ATMOSPHERE × EARTH × LIFE

The Second Rise of Oxygen

~800 to 540 million years ago

Proterozoic · Neoproterozoic · Tonian

Oxygen rose a second time, a billion and a half years after the first, and the second rise was not shaped like the first. The deep ocean, iron-rich and oxygen-free before the Gaskiers glaciation 580 million years ago, turns oxic in the sediments laid down just after it. But statistical analysis of the whole geochemical record finds no wholesale oxygenation of the seas. What it finds is a moderate, long climb in atmospheric oxygen and in how much life the ocean was fixing: enough to raise both the oxygen and the food in shallow water, which is where the first animals lived.

Why it matters

An animal is an expensive way to live: muscle, movement and hunting all run on oxygen, and they run on eating something that concentrated it first. Both arrived together, and neither arrived at once, so animals appear across tens of millions of years rather than on a date.

Dating & uncertaintyDisputed

That oxygen rose again is agreed; how far, how fast and when are not. Iron chemistry shows the deep ocean turning oxic after the Gaskiers glaciation at 580 Ma, while statistical analyses of the whole geochemical record find only a moderate rise before the Palaeozoic, with modern conditions reached much later.

Sources

1. Canfield et al. (2007), Science · Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life
2. Tostevin & Mills (2020), Interface Focus · Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic
3. Stockey et al. (2024), Nature Geoscience · Sustained increases in atmospheric oxygen and marine productivity in the Neoproterozoic and Palaeozoic eras

Earth history as one day

19:46:154 h 14 min before midnight

if Earth’s 4.54 billion years were compressed into a single day