The Snowball That Never Was

It’s a very cool idea that for years has sent shivers through the scientific community. But according to a group of Canadian geologists the popular Snowball Earth hypothesis deserves the deep freeze itself.

The Queen’s University scientists have entered a simmering international snowball fight: Did the Earth repeatedly freeze over into a planet-sized ice field about 650 million years ago – a freeze-thaw cycle that could have jump-started the first complex life in the oceans?

“The Snowball Earth hypothesis doesn’t jive with the evidence,” says Dr. Kurt Kyser, a geochemist at Queen’s and co-author of a series of technical papers – recently submitted for publication – that torch the Snowball Earth notion.

The idea of an ancient global deep freeze extends back to at least the late 1940s when an Australian geologist noted the global extent of glacial debris during the “Neoproterozoic” era – 650 million years ago. Most intriguing was the later discovery of glacial rocks that were formed near the equator.

Further research by a Canadian Harvard geologist, Dr. Paul Hoffman, appeared to round out the Snowball Earth hypothesis. His group measured a gradual drop in the ratio of carbon-13 to carbon-12 isotopes in sea water during the supposed big freeze, a change that was interpreted as a massive slowdown in marine plant photosynthesis during this period.

Dr. Kyser’s group doesn’t dispute the fact that the Earth went through a series of ancient big chills, but in what sounds like a Canadian winter discussion at the donut shop, it’s a question of just how cold was it?

According to Dr. Kyser, the cold, hard evidence to date sparks more questions than answers.

The Canadian evidence is based on a unique 650-million-year-old rock formation in the Mackenzie mountains in the southwest corner of the Northwest Territories, discovered by John Park of the Canadian Geological Survey. These fine-grained pink and white sedimentary rocks are the remains of an ancient continental shelf located on the equator.

“These rocks are extremely well preserved in relation to the other ones of this age in Australia and Namibia,” says Dr. Kyser.

This remarkable preservation has enabled the Canadian team to conduct a detailed “chemical fingerprinting” of the rocks. The tale they tell has left the researchers baffled and is cold comfort for Snowball Earth proponents.

The melting of massive glaciers around the globe would have released tonnes of fine glacial debris into the oceans. This would have dramatically boosted the levels of the isotope strontium-87 in relation to its sister isotope strontium-86.

“We looked at the ratio of strontium-87 to strontium-86 in the Canadian rocks and what we’ve been able to show is that there was no change before, during, or after the glaciation,” says Dr. Kyser.

The Queen’s group’s detailed analysis of the carbon isotopes in these rocks further puts the heat to the Snowball Earth hypothesis.

Rather than seeing a change in the carbon-13 to carbon-12 ratio in seawater during the ancient ice ages (when colder temperatures and ice cover might have slowed photosynthesis in the world’s ancient oceans), the scientists found a massive change in the carbon ratio between the two deep freeze cycles.

“Something happened to the carbon cycle that we haven’t seen at any other time – it slowed to a near standstill when the climate was relatively warm,” says Dr. Kyser, who, with Queen’s colleagues Drs. Guy Narbonne, Noel James, and Bob Dalrymple, is heading back into the Mackenzie mountains next summer to look for further clues.

So what did happen to the Earth’s climate about 650 million years ago?

Dr. Kyser says the climate models being used are too simplistic to accurately ice the question. Climate models for the Neoproterozoic currently involve only two or three of the more than a dozen variables – from ocean temperature and circulation, to atmospheric chemistry and volcano activity – that climatologists know goes into determining climate.

“If we had seen a change in the strontium ratio, then we would have said yes, this was an incredible change that occurred, and it really did screw up seawater, and then what follows tens of millions of years after this are the first complex animals,” says Dr. Kyser. “That would have been wonderful and would have all made sense. But, what we’re saying is sorry, that’s not the way it happened. So the advent of complex animals is probably related to something other than a Snowball Earth.”

Contact:

Dr. Kurt Kyser
Tel.: (613) 533-6179
E-mail: kyser@geol.queensu.ca
Web site: http://geol.queensu.ca/isotope_lab/