The greatest mass extinction in the 3.5-billion-year history of life on Earth probably occurred as a result of climate change - an ancient version of global warming - resulting from a series of huge volcanic eruptions.
A study of "the great dying", when 90 per cent of marine life and
three-quarters of land animals and plants died out, has failed to support earlier suggestions of a collision between the Earth and a giant asteroid.
However, scientists investigating volcanic ash sediments in South Africa and China - fallout from the same volcanic eruptions in Siberia - believe that a relatively slow rate of extinction occurred over millions of years.
Peter Ward of the University of Washington, a leader of the joint American-South African research team, said the volcanoes probably triggered the release of massive quantities of greenhouse gases, which led to catastrophic climate change.
"The marine extinction and the land extinction appear to be simultaneous, based on the geochemical evidence we found," Professor Ward said. "Animals and plants both on land and in the sea were dying at the same time, and apparently from the same causes - too much heat and too little oxygen."
The findings, published in the online version of the journal Science, suggest that if an asteroid did hit the Earth during this period it made only a minor contribution to the environmental changes caused by the volcanic eruptions. Fossil evidence clearly indicates that there was a mass extinction at the boundary between two geological periods called the Permian and the Triassic at a time when the landmass of the Earth was concentrated in one giant super-continent known as Pangea.
The scientists compared the chemical, biological and magnetic data from sediments dating back to this boundary from the Karoo basin in South Africa to similar sediment layers in China.
They found two patterns: a gradual extinction over 10 million years leading up to the Permian-Triassic boundary and a sharp increase in extinction rate at the boundary, which lasted a further 5 million years.
- INDEPENDENT
