Marine mineral-catalyzed NO and N2O formation on the anoxic early Earth

Microbial metabolisms were limited by available terminal electron acceptors in the anoxic environment of the early Archean. However, iron mineral phases in Fe2+-rich (ferruginous) oceans could have catalyzed reactions with abiotically fixed nitrogen leading to the formation of nitrous oxide (N2O), a potentially favorable terminal electron acceptor. We experimentally simulated anoxic surface-catalyzed reduction of nitrite and nitrate via green rust and magnetite. Besides N2O, we detected and quantified the formation of substantial amounts of nitric oxide (NO). While N2O escaped into the gas phase (63% of nitrite-nitrogen, with green rust as catalyst), NO remained associated with precipitates (7% of nitrite-nitrogen). Using diffusion and photochemical modeling, we show that marine N2O emissions could have sustained atmospheric N2O pools of 1-7 ppb. Although this concentration was insufficient to cause significant warming, the seawater enriched in N2O and NO could have critically affected early benthic life by providing the opportunity to conserve energy.