Development of a 1-D oxygen isotope photochemical model and its application to atmospheric O₂

Abstract

Oxygen is the second most abundant gas in the Earth’s atmosphere, but this has not always been the case. A suite of geochemical, palaeobiological, and geological proxies have been presented over the last few decades to better constrain the evolution of pO₂ over the history of our planet, but uncertainty remains. Here, we use numerical modelling with the 1-D photochemical model Atmos, firstly by exploring the boundary conditions of the model, and secondly by developing it to predict ∆¹⁷O values – a fairly novel proxy for Proterozoic and Phanerozoic pO₂. Our study of boundary conditions highlights the importance of choosing and describing boundary conditions carefully, as our flux-driven models produce somewhat different results to previous fixed mixing ratio-driven models. Our results provide a potential constraint on pO₂, suggesting that atmospheres with 6×10⁻⁷ < pO₂ < 2×10⁻³ may have been unlikely to exist for long periods of Earth history. We review these conclusions using our newly-developed oxygen isotope model, tuned to predict modern atmospheric ∆¹⁷O. Preliminary results predict the production and preservation of non-zero ∆¹⁷O in the geological record can occur for palaeo-atmospheres with pO₂ > 10⁻⁴, but even the minimum values observed at 1.4 Ga and 635 Ma do not require such low concentrations, especially if pCO₂ is higher than modern. The development of the oxygen isotope model allows the better prediction of ∆¹⁷O under various atmospheric conditions, and will be a useful tool in the interpretation of anomalous oxygen isotope compositions in the geological record.