Snowball Earth
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The Snowball Earth proposes that ice ages which took place in the Neoproterozoic was so severe that the Earth's oceans froze over completely, with only heat from the planetary core causing some liquid water to exist under ice more than a kilometer thick. The general hypothesis has been around for several decades but which has recently been reformulated by Paul F. Hoffman, Sturgis Hooper Professor of Geology at Harvard University.
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Overview
Since the 1960s, it has been hypothesized that the Earth's continents were subjected to glacial action between about 750 million and 580 million years ago. Paleontologist W. Brian Harland pointed out that contemporary glacial till deposits can be found on all continents, and first proposed that the Earth must have been in an ice age at this time. The problem is that they are found on all continents; even during the worst of the ice age just past, ice was still uncommon in equatorial continents. At first the then-new theory of plate tectonics seemed to offer an explanation, but in fact made the situation worse: studies of the magnetic orientations of the rocks of the period showed that the continents were clustered around the equator, as the supercontinent Rodinia, rather than being near the poles as might have been hoped.
The Snowball Earth theory argues backwards from the documented existence of tillites dropped by these glaciers, to suggest that the Earth must have frozen over. The mechanism by which it did so is still mysterious, but one suggestion is that the presence of the at-first ice free continents at the poles enhanced the natural process of carbon dioxide reabsorption through the erosion of silicate rocks, reducing the greenhouse effect and making the Earth colder until it reached a runaway point. However, the mechanism by which the Earth would unfreeze (as it has to have done if it did freeze at one point in the past) would leave distinctive traces.
White Earth is a name given to a theoretical equilibrium found in computer climate simulations whereby the model Earth undergoes complete glaciation. While this seems to have originally been considered a degenerate case, by the time James Gleick wrote his history of chaos theory Chaos, it was not dismissed but simply restated as something that probably just had not happened yet. The current evidence for the Snowball Earth would seem to back that theory up.
Evidence
Geological formations which "Snowball Earth" proponents point to as evidence of the hypothesis are iron-rich rocks and carbonate cap rocks. The association of the Snowball Earth event with the Cambrian Explosion is also of interest.
Lack of plants
There are two forms of carbon in sea water: carbon-12 and carbon-13. Ocean dwelling plants prefer carbon-12, so an ocean with plant life would have some carbonate rocks with carbon-13, while carbonate rocks in a dead ocean would have more carbon-12. During the supposed time of Snowball earth, there seemed to be no plant life in the water, which suggests a deep freeze killing off plant life in the water.
Iron-rich rocks
In the Earth's oxygen atmosphere, iron naturally rusts. Iron-rich rocks can only form in the absence of that oxygen, and these deposits are seen at the supposed time of the worst glaciation. Proponents of the theory point out that oxygen in the Earth's atmosphere is not naturally stable, and must receive continuous maintenance from the biosphere. A tremendous glaciation would curtail life on Earth, thus allowing the atmospheric oxygen to disappear, and iron-rich rocks to form. Detractors argue that this kind of glaciation would have made life extinct entirely, which did not happen. Proponents counter that it may have been possible for reservoirs of anaerobic and low-oxygen life powered by deep oceanic hydrothermal vents to have survived such an event within Earth's deep oceans and crust.
Carbonate cap rocks
The carbon dioxide levels necessary to unfreeze the Earth have been estimated as being 350 times what they are today, but would be able to accumulate due to the opposite of the effect mentioned earlier as a possible mechanism triggering the freeze in the first place; if the Earth was completely covered with ice, silicate rocks would not be exposed to erosion, and carbon dioxide would not then be removed from the atmosphere.
Eventually enough CO2 would accumulate from volcanic eruption that the oceans around the equator would finally melt, which would produce a band of open ice-free water, much darker than highly-reflective ice, with a characteristically lower albedo, which would absorb more energy from the sun. This would in turn heat the Earth more, melting more water to absorb more light, and so on. This positive feedback loop would melt the ice in geological short order, perhaps less than a millennium.
However, the carbon dioxide levels would still be two orders of magnitude higher than usual. Rain would wash it out of the atmosphere as a weak solution of carbonic acid, which would turn exposed silicate rock to carbonate rock, which would then erode easily, wash into the ocean and form deep layers of carbonate sedimentary rock. Thick layers of exactly this abiotic carbonate sediment can be found on top of the glacial till that first suggested the Snowball Earth.
Cambrian explosion
Proponents of this theory also point out that the frozen period may have ended only a few million years or so before the beginning of the Cambrian Explosion, at the beginning of the Vendian period. While not evidence per se, they consider the apparent sudden appearance of multicellular life suggestive of the removal of some great environmental stress holding life back, and propose that the cause of this stress was the deep freeze.
Other Snowball Earths
Another Snowball Earth has also been proposed for the first known ice age, 2.3 billion years ago. There the proposed mechanism is the first appearance of atmospheric oxygen, which would have absorbed any methane in the air. As methane is a powerful greenhouse gas, and as the Sun was notably weaker at the time, temperatures plunged. The evidence here is weaker, but a layer of iron-rich rock can also be found from this time.
One competing theory to explain the presence of ice on the equatorial continents was that the Earth's axial tilt was quite high, in the vicinity of 60°, which would place the Earth's land in high "latitudes". An even less severe possibility would be that it was merely the Earth's magnetic pole that wandered to this inclination, as the magnetic readings which suggested ice-filled continents depends on the magnetic and rotational poles being relatively similar (to be fair, there is some evidence to believe that this is the case). In either of these two situations, the freeze over would be limited to relatively small areas, as is the case today, and severe changes to the Earth's climate are not necessary.
References
- Gabrielle Walker, 2003, Snowball Earth, Bloomsbury Publishing, ISBN 0747564337
- Jenkins, Gregory, et al, 2004, The Extreme Proterozoic: Geology, Geochemistry, and Climate AGU Geophysical Monograph Series Volume 146, ISBN 0-87590-411-4
External links
- Snowball Earth Overview by Paul F. Hoffman and Daniel P. Schrag (http://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.html)
- One-page Snowball Earth Poster by Paul F. Hoffman (http://www-eps.harvard.edu/people/faculty/hoffman/snowball_poster.pdf) (pdf format, 7.22 MB).
- Scientific American article on snowball earth by Paul F. Hoffman and Daniel P. Schrag (http://www.sciam.com/article.cfm?articleID=00027B74-C59A-1C75-9B81809EC588EF21) (subscription required).
- Melting the Snowball Earth Theory, St. Andrews University news release on research by Dan Condon, Tony Prave and Doug Benn (http://calvin.st-andrews.ac.uk/external_relations/news_article.cfm?reference=264)de:Snowball Earth
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