Poison gas 'caused' great dying

The continent-wide nature of the event implies that it was caused by something in the atmosphere Dr Mark Sephton, Imperial College London

The Earth's greatest mass extinction was probably caused by poisonous volcanic gas, according to a study published in the journal Geology.

This "great dying" 250 million years ago killed off more than two-thirds of reptile and amphibian families.

But it also facilitated the rise of the dinosaurs, which would rule Earth for the next 185 million years.

New evidence comes from molecules preserved in rocks laid down at the time of the extinction.

Other causes suggested for the extinction have included an asteroid colliding with our planet or a massive release of methane from the deep sea.

Scientists already know the so-called end-Permian extinction coincided with a major volcanic eruption. This caused the biggest ever outpouring of basalt lava, covering vast swathes of land in what is now Siberia.

It created the Siberian Traps, a large province of igneous rocks centred around the city of Tura.

'Soil crisis'

Now, analysis of a unique set of molecules found in rocks taken from the Dolomite mountains in Italy sheds light on the environmental impact of this eruption.

The molecules are the remains of polysaccharides, large sugar-based structures common in plants and soil. However, these rocks are the remains of marine sediments.

Trilobites were one of the groups killed off in the extinction

The chemistry of the rocks revealed that the sugar molecules had originated on land, supporting the theory that massive soil erosion washed them into the sea.

The authors of the Geology paper believe volcanic gases from the eruption would have depleted Earth's protective ozone layer and acidified the land and sea, killed rooted vegetation.

This would have hindered the retention of soil and allowed it to be washed into the surrounding oceans.

Soil materials in the oceans would have blocked out light and soaked up oxygen. Analysis of rock chemistry suggests that after the so-called "soil crisis" on land, the marine ecosystem succumbed to the stresses of environmental change.

Oceanic life faltered, completing a global catastrophe.

"The cause of the end-Permian extinction has been highly controversial. We show that the terrestrial ecosystem was the first to suffer," said co-author Dr Mark Sephton of Imperial College London.

"The continent-wide nature of the event implies that it was caused by something in the atmosphere. The unique chemical data indicates that something fast and catastrophic happened on land."

The research was carried out by an international team of scientists from the UK, the Netherlands and the US.