The Great Oxidation Event

What happened

For well over a billion years, life on Earth ran without free oxygen. That changed when cyanobacteria mastered oxygenic photosynthesis, splitting water molecules with sunlight and releasing oxygen as a byproduct. NASA Astrobiology dates the resulting Great Oxidation Event to between 2.5 and 2.3 billion years ago, the point when oxygen first accumulated in Earth's atmosphere and has stayed present ever since. It was not instant. Newly released oxygen reacted with dissolved iron in the oceans first, rusting out of seawater as it went, before enough built up to escape into the air. NASA-funded research on ancient stromatolites in Australia's Shark Bay also found signs of oxygen in small pockets of shallow ocean water even before the main event, a more gradual warm-up than a single global switch flipping.

How we know

The main physical record is the banded iron formation: striped rock made of alternating iron oxide and silica layers, laid down as newly released oxygen reacted with iron dissolved in seawater and settled to the ocean floor. A U.S. Geological Survey study on iron-formation chemistry found that deposition of most of these formations ended abruptly about 1.85 billion years ago, right when the ocean's impact with the giant Sudbury asteroid appears to have mixed oxygen-poor deep water into the shallows, a strikingly specific full stop to a billion years of rusting oceans.

Sources

• NASA Astrobiology Institute. Clues of Earth's Early Rise of Oxygen (2019) · Reputable sourceastrobiology.nasa.gov · The domain "astrobiology.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
• U.S. Geological Survey. Extraterrestrial demise of banded iron formations 1.85 billion years ago · Reputable sourceusgs.gov · The domain "usgs.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)