Onset of persistent surface ocean oxygenation during the Great Oxidation Event

Free oxygen (O₂) first began accumulating in Earth's atmosphere shortly after the Archean-Proterozoic transition during the 'Great Oxidation Event' (GOE). The nature of surface ocean oxygenation at this time is, however, poorly quantified, limiting our understanding of planetary oxygenation thresholds. Geochemical records of global ocean redox may potentially shed light on this critical interval. Here, we show that vanadium (V) isotope ratios in 2.32-2.26-billion-year-old (Ga) shales from the Transvaal Supergroup, South Africa, capture a unidirectional transition in global ocean redox conditions shortly above the stratigraphic level marking the canonical rise of atmospheric O₂. Around 2.32 Ga, sedimentary sinks were dominated by anoxic environments that drove extensive seawater V drawdown. A positive shift in seawater V isotopic composition in the overlying stratigraphy indicates global expansion of marine settings with ≥ 10 μM dissolved O₂ in bottom water, likely restricted to shallow-water environments and attributable to widespread equilibration with an oxygenated atmosphere.