The classic interpretation of the Archaean banded iron formations (BIFs) presumes the presence of dissolved O₂ in the surface ocean to oxidize ferrous Fe. However, at least two alternative oxidation mechanisms are possible: UV photo-oxidation [1]; and the activity of anaerobic Fe(II)-oxidizing photosynthetic bacteria [2]. We are investigating Fe isotope fractionation as a means of differentiating amongst these mechanisms.

Photo-oxidation has been examined at pH ~3 and pH ~7 at 41°C in the presence of the following ligands: H₂O, OH^-, SiO₂, HCO₃^-, and Cl^- using UVA (316-400 nm) and UVC (200-280 nm) light sources. In these experiments, ferrous Fe was either oxidized and precipitated as ferric oxyhydroxide or precipitated as ferrous minerals, e.g. siderite (FeCO₃) with various ferrous silicate phases when silica was present. We find that isotopically heavy Fe was preferentially removed from solution. The fractionation factor (a)for the overall reaction in the pH ~ 3 experiments is ~ 1.0025. This value is comparable to the a between Fe²⁺ and Fe³⁺ hexaquo complexes [3], but larger than the effect seen during the overall process of ferrous Fe oxidation and precipitation at near-neutral pH [4].

The magnitude of isotope fractionation likely displays pH dependent variations for two reasons. First, ferric oxyhydroxide precipitation, which may impart a kinetic isotope effect, is faster at higher pH; however, when using a HCO3^- buffer system must compete with the formation of ferrous minerals. Second, the major UV-absorbing ferrous species in the ocean is the ferrous hydroxide ion [Fe(OH)⁺], the concentration of which is strongly pH dependent. Photo-oxidation experiments at realistic seawater pH are under current investigation.