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

The potent greenhouse gas nitrous oxide (N O) 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 N O from ferruginous Proterozoic seas. We empirically derived a rate law, , 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 Fe concentrations could have sustained a sea-air flux of 100-200 Tg N O-N year , if N fixation rates were near-modern and all fixed N was emitted as N O. A 1D photochemical model was used to obtain steady-state atmospheric N O concentrations as a function of sea-air N O flux across the wide range of possible pO values (0.001-1 PAL). At 100-200 Tg N O-N year and >0.1 PAL O , this model yielded low-ppmv N O, which would produce several degrees of greenhouse warming at 1.6 ppmv CH and 320 ppmv CO . These results suggest that enhanced N O 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 N O fluxes at intermediate O 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, N O is unlikely to have been an important greenhouse gas if Mesoproterozoic O was 0.001 PAL. At low O , N O 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.