US: Chilling evidence of super ice age

Scientists have found evidence that sea ice reached the equator during a super ice age more than 700 million years ago.

Though such an extensive freeze has been suspected, the new evidence - tropical rocks found in Canada's remote north-west - brings more precision about the time it occurred and also its extent.

The rocks from Canada's Yukon Territory showed glacial deposits and other signs of glaciation. But based on their magnetism and composition, scientists calculated that 716 million years ago the rocks were located at sea-level in the tropics, at about 10 degrees latitude.

"This is the first time the Sturtian glaciation has been shown to have occurred at tropical latitudes, providing direct evidence that this particular glaciation was a 'snowball Earth' event," says lead author Francis Macdonald, a geologist at Harvard University.

"Our data also suggest the Sturtian glaciation lasted a minimum of five million years."

Enriqueta Barrera, programme director in the US National Science Foundation's Division of Earth Sciences, which supported the research, said the Sturtian glaciation, along with the Marinoan glaciation right after it, were the greatest ice ages known to have taken place on Earth.

"Ice may have covered the entire planet then," said Barrera, "turning it into a 'snowball Earth'."

The survival of eukaryotes - life forms other than microbes such as bacteria - throughout the period suggests that sunlight and surface water remained available somewhere on Earth's surface. The earliest animals arose at roughly the same time.

"The fossil record suggests that all of the major eukaryotic groups, with the possible exception of animals, existed before the Sturtian glaciation," Macdonald said.

"The questions that arise from this are: If a snowball Earth existed, how did these eukaryotes survive? Did the Sturtian snowball Earth stimulate evolution and the origin of animals?"

Even during such an extreme event, Macdonald said there would be temperature gradients, and it was likely that sea ice move and thin, creating patches of open water where life could exist.

"Climate modelling has long predicted that if sea ice were ever to develop within 30 degrees latitude of the equator, the whole ocean would rapidly freeze over," Macdonald said.

"So our result implies quite strongly that ice would have been found at all latitudes during the Sturtian glaciation."

Scientists do not know exactly what caused the glaciation or what ended it, but Macdonald said its age closely matched that of a large area of rocks formed by cooled magma that stretched more than 1,500 kilometres from Alaska to Ellesmere Island in far north-eastern Canada.

This coincidence could mean the glaciation was either started or ended by volcanic activity.

The research was published in Science last week.