Great Oxidation Event
Hypotheses

Dissolved iron in oceans exemplifies O sinks. Free oxygen produced during this time was chemically captured by dissolved iron, converting iron Fe and Fe 2+ to magnetite (Fe 2+ Fe 3+ 2 O) that is insoluble in water, and sank to the bottom of the shallow seas to create banded iron formations. It took 50 million years or longer to deplete the oxygen sinks. The rate of photosynthesis and associated rate of organic burial also affect the rate of oxygen accumulation. When land plants spread over the continents in the Devonian, more organic carbon was buried and likely allowed higher O levels to occur. Today, the average time that an O molecule spends in the air before it is consumed by geological sinks is about 2 million years. That residence time is relatively short in geologic time; so in the Phanerozoic, there must have been feedback processes that kept the atmospheric O level within bounds suitable for animal life.
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