Possible Role of Microbial Communities of Lake Baikal Bottom Sediments in the Formation of Authigenic Siderites Abnormally Enriched in C Isotope

Precipitation of authigenic carbonates during diagenesis of bottom sediments is often a by-product of microbial activity, and the isotopic characteristics of such carbonate minerals are determined by the effects of carbon isotope fractionation during enzymatic reactions. The composition of microbial communities was estimated using the 16S rRNA gene metabarcoding method, and the chemical composition of pore waters of bottom sediments of three mud volcanoes of Lake Baikal containing siderites (FeCO) anomalously enriched in the C isotope (δC +7.24...+35.02‰ VPDB), the formation mechanisms of which are currently unknown, were analyzed. The obtained data are consistent with the hypothesis that siderite sedimentation may be associated with local zones of active biogenic destruction of organic matter, but it is suggested that the role of microbial communities in this process may be limited to the reduction of crystalline forms of Fe(III) oxyhydroxides to siderite or replenishment of the Fe(II) pool, rather than dissolved inorganic carbon (DIC). In this case, the factor causing the sedimentation of authigenic siderites enriched in the C isotope may be the supply of DIC or gaseous CO with a deep fluid flow, the isotopic characteristics of which will largely determine the δC values of carbonates in the sediments of mud volcanoes of Lake Baikal.