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The evolution of oxygenic photosynthesis was the most important geochemical event in Earth history, causing the Great Oxidation Event (GOE) ∼2.4 b.y. ago. However, evidence is mixed as to whether O₂ production occurred locally as much as 2.8 b.y. ago, creating O₂ oases, or initiated just prior to the GOE. The biogeochemical dynamics of possible O₂ oases have been poorly constrained due to the absence of modern analogs. However, cyanobacteria in microbial mats in a perennially anoxic region of Lake Fryxell, Antarctica, create a 1–2 mm O₂-containing layer in the upper mat during summer, providing the first known modern analog for formation of benthic O₂ oases. In Lake Fryxell, benthic cyanobacteria are present below the oxycline in the lake. Mat photosynthesis rates were slow due to low photon flux rate (1–2 µmol m⁻² s⁻¹) under thick ice cover, but photosynthetic O₂ production was sufficient to …