History
Sir Douglas Mawson (1882–1958), an Australian geologist and Antarctic explorer, spent much of his career studying the Neoproterozoic stratigraphy of South Australia where he identified thick and extensive glacial sediments and late in his career speculated on the possibility of global glaciation. Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and that of other continents where low-latitude glacial deposits are found, has remained constant through time. With the advancement of the continental drift hypothesis, and eventually plate tectonic theory, came an easier explanation for the glaciogenic sediments—they were deposited at a point in time when the continents were at higher latitudes.
In 1964, the idea of global-scale glaciation reemerged when W. Brian Harland published a paper in which he presented palaeomagnetic data showing that glacial tillites in Svalbard and Greenland were deposited at tropical latitudes. From this palaeomagnetic data, and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued for an ice age that was so extreme that it resulted in the deposition of marine glacial rocks in the tropics.
In the 1960s, Mikhail Budyko, a Russian climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global climate. Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions a feedback ensued where the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium. While Budyko's model showed that this ice-albedo stability could happen, he concluded that it had never happened, because his model offered no way to escape from such a scenario.
The term "Snowball Earth" was coined by Joseph Kirschvink, a professor of geobiology at the California Institute of Technology, in a short paper published in 1992 within a lengthy volume concerning the biology of the Proterozoic eon. The major contributions from this work were: (1) the recognition that the presence of banded iron formations is consistent with such a glacial episode and (2) the introduction of a mechanism with which to escape from an ice-covered Earth—the accumulation of CO2 from volcanic outgassing leading to an ultra-greenhouse effect.
Interest in the Snowball Earth increased dramatically after Paul F. Hoffman, professor of geology at Harvard University, and coauthors applied Kirschvink's ideas to a succession of Neoproterozoic sediments in Namibia, elaborated upon the hypothesis by incorporating such observations as the occurrence of cap carbonates, and published their results in the journal Science in 1998.
Currently, aspects of the hypothesis remain controversial and it is being debated under the auspices of the International Geoscience Programme (IGCP) Project 512: Neoproterozoic Ice Ages.
In the latest reported research in March 2010, the journal Science published an article "Calibrating the Cryogenian" which concluded that "Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent. Also in March 2010, a study was released that showed that Canadian rocks of glacial origin were created when that portion of Canada was at the equator approximately 716 Ma.
There is evidence for three separate "snowball earth" events. The first occurred about 2220 Ma, the second around 710 Ma, and the third around 640 Ma.
Evidence
The Snowball Earth hypothesis was originally devised to explain the apparent presence of glaciers at tropical latitudes. Modelling suggested that once glaciers spread to within 30° of the equator, an ice-albedo feedback would result in the ice rapidly advancing to the equator. Therefore, the presence of glacial deposits seemingly within the tropics appeared to point to global ice cover.
Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove two things:
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that a bed contains sedimentary structures that could only have been created by glacial activity;
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that the bed lay within the tropics when it was deposited.
During a period of global glaciation, it must also be demonstrated that
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3. glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence.
This latter point is very difficult to prove. Before the Ediacaran (before 630 Ma), the biostratigraphic markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at the same time. The best we can do is to estimate the age of the rocks using radiometric methods, which are rarely accurate to better than a million years or so.
The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the latitude of landmasses even as little as 200 million years ago can be riddled with difficulties.
Evidence #1: Palaeomagnetism
The Snowball Earth hypothesis was first posited in order to explain what were then considered to be glacial deposits near the equator. Since continents drift with time, ascertaining their position at a given point in history is far from trivial. In addition to considerations of how the continents would have fitted together, the latitude at which a rock was deposited can be constrained by palaeomagnetism.
When sedimentary rocks form, magnetic minerals within them tend to align themselves with the Earth's magnetic field. Through the precise measurement of this palaeomagnetism, it is possible to estimate the latitude (but not the longitude) where the rock matrix was deposited. Paleomagnetic measurements have indicated that some sediments of glacial origin in the Neoproterozoic rock record were deposited within 10 degrees of the equator, although the accuracy of this reconstruction is in question. This palaeomagnetic location of apparently glacial sediments (such as dropstones) has been taken to suggest that glaciers extended to sea-level in the tropical latitudes. It is not clear whether this can be taken to imply a global glaciation, or the existence of localised, possibly land-locked, glacial regimes.
Evidence #2: Low latitude glacial deposits
Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the Snowball Earth hypothesis many Neoproterozoic sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, it is worth remembering that many sedimentary features traditionally associated with glaciers can also be formed by other means. Thus the glacial origin of many of the key occurrences for Snowball Earth has been contested. As of 2007, evidence of equatorial glacial deposits was still debated. However, new data recently revealed in March 2010 is putting the Snowball theory on more solid ground. Evidence of possible glacial origin of sediment includes:
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Dropstones (stones dropped into marine sediments)
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Varves (annual sediment layers in periglacial lakes)
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Glacial striations (formed by embedded rocks scraped against bedrock)
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Diamictites (poorly sorted conglomerates, which can also form by other non-glacial methods)
Open Water Deposits
It appears that some deposits formed during the Snowball period could only have been formed in the presence of an active hydrological cycle. Bands of glacial deposits up to hundreds of meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers were melting and re-forming repeatedly; solid oceans would not permit this scale of deposition. It is considered possible that ice streams such as seen in Antarctica today could be responsible for these sequences. Further, sedimentary features that could only form in open water, for example wave-formed ripples, far-traveled ice-rafted debris and indicators of photosynthetic activity, can be found throughout sediments dating from the 'Snowball Earth' periods.
Carbon isotope ratios
There are two stable isotopes of carbon in sea water: carbon-12 (12C) and the rare carbon-13 (13C), which makes up about 1.109 percent of all carbon isotopes.
Biochemical processes, of which photosynthesis is one, tend to preferentially incorporate the lighter 12C isotope. Thus ocean-dwelling photosynthesizers, both protists and algae, tend to be very slightly depleted in 13C, relative to the abundance found in the primary volcanic sources of the Earth's carbon. Therefore, an ocean with photosynthetic life will have a higher 12C/13C ratio within organic remains, and a lower ratio in corresponding ocean water. The organic component of the lithified sediments will forever remain very slightly, but measurably, depleted in 13C.
During the proposed episode of Snowball Earth, there are rapid and extreme negative excursions in the ratio of 13C to 12C. This is consistent with a deep freeze that killed off most or nearly all photosynthetic life – although other mechanisms, such as clathrate release, can also cause such perturbations. Close analysis of the timing of 13C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic.
Banded Iron Formations
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with black-band ironstone
Banded iron formations (BIF) are sedimentary rocks of layered iron oxide and iron-poor chert. In the presence of oxygen, iron naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of the Earth's atmosphere during the Paleoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically-produced oxygen and precipitated out as iron oxide.
The bands were produced at the tipping point between an anoxic and an oxygenated ocean. Since today's atmosphere is oxygen rich (nearly 21 percent by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Paleoproterozoic (after 1.8 billion years ago) are associated with Cryogenian glacial deposits.
For such iron-rich rocks to be deposited there would have to be anoxia in the ocean, so that much dissolved iron (as ferrous oxide) could accumulate before it met an oxidant that would precipitate it as ferric oxide. For the ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the snowball earth argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea ice, while opponents suggest that the rarity of the BIF deposits may indicate that they formed in inland seas.
Cap carbonate rocks
Around the top of Neoproterozoic glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary limestone or dolostone metres to tens of metres thick. These cap carbonates sometimes occur in sedimentary successions that have no other carbonate rocks, suggesting that their deposition is result of a profound aberration in ocean chemistry.
These cap carbonates have unusual chemical composition, as well as strange sedimentary structures that are often interpreted as large ripples. The formation of such sedimentary rocks could be caused by a large influx of positively-charged ions, as would be produced by rapid weathering during the extreme greenhouse following a Snowball Earth event. The δ13C isotopic signature of the cap carbonates is near -5‰, consistent with the value of the mantle — such a low value is usually/could be taken to signify an absence of life, since photosynthesis usually acts to raise the value; alternatively the release of methane deposits could have lowered it from a higher value, and counterbalance the effects of photosynthesis.
The precise mechanism involved in the formation of cap carbonates is not clear, but the most cited explanation suggests that at the melting of a Snowball Earth, water would dissolve the abundant CO2 from the atmosphere to form carbonic acid, which would fall as acid rain. This would weather exposed silicate and carbonate rock (including readily-attacked glacial debris), releasing large amounts of calcium, which when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock. Such an abiotic "cap carbonate" sediment can be found on top of the glacial till that gave rise to the Snowball Earth hypothesis.
However, there are some problems with the designation of a glacial origin to cap carbonates. Firstly, the high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic, and dissolve any carbonates contained within — starkly at odds with the deposition of cap carbonates. Further, the thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin. An alternative mechanism, which may have produced the Doushantuo cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low — as low as 48‰ — δ13C values — as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments.
Changing acidity
Isotopes of the element boron suggest that the pH of the oceans dropped dramatically before and after the Marinoan glaciation. This may indicate a build up of carbon dioxide in the atmosphere, some of which would dissolve into the oceans to form carbonic acid. Although the boron variations may be evidence of extreme climate change, they need not imply a global glaciation.
Space dust
The Earth's surface is very depleted in the element iridium, which primarily resides in the Earth's core. The only significant source of the element at the surface is cosmic particles that reach Earth. During a Snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An iridium anomaly has been discovered at the base of the cap carbonate formations, and has been used to suggest that the glacial episode lasted for at least 3 million years, but this does not necessarily imply a global extent to the glaciation; indeed a similar anomaly could be explained by the impact of a large extra-planetary object, such as a meteor.
Cyclic climate fluctuations
Using the ratio of mobile cations to those that remain in soils during chemical weathering (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods. This pattern, if a true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to Pleistocene ice age cycles than to a completely frozen Earth.
What's more, glacial sediments of the Portaskaig formation in Scotland clearly show interbedded cycles of glacial and shallow marine sediments. The significance of these deposits is highly reliant upon their dating. Glacial sediments are difficult to date, and the closest dated bed to the Portaskaig group is 8 km stratigraphically above the beds of interest. Its dating to 600 Ma means the beds can be tentatively correlated to the Sturtian glaciation, but they may represent the advance or retreat of a Snowball Earth.
Further modelling shows that ice can in fact get as close as 25° or closer to the equator without initiating total glaciation.
End of Reading
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Source: Snowball Earth