Atmospheric CO 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 CO of micrometeorites entering a CO-rich atmosphere. We consider a CO-rich rather than an O-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 CO concentration of >70% by volume. Even if the early atmosphere was thinner than today, the elevated CO level indicated by our model result would help resolve how the Late Archean Earth remained warm when the young Sun was ~20% fainter.