Iron and oxygen isotope fractionation during photo-oxidation

For guidance on citations see FAQs. c [not recorded] Version: Version of Record Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online's data policy on reuse of materials please consult the policies page. Introduction. Ultraviolet (UV) photochemical oxidation of aqueous ferrous iron (Fe(II)aq) has been proposed as an effective pathway to the precipitation of banded iron formations (BIFs) [1, 2]. The rationale is that in the early Precambrian more high-energy UV sunlight could reach the seawater surface, because in absence of O2, the atmosphere was more transparent to UVs. The other two possible alternatives are: (i) O2-mediated oxidation, in which local " O2 oasis " are created during photosynthesis by microorganisms such as cyanobacteria [3, 4], and (ii) anoxygenic photosynthe-sis in which bacteria could use Fe(II)aq instead of H2O as the electron donor to produce the oxidized byprod-uct Fe(III) rather than gaseous oxygen [5, 6]. However, compared to these alternative mechanisms, UV photo-oxidation does not require involvement of any gaseous oxygen or biology, and the oxidation rate has been calculated to be high enough to account for the extensive occurrence of BIFs [2, 7]. The principal argument against photo-oxidation is based on the experimental observation that Fe(II)aq in solution tends to combine with bicarbonate and silicon to form insoluble minerals [8], but the experimental setup may not have been completely relevant to the conditions that prevailed at that time and it posed instead the question of Fe(II) solubility regardless of the process responsible for its oxidation. There is no direct evidence for the involvement of biology in the formation of BIFs as they lack microfossils [9]. The only indirect evidence comes from carbon and iron stable isotope signatures of BIF-associated carbonates, which point to dissimilatory iron reduction associated with respiration of organic carbon [10, 11]. To examine the role of UV photo-oxidation in BIF formation and to differentiate between the three scenarios, we performed photo-oxidation lab experiments and analyzed the isotopic compositions of the products to a very high precision. In particular, the mass fractionation law governing the photo-oxidation process was investigated. Methods. Our experimental procedure has already been described in detail previously [12] and so is only outlined here in brief. Oxygen is evacuated from a closed reaction system with high-purity Ar gas (O2 < 0.1 ppm). The system consists of …