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Iron formations

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Iron formations are Fe-enriched sedimentary rocks found in relative abundance between ∼3,000 and 1,800 million years ago (Ma), and again ∼800 Ma, but they are very scarce in the billion years in between). As IFs are composed of minerals thought to have ultimately derived from Fe(II)-containing marine waters, the lack of Fe formations between 1,800 and 800 Ma implies a lack of abundant seawater Fe(II) during this time interval. There have been two proposals to account for a lack of Fe(II). In one suggestion, atmospheric oxygen concentrations increased to levels sufficient to oxygenate the deep ocean, thus removing Fe(II) through oxidation In another suggestion, Fe(II) was, instead, removed through precipitation by sulfide In the latter scenario, an increase in sulfate flux to the ocean followed the great oxidation of the atmosphere (GOE) some 2,300 Ma An increase in sulfate availability increased rates of sulfate reduction in a dominantly anoxic ocean, removing Fe(II) from solution Recent work has uncovered substantial amounts of ocean anoxia in the time window between 1,800 and 800 Ma thus possibly favoring the second scenario. However, anoxic waters were frequently ferruginous, and thus, the lack of IFs between 1,800 and 800 Ma remains enigmatic.

Apart from issues related to the abundance of IFs through time, there is still much debate on the mechanisms of IF deposition IFs comprise a variety of different minerals including Fe oxides, Fe sulfides, Fe silicates, and Fe carbonates), and there is much discussion about which of these phases are primary precipitates and which are secondary minerals. There is also a broader debate as to how these minerals formed and the possible role of biology in their genesis

Understanding the genesis of the Fe minerals in IFs is one step toward understanding the relationship between IFs and the chemical and biological environment in which they formed. For example, the high Fe oxide content of many IFs is commonly explained by a reaction between oxygen and Fe(II) in the upper marine water column, with Fe(II) sourced from the ocean depths. The oxygen could have come from exchange equilibrium with oxygen in the atmosphere or from elevated oxygen concentrations from cyanobacteria at the water-column chemocline. The oxidation could have been strictly inorganic, given the rapid reaction kinetics between Fe(II) and oxygen or it could have been mediated by aerobic Fe(II)-oxidizing prokaryotes like Mariprofundus sp.. Indeed, there is some evidence that biological oxidation is favored over inorganic oxidation at low oxygen concentrations

Another possible route of iron oxide formation is the oxidation of Fe(II) to iron oxyhydroxides by bacterial anoxygenic photosynthesis. Notably, this pathway does not require oxygen. Anoxygenic photosynthesis was first proposed by and as having a possible role in IF formation, and the anoxygenic phototrophic oxidation of Fe(II) was subsequently discovered as a prokaryote metabolism by ref. . Since then, anoxygenic photosynthetic oxidation of Fe(II) has been found among both purple nonsulfur and green sulfur bacterial lineages and these bacteria have been found active in modern ferruginous settings These observations support the idea that anoxygenic photosynthesis could have driven Fe(II) oxidation in ancient IF settings Still, despite these recent insights, the role of anoxygenic phototrophic bacteria in IF formation remains uncertain

While many IFs are characterized by iron oxide phases, many also contain abundant, if not dominant, iron carbonates. These minerals precipitated either directly from the anoxic Fe(II)-containing water column or during early diagenesis in the sediments. In both cases, microbial reduction of ferric oxides produced in the upper water column would have elevated the concentration of Fe(II) as well as the pH and bicarbonate concentration, all favorable for siderite precipitation However, the metabolic products of microbial respiration reactions typically accumulate to much higher concentrations in sediments compared with the water column so that mineral formation driven by respiration is more favored in sediments. The carbonate associated with siderite in siderite-rich IFs is typically depleted in 13C, sometimes greatly so, compared with likely values for seawater These large 13C-depleted values are consistent with the role of Fe reduction in carbonate formation, most likely in sediments but without further proof, the role of Fe reduction in IF generation remains uncertain.

We report here on a substantial IF in the ∼1,400 Ma Xiamaling Formation of the North China Craton. Within the current boundaries of the Xiamaling Formation, this IF likely contained ∼520 gigatons (Gt) of authigenic Fe, placing it in the same order of magnitude as many Archean, Paleoproterozoic, and Neoproterozoic IFs The Xiamaling IF also contains biomarker and geochemical evidence for a dynamic biological Fe cycle, including extensive and efficient dissimilatory Fe reduction and anoxygenic phototrophic Fe oxidation. Much of this evidence is only possible because of the low thermal maturity of the Xiamaling rocks uniquely preserving biomarkers and other organic geochemical indicators. Thus, the Xiamaling IF provides the best-resolved insights to date as to the role of the ancient Fe cycle in IF deposition and diagenesis, and during a time when IF deposition has been considered unimportant.

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