Mineral-catalysed formation of marine NO and N 2 O on the anoxic early Earth

Microbial denitrification converts fixed nitrogen species into gases in extant oceans. However, it is unclear how such transformations occurred within the early nitrogen cycle of the Archaean. Here we demonstrate under simulated Archaean conditions mineral-catalysed reduction of nitrite via green rust and magnetite to reach enzymatic conversion rates. We find that in an Fe 2+ -rich marine environment, Fe minerals could have mediated the formation of nitric oxide (NO) and nitrous oxide (N 2 O). Nitrate did not exhibit reactivity in the presence of either mineral or aqueous Fe 2+ ; however, both minerals induced rapid nitrite reduction to NO and N 2 O. While N 2 O escaped into the gas phase (63% of nitrite nitrogen, with green rust as the catalyst), NO remained associated with precipitates (7%), serving as a potential shuttle to the benthic ocean. Diffusion and photochemical modelling suggest that marine N 2 O emissions would have sustained 0.8–6.0 parts per billion of atmospheric N 2 O without a protective ozone layer. Our findings imply a globally distributed abiotic denitrification process that feasibly aided early microbial life to accrue new capabilities, such as respiratory metabolisms.