Biogeochemical cycling of iron in the Archean-Paleoproterozoic Earth: Constraints from iron isotope variations in sedimentary rocks from the Kaapvaal and Pilbara Cratons

Siderite-rich samples have δ56Fe values of − 0.5 ± 0.5‰, and experimentally determined Feaq2+-siderite fractionation factors suggest that these rocks formed from Feaq2+ that had similar or slightly higher δ56Fe values. The δ56Fe values calculated for Feaq2+ overlaps those of modern submarine hydrothermal fluids, but it is also possible that Feaq2+ had δ56Fe values higher than those of modern hydrothermal fluids, depending upon the Feaq2+-Fe carbonate fractionation factor that is used. In contrast, Corg-rich samples and magnetite-rich samples have strongly negative δ56Fe values, generally between − 2.3‰ and − 1.0‰, and available fluid-mineral fractionation factors suggest that the Fe-bearing minerals siderite and magnetite in these rocks formed in the presence of Feaq2+ that had very low δ56Fe values, between − 3‰ and − 1‰. Reduction of Fe3+ hydroxide by sulfide, precipitation of sulfide minerals, or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low-δ56Fe Feaq2+. We interpret microbial dissimilatory Fe3+ reduction (DIR) as the best explanation for producing such low δ56Fe values for Feaq2+, and our results suggest that DIR was a significant form of respiration since at least 2.9 Ga.