ATMOSPHERE × EARTH
The Proterozoic Oxygen Crash
~2.06 to 1.95 billion years ago
Proterozoic · Paleoproterozoic · Rhyacian
Oxygen in the atmosphere, after a first rise, fell back to a fraction of its peak between about 2.06 and 1.95 billion years ago and stayed low for more than a billion years, an interval called the boring billion.
The oxygen crash is the fall in the concentration of oxygen in the air after the burst that followed the first oxygenation. The concentration fell back to a fraction of its peak and remained at that level for more than a billion years, an interval known as the boring billion. One explanation attributes the fall to a shortage of phosphorus. Phosphorus is the limiting nutrient of photosynthesis, meaning the nutrient whose scarcity sets the ceiling on production whatever else is available. As the supply of phosphorus ran down, less organic carbon, the carbon of living matter, was produced and buried. Oxygen accumulates in the air only when carbon is buried: photosynthesis releases oxygen when it makes organic carbon, and carbon left to decay consumes that oxygen again.
Why it matters
Oxygen rose to a peak, collapsed, and then barely varied for a billion years. It therefore did not reach its present level by a gradual rise.
Dating & uncertainty
During the crash, carbon isotopes, the light and heavy forms of carbon, return to their baseline ratio. Marine sulfate evaporites, salts left by evaporating seawater, and phosphorites, phosphate-rich rocks, cease to form. Shales, rocks formed from mud, become less oxidised. The cause is not established. One explanation is that the nutrient supply feeding the rise ran out. Eguchi and colleagues (2019) tie the whole episode to a change in volcanic carbon degassing, the release of carbon by volcanoes. A third view questions whether the isotopic excursion, the shift in carbon isotopes on which this account rests, was global.
Earth history as one day
13:06:3610 h 53 min before midnight
if Earth’s 4.54 billion years were compressed into a single day