SIGNS of a sulphur cloud that swamped the Earth 250 million years ago may add
to the evidence that an impact by an extraterrestrial object caused the biggest
mass extinction in the planet鈥檚 history, at the end of the Permian period.
Geologist Kunio Kaiho of Tohoku University in Sendai, Japan, found sharp
changes in the ratios of sulphur and strontium isotopes in rocks in northern
China at the time of the extinction (Geology, vol 29, p 815). He thinks
a large comet or asteroid hit the ocean, forming a crater 600 to 1200 kilometres
across and vaporising sulphur from the underlying mantle. The sulphur would have
reacted with oxygen, removing it from the atmosphere and producing very acidic
rain, helping to kill off life.