Nitrous oxide from chemodenitrification: A possible missing link in the Proterozoic greenhouse and the evolution of aerobic respiration

The potent greenhouse gas nitrous oxide (N₂O) may have been an important constituent of Earth's atmosphere during Proterozoic (~2.5–0.5 Ga). Here, we tested the hypothesis that chemodenitrification, the rapid reduction of nitric oxide by ferrous iron, would have enhanced the flux of N₂O from ferruginous Proterozoic seas. We empirically derived a rate law, (Formula presented.), and measured an isotopic site preference of +16‰ for the reaction. Using this empirical rate law, and integrating across an oceanwide oxycline, we found that low nM NO and μM-low mM Fe²⁺ concentrations could have sustained a sea-air flux of 100–200 Tg N₂O–N year⁻¹, if N₂ fixation rates were near-modern and all fixed N₂ was emitted as N₂O. A 1D photochemical model was used to obtain steady-state atmospheric N₂O concentrations as a function of sea-air N₂O flux across the wide range of possible pO₂ values (0.001–1 PAL). At 100–200 Tg N₂O–N year⁻¹ and >0.1 PAL O₂, this model yielded low-ppmv N₂O, which would produce several degrees of greenhouse warming at 1.6 ppmv CH₄ and 320 ppmv CO₂. These results suggest that enhanced N₂O production in ferruginous seawater via a previously unconsidered chemodenitrification pathway may have helped to fill a Proterozoic “greenhouse gap,” reconciling an ice-free Mesoproterozoic Earth with a less luminous early Sun. A particularly notable result was that high N₂O fluxes at intermediate O₂ concentrations (0.01–0.1 PAL) would have enhanced ozone screening of solar UV radiation. Due to rapid photolysis in the absence of an ozone shield, N₂O is unlikely to have been an important greenhouse gas if Mesoproterozoic O₂ was 0.001 PAL. At low O₂, N₂O might have played a more important role as life's primary terminal electron acceptor during the transition from an anoxic to oxic surface Earth, and correspondingly, from anaerobic to aerobic metabolisms.