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 ancient 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
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
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