Fe isotope fractionation during Fe(II) oxidation by the marine photoferrotroph Rhodovulum iodosum in the presence of Si – Implications for Precambrian iron formation deposition

Abstract

The iron (Fe) isotopic composition of Precambrian iron formations (IFs), besides providing geological context through its mineralogical properties, was suggested to function as a biosignature that can be used to infer a potential microbial role in the formation of the deposited Fe minerals. Anoxygenic phototrophic Fe(II)-oxidizing bacteria (photoferrotrophs), capable of oxidizing Fe(II) anoxically using light energy, were potentially involved in Fe(II) oxidation in anoxic or suboxic Precambrian oceans. The effect of Si on Fe isotopic fractionation between aqueous Fe(II) and Fe–Si-co-precipitates has been investigated before. However, it is currently unknown how stable Fe isotopes are fractionated during enzymatic Fe(II) oxidation under marine hydrogeochemical conditions, and particularly how the presence of Si affects the Fe isotope composition and the isotopic exchange among different Fe phases. We therefore studied Fe isotope fractionation during Fe(II) oxidation by the marine photoferrotroph Rhodovulum iodosum in simulated Precambrian seawater amended with 1mM dissolved Si. Our results show that the change in the Fe isotope compositions over time for both the initial aqueous Fe(II) (Feaq) and the Fe(III) precipitates (Feppt) follow a Rayleigh distillation model. Moreover, the fractionation (ε56Feppt-aq) determined independently from either δ56Feaq or δ56Feppt data resulted in a value of 2.3±0.3 (2SD, N=6). This value differs from the fractionation factor determined previously for Fe(II) oxidation by R. iodosum in the absence of Si, where the fractionation calculated from δ56Feaq (i.e. 0.96–1.18) was different from that calculated from δ56Feppt (1.96–1.98). This difference was attributed to isotopic exchange processes with soluble and sorbed Fe species. The present study suggests that Si present in Precambrian oceans retards Fe isotopic exchange, likely through combined effects of complexation of dissolved Fe species by Si and sorption of Si to Fe(III) minerals, thus lowering sorption of Fe(II) to the Fe(III) minerals, which is necessary for isotopic exchange. In summary our data suggests that Si in ancient oceans played a key role for the Fe isotope composition of Fe(III) minerals that were deposited by photoferrotrophic iron oxidation in Precambrian oceans by minimizing subsequent isotope exchange and recrystallization processes with aqueous Fe(II).