Micropyrite in stromatolite : a time capsule of ancient metabolisms

The identification of ancient microbial signatures preserved in the geological record is crucial for understanding life evolution during the Early Earth. Stromatolites and microbialites are considered among undisputable oldest trace of life but their biogenicity is sometimes disputed. Archean stromatolite contain small sulfides hereafter designed as micropyrite, often enclosed with organic material, that can be formed either by abiotic processes (reaction between H2S and Fe(II)) or by metabolic activity like microbial sulfate reduction (MSR) or dissimilatory iron reduction (DIR). Iron and sulfur isotope compositions offer the most direct means to track the biogeochemical cycling of these elements through time, but their joint use as biosignatures of specific metabolic activity has been relatively limited to date. We have developed a microscale approach using correlative microscopy and SIMS and NanoSIMS analyses that allow to document Fe and S isotope composition at the mineralogical scale of the pyrite grains. Here we will present results obtained on the Tumbiana Formation (2.7 Ga, Western Australia), on the Buck Reef chert (3.41 Ga, basal member of the Kromberg Formation, South Africa), and on the Moodies Group (3.2 Ga, South Africa). Our approach allows to decipher the post-depositional metasomatic influence from the primary microbial signatures inherited during diagenesus. Our results also highlight that pyrite formation record very local conditions at the sediment-interface or at the microbial mats. Our results demonstrate the existence of microbial iron respiration as early as 3.26 Gyr ago, providing new time calibration of the tree of life for this type of metabolism.