The role of bacteria in the formation of cold seep carbonates: geological evidence from Monferrato (Tertiary, NW Italy)

Abstract

Abstract Methane-derived carbonate rocks (Lucina limestone and Marmorito limestone) crop out in Monferrato (NW Italy) and represent one of the first described examples of rocks produced at fossil cold seeps. These rocks, of Miocene age, consist of strongly carbonate-cemented siliciclastic sediments ranging in grain size from mud to coarse sand. The methane-related origin of Monferrato carbonates is based on: (a) outcrop-scale evidence: patchiness of cementation, chemosymbiotic fossil communities, presence of a network of polyphase carbonate-filled veins not related to tectonics; (b) isotope geochemistry: very depleted δ13C values, as low as −50‰ PDB; (c) peculiar petrographic features. Diverse microbial communities have been observed in present-day cold seeps. These communities include sulphate-reducing, sulphur-oxidizing and methane-oxidizing bacteria. The present work is focused on the identification and description of fossil evidence of such microbial activity in the Monferrato carbonates. Examples of fossilization of microbial structures are probably represented by pyritic rods and dolomite tubes referable to sulphur-oxidizing and to unspecified bacteria, respectively. Less direct but more abundant evidence has been found through petrographic and SEM studies of seep carbonates. Many features point to the presence of organic clumps or mats capable of trapping sediment and promoting carbonate precipitation: microcrystalline calcite peloids; dolomite crystals with irregular hollow cores; dolomite spheroids with dumbbell-shaped cores; laminated internal sediments lining cavities completely. All these features are interpreted to result from bacterially mediated, sedimentary and diagenetic processes and can therefore be considered as an additional evidence of ancient methane seeps.