Anaerobic oxidation of methane in hypersaline Messinian environments revealed by 13C-depleted molecular fossils

The Messinian sequence of evaporitic deposits in Italy includes authigenic carbonates that have been suggested to derive from the microbial degradation of organic compounds, but the biogeochemical mechanisms that led to their formation remained unknown. To unravel these mechanisms, 13C-depleted carbonate rocks including native sulphur-bearing limestone (Calcare Solfifero) from two locations in Sicily and bedded limestone from the northern Apennines have been studied. Their δ13C values as low as − 49‰ reflect incorporation of carbon derived from the oxidation of methane, which was previously suggested to have been linked to bacterial sulphate reduction. Molecular fossils extracted from the limestones reveal that methane was indeed oxidized in an anaerobic process by archaea and sulphate-reducing bacteria. The observed biomarker patterns resemble those of methane-seep carbonates, which form as a consequence of the anaerobic oxidation of methane (AOM). AOM-specific, 13C-depleted archaeal biomarkers such as PMI (δ13C: − 101 to − 75‰) as well as compounds derived from sulphate-reducing bacteria (iso- and anteiso-fatty acids; δ13C as low as − 85‰) were identified in the Messinian carbonates. Although the biomarker results clearly point to AOM, the compound inventory also revealed distinct differences to patterns found at marine methane-seeps. For example, sn3-hydroxyarchaeol is in some samples the only hydroxyarchaeol present, something usually not found at seeps. Archaeol is less depleted in 13C than PMI at most sites (δ13C: − 55 to − 30‰), pointing to the co-occurrence of methanogenic or halophilic archaea with archaea involved in AOM. (1) The presence of calcite pseudomorphs after lenticular gypsum, (2) 18O-enrichment in carbonates, and (3) the biomarkers O-phytanyl-O-sesterterpanyl glycerolether (extended archaeol), tetrahymanol, and possibly phytanylglycerol monoethers as well as non-isoprenoidal macrocyclic glycerol diethers confirm that carbonate formation took place under evaporitic conditions. AOM has previously been shown to be inhibited in some brine pools on the modern seafloor. Our observations, however, demonstrate that AOM functions in hypersaline environments as well.