Ocean disaster will take a million years to recover

The fear of a dramatic drop in world ocean oxygen content has been heightened as new work suggests it could take a million years for oceans to recover.

Scientists have warned that we are "on the edge of anoxia", which could cause widespread extinction of marine life. Now a study from the University of Exeter suggests while dramatic drops in oceanic oxygen, which cause mass extinctions of sea life, do come to a natural end – it takes about a million years.

Sarah Baker, lead researcher from the University, said: “Once you get into a major event like anoxia, it takes a long time for the Earth's system to rebalance. This shows the vital importance of limiting disruption to the carbon cycle to regulate the system and keep it within habitable bounds."

The scientists studied the Toarcian Oceanic Anoxic Event, which occurred 183 million years ago. This was accompanied by a large disturbance to the global carbon cycle, depleting both ocean oxygen levels and large numbers of marine life.

Assisted by computer models, scientists predict after effects of an anoxia event – increased burial of organic carbon, due to less decomposition, and more plant and marine productivity in the warmer, carbon-rich environment – should drive a rise in atmospheric oxygen. To test the theory, scientists examined fossil charcoal samples to see evidence of wildfires which would have occurred more frequently during oxygen-rich times.

Researchers found a period of increased wildfire activity started one million years after the beginning of the anoxic event, lasting for approximately 800,000 years. Baker said: “We argue that this major increase in fire activity was primarily driven by increased atmospheric oxygen. Our study provides the first fossil-based evidence that such a change in atmospheric oxygen levels could occur in a period of one million years.”

The increased proliferation of wildfires may have helped end ocean anoxia by reducing the numbers of plants on land. This creates a domino effect as these plants would help erode rocks that could support marine life in the oceans. Due to less marine life in the oceans – which would breathe oxygen – the oceans build up higher oxygen content, ending anoxia.

It may be essential, say the researchers, to maintain the natural functioning of wildfire activity to help regulate the Earth system in the long-term.

The paper was published in Nature Communications.