FIG. 1.
Overview of the major processes controlling atmospheric O2 and CH4 levels (left panels). Red circles/arrows denote net biogenic sources (e.g., after accounting for biological consumption), gray ovals/arrows denote potential abiotic sources, and blue squares/arrows denote net sink fluxes. Schematic depiction of the evolution of atmospheric O2 and CH4 throughout Earth's history (right panels). Using results from Reinhard et al. (2017), shaded boxes show ranges based on geochemical proxy or model reconstructions, whereas red curves show possible temporal trajectories through time. (Credit: Chris Reinhard)