Methane-related authigenic carbonates from the Black Sea: geochemical characterisation and relation to seeping fluids

During TTR11 Cruise (2001), three areas of active fluid venting and mud volcanism were investigated in the Black Sea below the oxic zone at depths varying between 800 and 2200 m. Authigenic carbonates often associated with microbial mats were recovered from the sea floor and the shallow subsurface. Structural and petrographic observations allowed the distinction of five different types of authigenic carbonates; three of these consist of carbonate-cemented layered hemipelagic sedimentary units, while the other two consist of carbonate-cemented mud breccia sediment and authigenic micrite slabs. The carbonate cements consist predominantly of micritic Mg calcite. Their δ13CCaCO3 varies between −8.5‰ and −46.9‰ at the different sampling locations, indicating that authigenic carbonates incorporate variable proportions of carbon derived from the anaerobic oxidation of methane (AOM), the oxidation of organic matter and from sea water. Methane is the dominant component among other hydrocarbon gases in these sediments. Its relative amount varies from 99.9% to 95.1% of total hydrocarbon gases and its δ13C values range from ≅−40‰ to ≅−74‰. Methane in sediments associated with the carbonate crusts shows carbon isotopic values 25–30‰ lighter than the authigenic carbonates at all the studied sites, indicating that methane present in the seeping fluids confers a distinct isotopic signature to the carbonate deposits at each location. Models proposed for the formation of carbonate slabs in the subsurface imply methane seepage impeded by homogenous clayey laminae or by pre-existing slabs, coupled with microbial activity oxidising methane and organic matter present in the sediment. Mud breccia crust pavements on the sea floor form by carbonate cementation of methane-charged sediment. Gas saturation of the sediment is confirmed by the presence of gas hydrates, whose shape indicates an association with authigenic carbonates, supporting the idea that sedimentary structures can control gas distribution.