Iron-bacterial mediation in Phanerozoic red limestones: State of the art Bernard Mamet, Alain Pre ´at * University of Brussels, Department of Earth and Environmental Sciences, CP160/02-Geology, 50 av F-D Roosevelt, B-1050, Brussels, Belgium Abstract 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 dred limestonesT 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. d 56 Fe values are systematically lower in the red residues (varying from À 1.19x to À 0.34x) compared to the grey ones (0.27x to À 0.09x). This indicates a significant accumulation of lighter isotopes in the red parts suggesting that hematite have been formed via a biological pathway. D 2005 Elsevier B.V. All rights reserved. Keywords: Hematite; Red pigmentation; Iron-bacteria; Diagenesis 1. Introduction Three different hypotheses can be considered to explain the origin of pigmentation of red limestones which are found sporadically throughout Phanerozoic time. Whatever may be the relation with the iron, the following three scenarios are possible: (1) telogenetic (post-sedimentary) superficial alteration; (2) detrital origin of the iron derived from continental weathering during sedimentation; (3) presence of iron-bacteria at the time of sedimentation. The first hypothesis concerns surface contamination. Superficial alteration or dcontaminationT can be related to simple weathering or iron-bacteria. Hofmann and Farmer (2000) have described various filamentous fab- rics in macroporous rocks such as volcanics, oxidized ores, limestone solution cavities and macrofossils. These microbial filaments grew in subsurface environ- ments affecting the original rocks. For example, Fe- encrusted filaments were identified in low-temperature quartz deposited in solution cavities in massive Devo- nian limestones in Germany (Kretzchmar, 1982). The age of this occurrence, and others located close by, is 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.12.009 * Corresponding author. Fax: +32 2 6502226. E-mail address: apreat@ulb.ac.be (A. Pre ´at). Sedimentary Geology 185 (2006) 147 – 157 www.elsevier.com/locate/sedgeo