Snowball Earth Cause Debated
The hypothesis that Earth was completely covered in ice (Snowball Earth) 635 million years may not be so. The atmospheric concentration of carbon dioxide during that period was much lower than previously thought, according to a team of French researchers from the Institut de Physique du Globe de Paris (CNRS/IPGP/Université Paris Diderot), working in collaboration with scientists from Brazil and the U.S. The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once. Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes, and other otherwise enigmatic features in the geological record.
The initiation of a Snowball Earth event would involve some initial cooling mechanism, which would result in an increase in the Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase the Earth's albedo, which would result in positive feedback for cooling. If enough snow and ice accumulates, runaway cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where solar radiation is most direct.
Many possible triggering mechanisms could account for the beginning of a Snowball Earth, such as the eruption of a Supervolcano, a reduction in the atmospheric concentration of Greenhouse gases such as methane and/or carbon dioxide, changes in Solar energy output, or perturbations of the Earth's orbit. Regardless of the trigger, initial cooling results in an increase in the area of the Earth's surface covered by ice and snow, and the additional ice and snow reflects more solar energy back to space, further cooling the Earth and further increasing the area of the Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen equator as cold as modern-day Antarctica.
Global warming associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a Snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of the Earth's surface would require as few as 1,000 years.
The atmospheric concentration of CO2 during that period was much lower than previously thought, according to a team of French researchers from the Institut de Physique du Globe de Paris (CNRS/IPGP/Université Paris Diderot), working in collaboration with scientists from Brazil and the US. Their work, which is published in the journal Nature dated 6 October 2011, challenges part of the so-called Snowball Earth hypothesis and rekindles the debate about the origins of the deglaciation mechanism.
The Earth has experienced several extreme glacial events, two of which took place during the aptly named Cryogenian period (710-630 million years ago). In 1992 and 1998 scientists hypothesized that around 635 million years ago our planet underwent a major glacial episode that left it entirely smothered in ice. Today still, the question of how this episode came to an end remains unanswered, given that ice reflects more solar radiation back into space than rocks do. In the Snowball Earth hypothesis, it is assumed that enough CO2 of volcanic origin had built up in the atmosphere for this greenhouse gas to warm up the surface of the planet and cause the ice to melt. According to this scenario, CO2 concentrations must have fluctuated around 120 000 ppmv (i.e.12%), which is 300 times greater than CO2 concentrations today.
In order to assess the atmospheric concentration of CO2 at that time, the French, Brazilian and US researchers studied carbonates deposited 635 million years ago (the Marinoan glaciation). These sediments cap the glacial deposits of that period, believed to have witnessed a global glaciation known as Snowball Earth. The study is based on the difference in carbon isotopic composition between carbonates and organic matter in fossilized organisms, which reflects atmospheric concentrations of CO2. The results show that CO2 concentrations were very close to what they are today (less than 200 ppmv), which is far from being sufficient to bring about the end of a glacial episode of this magnitude.
This work not only challenges part of the Snowball Earth hypothesis, but also implies that these glacial episodes were not as intense as previously suggested. Moreover, this data is consistent with the idea that the atmosphere at the same period was much more oxygen-poor, around 1%, as compared to today's levels of approximately 20%. Scientists will therefore need to examine alternative deglaciation mechanisms or gases other than CO2, such as methane, which has also been suggested as part of this hypothesis.
For further information: http://www2.cnrs.fr/en/1920.htm or http://www.nature.com/nature/journal/v478/n7367/full/nature10499.html