EARTH × LIFE × ATMOSPHERE
The Great Oxidation Event
~2.4 billion years ago
Proterozoic · Paleoproterozoic · Siderian
Oxygen accumulated in the Earth's atmosphere and stayed there for the first time about 2.4 billion years ago, after hundreds of millions of years in which iron, volcanic gases and rock absorbed it as fast as microbes produced it.
The Great Oxidation Event is the lasting accumulation of oxygen in the Earth's atmosphere, about 2.4 billion years ago. Microbes had been producing oxygen for hundreds of millions of years, but three kinds of matter absorbed it as fast as it appeared: dissolved iron, volcanic gases and reduced rock, which is rock that can still combine with oxygen. Such absorbers are called sinks. Oxygen began to accumulate when the sinks could no longer absorb all that the microbes produced. The evidence of this change comes from sulfur in rocks. Sulfur has several isotopes, which are atoms of the same element with different masses. A particular proportion of these isotopes, called an isotopic signature, forms only in air free of oxygen and ozone, a gas made of oxygen atoms. This signature disappears from rocks across the whole planet and afterwards reappears only briefly. The whole event took about two hundred million years, during which the oxygen concentration rose and fell back before stabilising.
Why it matters
The Great Oxidation Event is the widest upheaval that living organisms have caused on the Earth, and the Earth has not returned to its earlier state. Oxygen was toxic to a large part of the organisms then alive. It probably ended the greenhouse effect of methane, a gas that traps heat, and probably allowed the glaciation of the world, in which ice spread across the planet. At the same time, oxygen made respiration possible. Respiration is a slow oxidation, a chemical reaction with oxygen, which extracts energy from food. Every later organism that respires, the human species included, lives in the world that the Great Oxidation Event produced.
Dating & uncertainty
The Great Oxidation Event began between 2.46 and 2.33 billion years ago. Researchers debate whether the oxygen concentration then remained high, or fell back, with oxygen becoming permanent only about 2.22 billion years ago, as Poulton and colleagues (2021) argue. The answer changes the interpretation of the first global glaciations. The debate rests on records of sulfur isotopes in rocks, and these records disagree.
Earth history as one day
11:18:4512 h 41 min before midnight
if Earth’s 4.54 billion years were compressed into a single day