Photoferrotrophy, BIF deposition, and iron-free oxygen oases in the Archean surface ocean

Abstract Banded Iron Formations (BIFs) are both the world’s largest ore deposits and important geological archives that record the early evolution of the Earth-Life system. BIFs were likely deposited as the result of ferrous iron [Fe(II)] oxidation, precipitation, and sedimentation from iron-rich (ferruginous) seawater, mostly during the Archean Eon. Proposed mechanisms for iron oxidation include abiotic reactions with photosynthetic oxygen, reaction with oxygen catalyzed by iron-oxidizing bacteria (IOB), and anoxic oxidation by anoxygenic iron-oxidizing phototrophic bacteria (photoferrotrophs). These iron oxidation processes may have operated concurrently, but their relative contributions to BIF deposition have not been considered. Here, we developed a 1-D ferruginous ocean model incorporating abiotic iron cycling and the physiology of oxygenic phototrophs, microaerophilic IOB, photoferrotrophs, and iron-reducing bacteria. Our model shows that, under Archean ocean conditions, most iron oxidation and precipitation would have been driven by photoferrotrophy, with a small fraction by microaerophilic IOB and a negligible contribution from abiotic reactions. The combined activities of these pathways led to BIF deposition at rates in line with geological records and, importantly, allowed the development of an Fe(II)-free surface ocean conducive to the formation of oxygen oases and the proliferation of oxygenic phototrophs. Teaser Archean ocean simulation shows that photoferrotrophs dominated the precipitation of BIFs and promoted the formation of marine oxygen oases.