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On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28- to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ 56 Fe values of -1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ 33 S values > +3‰, while the other groups display more variable and closer to 0‰ Δ 33 S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ 33 S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass-independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro-organisms closely related to the last common ancestor had the ability to reduce Fe(III)., (© 2020 The Authors. Geobiology published by John Wiley & Sons Ltd.)

Microbial sulfate reduction (MSR) is thought to have operated very early on Earth and is often invoked to explain the occurrence of sedimentary sulfides in the rock record. Sedimentary sulfides can also form from sulfides produced abiotically during late diagenesis or metamorphism. As both biotic and abiotic processes contribute to the bulk of sedimentary sulfides, tracing back the original microbial signature from the earliest Earth record is challenging. We present in situ sulfur isotope data from nanopyrites occurring in carbonaceous remains lining the domical shape of stromatolite knobs of the 2.7-Gyr-old Tumbiana Formation (Western Australia). The analyzed nanopyrites show a large range of δ 34 S values of about 84‰ (from -33.7‰ to +50.4‰). The recognition that a large δ 34 S range of 80‰ is found in individual carbonaceous-rich layers support the interpretation that the nanopyrites were formed in microbial mats through MSR by a Rayleigh distillation process during early diagenesis. An active microbial cycling of sulfur during formation of the stromatolite may have facilitated the mixing of different sulfur pools (atmospheric and hydrothermal) and explain the weak mass independent signature (MIF-S) recorded in the Tumbiana Formation. These results confirm that MSR participated actively to the biogeochemical cycling of sulfur during the Neoarchean and support previous models suggesting anaerobic oxidation of methane using sulfate in the Tumbiana environment., (© 2018 John Wiley & Sons Ltd.)

The significant degradation that fossilized biomolecules may experience during burial makes it challenging to assess the biogenicity of organic microstructures in ancient rocks. Here we investigate the molecular signatures of 1.88 Ga Gunflint organic microfossils as a function of their diagenetic history. Synchrotron-based XANES data collected in situ on individual microfossils, at the submicrometre scale, are compared with data collected on modern microorganisms. Despite diagenetic temperatures of ∼150-170 °C deduced from Raman data, the molecular signatures of some Gunflint organic microfossils have been exceptionally well preserved. Remarkably, amide groups derived from protein compounds can still be detected. We also demonstrate that an additional increase of diagenetic temperature of only 50 °C and the nanoscale association with carbonate minerals have significantly altered the molecular signatures of Gunflint organic microfossils from other localities. Altogether, the present study provides key insights for eventually decoding the earliest fossil record.