Microbial processes during deposition and diagenesis of Banded Iron Formations.

Banded Iron Formations (BIFs) are marine chemical sediments consisting of alternating iron (Fe) - rich and silica (Si)-rich bands which were deposited throughout much of the Precambrian era. BIFs represent important proxies for the geochemical composition of Precambrian seawater and provide evidence for early microbial life. Iron present in BIFs was likely precipitated in the form of Fe 3+ (Fe(III)) minerals, such as ferrihydrite (Fe(OH) 3 ), either through the metabolic activity of anoxygenic photoautotrophic Fe 2+ (Fe(II))-oxidizing bacteria (photoferrotrophs), by microaerophilic bacteria, or by the oxidation of dissolved Fe(II) by O 2 produced by early cyanobacteria. However, in addition to oxidized Fe-bearing minerals such as hematite (Fe III 2 O 3 ), (partially) reduced minerals such as magnetite (Fe II Fe III 2 O 4 ) and siderite (Fe II CO 3 ) are found in BIFs as well. The presence of reduced Fe in BIFs has been suggested to reflect the reduction of primary Fe(III) minerals by dissimilatory Fe(III)-reducing bacteria, or by metamorphic (high pressure and temperature) reactions occurring in presence of buried organic matter. Here, we present the current understanding of the role of Fe-metabolizing bacteria in the deposition of BIFs, as well as competing hypotheses that favor an abiotic model for BIF deposition. We also discuss the potential abiotic and microbial reduction of Fe(III) in BIFs after deposition. Further, we review the availability of essential nutrients (e.g. P and Ni) and their implications on early Earth biogeochemistry. Overall, the combined results of various ancient seawater analogue experiments aimed at assessing microbial iron cycling pathways, coupled with the analysis of the BIF rock record, point towards a strong biotic influence during BIF genesis.