Fig. 4. An emergent view of Archean terrestrial O₂ production.

It is likely that oxidative weathering signatures such as Mo enrichments in shales preceded atmospheric oxygenation due to local O₂ production and consumption in terrestrial environments. Shallow soils in proximity to microbial communities (green in the figure) may have experienced greater than ~10⁻⁷ PAL PO₂, which was capable of partially oxidizing sulfide grains and delivering trace metals such as Mo to rivers and marine environments in excess of hydrothermal or detrital contributions. Until the late Archean, these O₂ fluxes had very little effect on atmospheric redox, which remained <10⁻⁶ PAL PO₂ in the presence of rapidly supplied reductants (O₂ sinks).