FIG. 3.

Evolution of Earth's atmospheric oxygen content through time. Red curve shows the authors' preferred model for long-term evolution (see text for details), while the blue shaded region reflects alternative views based on numerical simulations that predict O₂ stability only at higher pO₂ (Daines et al., 2017). Solid red arrows denote possible transient increases in pO₂ for which geochemical evidence exists; pink dashed arrows indicate less certain events. Blue arrows in the late Archean indicate hypothetical “whiffs” of O₂ and are included to highlight schematically the oscillatory system behavior assumed to accompany transitions from one steady state to another. Supporting details are provided in the supplementary material (see Supplementary Fig. S2). PAL = present atmospheric level.