The role of microaerophilic Fe-oxidizing micro-organisms in producing banded iron formations

摘要

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe-oxidizing micro-organisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder whether similar organisms played a role in producing BIFs. To evaluate this, we review the current knowledge of modern microaerophilic FeOM in the context of BIF paleoenvironmental studies. In modern environments wherever Fe(II) and O-2 co-exist, microaerophilic FeOM proliferate. These organisms grow in a variety of environments, including the marine water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O-2 concentrations, measured as low as 2m to date, although lower concentrations have not been tested. While some extant FeOM can tolerate high O-2 concentrations, many FeOM appear to prefer and thrive at low O-2 concentrations (similar to 3-25m). These are similar to the estimated dissolved O-2 concentrations in the few hundred million years prior to the Great Oxidation Event' (GOE). We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O-2 and show that microaerophilic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. Based on this synthesis, we propose that microaerophilic FeOM were capable of playing a significant role in depositing the largest, most well-known BIFs associated with the GOE, as well as afterward when global O-2 levels increased.