A MASS extinction which wiped out more than 90 per cent of prehistoric marine species and two thirds of animals living on land was triggered by chemical changes in the ocean brought about by massive volcanic eruptions, a new study suggests.

Huge amounts of carbon dioxide released by the volcanic eruptions were absorbed by the Earth's oceans, causing them to become much more acidic.

This had catastrophic consequences for life of Earth, according to scientists at Edinburgh University.

The study is the first to show that that highly acidic oceans were to blame for the Doomsday-like decimation of life on Earth 252 million years ago - around 20 million years before the first dinosaurs emerged.

The event, known as the Permian-Triassic Boundary extinction, took place over a 60,000 year period. Acidification of the oceans lasted for around 10,000 years, according to the study.

The researchers said the decline in the PH of the oceans was the driving force behind the deadliest phase of the extinction, which dealt a final blow to an already unstable ecosystem.

Increased temperatures and widespread loss of oxygen in the oceans as a result of volcanic activity had already put the environment under pressure.

Oceans can absorb some carbon dioxide but the huge volume released by the eruptions - at such a fast rate - was impossible to cope with.

The findings are helping scientists understand the threat posed to marine life by modern-day ocean acidification caused by carbon dioxide emissions from fossil fuels.

The amount of carbon added to the atmosphere by the prehistoric volcanic eruptions was probably greater than the entire store of carbon in today's fossil fuel reserves, the researchers said.

However, the carbon is thought to have been released at a rate similar to modern emissions, and it was this fast pace of release which played a critical role in driving ocean acidification.

Dr Matthew Clarkson, of Edinburgh University's School of GeoSciences, who co-ordinated the study, said: "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. 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 conducted by analysing rocks unearthed in the United Arab Emirates which were on the ocean floor at the time of the cataclysmic eruptions. The rocks preserve a detailed record of changing oceanic conditions at the time.

They used these to develop a climate model to work out what drove the extinction.

The study, published in the journal Science, was carried out in collaboration with the University of Bremen, Germany, and the University of Exeter, together with the Universities of Graz, Leeds, and Cambridge.

Professor Rachel Wood, also of Edinburgh University's School of GeoSciences, said: "This work was highly collaborative and the results were only possible because we assembled a unique team of geochemists, geologists and modellers to tackle an important and long-standing problem."

The type of volcanic activity described in the study would have been no ordinary eruptions, but rather the work of so-called "supervolcanoes".

The Earth has been shattered by a number of supervolcanic eruptions throughout its history, but none have occurred during recorded human history.

The most recent occurred around 640,000 years ago when the infamous supervolcano beneath what is now Yellowstone National Park exploded, propelling debris as far as the Gulf of Mexico and plunging the planet into a devastating nuclear-type winter.

A Yellowstone eruption today would equal the force of around 1000 Hiroshima bombs and, worryingly, scientists believe it is around 40,000 years overdue - with some studies pointing to ominous signs of activity in the last decade alone.