Acidified Earth's oceans, caused by extreme volcanic activity, triggered the greatest extinction in the planet's history, a study suggested Thursday.
The event, which took place 252 million years ago, wiped out more than 90 percent of marine species and more than two-thirds of the animals living on land.
It happened when Earth's oceans absorbed huge amounts of carbon dioxide from volcanic eruptions, which made them acidic with catastrophic consequences for life on Earth, researchers reported in the U.S. journal Science.
The amount of carbon added to the atmosphere that triggered the mass extinction was probably greater than today's fossil fuel reserves, according to the study coordinated by the University of Edinburgh.
The researchers analyzed rocks unearthed in the United Arab Emirates, which were on the ocean floor at the time, to develop a climate model to work out what drove the extinction. The rocks preserve a detailed record of changing oceanic conditions at the time.
They found volcanism related to the formation of the Siberian Traps likely injected two pulses of carbon dioxide into the atmosphere during the Permian-Triassic Boundary extinction, which lasted about 60,000 years.
That included a 50,000-year slow leak of the gas into the atmosphere and then a 10,000-year large, rapid injection.
The researchers suggested that Earth's oceans were highly alkaline at first, buffering them against the first pulse of the carbon but that the second, larger pulse of gas triggered a widespread ocean acidification event.
It was probably this abrupt change in seawater pH after the second pulse of carbon dioxide that eliminated the vast majority of heavily calcified marine life from the seas, they said.
And the mass extinction of both marine and land-based animals demonstrated that extreme change took place in all of Earth's ecosystems.
"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," said Matthew Clarkson, of the University of Edinburgh's School of GeoSciences, who coordinated the study.
"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."
The study was carried out in collaboration with the University of Bremen, Germany, and the Universities of Exeter, Graz, Leeds, and Cambridge.