First discovery and formation process of authigenic siderite from gas hydrate–bearing mud volcanoes in fresh water: Lake Baikal, eastern Siberia

Abstract

We report on the first authigenic siderite (FeCO 3 ) 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 δ 13 C 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 18 O (−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).