This week, as an exciting opening to the course, we're reading about the Snowball Earth hypothesis. This hypothesis states that, at three or four time periods between 2220 million years ago (mya) and 560 mya, the earth was completely covered in ice. There are multiple lines of evidence supporting the Snowball Earth hypothesis, the most important of which is glacial deposits from this time period on all continents, even those which were located in the tropics. These deposits are often covered by carbonate rocks, called cap carbonates, that could have been formed during rapid warming events following long periods of glaciation. Ratios of C13 (a stable carbon isotope) to C12 in these rocks are consistent with a decrease in biological activity during each glaciation. There are several proposed causes of these glaciations. One of these is the breakup of the supercontinent, Rodinia, and the migration of the smaller continents into tropical regions. (Hoffman and Schrag 2000). This hypothesis is tested in our primary research paper for the week, Donnadieu et al. 2004.
The breakup of Rodinia could have caused an increase in rainfall, because smaller continents with greater shoreline could be exposed to more rainfall. Rainfall, in turn, causes weathering of CO2 from land into the ocean, leading to decreased CO2 in the atmosphere. This could eventually result in ice forming at the poles, spreading slowly toward the equator. Ice reflects sunlight from the Earth's surface back into space, which would cool the Earth further, a phenomenon known as albedo. This positive feedback loop could result in complete or near-complete glaciation of the earth. Donnadieu et al. (2004) use climate models to test the hypothesis that the breakup of Rodinia could decrease the levels of atmospheric CO2 enough to initiate this process. They find that increased weathering resulting from changes in geography could have lowered Earth's average temperature by around 8 C, a level able to trigger a full glaciation.
In your comments, and for class on Thursday, think critically about this hypothesis. Do you think evidence is sufficient to support a Snowball Earth? What evidence is there against a Snowball Earth? What are your thoughts on using a modeling approach to study this problem? How would you study this problem? How do you think life could have survived during a Snowball Earth climate? How could the climate have escaped from a Snowball Earth glaciation? Most importantly, what questions do you have about the Snowball Earth hypothesis?
Citations:
Donnadieu, Yannick, Yves Godderis, Gilles Ramstein, Anne Nedelec, & Joseph Meert. 2004. A 'snowball Earth' climate triggered by continental break-up through changes in runoff. Nature 428: 303-306.
Hoffman, Paul F. & Daniel P. Schrag. 2000. Snowball Earth. Scientific American 282: 68-75.