Oxygenating Earth

This chapter summarizes the current understanding of Earth's initial oxygenation. Oxygenic photosynthesis seems to have evolved very early on, perhaps no more than one-and-a-half billion years after Earth's formation, according to available data. But it took another billion years, roughly half of Earth's existence at that point, before the molecular oxygen (O2) produced during photosynthesis began to accumulate to meaningful levels in the atmosphere and oceans. The reason for this long time-lag is the O2 mass-balance. Earth needed time to evolve biologically, geologically, and chemically before O2 production sufficiently outpaced O2 destruction. We discuss the most compelling evidence of early O2 production and accumulation. We discuss the most strongly evidenced and seemingly important changes to Earth's mantle, crust, and surface that led to oxygenation. We discuss the apparently fitful nature of the “Great Oxidation Event”. And we discuss current best estimates for how much O2 accumulated in earlyEarth’s atmosphere and oceans. Our discussion ends with a three-part working hypothesis for the initial oxygenation of Earth.