carbonates. The composition of the dispersed organic matter and individual hydrocarbon molecular markers (n-alkanes, dimethyl alkanes, isoprenoids, steranes, terpanes and polycyclic aromatic hydrocarbons) testify to the input from predominantly allochthonous terrestrial and autochthonous microbial and algal sources. The studied sources, maturity and biodegradation parameters of organic matter vary significantly for areas with different fluid discharge. The composition of specific biomarkers including isoprenoids and immature hopanoids reflects the lateral and vertical changes of microbial activity in sediments associated with various environmental conditions. The identified types of terpanes distribution (mature, mixed and immature) correlate well with types of fluid discharge and attest to the development of various methanogenic and methanotrophic microbial communities in sediments. Moreover, the revealed specificity of microbial molecular markers distribution allowed us to suggest the fluid discharge processes in zones where they were not previously detected.

Comparative studies of the composition and distribution of the dispersed organic matter (DOM) were performed for the bottom sediments from active areas of the Ashadze hydrothermal field (the Mid-Atlantic Ridge, 13° N) and for background sediments from the area treated (sampled during the cruises of R/V Professor Logachev in 2003 and 2007). The content of hydrocarbons (HCs) in the hydrothermal samples is about 20 times as high as the background values. The HCs composition includes both low-transformed (isoprenoids, hopenes, ββ-hopanes, and ααα27R-steranes) and geologically mature chemofossils (C₁₆-C₃₅ n-alkanes, geohopanes, moretanes, and polyaromatic hydrocarbons). The polyaromatic hydrocarbons (PAHs) are mainly represented by phenanthrene and its alkyl homologues, i.e., by possible products of the diagenetic transformation of biogenic precursors. The results obtained point to the mixed genesis of the hydrothermal DOM, which is caused first by the diversity of the biota composition in the considered region, as well as by the specificity of the processes of the DOM’s maturing under the extreme environmental conditions.

Study of the molecular composition of the dispersed organic matter (OM) in bottom sediments of Lake Baikal was conducted (supported by RSF #19-17-00226). Sediments (11 gravity cores - 28 samples) were collected during research expeditions of the R/V “G.U. Vereschagin” (LIN SB RAS, Irkutsk) in 2016-2018.Variations in composition and ratios of aliphatic and aromatic components reflect changes of OM sources. Most n-alkane profiles show the distinguishable predominance of terrigenous components C27-C31. The highest biodegradation degree and increased content of isoprenoids is detected near the Gorevoy cliff where the active oil discharge was observed. Biogenic hopanes (ββ-hopanes and hopenes) predominate in most samples and diagenetic type of distribution is identified only in sediments with oil inclusions. Steranes are the minor components with ethylcholestanes as the main peaks attesting to the input of land plants. Increased values of perylene and phenanthrene in polycyclic aromatic HCs composition indicate the mixed biogenic-petrogenic nature of OM of the studied Lake Baikal sediments, while the oily samples contain only trace amounts of perylene.The branched 2,7-dimethyl alkanes (m/z 127) have been identified in mudstone samples from the Vendian Marna Formation from the Sayan-adjacent Biryusa area and in Permian and Upper Carboniferous coal-bearing rocks from superdeep well SV-27 (Vilui syneclise) [1]. Their precursors most likely are the analogues of branched methylenated acids detected in lipids of modern bacteria (9,10-methylene hexadecane, 9,10-methylene octadecane, and 11,12-methylene octadecane acids). Decarboxylation of the methylenated acids branched at the second and seventh carbon atoms during diagenesis and catagenesis should have resulted in 2,7-dimethyl alkanes that were detected in all immersed sediments of the southern, central and northern parts of Lake Baikal.Trace amounts of the other poorly studied group of compounds – monoaromatic steroids (MAS) were identified in bottom sediments near the mud volcano Kedr in the southern part of the lake. These structures can be formed during diagenetic transformations of sediments at the contacts of OM with clays (catalyzers) together with the formation of regular steranes and diasteranes (C27-C29). They have been previously detected in apocatagenetic rocks of the East Siberian sedimentary basin (ultradeep hole SV-27 from the Middle Vilyui area of the Vilyui syneclise) [2]. The absence of the main fragmental ion m/z 253 in the analyzed samples points to the migration of methyl alternate from C-17 to C-23 alkyl-chain position and agrees with distribution of the similar structures (m/z 281, 309, 366) in rocks of the hole SV-27. The detected 17-desmethyl-23-methylmonoaromatic steroids appear and exist at high temperatures and pressures and are very thermodynamically stable.Thus, the input of the OM of catagenetic maturity degree to the bottom sediments of Lake Baikal is likely associated with the deep fluid migration and mud volcanic breccia uplift to the surface.References[1] Kashirtsev V.A. et al., 2009. New homologous series of biomarker molecules from Vendian deposits of the Sayan-adjacent Biryusa area. Russian Geology and Geophysics 50, 541–545.[2] Kashirtsev V.A. et al., 2016. New monoaromatic steroids in organic matter of the apocatagenesis zone. Doklady Earth Sciences 469, 815–818.

Comparative study of genesis and structure of dispersed organic matter (DOM) from background pelagic bottom sediments and sediments inside an active hydrothermal field Ashadze-1 collected during the cruise of the R/V “Professor Logachev” in 2003 was carried out. The received results allow to speak about an essential originality of structure and distribution of DOM in bottom sediments of the field Ashadze-1, according to unique physical and chemical conditions and facial specificity of sedimentation in hydrothermal zones. At the same time, attributes of petroleum hydrocarbons abiogenic synthesis hasn’t been fixed. Opposite, the received data allow to consider the process of fast maturing of biogenic OM in hydrothermal systems as a major factor of HC formation.