Oxidized micrometeorites suggest either high <i>p</i>CO₂ or low <i>p</i>N₂ during the Neoarchean.

Abstract

Tomkins et al. [A. G. Tomkins <i>et al.</i>, <i>Nature</i> 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 <i>p</i>N₂ 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.