The early evolution of the biogeochemical nitrogen cycle.

Résumé: For the early (Precambrian) part of Earth's history, very little data constrains the evolution of the nitrogen cycle, a key control on nutrient availability and thus on the productivity and size of the biosphere. We studied nitrogen isotopes in a drill-core from the 2.5 billion year old Mt McRae Shale in the Pilbara Craton of Australia, at high stratigraphic resolution in a homogeneous lithology. Organic d15N changed from +1 per mille to +8 over 25 metres upwards, returning to +2 another 25 metres upwards. As other environmental indicators (Mo and S isotopes, Fe/S systematics) point towards a temporary "whiff" of environmental oxygen coincident with this N isotope excursion, the data is best interpreted as recording a shift from anaerobic to aerobic nitrogen cycling, returning to an anaerobic state as the oxygen pulse ended and euxinic ocean conditions started. During the prolonged marine euxinic interval of the mid-Proterozoic, a similarly disrupted nitrogen cycle may have maintained equable temperatures, as copper limitation curtailed denitrification by suppressing nitrous oxide reductase activity. This could have increased the N2O/N2 ratio of nitrogenous gases returning to the atmosphere, causing a "laughing gas" greenhouse to counteract the Faint Young Sun after atmospheric oxygenation had removed the methane greenhouse effect in the Early Proterozoic.