Period· The columns of time
Siderian
2.5 Ga → 2.3 Ga
The period in which the air acquires oxygen for good, and the planet, stripped of its methane blanket, freezes over.
What the evidence shows
Iron rains out of the sea. Dissolved iron meets the first free oxygen and settles to the seabed, laying down banded formations on a scale never matched since, and they give the period its name, from the Greek word for iron. The oxygen itself arrives around 2.4 billion years ago, and this time it stays. It costs the planet its warmth. Methane cannot survive in air that holds oxygen, and the greenhouse it sustained under a Sun some 15 per cent fainter than today's vanishes with it: the Huronian glaciations follow, and the ice reaches the tropics.
Bounds and conventions
Nothing in the rock marks either bound: 2,500 and 2,300 million years are round numbers, fixed by agreement. The oxygenation they are meant to frame is a process and not an instant, and the sulfur isotopes that record it have not settled on a date. One reading puts the irreversible step near 2.33 billion years, some 170 million years after the period opens; another puts the same transition between 2,502 and 2,434 million years, on the opening itself. The gap decides whether the period contains the event or merely begins with it.
Sources
- Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
- Gumsley et al. (2017), PNAS · Timing and tempo of the Great Oxidation Event
- Luo et al. (2016), Science Advances · Rapid oxygenation of Earth's atmosphere 2.33 billion years ago
- Kopp et al. (2005), PNAS · The Paleoproterozoic snowball Earth: a climate disaster triggered by the evolution of oxygenic photosynthesis
- Warke et al. (2020), PNAS · The Great Oxidation Event preceded a Paleoproterozoic snowball Earth