Scientists have ruled out a key hypothesis to explain Earth's greatest extinction, when 95 percent of marine species and 70 percent of land species were wiped out at the end of the Permian 250 million years ago.From Physorg.com:
Researchers have now ruled out a leading theory that the oceans became starved of oxygen and rich with sulphide, causing marine life to die out. This theory suggests that clouds of hydrogen sulphide, produced as a by-product of intense vulcanism, poisoned life, and by attacking the ozone layer, allowed solar radiation to destroy its DNA.
According to their calculations, the lower levels of the atmosphere in the tropics would have acted as an oxidising buffer, preventing the hydrogen sulphide from seriously damaging the ozone layer.
"These gases seem unlikely to be the cause of coincident terrestrial biotic extinctions," the paper says.
Other theories for the extinction include an asteroid or a long period of vulcanism which caused a lethal mixture of acid rain and global warming.