The first oxygen oases
Whiffs of oxygen long before the air filled up
See it on the timeline →In depth
Trace elements such as molybdenum and chromium in shales from Mount McRae in Australia show oxygen existed locally as early as three billion years ago, over 500 million years before the Great Oxidation. This changes the picture from a switch to a threshold: production came first, and the sinks — dissolved iron, volcanic gases, weathering sulphides — had to be saturated before it could accumulate globally. The distinction between "when did it start" and "when did it become visible" is one of the most general lessons in Earth history.
For children
Bacteria had been making oxygen for hundreds of millions of years before the air filled with it. Where they grew thickest, in shallow lagoons, the water around them became rich in oxygen — small oases in a world without any. The rest was swallowed up by iron and rock.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org