Oxygen production and rapid iron oxidation in stromatolites immediately predating the Great Oxidation Event

Earth's surface became permanently oxygenated during the Great Oxidation Episode, a geochemical transition between ∼2.43 and 2.22 billion years ago, but shallow-water cyanobacterial oases with molecular oxygen production likely existed for hundreds of millions of years before, in the otherwise anoxic and iron-rich oceans. Despite abundant geochemical evidence for elevated ambient oxygen in the Archean upper ocean, sites of active microbial oxygen production have not been geochemically characterized. We report geochemical and iron isotopic data for a horizon of iron-rich stromatolites in the 2.46–2.43 Ga Griquatown Iron Formation in South Africa deposited on the margin of an anoxic ferruginous basin. Bulk-rock and micro-sampled iron isotope data for the stromatolites indicate quantitative oxidation of iron delivered by deep upwelling currents, which is most readily explained by cyanobacterial communities inhabiting the stromatolites and producing local oxygen enrichments near the fair-weather wave-base. Modest enrichments in Mn and Ce indicate high oxidation potential in this stromatolitic setting. The iron-rich nature of the stromatolites indicates that upwelling iron sources in the early Paleoproterozoic oceans overwhelmed established iron-precipitation mechanisms in deeper basins that had generally maintained iron-poor conditions in shallow-marine peritidal zone during the Neoarchean.