The Great Oxidation Event

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The irreversible appearance of free oxygen in Earth's atmosphere, and the largest change any organism has ever made to the planet. Oxygenic photosynthesis in cyanobacteria had probably been operating for some time before this, but the oxygen produced was consumed by reduced iron and sulfur in the oceans and by volcanic gases; the event is the point at which production finally exceeded that sink. The evidence is not the presence of oxygen but the disappearance of a signature that requires its absence: the mass-independent fractionation of sulfur isotopes vanishes from the sedimentary record, and it cannot be regenerated once an ozone layer exists. Concentrations after the transition are poorly constrained — probably somewhere between a fraction of a per cent and a few per cent of present atmospheric levels, far below modern values — and the rise was not monotonic, with evidence of transient oxygen pulses before it and a substantial excursion after. The consequences include the loss of most of the anaerobic biosphere to what was, for it, a poison.

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