Australia: The Land Where Time Began

A biography of the Australian continent 

Snowball Earth - Was it Actually a Profound Wintry Mix

It was suggested in 1998 by scientists from Harvard University that the Earth Froze from pole to pole more than 500,000 Ma, an event that threatened all of life with extinction, though they also suggest this big freeze may have pushed it to a great burst of evolution (Science 28 August 1998 p.1342). Evidence has now been found that there were glaciers in the tropics more than 100 My before the proposed global freeze (Science, 27 December 2012). A hall-mark of so-called hard snowball Earth scenarios is glaciation at low latitudes, at which times the world ocean was sealed from the atmosphere by ice that may have reached a kilometre in thickness.

According to Michael Arthur, a geochemist at Pennsylvania State University, in spite of the new work, the snowball Earth hypothesis, that has been widely studied, is also widely disputed. The author¹ suggests that "the Earth was profoundly cold in those geologically weird days"¹, but while many agree that the global glaciation was more like a Slushball Earth, rather than the much less likely scenario of a hard snowball Earth in which the entire Earth was sealed in ice. Francis McDonald, a geologist from Harvard, and colleagues from Harvard and elsewhere, have presented new evidence of the snowball Earth hypothesis. They dated layered volcanic ash that was deposited at about the time of the Sturtian Glaciation, about 716.5 Ma. That is the time when, according to palaeomagnetic records, the rocks and the glaciers of the Sturtian were located in the tropics.

Several decades prior to the coining of the term "snowball Earth" in 1992, by Joseph Kirschvink, a geobiologist at Caltech, Pasadena, researchers had speculated about the concept of a hard snowball, but was more widely accepted only after Paul Hoffman, a geologist at Harvard, together with 3 colleagues boosted it in the Science paper in 1998). The authors of the 1998 paper concluded, based on simple climate modelling that during the Marinoan Glaciation, about 650 Ma, any ice reaching the tropical latitudes would have continued on to the equator. They suggested that once the highly reflective ice cover reached a critical area climatic feedbacks would inevitably extend the ice to the equator, resulting in the formation of a global glaciation - a hard snowball.

It is suggested by more recent palaeoclimate modelling that to achieve a hard snowball from low latitude glaciation may be difficult, if not impossible. According to Mark Chandler, a climate modeller of the Godard Institute for Space Studies, says the oceans can't be frozen over, as indicated by model oceans, as oceans can hold a lot of heat, which is moved around in currents, and it is this that prevents the ocean from completely freezing over. According to Chandler, a few years ago a pattern became established that the more sophisticated the model used, the less likely it became that a hard snowball would form.

A "thin ice "snowball has been suggested by James Kasting, an atmospheric physicist at PSU. Together with David Pollard, also at PSU, he considered how a continent on the poleward side of an inland sea might prevent thick ice from intruding from higher latitudes and preserve areas of ocean ice thin enough to allow enough sunlight to pass through for marine photosynthesis to continue beneath the ice. They believe a thin ice scenario better satisfies all the constraints than other models.

According to Philip Allen, Imperial College London, a worldwide glaciation is rejected by almost all geologists, suggesting that 90 % of geologists are "quite hostile" towards it. Beginning about the time the Marinoan Glaciation hard snowball was first proposed, geologists, including Allen, carried out field studies of glacial deposits. The geologists found that the sediments recorded moving water and ice, flowing ocean currents and waves that were moving on open seawater, and not the stagnation that they expected, Allen saying "We do not have a hard snowball Earth"¹. Hoffman has argued that the conditions the group found could reflect the conditions that were present either prior to, or following, a hard snowball, though he doesn't dispute their interpretations.

According to the author¹ a hard snowball is not accepted by most geochemists. The contention that a bizarre chemical deposit that was found on the top of glacial deposits, the cap carbonate formation, is the key to the argument of the Harvard group, the formation of which could only have occurred if the world ocean had been sealed from the atmosphere for a period of millions of years. The group maintained that the marine biota continued to function because of cracks, that were rare, and volcanic hot spots on the ocean floor. A co-author of the 1998 Science paper, Alan Jay Kaufman, a geologist at University of Maryland, College Park, has now changed his stance, supporting the slushball hypothesis, following his study of the carbon, strontium and sulphur isotopic records. Kaufman says, particularly the sulphur isotopes, suggest there was more open water than mere cracks in the ice.

Resistance to acceptance of the hard snowball is, according to Hoffman, typical of scientific controversy, suggesting "The problem is the experts reach a quick judgment and dig themselves into a position." He notes that the idea of a recent ice age took 40 years and a new generation of scientists to be accepted in the 19th century, and he suggests the evidence for a hard snowball is getting increasingly strong, citing recent evidence from oxygen isotope findings that support the atmospheric carbon dioxide concentrations that are predicted by the hard snowball.

Sources & Further reading

Kerr, Richard A. "Snowball Earth Has Melted Back to a Profound Wintry Mix." Science 327, no. 5970 (March 5, 2010 2010): 1186.