AMONG EARTH SCIENTISTS, there now appears to be general agreement on three glaciations that potentially could have led to a snowball Earth.
The first happened about 2.2 billion years ago, the other two in the late Neoproterozoic era; the Sturtian (or Cryogenian) period, about 700 million years ago, and the Elatina (or Marinoan) period, about 635 million years ago. But there could have been many more, perhaps even a mix of snowball with slushball glaciations, severe but not complete.
The main concern is the lack of reliable dates. "We need a better handle on the absolute dating of [deposits]," says Kath Grey. "You need volcanics to date, and we just don't have that many in the succession. Finding them is difficult. They are usually little ash beds, very thin, and it's like trying to find a needle in a haystack."
And that's a real problem for many of the doubters. While Hoffman is working from his broad hypothesis down, they are working from the bottom up, trying to fit the evidence to the theory. Most are using forms of stratigraphy, tracing geological history by matching bands or strata of rock in different areas with each other. Presumably for a complete snowball Earth glaciation, you should find a matching band of glacial deposit all over the world.
Grey has spent over 30 years tracking the distributions of fossils in Australia, and matching them across the central and southern sedimentary basins of the country. Thanks to extensive petroleum and mineral exploration, Australia has a reasonably complete record in the form of drillcores from its great sedimentary basins.
"I'm urging caution about snowball Earth, for the simple reason that the Australian data that we've got so far – and the data's limited – doesn't necessarily fit with data that's coming in from overseas. And if it turns out that these glaciations are not synchronous, then we've got quite a big problem."
Within Australia everything seems to correlate quite nicely, she says, but there are issues about the actual age of the Elatina glaciation.
If you place this band at 635 million years ago, which correlates with what has been found in China, she says, you end up with four significant glaciations in Australia, whereas if you place it later, together with a well-dated glaciation from northeast Tasmania and King Island in Bass Strait at 580 million years ago, you only have three. There's no good evidence one way or the other, and either way you end up with more than the two generally accepted glaciations.
Grey sees other significant differences. She has been particularly interested in 'acritarchs', microscopic organic structures in the fossil record, which she thinks are probably planktonic green algae. They can be tracked for more than a billion years, but during the Neoproterozoic era, a new, larger, spiny form appears.
In China, this happens straight after their last major glaciation, a match with sites in Norway, Svalbard and the Himalayas. But in Australia there are different species, and they come in rapidly, long after glaciation and close to the debris from a huge asteroid impact in South Australia known as the Acraman event. This matches sites in East Siberia and Eastern Europe.
Another issue is the publication of a precise radiometric dating of the older Sturtian glaciation in South Australia by Mark Fanning of the Australian National University, in Canberra, and Paul Link of Idaho State University in Pocatello, USA. They put it at 659 million years ago, at odds with the estimated 700 million years that has been accepted until now.
"It certainly creates a problem," says Hoffman. "It doesn't leave much time between the two glaciations. I think it will eventually sort itself out, but it's certainly a bit of a complication at the moment."
