The effects of microbial iron reduction and silica on green rust in banded iron formations

Abstract

The geochemical composition of Earth’s oceans was different from that of modern oceans prior to the Great Oxidation Event (2.4 Ga), with negligible free oxygen and abundant iron dissolved in both water columns and sediments. Deposition of these anoxic, iron-rich marine sediments, mainly in the Archean eon (2.5-4 Ga), gave rise to banded iron formations (BIFs). The first life on Earth, in the form of microbes, is thought to have evolved under the same conditions as BIFs. Thus, BIFs may contain a record of those microbes’ metabolisms. Archean BIFs are composed of iron-rich oxides, carbonates, and silicates. These minerals are thought to be secondary minerals altered from an unknown primary phase by a combination of abiotic and possibly biotic processes. One proposed primary BIF phase is carbonate green rust, an Fe(II,III) salt that could have been transformed via microbial iron reduction into Fe(II)-rich secondary BIF minerals. The two main goals of this study were 1) to explore the effects of microbial iron reduction on green rust and 2) to explore the effects of silica on green rust transformation. Silica concentrations are thought to have been high in Archean oceans, but silica is also known to stabilize green rust and slow its transformation into other mineral phases, raising questions about how the high marine silica and hypothesized green rust BIF sediments interacted to form the BIF minerals seen today. Using scanning electron microscopy, Raman spectroscopy, and X-ray diffraction, we found that after hydrothermal aging, green rust transformed into siderite and subsequently magnetite, a common BIF carbonate and oxide, respectively, independent of the presence of S. putrefaciens. These results provide support for green rust as a primary BIF phase. However, the results suggest that siderite is not a reliable biosignature in the context of green rust as it forms in both biotic and abiotic conditions. Siderite was only seen in the absence of silica, while iron-silica coprecipitates dominated when silica was present. Our findings potentially constrain BIF mineral formation conditions, suggesting that iron silicates were the predominant minerals forming when marine silica concentrations were high and iron carbonates appeared only when silica levels were low. Further exploration of biotic and abiotic iron reduction pathways under these conditions would improve our understanding of early life on Earth and could help identify signs of past or present life elsewhere in our solar system and beyond.