Does the Calcare di Base (Messinian, Northern Calabria) represent a bacterial induced deposit?

Geochemical and petrographic studies of organic matter have been carried out on the MessinianCalcare di Base Formation cropping out in the Rossano basin, northern Calabria. This approach allowed toelucidate the depositional conditions under which these carbonates formed, namely the physicochemicalproperties of the water column and the possible role of microbes in the mineralization processes.The biological evidence in the microfacies, like thrombolites mainly constitute of clotted peloidal micrites andfaecal pellets, the primary microstructure, the absence of mould and pseudomorphs after evaporitic minerals,exclude an evaporitic deposition for these sediments as well as the result of diagenetic processes replacing Casulphatesinto carbonates (Guido et al., this volume). These considerations are confirmed by the absence ofmolecular fossils indicative of anoxic or hypersaline environment as pregnanes and homopregnanes, extendedhopanes (>C33), gammacerane and isorenieratane. The study of carbonaceous remains emphasizes a wide varietyof the organic input. Geochemical data (Rock-Eval pyrolysis) indicate a mixed (marine and continental) organicinput. These data have been confirmed by organic petrographic observations (palynofacies) that revealed thepresence of phytoclasts derived from continental plant tissues, amorphous organic matter, and variable proportionsof zooclasts, pollens, spores, phytoplanktonic organisms and filaments, dubitatively attributable to cyanobacteria.The constant and strong bacterial signal of the molecular fossils, represented by the n-alkanes with mode in nC26-nC28 and with no odd-even carbon number predominance, branched alkanes, hopanes and unsatured fatty acids,together with the widespread presence of amorphous organic matter in the palynofacies, corroborates theinterpretation that the clotted peloidal micrite represents a bacterial induced deposit.