A series of global ice ages may have nearly snuffed out life about 700 million years ago, researchers report in tomorrow's Science . Coated by ice and snow from pole to pole, the planet slept on for millions of years, according to this theory, until it was finally roused by its own volcanic emanations.
Geologists Paul Hoffman and Galen Halverson and geochemist Daniel Schrag, all of Harvard University, and geochemist Alan Kaufman of the University of Maryland, College Park, present isotopic and geological evidence from Namibia for an ancient "snowball" Earth that threatened the diverse but still simple organisms that then constituted life on Earth. "It's mind-boggling that such events may have happened," says Hoffman.
The data come from rock deposited about 700 million years ago on the edge of a long-vanished ocean, in what is now Namibia in southwest Africa. Well before a layer of glacial debris was deposited, the isotopic ratio suggests that carbon was removed from the world ocean through about half chemical and half biological processes, says Hoffman. But as the ice age approached, the portion due to biology began to decrease, as would happen if the ice were gradually spreading across the globe. The glacial deposit itself doesn't preserve a faithful isotopic record. But an overlying carbonate layer suggests that biological productivity had dropped all the way to zero, and recovered only slowly. "It's difficult to imagine any other mechanism that would shut down productivity on that scale other than global glaciation," says Hoffman.
Hoffman assumes that there were at least a few breaks in the ice or patches of bare ground where microbes and multicellular algae survived to later give rise to all life today. The group suggests that eventually, volcanic carbon dioxide oozing from the interior over millions of years created a greenhouse effect powerful enough to break the ice's grip. Then, this high carbon dioxide level drove the deposition of the cap carbonate.
Not everyone is convinced that the glaciation was so extreme. "This is really interesting, but it's really speculative," says geochemist Louis Derry of Cornell University. "There are significant questions about the data." For example, geochemist Martin Kennedy of the University of California, Los Angeles, has carbon isotopic data from Namibia, but they show no deep productivity decline before the Namibian glacial deposits. It remains to be seen whether this snowball can take the heat.