'Snowball Earth': Glaciers, ice packs once met at Equator
Recent piddling glaciations were luxury
American boffins say they have discovered evidence that almost the entire world was covered in sea ice and glaciers at certain points in the remote past, during so-called "snowball Earth" periods where the polar ice sheets met at the Equator.
It were grim in the old days.
Geologists probing conditions seen in the ancient world have long considered that there was a cold spell known as the Sturtian Glaciation about 716 million years ago. However there has been disagreement in boffinry circles as to just how severe this glaciation was.
Now, researchers from Harvard uni in the States, funded by the US government, say they have found ironclad proof that there were glaciers right down on the Equator at that time. Tropical rocks from the Sturtian, which have since migrated up to remote northwestern Canada, show unmistakable signs of having been covered in big ice back then.
"This is the first time that 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 geology brainbox Francis Macdonald of Harvard.
"Our data also suggest that the Sturtian glaciation lasted a minimum of five million years," he adds.
"Ice may have covered the entire planet then," comments Enriqueta Barrera, federal science bigwig, "turning it into a 'snowball Earth'."
The new rock data is considered to add support to the theory that the planet has seen two runaway Snowball-Earth glaciations in the past, the Sturtian and the following Marinoan. There were also two similarly-extreme "hothouse" spells where all the ice melted, perhaps 670 and 630 million years ago. Thereafter more moderate glaciations and warmings have been seen.
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Macdonald says that glaciers on land at the Equator means solid pack ice at sea in the tropics, too.
"Climate modeling 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," he says. "So our result implies quite strongly that ice would have been found at all latitudes during the Sturtian glaciation."
Even on Snowball Earth, however, it's thought that there must have been "refugia" where liquid surface water and at least occasional sunlight remained, as proto-animal life above the bacterial level is believed to have arisen and survived at around the time of the first, Sturtian snowballing. Macdonald considers that the big freeze may actually have stimulated early, primitive life to get its act together and begin the long process which has led to the present-day biosphere.
"The fossil record suggests that all of the major eukaryotic [other than microbial] groups, with the possible exception of animals, existed before the Sturtian glaciation," says the prof. "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?"
"From an evolutionary perspective," he adds, "it's not always a bad thing for life on Earth to face severe stress."
Macdonald and his colleagues aren't sure what triggered the Sturtian snowball effect, but note that a lot of lava came to the earth's surface at around that point, suggesting that the big chill could have been kicked off by volcanic dust darkening the skies.
The Harvard team publish their results this week in Science. ®
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