Abstract
Geochemical evidence for local redox transitions is observed 300 Ma years prior to the Great Oxygenation Event (GOE ca. 2400 Ma) and suggests evolving environmental conditions through a continuous sequence where euxinia (anoxic and sulfidic) preceded the expansion of more oxidizing conditions. Based on multiproxy data such as iron speciation (FePy/FeHR > 0.7) and Mo enrichments (varying from 3 to 64), we present one of the oldest records of euxinia from the Archean Pitangui greenstone belt (~2700 Ma), Minas Gerais, Brazil. The sulfur isotopic composition of sedimentary sulfides indicates early inputs of sulfate from oxidative weathering and an attenuation of sulfur mass independent fractionation that combined with other geochemical signatures (FeHR/FeT, Ce anomalies and rare earth element fractionation) is consistent with the expansion of oxygen availability, and suggests localized oxic conditions before the GOE.
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