Snowball Earth

IN THE EARLY 1960s, Brian Harland, a geologist at Cambridge University, observed that rocks on several continents, dating from the Neoproterozoic era (approximately 800–680 million years ago), contain glacial debris. Some of the glacial debris included carbonate rocks, which are known to form in the tropics (e.g., in the present-day Bahama Banks). This conclusion later gained additional support from paleo-magnetic data. One potential explanation is that the [Page 904]Earth entire Earth was covered by ice and snow during the Neoproterozoic. This has come to be known as the “Snowball Earth” hypothesis.

The Snowball Earth hypothesis proposes that the Earth was entirely covered by ice in part of the Cryogenian period.

One early problem was understanding how a global ice age could have commenced. During the 1960s, the Russian climate scientist Mikhail Budyko used a computer simulation to establish that a runaway ice-albedo feedback effect could lead to global glaciation. The term albedo refers to the amount of the sun's energy that is reflected by the Earths surface. As glaciers grow in extent, they reflect more of the sun's energy, which causes the atmosphere to cool. This in turn causes the glaciers to grow. Budyko showed that if the glaciers extended beyond a certain critical point, this ice-albedo feedback could lead to a global ice age.

A second obstacle was understanding how a global ice age could ever end once it began. In the early 1990s, Joseph Kirschvink of the California Institute of Technology observed that during a global ice age, the carbon cycle would shut down. Volcanoes sticking up through the ice cover would continue to add carbon dioxide to the atmosphere. Having nowhere else to go, the carbon dioxide would then accumulate over millions of years until a runaway greenhouse effect caused the ice to melt.

One important rival to the Snowball Earth hypothesis is the high obliquity hypothesis. If the tilt of the earth's axis had been much different during the Neoproterozoic, the poles could have received more solar energy than the tropics. If so, it would be possible to explain the evidence for glaciers in the tropics without supposing that the entire planet had frozen over.

In his widely cited 1992 paper, Kirschvink also proposed an explanation for banded iron deposits observed in Neoproterozoic glacial debris. Iron is not soluble in seawater in the presence of oxygen. During a true Snowball Earth episode, the oceans would have become deoxygenated over time. Iron from thermal vents would build up in the seawater. Then, when the ice finally melted, and oxygen was once again exchanged between the oceans and atmosphere, oxidized iron would have been left along with the debris from the retreating glaciers.

During the 1990s, two Harvard scientists, Paul Hoffman and Daniel Shrag, gathered additional, highly suggestive evidence that seemed to favor the Snowball Earth theory. They found that in many places, the Neoproterozoic glacial debris occurs right below thick layers of carbonate rock (which are known as “cap carbonates”), and they showed how Kirschvink's proposal could account for this. During a Snowball Earth episode, very large amounts of carbon dioxide would have built up in the atmosphere. As the ice receded and the carbon cycle resumed, large amounts of carbon would have been washed out of the atmosphere during storms and ended up in the form of carbonate rock on the ocean floor. More controversially, Hoffman and Shrag also studied the ratio of carbon-12 to carbon-13 isotopes in the cap carbonates. They argued that an unusual dip in the carbon isotope ratio signified a temporary shutdown of photosynthetic activity in the earth's oceans.

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