Atmospheric CO₂ levels from 2.7 billion years ago inferred from micrometeorite oxidation

Earth's atmospheric composition during the Archean eon of 4 to 2.5 billion years ago has few constraints. However, the geochemistry of recently discovered iron-rich micrometeorites from 2.7 billion-year-old limestones could serve as a proxy for ancient gas concentrations. When micrometeorites entered the atmosphere, they melted and preserved a record of atmospheric interaction. We model the motion, evaporation, and kinetic oxidation by CO₂ of micrometeorites entering a CO₂-rich atmosphere. We consider a CO₂-rich rather than an O₂-rich atmosphere, as considered previously, because this better represents likely atmospheric conditions in the anoxic Archean. Our model reproduces the observed oxidation state of micrometeorites at 2.7 Ga for an estimated atmospheric CO₂ concentration of >70% by volume. Even if the early atmosphere was thinner than today, the elevated CO₂ level indicated by our model result would help resolve how the Late Archean Earth remained warm when the young Sun was ~20% fainter.