Several lines of evidence indicate that the advent of oxygenic photosynthesis preceded the oxygenation of the atmosphere—perhaps by as much as 300 million years. The fate of biogenic O₂ prior to its appearance in the atmosphere remains speculative, but recent work suggests that O₂ was locally available within the surface ocean to support aerobic microbial ecosystems. Simple mass balance predicts that locally oxygenated environments (oxygen oases) could exist in areas of high productivity if the local rate of O₂ production by oxygenic photosynthesis exceeded the combined rate of O₂ loss by a number of processes (e.g., exchange with the atmosphere, transport within the ocean, reaction with reduced aqueous species, biological consumption). The areal extent of these environments and the dissolved O₂ concentrations that could have persisted in an otherwise anoxic ocean, however, are key uncertainties in our understanding of the spatiotemporal redox-evolution of the early earth system.

We use an earth system model of intermediate complexity that has been modified to simulate a theoretical Archean biosphere in order to explore redox heterogeneity in the late Archean surface ocean. We demonstrate that oxygen oases are an expected consequence of oxygenic photosynthesis beneath an essentially O₂-devoid atmosphere—and that oxygenated surface waters need not be restricted to shallow coastal environments or microbial mats. Within oxygen oases, O₂ concentrations locally approach ~ 1–10 μM for a large range of plausible Archean conditions. Although O₂ concentrations in the open ocean are exceedingly low, biologically relevant dissolved O₂ concentrations are widespread in our hypothetical surface ocean.