Further evidence for ‘snowball Earth’

Posted By Ian Randall on 5 March 2010

Sea ice reached as far as the equator during the Sturtian glaciation, 716.5 million years ago, geologists have determined. The research, which is to be published in this week’s Science, has impacts on our perceptions of the conditions under which early animals evolved.

The researchers, led by a team from Harvard University, studied tropical deposits from the Cryogenian Period which are found now outcropping in northwest Canada – the rocks demonstrated many signs of glaciation, including striated clasts, deformed softer sediments and debris from ice rafts. Palaeomagnetic analysis determined that during the Sturtian, these rocks were located at 10 degrees latitude, at sea level.

Canada: an iron-rich layer of 716.5 Ma glacial deposits (maroon) lies on top of the older carbonate reef (grey) that formed in the tropics. Picture credit - Francis A. Macdonald/Harvard University

“This is the first time that the Sturtian glaciation has been shown to have occurred at tropical latitudes, providing direct evidence that this particular glaciation was a ‘snowball Earth’ event,” said Francis A. Macdonald, of Harvard’s Department of Earth and Planetary Sciences.

He added: “Because of the high albedo of ice, climate modeling has long predicted that if sea ice were ever to develop within 30 degrees latitude of the equator, the whole ocean would rapidly freeze over.”

As eukaryotic life (cells which contain a nucleus) is known to have endured this global cooling event, palaeontologists must now face the question of how they survived. Macdonald believes that even on a glaciated Earth, eukaryotes would have been able to find local areas of surface waters and sunlight needed for life; these may have been generated by temperature gradients causing the ice to thin out in places.

As the earliest animals appear around this time, after a rise in the diversity of eukaryotes, the possibility also arises that the snowball conditions played a part in forcing the development of animal life.

Macdonald added: “From an evolutionary perspective, it’s not always a bad thing for life on Earth to face severe stress.”

Artist's impression of a snowball Earth

The study also enabled better confines to be placed on the duration of the Sturtian glaciation, which is now thought to have lasted at least 5 million years. The cause of the cooling, however, is not so well understood.

One theory, supported by the new dating of the event, ties it to the presence of a large (932 metres wide) igneous province in Canada, which could potentially have been responsible for initiating or ending the glacial period.


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