Nitrogen isotope signatures of microfossils suggest aerobic metabolism 3.0 Gyr ago

There is compelling evidence for early oxygenation of mid-Archean oceans. However, the biological use of molecular oxygen is still not ascertained. Here we report the nitrogen isotope composition measured in isolated microfossils ( 15 N m ) from the 3.0 billion years old Farrel Quartzite metasediments. We show that the quasi-null bulk 15 N values of Farrel Quartzite organic matter encompass a large 15 N isotopic heterogeneity at the scale of isolated microfossils (-21.6 < 15 N m < +30.7 ). Rayleigh fractionation is required to yield such large 15 N variations. Based on these data, we propose a model in which negative 15 N m values determined on film-like and on spheroidal microfossils are explained by ammonia assimilation in the anoxic deeper levels of the water column, whereas positive 15 N m values determined on lenticular microfossils were driven by both ammonia assimilation and aerobic oxidation close to the sea surface. Since ammonium aerobic oxidation requires the presence of free molecular O 2 within the water column, we further suggest that positive 15 N m values reflect an ocean redox stratification tightly related to O 2 production by oxygenic photosynthesisers in a mid-Archean ocean 3.0 Gyr ago.