State of the planet· The columns of time
Great Oxidation Event
2.46 Ga → 2.22 Ga
Free oxygen accumulates in the atmosphere, in fits and starts, alongside the first global glaciations.
What the evidence shows
Two hundred and forty million years of transition, which is why this is a band and not a line. Oxygen begins to accumulate between 2.46 and 2.43 billion years ago, and a separate line of evidence places irreversible accumulation at 2.33 billion. The two dates do not contradict each other: they measure different moments of the same slow crisis. Glaciations accompany the change, and that is not a coincidence, since oxygen destroys methane, and methane was what kept the planet warm under a fainter Sun.
Bounds and conventions
Oxygen does not arrive and stay: between the first accumulation and the last it rises and falls repeatedly, and the oscillation does not settle until about 2.25 billion years ago. Matching one continent's rock layers to another's over this interval is still argued, which is why the edges of the band are drawn soft.