Intense biogeochemical iron cycling revealed in Neoarchean micropyrites from stromatolites

Iron isotope compositions of sedimentary pyrites (FeS2) are used to constrain the redox evolution Precambrian ocean and early Fe-based metabolisms such as Dissimilatory Reduction (DIR). Sedimentary can record biotic abiotic iron reduction, which have similar ranges Fe isotopic fractionation, well post-depositional histories metamorphic overprints that modify compositions. However, some exceptionally well-preserved records, stromatolite-bearing Tumbiana Formation (ca. 2.7 Ga, Western Australia) been proven retain primary information on Early Neoarchean microbial ecosystems associated metabolic pathways. Here, we present in situ measurements micropyrites included four stromatolites from order assess respiration metabolism using signatures. A set 142 has analyzed three lamina types, i.e. micritic, organic-rich fenestral laminae, by Secondary Ion Mass Spectrometry (SIMS), a Hyperion radio-frequency plasma source. The diversity laminae is attributed specific depositional environments, leading formation Type 1 (micritic laminae) 2 (organic-rich diagenetic effects (Type 3, laminae). preserved comparable ?56Fe ranges, respectively ?1.76‰ +4.15‰ ?1.54‰ +4.44‰. 3 recorded range, although slightly more negative values between ?2.20‰ +2.65‰. Globally, our data show large range values, +4.44‰, with unimodal distribution differs bimodal previously reported stromatolites. Such cannot be explained unique process (e.g., biotic/abiotic reduction or pyrite only controlled precipitation rate). It rather could reflect two-step cycling sediment pore water including i) partial oxidation forming Fe(OH)3 positive followed ii) partial, possibly microbially induced, Fe2+ availability for sulfate reducers carrying both ?34S In this model, buildup subsequent through time residual reservoir arising activity methanotrophs, explain strongly ?56FeFe(OH)3 up 4‰. These results indicate Archean mats site interaction several closely linked biogeochemical cycles involving Fe, S C.