Period· The columns of time

Siderian

2.5 Ga → 2.3 Ga

In the Siderian, from 2.5 to 2.3 billion years ago, oxygen became durably established in the atmosphere and the planet, stripped of its methane greenhouse, froze for the first time.

Period in which oxygen becomes durably established in the atmosphere and the planet, deprived of its methane greenhouse, undergoes its first global glaciations.

What the evidence shows

Dissolved iron meets the first free oxygen, oxidises and precipitates, depositing banded formations on a scale never since matched; these deposits give the period its name, formed on the Greek sideros, iron. Atmospheric oxygen establishes itself around 2.4 billion years and does not disappear again. The climatic consequence is immediate: methane does not persist in oxygenated air, and the greenhouse it maintained under a Sun about 15 per cent less luminous than today disappears with it. The Huronian glaciations follow, and ice reaches the tropics.

Bounds and conventions

Both limits, 2,500 and 2,300 million years, are round numbers fixed by convention and correspond to no marker. The oxygenation they enclose is a process and not an instant, and the sulfur isotopes that record it do not supply a single date. One reading places the irreversible step around 2.33 billion years, some 170 million years after the period opens; another places it between 2,502 and 2,434 million years, that is, at that opening itself. The difference determines whether the period contains the event or begins with it.