Mineralogical, geochemical and isotopic characterization of authigenic carbonates from the methane-bearing sediments of the Bering Sea continental margin (IODP Expedition 323, Sites U1343–U1345)

• The authigenic carbonates from the Bering Sea were precipitated within the clayey/diatom-rich sediments since at least 2.4 Ma.
• Authigenic carbonates precipitated sequentially during progressive burial, with Mg-calcite at shallow, and dolomite and Fe-rich carbonates at deeper burial depths.
• Mg-calcite precipitated from 13C-depleted pore water dissolved inorganic carbon (DIC) above and within the sulfate–methane transition zone (SMTZ).
• Dolomite and Fe-rich carbonates precipitated from 13C-enriched pore water DIC below the SMTZ.
• The microbial reactions (sulfate reduction, anaerobic oxidation of methane and methanogenesis) enhanced both carbonate and silicate mineral diagenesis in methane-bearing sediments.

During Expedition 323 of the Integrated Ocean Drilling Program to the Bering Sea (July 5–September 4, 2009), three sites were drilled along the Bering Sea northeastern continental margin [U1343 down to 745 meters below sea floor (mbsf), U1344 (745 mbsf), U1345 (150 mbsf)]. Diagenetic carbonates are present at all sites within the clayey, diatom-rich oozes of the Bering Sea, where pore waters are also characterized by extremely high methane concentrations. We here present mineralogical, elemental and isotopic data obtained from the authigenic carbonate-rich intercalations within the clay-rich Pleistocene sediments deposited along the Bering Sea continental margin. The mineralogy of the authigenic carbonates is generally represented by composite mixtures of very small crystals of magnesian calcite, dolomite, and iron-rich carbonates, with the latter phases occurring below 260 mbsf at Site U1343, below 200 mbsf at Site U1344, and below 130 mbsf at Site U1345. Element geochemistry shows that Ca, Mg, Fe, Ba, Mn, Sr and U are enriched in the carbonate-rich intercalations relative to the background sediments due to their incorporation into the carbonates and into other authigenic phases (e.g., barite and pyrite). The oxygen and carbon isotopic compositions of the authigenic carbonate minerals show that they were sequentially precipitated from pore waters at different temperatures (i.e., different burial depths) and with different isotopic compositions of dissolved inorganic carbon (DIC). The authigenic Mg-calcite precipitated early during diagenesis and shallow burial from a 13C-depleted DIC pool, whereas dolomite and Fe-rich carbonates formed during later diagenesis and deeper burial from a 13C-enriched DIC pool. These authigenic carbonate occurrences are interpreted as resulting from microbial sulfate reduction combined with anaerobic oxidation of methane, and methanogenesis that was intimately linked to the alteration of silicates, especially iron-rich clay minerals.