Messinian carbonate-rich beds of the Tertiary Piedmont Basin (NW Italy): Microbially-mediated products straddling the onset of the salinity crisis

The seven Messinian microbial carbonate-rich layers cropping out in the Pollenzo section (TertiaryPiedmont Basin, NW Italy) are interbedded with a precession-related cyclic succession composedof euxinic shale/marl couplets and straddle the onset of the Messinian salinity crisis (MSC). A sharpchange of sedimentological, compositional and geochemical characteristics was observed from theonset of the MSC onward, suggesting strong differences in the mechanisms responsible forcarbonate precipitation. Pre-MSC beds are mainly composed of dolomite and are interpreted as theproduct of early diagenesis, formed by bacterial sulphate reduction in the shallow subsurface alongthe sulphate-methane interface. Dolomite precipitation was temporarily enhanced by an upward fluxof methane-rich fluids, possibly sourced by gas hydrate destabilisation. Conversely, from the onsetof the MSC onward, carbonate beds are thinly laminated and show abundant filaments interpretedas Beggiatoa-like bacteria, while calcite (and aragonite) are the dominant authigenic phases. Thesebeds are interpreted as microbialites, resulting from the lithification of chemotrophic bacterial matsdominated by sulphide-oxidizing bacteria. The carbonate necessary for their preservation wasprovided by bacterial sulphate reduction. These microbialites, that appear as the deeper watercounterpart of bottom-grown selenite layers deposited in the marginal part of the basin, formed onan anoxic sea bottom under a density stratified water column. The high sulphate concentration inthe pore waters, related to the presence of concentrated brines and to the regeneration of sulphate bysulphide-oxidizing bacteria, is considered as the driving factor controlling the mineralogical changefrom dolomite to calcite and aragonite. Finally, the sharp shift toward negative 18O values ofcarbonates, observed from the onset of the MSC onward, was probably related to fractionationprocesses operated by sulphate-reducing bacterial communities in the pore waters. This suggeststhat care must be taken, when dealing with microbial carbonates, in the interpretation of the oxygenisotope values in term of fluctuating salinity condition of the water mass. In conclusion, this studysuggests that the onset of the MSC had a strong impact on microbial populations governing carbonate precipitation, in a sector of the basin dominated by depositional conditions not suitablefor gypsum precipitation.