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Oxygen-starved oceans can take a million years to recover


A new model may explain why an ancient crash in the ocean’s oxygen levels lasted a million years and led to almost another million years of catastrophic fires, writes Andrew Masterson.


A drop in ocean oxygen levels is bad news for marine life, and new research suggests a recovery will not be quick.
Dave Fleetham / Getty

About 183 million years ago something disrupted the Earth’s carbon cycle, which triggered a sudden drop in the oxygen levels of the world’s oceans, initiating a mass extinction of marine species.

Depleted oxygen levels – known as anoxia – continued for one million years. When it returned to pre-crisis levels, it was accompanied by an upsurge in forest fires, which lasted for around 800,000 years.

Understanding what processes govern the slow recovery from anoxic events is now a little easier, thanks to a fossil-based study conducted by a team led by geographer Sarah Baker from the UK’s University of Exeter.

The work is published in the journal Nature Communications.

Baker and colleagues first used computer simulations to model the likely effects of the Jurassic oxygen plunge, known as the Toarcian Oceanic Anoxic Event.

They deduced that a decrease in the level of oxygen in the oceans would result in the slowing down of decomposition of dead plants and animals, and thus an increase in the amount of organic carbon ending up buried in the sediment on the sea floor.

The low levels of oxygen – and thus proportionately higher levels of carbon dioxide – would also drive greater photosynthetic plant activity in the water and on the land, which, in turn, would lead to a rise in oxygen levels.

Eventually, after a million years, the proportion of oxygen in the ocean and the atmosphere would return to pre-anoxic levels. Since higher levels of oxygen making combustion easier, any such increase, the modeling suggested, would produce a surge in forest fires.

To test the computer predictions, Baker and her team sought out fossil carbon samples from Wales and Portugal and tested them for evidence of wildfires. They found it was abundant, and dated within a range of 800,000 years – exactly as the modelling had predicted.

“We argue that this major increase in fire activity was primarily driven by increased atmospheric oxygen,” says Baker.

The team’s paper concludes that the wildfire activity constitutes the first fossil-based evidence of “fire-feedback” playing a role in ending the Toarcian event.

The discovery that oceanic anoxia lasts a million years and ends in a conflagration only slightly shorter carries considerable contemporary resonance, with many climate scientists believing global warming has pushed the world to the edge of another anoxic catastrophe.

“This shows the vital importance of limiting disruption to the carbon cycle to regulate the Earth system and keep it within habitable bounds,” says Baker.

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Andrew Masterson is news editor of Cosmos.
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