Oceans '˜on verge of mass extinction event', scientists warn

The world's oceans are at the tipping point of being starved of oxygen leading to mass sea life extinction which could last a million years.

Mediterranean bream fish in the Mediterranean Sea, off the Sormiou creek near Marseille in southern France. Scientists fear the modern ocean is on the edge of anoxia. Picture: Boris Horvat/AFP/Getty

University of Exeter scientists fear the modern ocean is “on the edge of anoxia”, when the seas are depleted of oxygen.

And while this dramatic drop in oceanic oxygen does comes to a natural end, the process takes about a million years.

Studying what happened during the Jurassic period, researchers found the drop in oxygen causes more organic carbon to be buried in sediment on the ocean floor.

This eventually leads to rising oxygen in the atmosphere which ultimately re-oxygenates the ocean. However, it took a million years to re-establish the balance.

Lead researcher Sarah Baker said it was now “critical” for modern humans to limit carbon emissions to prevent this.

She 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 Earth system and keep it within habitable bounds.”

The researchers studied the Toarcian Oceanic Anoxic Event, which took place 183 million years ago.

This was characterised by a major disturbance to the global carbon cycle, depleted oxygen in the Earth’s oceans and mass extinction of marine life.

Numerical models predicted that 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, causing the end of an anoxic event after one million years.

Testing the theory, they examined fossil charcoal samples to see evidence of wildfires, which would be more common in oxygen-rich times.

These were taken at Mochras in Wales and Peniche, Portugal.

Experts found a period of increased wildfire activity which started one million years after the onset of the anoxic event and lasted for about 800,000 years.

Ms Baker added: “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 increase in fire activity may have also helped end ocean anoxia by burning and reducing the amount of plants on land.