Premium
Nitrous oxide from chemodenitrification: A possible missing link in the Proterozoic greenhouse and the evolution of aerobic respiration
Author(s) -
Stanton Chloe L.,
Reinhard Christopher T.,
Kasting James F.,
Ostrom Nathaniel E.,
Haslun Joshua A.,
Lyons Timothy W.,
Glass Jennifer B.
Publication year - 2018
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12311
Subject(s) - proterozoic , nitrous oxide , ferrous , snowball earth , greenhouse gas , flux (metallurgy) , chemistry , atmosphere (unit) , atmospheric sciences , environmental chemistry , geology , oceanography , thermodynamics , physics , paleontology , organic chemistry , glacial period , tectonics
Abstract The potent greenhouse gas nitrous oxide (N 2 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 2 O from ferruginous Proterozoic seas. We empirically derived a rate law,d N 2 Od t = 7.2 × 10 − 5[ Fe 2 + ] 0.3[ NO ] 1, 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 2+ concentrations could have sustained a sea‐air flux of 100–200 Tg N 2 O–N year −1 , if N 2 fixation rates were near‐modern and all fixed N 2 was emitted as N 2 O. A 1D photochemical model was used to obtain steady‐state atmospheric N 2 O concentrations as a function of sea‐air N 2 O flux across the wide range of possible p O 2 values (0.001–1 PAL ). At 100–200 Tg N 2 O–N year −1 and >0.1 PAL O 2 , this model yielded low‐ppmv N 2 O, which would produce several degrees of greenhouse warming at 1.6 ppmv CH 4 and 320 ppmv CO 2 . These results suggest that enhanced N 2 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 2 O fluxes at intermediate O 2 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 2 O is unlikely to have been an important greenhouse gas if Mesoproterozoic O 2 was 0.001 PAL . At low O 2 , N 2 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.