Oceanic Anoxic Event 1a
The oceans lose their oxygen, bury a continent's worth of carbon, and turn corrosive.
- When
- —
- Duration
- 1.2 million years
- Known to
- ± 500 thousand years
- Era
- Cretaceous, Mesozoic
The Selli Event, around 120 million years ago, is the first of the great Cretaceous ocean crises. Volcanic carbon warms the climate, slows deep circulation and fertilises the surface ocean; organic matter rains down faster than it can be oxidised, and black shales accumulate across ocean basins. Carbonate producers suffer as the water acidifies, and reef systems are hit hard. The buried carbon eventually draws carbon dioxide back down, ending the event — a self-terminating crisis, but one that takes over a million years to resolve.