Oxidized micrometeorites suggest either high p CO 2 or low p N 2 during the Neoarchean

Tomkins et al. [A. G. Tomkins et al., Nature 533, 235–238 (2016)] suggested that iron oxides contained in 2.7-Ga iron micrometeorites can be used to determine the concentration of O2 in the Archean upper atmosphere. Specifically, they argued that the presence of magnetite in these objects implies that O2 must have been near present-day levels (∼21%) within the altitude range where the micrometeorites were melted during entry. Here, we reevaluate their data using a 1D photochemical model. We find that atomic oxygen, O, is the most abundant strong oxidant in the upper atmosphere, rather than O2. But data from shock tube experiments suggest that CO2 itself may also serve as the oxidant, in which case micrometeorite oxidation really constrains the CO2/N2 ratio, not the total oxidant abundance. For an atmosphere containing 0.8 bar of N2, like today, the lower limit on the CO2 mixing ratio is ∼0.23. This would produce a mean surface temperature of ∼300 K at 2.7 Ga, which may be too high, given evidence for glaciation at roughly this time. If pN2 was half the present value, and warming by other greenhouse gases like methane was not a major factor, the mean surface temperature would drop to ∼291 K, consistent with glaciation. This suggests that surface pressure in the Neoarchean may need to have been lower—closer to 0.6 bar—for CO2 to have oxidized the micrometeorites. Ultimately, iron micrometeorites may be an indicator for ancient atmospheric CO2 and surface pressure; and could help resolve discrepancies between climate models and existing CO2 proxies such as paleosols.