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

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 ag e, consist of strongly carbonate-cemented siliciclastic sediments ranging i n grain size from mud to coarse sand. The methane-related origin of Monferr ato carbonates is based on: (a) outcrop-scale evidence: patchiness of cemen tation, chemosymbiotic fossil communities, presence of a network of polypha se carbonate-filled veins not related to tectonics; (b) isotope geochemistr y: very depleted delta(13)C values, as low as -50 parts per thousand PDB; ( c) peculiar petrographic features. Diverse microbial communities have been observed in present-day cold seeps. These communities include sulphate-redu cing, sulphur-oxidizing and methane-oxidizing bacteria. The present work is focused on the identification and description of fossil evidence of such m icrobial activity in the Monferrato carbonates. Examples of fossilization o f microbial structures are probably represented by pyritic rods and dolomit e tubes referable to sulphur-oxidizing and to unspecified bacteria, respect ively. Less direct but more abundant evidence has been found through petrog raphic and SEM studies of seep carbonates. Many features point to the prese nce of organic clumps or mats capable of trapping sediment and promoting ca rbonate precipitation: microcrystalline calcite peloids; dolomite crystals with irregular hollow cores; dolomite spheroids with dumbbell-shaped cores; laminated internal sediments lining cavities completely. All these feature s are interpreted to result from bacterially mediated, sedimentary and diag enetic processes and can therefore be considered as an additional evidence of ancient methane seeps. (C) 1999 Elsevier Science B.V. All rights reserve d.