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Neoproterozoic Oxygenation Event
c. 800–540 million years ago (debated)· Event· Earth, climate & extinctions
DebatedScientists disagree about when, how or why.
A second major rise in oxygen, roughly 800 to 540 million years ago, that may have brought air and sea closer to modern levels.
See it on the Life map →What happened
After the Boring Billion, chemical signals in rocks point to oxygen rising again in the atmosphere and spreading into deeper ocean water. Proposed drivers include nutrients washed into the sea after the Snowball Earth glaciations, more algae burying carbon, and tectonic changes. Rather than one step, many researchers now see several pulses, with the deep ocean perhaps not fully oxygenated until about 420 million years ago.
Why it matters
Large, active animals need far more oxygen than microbes, so this rise is a leading candidate for why animals appeared when they did.
The evidence (fossils, genes, rocks)
Trace metals such as molybdenum and uranium in black shales, chromium and selenium isotope shifts, the iron chemistry of ocean sediments, and large carbon isotope swings.
Still debated
Its timing, size and cause are all debated; some argue oxygen rose after animals evolved, possibly partly because of them.
Connections (3)
▸Snowball Earth may have boosted Neoproterozoic Oxygenation EventDebated
The connection
Glaciers grinding over continents produce fine rock flour rich in phosphorus, a key nutrient. When the ice melted, that flood of nutrients may have fed algae blooms; buried algae locked away carbon, leaving more oxygen in the air. Some researchers link the oxygen rise directly to the Snowball glaciations.
When and how
After each Cryogenian glaciation, about 660 and 635 million years ago; Noah Planavsky and colleagues proposed the phosphorus link in 2010.
How sure are we?
Phosphorus is abundant in iron-rich rocks deposited around the glaciations, and some oxygen indicators rise afterward, but the timing is imprecise.
Debates and open questions
Oxygen may have begun rising before the Snowball, so the glaciations might be part of a wider change rather than its cause.
▸Neoproterozoic Oxygenation Event supplied oxygen for Ediacaran biotaDebated
The connection
Large organisms need more oxygen than microbes, because oxygen must diffuse into bigger bodies. Rising oxygen in shallow and deep seas may have allowed Ediacaran organisms to reach meter scale and live on deep seafloors such as Mistaken Point.
When and how
About 580–539 million years ago; Don Canfield and colleagues linked deep-ocean oxygenation to the Newfoundland fossils in 2007.
How sure are we?
Iron chemistry in Newfoundland sediments suggests deep water became oxygenated after the Gaskiers glaciation, shortly before large fossils appear there.
Debates and open questions
Oxygen seems to have swung up and down; some argue Ediacaran organisms needed little oxygen and other factors mattered more.
▸Neoproterozoic Oxygenation Event is one candidate in Why the Cambrian explosion happenedDebated
The connection
The oxygen hypothesis says animal bodies could not grow large, active or predatory until oxygen rose enough. In this view the Neoproterozoic Oxygenation Event lifted a ceiling, and the Cambrian explosion followed once oxygen passed a threshold, perhaps in pulses.
When and how
Oxygen rose about 800–540 million years ago; the idea goes back to J. R. Nursall in 1959 and remains under active study.
How sure are we?
Rock chemistry shows rising and fluctuating oxygen around the Cambrian, and some pulses of animal diversity seem to match oxygen pulses.
Debates and open questions
Sponges and simple animals need little oxygen, and some studies find no rise right before the explosion. Is oxygen cause or consequence?