State of the planet· The columns of time

Oxygenated world

2.22 Ga → 717 Ma

For a billion and a half years, from 2.22 billion to 717 million years ago, the atmosphere held oxygen in very small quantity, a ceiling that is one explanation offered for the late arrival of large animals.

Regime of a billion and a half years during which the atmosphere contains oxygen in very small quantity, that ceiling being one of the explanations advanced for the late appearance of large animals.

What the evidence shows

Oxygen had been produced for hundreds of millions of years before this regime opens; what changes is that it ceases to be consumed at the rate of its production. The situation then remains unchanged for a billion and a half years, from 2.22 billion to 717 million. Concentrations remain far below present ones, probably below a few per cent of them, which suffices for a cell but not for a large organism: an active animal must convey oxygen from its surface to its internal tissues, and below a certain concentration diffusion no longer suffices. This constraint is one of the explanations long advanced for the appearance of animals at the end of this regime and not during the billion years preceding it.

Bounds and conventions

The regime marks a presence and not an abundance, a distinction that carries most of the discussion: to state that the Proterozoic atmosphere contained oxygen is accurate, to suppose it breathable is not. The quantity is estimated from trace metals and sediment isotopes, methods that more readily supply a ceiling than a value, and the published ceilings differ from one another by a factor of ten.