Résumé
Stromatolites are considered to be the major drivers for the oceanic oxygenation during Archean time. However, respective rolesof photosynthetic and chemolithotroph organisms in these Archean rocks is not precisely demonstrated. Primarysub-micrometer pyrites associated with pristine organic matter are widespreadin Archean and modern sediments. Iron isotopes are fractionated by redox processesand biological activitiesand are widely used for tracing paleoredox conditions, diagenetic processes ormetabolic signaturesas iron respiration (DIR). Sub micrometer δ56Fevalues ofmicropyritesassociated with organic matter-laminae and CaCO3-SiO2matrixin stromatolites from the 2.7Ga Tumbiana Formationhave been measured by SIMS using the new Hyperion RF Plasma source. Iron isotope compositions of micropyritesdisplaya wide rangeof valuesfrom-2.20‰ to +4.44‰,comparable to the entire rangeof known terrestrialiron isotopevalues (between-2.4‰ and +2‰). Our data show a large and continuous range defined by more positive values compared to thebimodal distributionpreviously described(Yoshiya et al., 2012). Our results areinterpreted as an interplay between abioticoxidationand biological induced reductionprocessesduring iron cycling at the biofilm scale.