From correspondents in Oslo | February 29, 2008
FOSSIL evidence of a cooling of the oceans 35 million years ago may have solved a mystery about how Antarctica froze over in one of the big climate shifts in Earth's history, scientists said overnight.
The fossil signs of a 2.5 celsius fall in ocean temperatures, enough to trigger the formation of Antarctica's ice sheet, may also help understand whether the continent will thaw because of modern global warming.
A full melt of Antarctica would raise world sea levels by about 57 metres over thousands of years. Even a small melt would threaten coastal cities from Shanghai to New York or low-lying islands.
"New evidence could solve the puzzle of why Antarctica went into the deep freeze," the University of Cardiff said of a study by scientists in Wales and the United States and published in the Geological Society of America's journal Geology.
"Now we understand the system better," Caroline Lear, of Cardiff University and lead author of the study, said.
"Some other records had suggested there was even a warming at that time, which was really confusing."
The study, of pin-head sized fossil animals known as foraminifera found in mud in Tanzania, showed that the oceans cooled 35 million years ago, perhaps after shifts in the earth's orbit around the sun.
In cooler temperatures the shells of foraminifera contain less magnesium than in warmer water. The sediments originally had been part of the Indian Ocean.
The new evidence could reinforce modern climate models that had struggled to explain the ancient behaviour of ice sheets.
"Now we can have more confidence in what the climate models predict," she said.
Records indicated that Antarctica's ice formed when concentrations of carbon dioxide, naturally produced by living organisms and now the main modern industrial greenhouse gas, were about twice current levels in the atmosphere.
"But you can't simply ... say that if CO2 levels are twice what they are today the Antarctic ice sheet will melt," she said, adding that the vast block of ice acted as a natural deep freeze that slowed any thaw.
Before 35 million years ago there were probably only small ice sheets.
"If you go back 50 million years there was no ice anywhere on the planet, carbon dioxide levels were higher and the south coast of Britain had mangroves," she said.
