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Extreme 15N-enrichments in 2.72-Gyr-old sediments: evidence for a turning point in the nitrogen cycle

Authors

  • C. THOMAZO,

    1. Laboratoire de géochimie des isotopes stables, Institut de Physique du Globe de Paris, Université Paris Diderot, CNRS, PRES Sorbonne Paris Cité, France
    2. UMR CNRS 5561 Biogéosciences, Université de Bourgogne, Dijon, France
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  • M. ADER,

    1. Équipe de géochimie des isotopes stables, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France
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  • P. PHILIPPOT

    1. Équipe de geobiosphère actuelle et primitive, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France
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Corresponding author: C. Thomazo. Tel.: +33 38039 35 78; fax: +33 38039 63 67; e-mail: christophe.thomazo@u-bourgogne.fr

Abstract

Although nitrogen is a key element in organic molecules such as nucleic acids and proteins, the timing of the emergence of its modern biogeochemical cycle is poorly known. Recent studies on the antiquity of the nitrogen cycle and its interaction with free oxygen suggests the establishment of a complete aerobic N biogeochemical cycle with nitrification, denitrification, and nitrogen fixation at about 2.68 Gyr. Here, we report new bulk nitrogen isotope data for the 2.72 billion-year-old sedimentary succession of the Tumbiana Formation (Pilbara Craton, Western Australia). The nitrogen isotopic compositions vary widely from +8.6‰ up to +50.4‰ and are inversely correlated with the very low δ13C values of associated organic matter defining the Fortescue excursion (down to about −56‰). We propose that this 15N-enrichment records the onset of nitrification coupled to the continuous removal of its derivatives (nitrite and nitrate) by denitrification. This finding implies an increase in the availability of electron acceptors and probably oxygen in the Tumbiana depositional environment, 300 million years before the oxygenation of the Earth’s atmosphere.

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