Potential Role of Fe(II)-Oxidizing Photoautotrophic Bacteria in the Deposition of Precambrian Banded Iron Formations

Banded iron formations (BIFs) are Precambrian sedimentary deposits that consist of alternating layers of silica and Fe(II) or Fe(III) minerals. How these deposits formed at different periods in Earth history, in particular how the Fe(III) minerals in the BIFs were formed from the Fe(II) that was present in ancient oceans, has not been resolved, despite intensive investigation over the last century. Central to this enigma is the controversy over the following questions: When did O2 evolve on our planet? Was O2 present in sufficient concentrations to be responsible for the deposition of the earliest BIFs (ca. 3.8-2.2 Ga)? The discovery of anoxygenic Fe(II)-oxidizing bacteria provides an alternative explanation for BIF deposition in the absence of O2. Here we present calculations based on Fe(II)-oxidation rates determined experimentally with different modern Fe(II)-oxidizing organisms under light regimes representative of water depths of a few hundred meters. Additionally we provide a new hypothesis based on geochemical and microbiological data to explain the alternating layering of silica and iron in BIFs. Our experimental results and theoretical considerations suggest that, in a stratified Precambrian ocean, a layer of anoxygenic phototrophic bacteria living beneath the wind-mixed surface layer was the most likely catalyst for BIF deposition, even in the presence of O2-producing cyanobacteria.