Ocean waters made lethal by carbon dioxide from massive volcanic eruptions are being blamed for the greatest mass extinction in Earth's history.

More than 250 million years ago, nearly all marine life disappeared when global volcanic activity spewed carbon dioxide that settled into the oceans and made them acidic at levels that were deadly for most marine species alive at the time, researchers suggest.

Ninety percent of marine species and more than two-thirds of animals living on land perished at the end of the Permian period in what scientists have taken to calling the Great Dying.

Chemical evidence in rocks from that time has shown just how quickly the chemical makeup of the world's oceans was altered, researchers report in the journal Science.

The killer punch of CO2 could have been delivered by massive episodes of volcanic activity in what is known as the Siberian Traps, existing today as an immense region of volcanic rock occupying northern Eurasia, the researchers say.

Those episodes may have been ongoing for 60,000 years and led to an increase of ocean acidification that could have lasted for as long as 10,000 years, researchers say.

A study team headed by Matthew Clarkson at the University of Edinburgh went looking for the evidence in rocks from the United Arab Emirates, rocks that would have been on the sea floor 250 million years ago.

In those rocks they found signs of the changing conditions in the oceans of the world in that distant past.

"Scientists have long suspected that an ocean acidification event occurred during the greatest mass extinction of all time, but direct evidence has been lacking until now," Clarkson says.

Scientists say the findings could increase our understanding of the risks modern-day ocean acidification caused by climate change presents to marine life today.

While the total amount of carbon added to the atmosphere 250 million years ago is greater than all of today's fossil fuel reserves represent, modern emissions are adding carbon at a similar rate.

That's worrying, scientists say, because a fast rate of release was a critical factor in driving up the ocean acidification that triggered the Great Dying.

"This is a worrying finding, considering that we can already see an increase in ocean acidity today that is the result of human carbon emissions," Clarkson says.

Oceans can absorb some carbon dioxide but large volumes released at a fast rate can quickly change the chemistry of the oceans, the researchers say.

"It's such a rapid change, the ocean can't buffer the CO2 increase," Clarkson warns.