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Snowball Earth Part 3: Evidence Against

© Geoff Habiger

A group of scientists led by Joseph Kirschvink hypothesized that the Earth was once encased in a global spanning glaciation. Ice covered the continents and even the tropical oceans were covered in thick sea ice. The global glaciation, a "Snowball Earth", lasted for millions of years and was finally broken by the accumulated build up of carbon dioxide in the atmosphere that eventually led to a runaway greenhouse event that melted the glaciers in the span of only a few hundred years.

A growing group of scientists are now finding fault with the "Snowball Earth" theory. They present evidence that the Earth was not covered in thick sheets of ice, instead that the tropical oceans remained mostly ice-free. Many have dubbed this hypothesis the "Slushball Earth".

Carbon Ratios

The evidence presented by the detractors of the "Snowball Earth" follows many of the lines of evidence present by Kirschvink and others, with different conclusions. Martin Kennedy, a geologist from University of California at Riverside has examined rocks deposited during the glacial periods. Kennedy looked at limestones and dolomites from Precambrian glacial deposits with very stringent criteria. The rocks had to be deposited in situ (they could not have been transported from another location), and he looked at rocks in many different locations. This was done to rule out the possibility that any one site might have been an aberrant locality, such as a lacustrine (lake) deposit of limestone. Kennedy took samples from these localities and measured the ratio of 13C to 12C in the rocks. He found that the ratio of 13C to 12C was higher in the rocks deposited during the glaciation and then dropped after the glacial periods ended. This higher ratio of 13C to 12C suggests that the marine ecosystems were very healthy and is evidence, Kennedy says, that the oceans in low latitudes were ice-free. If the oceans were covered with ice, Kennedy states, then the ratios of 13C to 12C would be closer to levels seen in the Earth's mantle.

Climate Modeling

Linda Sohl and March Chandler from NASA undertook another approach to disprove the "Snowball Earth" theory. They used advanced climate modeling techniques that were not available to Brian Harland and Mikhail Budyko in the 1960's. Sohl and Chandler's model used four main criteria to adjust their climatic model for the time of the glaciations: solar luminosity (the strength of the suns solar output), the geographic distribution of the continents, atmospheric carbon dioxide levels, and ocean heat transport.