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

While microbial denitrification is the major pathway converting fixed nitrogen species into gases in extant oceans, it has been unclear how such transformations occurred within the early nitrogen cycle of the Archean. Here we report that in a Fe2+-rich (ferruginous) marine environment, Fe minerals could have catalyzed abiotic denitrification reactions leading to the formation of nitric oxide (NO) and nitrous oxide (N2O) at rates comparable to those measured in modern oxygen minimum zones. We experimentally simulated anoxic surface-catalyzed reduction of nitrite and nitrate via green rust and magnetite. Nitrate did not exhibit reactivity in the presence of either mineral or aqueous Fe2+, however, both minerals induced rapid nitrite reduction to NO and N2O. While N2O escaped into the atmosphere (63% of nitrite-nitrogen, with green rust as catalyst), NO remained associated with precipitates (7%). Using diffusion and photochemical modeling, we determine that marine N2O emissions would have sustained 0.8-6 ppb of atmospheric N2O without a protective ozone layer. Simultaneously, sinking of green rust particles constituted an effective shuttle for mineral-associated NO to reach benthic ocean layers. Seawater NO and N2O were either available as energy sources for early microbial metabolisms (~100 to 300 kcal e–1) or acted as cell toxins. Our findings add critical detail to the as yet incompletely documented Archean nitrogen cycle, implying a globally distributed process driven by chemical kinetics similar to those of modern enzymatically mediated conversions.