LONDON - Scientists may have to rethink accepted theories of how the prehistoric earth's atmosphere developed after new discoveries in ancient sulphur raised serious questions, researchers said on Wednesday.
Up to now it has been generally accepted that the earth's atmosphere was devoid of oxygen for some 80 percent of its existence.
"The popular model is that there was little oxygen in the earth's atmosphere before about 2.4 billion years ago," said Hiroshi Ohmoto of Pennsylvania State University.
But Ohmoto's team has cast doubt on the theory after finding sulphur isotopes, indicating prevalent oxygen, that predate the accepted start of atmospheric oxygenation.
The key lies in the fact that while all isotopes of sulphur behave the same chemically, they have slightly differing masses according to the amount of atmospheric oxygen at the time.
Isotopes from two sulphur samples the team analysed -- one 2.76 billion years old from a lake bed and the other 2.92 billion years old from the sea bed -- did not indicate an oxygen-starved atmosphere.
"We analysed the sulphur composition and could not find the abnormal sulphur isotope ratio (indicating no oxygen)," Ohmoto said. "This is the first time that sediment that old was found to contain no abnormal sulphur isotope ratio."
The team concluded that there were two possible explanations -- either that prehistoric atmospheric oxygen levels fluctuated wildly over the millennia, or that sulphur showing no oxygen might have been produced in an oxygenated atmosphere as long ago as 3.8 billion years by violent volcanic activity.
Either way, they said, the accepted theories needed to be re-evaluated.
The findings were published in the science journal Nature.
REUTERS NEWS SERVICE
