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

Abstract

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. The δ 56Fe values for low-C org, low-C carb, and low-S sedimentary rocks are close to 0‰, the average of igneous rocks. This range is essentially the same as that of C org-poor late Cenozoic loess, aerosol, river loads, and marine sediments and those of C org-poor Phanerozoic–Proterozoic shales. That these δ 56Fe 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 O 2 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 (Fe aq 2+) and Fe-bearing minerals must have been negligible. In contrast, if the Archean atmosphere was oxic, Fe would have been retained as Fe 3+ hydroxides during weathering as it is today, which would produce minimal net isotopic fractionation in bulk detrital sediments. Siderite-rich samples have δ 56Fe values of − 0.5 ± 0.5‰, and experimentally determined Fe aq 2+-siderite fractionation factors suggest that these rocks formed from Fe aq 2+ that had similar or slightly higher δ 56Fe values. The δ 56Fe values calculated for Fe aq 2+ overlaps those of modern submarine hydrothermal fluids, but it is also possible that Fe aq 2+ had δ 56Fe values higher than those of modern hydrothermal fluids, depending upon the Fe aq 2+–Fe carbonate fractionation factor that is used. In contrast, C org-rich samples and magnetite-rich samples have strongly negative δ 56Fe 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 Fe aq 2+ that had very low δ 56Fe values, between − 3‰ and − 1‰. Reduction of Fe 3+ hydroxide by sulfide, precipitation of sulfide minerals, or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low- δ 56Fe Fe aq 2+. We interpret microbial dissimilatory Fe 3+ reduction (DIR) as the best explanation for producing such low δ 56Fe values for Fe aq 2+, and our results suggest that DIR was a significant form of respiration since at least 2.9 Ga.