Archean to early Paleoproterozoic iron formations document a transition in iron oxidation mechanisms
Résumé
It is generally accepted that photosynthetic marine planktonic bacteria were responsible for the oxidation of dissolved ferrous iron (Fe(II)) and the subsequent deposition of iron formations (IFs) throughout the Archean and early Paleoproterozoic. However, the relative roles of the different biological Fe oxidation mechanisms in driving IF deposition-such as anoxygenic photosynthesis (photoferrotrophs) and oxygenic photosynthesis (cyanobacteria)-remain poorly resolved. Here, we present coupled bulk-rock Fe isotope and manganese (Mn) versus Fe ratios from Archean to early Paleoproterozoic IFs in order to provide a new perspective on Earth's early redox history and processes leading to IF deposition. Based on this updated IF geochemical record, we bolster the case that the partial oxidation of Fe(II) to Fe(III) was central to IF genesis, arguing against extensive water column Fe(II) silicate formation as the main process driving IF deposition. The geochemistryof IFs deposited prior to the Great Oxidation Event (GOE) shows that partial Fe(II) oxidation was a common feature in either anoxic or low oxygen (O2) conditions, where metabolic Fe(II) oxidation by photoferrotrophs is likely to have prevailed over ambient Fe(II)
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