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doi: 10.7185/geochemlet.1816 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 (δNμm) from the 3.0 billion years old Farrel Quartzite metasediments. We show that the quasi-null bulk δ15N values of Farrel Quartzite organic matter encompass a large 15N isotopic heterogeneity at the scale of isolated microfossils (-21.6 ‰ < δNμm < +30.7 ‰). Rayleigh fractionation is required to yield such large δ15N variations. Based on these data, we propose a model in which negative δNμm 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 δNμm 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 O2 within the water column, we further suggest that positive δNμm values reflect an ocean redox stratification tightly related to O2 production by oxygenic photosynthesisers in a mid-Archean ocean 3.0 Gyr ago. Received 10 July 2017 | Accepted 10 June 2018 | Published 24 July 2018 1. Muséum National d’Histoire Naturelle, Sorbonne Université, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France 2. Department of Environmental Engineering and Architecture, Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan 3. School of Earth and Environmental Sciences, University of Manchester, M13 9PL Manchester, UK 4. Sorbonne Université, UPMC, CNRS, EPHE, PSL, UMR 7619 METIS, 4 place Jussieu, F-75005 Paris, France * Corresponding author (email: fdelarue@mnhn.fr) Introduction The isotopic composition of nitrogen (N) has been investigated in order to tackle questions related to Archean life (Beaumont and Robert, 1999; Pinti et al., 2001; Thomazo et al., 2011; Stueken et al., 2015a). In the present day natural environment, N is mostly present as di-nitrogen (N2), ammonium (NH4), organic N, nitrite (NO2) and nitrate (NO3). Within the marine N cycle, N compounds are closely coupled in a redox reaction network including specific isotope fractionation processes (Sigman et al., 2009). Accordingly, δ15N of sedimentary organic matter (OM) has been used to infer the evolution of the redox conditions of Archean oceans. For instance, negative δ15N values ranging between -7 and 0 ‰ in the OM isolated from early Archean rocks (3.6 to 3.2 Ga) have been interpreted as reflecting (i) anoxic Archean oceans in which microorganisms consumed N2 and NH4 (Beaumont and Robert, 1999) or (ii) a consumption of N2 and NH4 by chemosynthetic bacteria flourishing close to 15N-depleted hydrothermal vents (Pinti et al., 2001; Shen et al., 2006). The late Archean era (2.8 to 2.5 Ga) is characterised by the occurrence of positive OM δ15N values (up to ca. +5 ‰) presumably reflecting a rise in the abundance of oxidised N species (Beaumont and Robert, 1999; Garvin et al., 2009; Godfrey and Falkowski, 2009). This suggests that significant free O2 was present in the oceans at that time, which is also supported by several inorganic palaeoredox proxies (Anbar et al., 2007; Reinhard et al., 2009; Stueken et al., 2015b). Although early and late Archean sedimentary δ15N values suggest a progressive rise in the oxygenation level of Archean oceans, mid-Archean times (3.2 to 2.8 Ga) are characterised by a narrow range of δ15N values close to the N isotope composition of the present day atmosphere (0 ‰), suggesting the dominance of N2 fixing microorganisms (Stueken et al., 2015a). However, complementary geochemical evidence points toward oxygenic photosynthesis occurring as early as ca. 3.0 Ga (Crowe et al., 2013; Lyons et al., 2014; Planavsky et al., 2014) and ocean redox stratification ca. 3.2 Gyr ago (Satkoski et al., 2015). If correct, such early oxygenation and ocean redox stratification should be recorded in redox-dependent sedimentary δ15N values, implying in turn that the δ15N values around 0 ± 2 ‰ measured on bulk samples might mask isotopic heterogeneities present at smaller scales. To investigate this issue, we have characterised the N isotope composition of organic microfossils (noted δNμm) isolated from the mid-Archean Farrel Quartzite formation (3.0 Ga; Sugitani et al., 2007) using nanoscale secondary ion mass spectrometry (NanoSIMS).

Nitrogen isotope signatures of microfossils suggest aerobic metabolism 3.0 Gyr ago