New empirical kinetics of iron oxidation by CO2 applied to micrometeorites imply a CO2-rich Archean atmosphere

The oxidation state of iron-type micrometeorites preserved in 2.7 Ga sedimentary rocks offers a potential proxy for reconstructing Archean atmospheric CO 2 levels. However, the reaction kinetics of CO 2 -driven oxidation during atmospheric entry lack empirical constraints. Here, we present high-temperature experiments using a custom-built, inductively heated vacuum furnace to quantify the reaction kinetics between heated iron and CO 2 under low-pressure anoxic conditions. The newly measured kinetic parameters were incorporated into a one-dimensional micrometeorite entry model, updated with Archean atmospheric profiles from photochemical simulations. Despite being a weaker oxidant than O 2 , our results demonstrate that CO 2 alone can oxidize micrometeorites efficiently in the early upper atmosphere. Moreover, photolysis of CO 2 generates atomic oxygen, and subsequent recombination produces molecular O 2 , both of which could also contribute to micrometeorite oxidation. By integrating experimentally derived reaction rates with modeled entry trajectories, we estimate that surface CO 2 concentrations in the Archean atmosphere exceeded 9 vol%, with an upper bound between 21 vol% and 60 vol%, to reproduce the oxidation observed in one best-preserved Archean micrometeorite. Our results also indicate that when CO 2 levels are below 20 vol%, nearly all incident CO 2 molecules during atmospheric entry are kinetically able to react with micrometeorites, using extrapolated laboratory-derived kinetic parameters. These findings suggest that high CO 2 levels in the Archean atmosphere drove observed micrometeorite oxidation, which is consistent with an inferred need for strong greenhouse warming to counter a faint, young Sun.