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(1) (i.e.12%), which is 300 times greater than CO2
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 3 200 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.
© Pierre Sans-Jofre
Outcrop in the Terconi quarry, Mato Grosso, Brazil. The lower part shows a pink dolomite layer overlain by grey limestone, richer in organic matter. These carbonates lie directly above Marinoan glacial sediments.
(1) ppmv: parts per million by volume
A carbon isotope challenge to the snowball Earth, P. Sansjofre, M. Ader, R. I. F. Trindade, M. Elie,
J. Lyons, P. Cartigny and A. C. R. Nogueira – Nature, 6 October 2011