Iron-bacterial mediation in Phanerozoic red limestones: State of the art

Different pathways can be proposed to explain the origin of the red pigmentation present in many Phanerozoic limestones. We investigate three possible models: (1) telogenetic alteration; (2) detrital input and (3) role of iron-bacteria. The third hypothesis is supported by our data for a number of Phanerozoic series. By this hypothesis not only can we highlight the importance of the microenvironments at the dysoxic–anoxic water sediment interfaces for the bacterial Fe-oxidation but also can conclude that the ‘red limestones’ have no particular paleogeographic meaning. The seven studied red successions are rather poor in iron (350 ppm in the Ammonitico Rosso) without large fluctuations. The pigmentation is due to the dispersion of submicrometric (hydro)oxides (now hematite) formed by bacterial mediation during early diagenesis in various microaerophilitic environments. Different ferruginous morphs, e.g., filaments, coccoids, microstromatolites, suggest the presence of iron-bacteria. The limiting factor is the oxygen content which was low in these very quiet and relatively deep environments. The presence of iron-bacteria is also confirmed by iron isotope analysis on the Ammonitico Rosso. Fe isotopic compositions have been analyzed on residues as well as on leachates of red and grey facies of the same interval. δ56Fe values are systematically lower in the red residues (varying from − 1.19‰ to − 0.34‰) compared to the grey ones (0.27‰ to − 0.09‰). This indicates a significant accumulation of lighter isotopes in the red parts suggesting that hematite have been formed via a biological pathway.