Neoarchaean oxygen-based nitrogen cycle en route to the Great Oxidation Event.

The nitrogen isotopic composition of sedimentary rocks (δ 15 N) can trace redox-dependent biological pathways and early Earth oxygenation 1,2 . However, there is no substantial change in the sedimentary δ 15 N record across the Great Oxidation Event about 2.45 billion years ago (Ga) 3 , a prominent redox change. This argues for a temporal decoupling between the emergence of the first oxygen-based oxidative pathways of the nitrogen cycle and the accumulation of atmospheric oxygen after 2.45 Ga (ref.  3 ). The transition between both states shows strongly positive δ 15 N values (10-50‰) in rocks deposited between 2.8 Ga and 2.6 Ga, but their origin and spatial extent remain uncertain 4,5 . Here we report strongly positive δ 15 N values (>30‰) in the 2.68-Gyr-old shallow to deep marine sedimentary deposit of the Serra Sul Formation 6 , Amazonian Craton, Brazil. Our findings are best explained by regionally variable extents of ammonium oxidation to N 2 or N 2 O tied to a cryptic oxygen cycle, implying that oxygenic photosynthesis was operating at 2.7 Ga. Molecular oxygen production probably shifted the redox potential so that an intermediate N cycle based on ammonium oxidation developed before nitrate accumulation in surface waters. We propose to name this period, when strongly positive nitrogen isotopic compositions are superimposed on the usual range of Precambrian δ 15 N values, the Nitrogen Isotope Event. We suggest that it marks the earliest steps of the biogeochemical reorganizations that led to the Great Oxidation Event.