Assessing the Importance of Iron Photo-oxidation on Ancient Earth and Beyond

Abstract:

Every planet orbiting a star is subject to photochemical reactions that can potentially alter its habitability. Unfortunately, we do not understand photochemistry in complex aqueous systems well enough to infer its importance for other worlds. This is a concern for exoplanets orbiting M and K type stars because a higher fraction of the energy emitted by these stars is in the UV wavelength range as compared to the Sun. Consequently, the ocean chemistry of these worlds could be very different from our own.

We are investigating the photo-oxidation of dissolved Fe²⁺ to Fe³⁺ in experimental simulations of Earth’s early oceans. Earth’s oceans from > 2.5 Ga are an analog for other worlds because of the larger UV flux before the development of an atmospheric ozone layer. The photochemical oxidation of Fe²⁺ can remove Fe from anoxic oceans via production of insoluble Fe³⁺-oxyhydroxides. Fe²⁺ photo-oxidation can also drive the formation of H₂ when H⁺ is the e^- acceptor. Published photogeochemical models suggest that on the early Earth, Fe photo-oxidation rates could have been high enough to account for the deposition of Precambrian Banded Iron Formations¹ and to produce H₂ at rates comparable to volcanism or serpentinization^2,3. However, these rates depend on Fe²⁺ photo-oxidation having a significant quantum yield at wavelengths > 300 nm, where the solar flux is greater than at shorter wavelengths. More recent experimental studies conflict on this point⁴.

We are revisiting this question with an improved design, including a full-spectrum Xe lamp and direct determinations of [Fe²⁺], [Fe³⁺], [Fe_T]. When our experimental solutions are exposed to full spectrum UV radiation, Fe²⁺ oxidizes at a rate of ~2%/day. Fe³⁺ forms and Fe³⁺-oxyhydroxides precipitate. However, when light < 325 nm is optically blocked, [Fe²⁺] and [Fe³⁺] remain constant within analytical error, and no precipitate forms. The reduced wavelength range able to induce Fe²⁺ photo-oxidation reduces the Fe²⁺ photo-oxidation rate, as well as photochemically induced H₂ production rate by at least a factor of 5. The significant downward revision of estimated Fe²⁺ photo-oxidation rates on early Earth can inform the development of photogeochemical models for exoplanetary oceans.

¹Braterman et al., 1983, ²Braterman & Cairns-Smith, 1987, ³Catling, 2014, ⁴Konhauser et al., 2007.