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More evidence for snowball Earth

Two new studies support the long-debated "snowball Earth" hypothesis—the idea that Earth was encased in ice one or more times between about 760 and 600 million years ago.

To kick-start a global glaciation, vast amounts of carbon dioxide must be removed in some way from the atmosphere, thus cooling the globe. If iceancient land masses were clustered at low latitudes, ice sheets atop them would have far more sunshine to reflect than do polar ice sheets; thus, they would help reduce global temperature further and keep the glaciers expanding. The freeze cycle would end only when volcanoes replenish enough CO2 in the air above the frozen globe.

Geologic evidence hints that at least two global glaciations may have occurred: the Sturtian (from about 760 to 700 million years ago) and the Marinoan (650–600 Ma). A third event, the Gaskiers (580–565 Ma), has been detected regionally but not yet proven to be global.

A modeling study depicts the Sturtian glaciation in the 18 March issue of Nature. Scientists at France's National Center for Scientific Research and the University of Florida used a coupled climate-geochemical model called GEOCLIM. In it, they specify the breakup of a low-latitude supercontinent, believed to have occurred around 800 to 700 Ma. The newly formed landmasses—with far more collective coastline than before—trigger a global rise in precipitation. These processes draw enough CO2 out of the air to change the global greenhouse to an icehouse.

It's unclear how subsequent snowball Earths might have formed. "Whether a continental configuration similar to the Sturtian one still applies to the younger glaciations is a matter of debate," write the authors.

In the May issue of Geology, a study by a U.S.–Chinese team supports the global extent of the Marinoan event by clarifying the age of a vast formation in the Yangtze region of southern China. Analyzing deposits of zircon—a highly stable mineral—the team found that two sets of glacial deposits must have occurred on either side of a period around 663 Ma, thus placing the latter deposits after the Sturtian event.

Biologists are keenly interested in these glaciations, since biodiversity expanded dramatically just after the Gaskiers period. "Our contribution can clarify the global picture of geological and biological evolution in the ancient past," says Geology coauthor Shuhai Xiao (Virginia Polytechnic Institute), "but a lot of uncertainty remains ".


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