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. Before and during the Gaskiers glaciation 580 million years ago, the iron in deep-sea sediments records a deep ocean rich in iron and free of oxygen; in the sediments laid down after it, the same chemistry records an oxygenated one. But statistical analysis of the whole geochemical record finds no evidence that the seas were oxygenated wholesale in the late Neoproterozoic. What it does find is a moderate, long climb in atmospheric oxygen and in how much living matter the ocean produced: changes that would have raised both the dissolved oxygen and the food supply in shallow water, 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 on food that other living things had to build first. The two came together, and neither came all 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, long-term rise, with stable modern conditions reached in the Late Palaeozoic, long after the first animals.
Sources
- Canfield et al. (2007), Science · Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life
- Tostevin & Mills (2020), Interface Focus · Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic
- 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
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