CO2 and O2 oxidized 2.7 Ga micrometeorites in two stages suggesting a >32% CO2 atmosphere

It is widely accepted that atmospheric pO2 < 1 ppm before the Great Oxidation Event. Yet a recent study found fossil micrometeorites (MMs) containing oxidized iron species wüstite (FeO) and magnetite (Fe3O4) formed 2.7 billion years ago (Ga). How these MMs became uncertain. Abundant O2 in upper atmosphere oxidation by CO2 have been suggested. However, photochemical reactions cannot produce sufficient O2, can only FeO, each individually failing to explain formation of Fe3O4-only MMs. Using an model including photochemistry, we show >32% Archean different entry angles generate Fe-FeO mixed composition discovered. happens two stages: under brief melting, then O2. Our results challenge existing constraints on Earth’s concentration at Ga support warm Late despite ‘faint young Sun’.