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

The potent greenhouse gas nitrous oxide (N2 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 N2 O from ferruginous Proterozoic seas. We empirically derived a rate law, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mfrac><mml:mrow><mml:mi>d</mml:mi> <mml:mfenced><mml:msub><mml:mi>N</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>O</mml:mi></mml:mfenced> </mml:mrow> <mml:mrow><mml:mi>d</mml:mi> <mml:mi>t</mml:mi></mml:mrow> </mml:mfrac> <mml:mo>=</mml:mo> <mml:mn>7.2</mml:mn> <mml:mo>×</mml:mo> <mml:msup><mml:mn>10</mml:mn> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>5</mml:mn></mml:mrow> </mml:msup> <mml:msup><mml:mrow><mml:mo>[</mml:mo> <mml:msup><mml:mtext>Fe</mml:mtext> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> <mml:mo>]</mml:mo></mml:mrow> <mml:mrow><mml:mn>0.3</mml:mn></mml:mrow> </mml:msup> <mml:msup><mml:mrow><mml:mo>[</mml:mo> <mml:mtext>NO</mml:mtext> <mml:mo>]</mml:mo></mml:mrow> <mml:mn>1</mml:mn></mml:msup> </mml:mrow> </mml:math> , 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 N2 O-N year-1 , if N2 fixation rates were near-modern and all fixed N2 was emitted as N2 O. A 1D photochemical model was used to obtain steady-state atmospheric N2 O concentrations as a function of sea-air N2 O flux across the wide range of possible pO2 values (0.001-1 PAL). At 100-200 Tg N2 O-N year-1 and >0.1 PAL O2 , this model yielded low-ppmv N2 O, which would produce several degrees of greenhouse warming at 1.6 ppmv CH4 and 320 ppmv CO2 . These results suggest that enhanced N2 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 N2 O 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, N2 O is unlikely to have been an important greenhouse gas if Mesoproterozoic O2 was 0.001 PAL. At low O2 , N2 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.