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

Iron isotope compositions of low-metamorphic grade samples of Archean–Paleoproterozoic sedimentary rocks obtained from fresh drill core from the Kaapvaal Craton in South Africa and from the Pilbara Craton in Australia vary by ~ 3‰ in 56Fe/54Fe ratios, reflecting a variety of weathering and diagenetic processes. Depositional ages for the 120 samples studied range from 3.3 to 2.2 Ga, and Fe, C, and S contents define several compositional groups, including samples rich in Fe, organic carbon, carbonate, and sulfide. 6Fe values for low-Corg, low-Ccarb, and low-S sedimentary rocks are close to 0‰, the average of igneous rocks. This range is essentially the same as that of Corg-poor late Cenozoic loess, aerosol, river loads, and marine sediments and those of Corg-poor Phanerozoic–Proterozoic shales. That these δ56Fe values are the same as those of igneous rocks suggests that Fe has behaved conservatively in bulk sediments during sedimentary transport, diagenesis, and lithification since the Archean. These observations indicate that, if atmospheric O2 contents rose dramatically between 2.4 and 2.2 Ga, as proposed by many workers, such a rise did not produce a significant change in the bulk Fe budget of the terrestrial sedimentary system. If the Archean atmosphere was anoxic and Fe was lost from bedrock during soil formation, any isotopic fractionation between aqueous ferrous Fe (Feaq2+) and Fe-bearing minerals must have been negligible. In contrast, if the Archean atmosphere was oxic, Fe would have been retained as Fe3+ hydroxides during weathering as it is today, which would produce minimal net isotopic fractionation in bulk detrital sediments. te-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.