State of the planet· The columns of time

Great Oxidation Event

2.46 Ga → 2.22 Ga

Between 2.46 and 2.22 billion years ago, free oxygen accumulated in the atmosphere in pulses, at the same time as the first global glaciations.

Transitional regime during which free oxygen accumulates in the atmosphere in pulses, simultaneously with the first global glaciations.

What the evidence shows

The transition covers two hundred and forty million years, which warrants a band and not a line. Oxygen begins to accumulate between 2.46 and 2.43 billion years, and another line of evidence places the irreversible accumulation at 2.33 billion. The two datings do not conflict: they measure distinct moments of the same transition. Glaciations accompany the change, and the relation is not fortuitous: oxygen destroys methane, whose greenhouse effect kept the planet above freezing under a less luminous Sun.

Bounds and conventions

Oxygen does not establish itself at once: between the first accumulation and the last it rises and falls several times, the oscillation subsiding only around 2.25 billion years. Correlating layers between continents across this interval remains debated, hence limits drawn as gradients.