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The evolution of oxygenic photosynthesis was the most important geochemical event in Earth history, causing the Great Oxidation Event (GOE) similar to 2.4 b.y. ago. However, evidence is mixed as to whether O-2 production occurred locally as much as 2.8 b.y. ago, creating O-2 oases, or initiated just prior to the GOE. The biogeochemical dynamics of possible O-2 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 O-2-containing layer in the upper mat during summer, providing the first known modern analog for formation of benthic O-2 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 mu mol m(-2) s(-1)) under thick ice cover, but photosynthetic O-2 production was sufficient to sustain up to 50 mu mol O-2 L-1, sandwiched between anoxic overlying water and anoxic sediments. We hypothesize that Archean cyanobacteria could have similarly created O-2 oases in benthic mats prior to the GOE. Analogous mats may have been at least partly responsible for geological evidence of oxidative weathering prior to the GOE, and habitats such as Lake Fryxell provide natural laboratories where the impact of benthic O-2 oases on biogeochemical signatures can be investigated.