Abstract
[1] We report on the first authigenic siderite (FeCO3) concretions recovered from near-bottom sediments at gas hydrate-bearing mud volcanoes in fresh water (Lake Baikal, Eastern Siberia). The carbonates appear as firm ‘plate-type’ formations at the Malenky mud volcano (Southern Baikal Basin) and as soft nodules at the K-2 mud volcano (Central Baikal Basin). Calcium is the main divalent component which substitutes iron in the carbonate lattice (7 to 20 mol%). The δ13C values of the carbonates (+3.3 to +6.8‰ at Malenky, and +16.5 to +21.9‰ PDB at K-2) indicate that their formation is due to methanogenesis. The latter was most likely caused by the microbial methyl-type (acetate) fermentation that is suggested from the isotopic composition of the accompanying methane hydrates and dissolved methane. General depletion of the siderites in 18O (−11.6 to −9.9 ‰ at Malenky, and −13.9 to −12.3‰ PDB at K-2) is mainly inherited from the isotope composition of pore water (−15.2 to −15.4‰ SMOW) at ambient temperature (3.5°C).
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