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Scientists say they've ruled out a key hypothesis to explain Earth's greatest extinction, when 95 per cent of marine species and 70 per cent of land species were wiped out.
Dubbed the "Great Dying" or "the mother of all mass extinctions", the catastrophe occurred around 250 million years ago at the end of the Permian era.
The event may have unfolded over millions of years, and an increasing number of clues testify to its severity, including the discovery worldwide of eerie, fossilised, mutant plant spores. What is unclear, though, is what caused it.
British researchers, reporting in the journal Nature Geoscience, rule out a leading theory that the oceans became starved of oxygen and rich with sulphide, causing marine life to die out.
Clouds of hydrogen sulphide - the same chemical that comes from rotting eggs - rose from the seas and, abetted by methane released as a byproduct of intense vulcanism, attacked the ozone layer, which filters ultraviolet-B light from the sun.
On the ground, life was ravaged, goes this theory. Living things were poisoned by toxic levels of hydrogen sulphide and their DNA was shredded by solar radiation.
A team led by David Beerling of the University of Sheffield in northern England created a two-dimensional computer model of atmospheric chemistry to test this notion.
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 prevailing theories include an impact, or series of impacts, by an asteroid; and a brief but fierce period of vulcanism which caused a lethal mixture of acid rain and global warming.
- NZPA
