More than half a billion years ago, our planet was a giant snowball hurtling through space. Glaciers blanketed the globe all the way to the equator in one of the mysterious "Snowball Earth" events geologists think at least twice in the Earth's ancient past. Now, the scientists have found that the final snowball episode probably ended in a flash about 635 million years ago, a geologically fast event that could have implications for today's human-driven global warming.
The ice, which built over several thousand years , "Melted in no more than 1 million years," says Shuhai Xiao, a paleobiologist at the Virginia Polytechnic Institute and State University in Blacksburg who was part of the team that made the discovery. That's the blink of an eye in our planet's 4.56 billion-year history, suggesting the globe has reached a sudden tipping point, Xiao says. Although the team does not know for sure what caused it, the carbon dioxide emitted by ancient vvcanoes may have triggered a greenhouse event, causing the ice sheets to thaw rapidly.
It shines light on the deglaciation rate, Xiao and colleagues dated volcanic rocks from southern China's Yunnan province. These were embedded under another kind of rock called a cap carbonate-unique deposit of limestone and dolostone that formed during Snowball Earth shutdown in response to high levels of carbon dioxide in the atmosphere. Using radiometric dating techniques, the team found that volcanic rocks were 634.6 million years old, giving or taking about 880,000 years. Alone, this single new date could not reveal the speed at which the melting happened. But in 2005, a different team of scientists from volcanic rocks from above a similar cap at a different location in China's Guizhou Province. They were dated to 635.2 million years, give or take 570,000 years.
Together, the two samples suggest that the melting event was a quick thaw of about 1 million years, Xiao and his colleagues wrote last month in Geology The key, Xiao explains, is that these two dates are far more accurate than those of past samples, with error bars of less than 1 million years. Those errors essentially bracket the period in which the cap carbonates formed-and, thus, bound the period of the final Snowball Earth thawing event. Because previously discovered samples have a bunch of several million years or more, Xiao says these new dates are the first one that can be used to calculate the rate of melting with any certainty.
However, since the two new samples come from the southern China , they do not paint a global picture of the ancient thaw, says Carol Dehler, a geologist at Utah State University in Logan. To do that, scientists would need to find datable volcanic rocks from other parts of the world, which are about "as common as unicorns," she jokes.
Meanwhile, understanding the nature of these ancient glaciations could help scientists dealing with climate change today: "I think one of the biggest messages that Snowball Earth can Send humanity, "Dehler says," is that it shows the Earth's ability to change extreme ways in short and long time scales. "