Alternatively, changes in solar energy output or perturbations of Earth's orbit could act as a trigger. However the initial cooling comes about, resultant ice accumulation would reflect solar energy back to space, further cooling the atmosphere and generating more ice cover.
This feedback loop could eventually produce a frozen equator as cold as modern-day Antarctica. To break out of this icy condition either the level of solar energy incident on Earth would have to increase significantly, or huge quantities of greenhouse gases, emitted primarily by volcanic activity, would have to accumulate over millions of years. The eventual melting would perhaps take as little as 1,000 years.
Attempts to construct computer models of a Snowball Earth have also struggled to accommodate global ice cover without fundamental changes in the laws and constants which govern the planet.
The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the Cryogenian period.
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:
During a period of global glaciation, it must also be demonstrated that
This latter point is very difficult to prove. Before the Ediacaran, 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 can be riddled with difficulties.
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. Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator.
Skeptics suggest that the palaeomagnetic data could be corrupted if the Earth's magnetic field was substantially different from today's. Depending on the rate of cooling of the Earth's core, it is possible that during the Proterozoic, its magnetic field did not approximate a dipolar distribution, with a North and South pole roughly aligning with the planet's axis as they do today. Instead, a hotter core may have circulated more vigorously and given rise to 4, 8 or more poles. Paleomagnetic data would then have to be re-interpreted as particles could align pointing to a 'West Pole' rather than the North Pole.
Another weakness of reliance on palaeomagnetic data is the difficulty in determining whether the magnetic signal recorded is original, or whether it has been reset by later activity. For example, a mountain-building releases hot water as a by-product of metamorphic reactions; this water can circulate to rocks thousands of km away and reset their magnetic signature. This makes the authenticity of rocks older than a few million years difficult to determine without painstaking mineralogical observations.name=Meert1994pm>
There is currently only one deposit, the Elatina deposit of Australia, that was indubitably deposited at low latitudes; its depositional date is well constrained, and the signal is demonstrably original.
Biochemical processes, of which photosynthesis is one, tend to preferentially incorporate the lighter isotope. Thus ocean-dwelling photosynthesizers, both protists and algae, tend to be very slightly depleted in , 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 / 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 .
During the proposed episode of Snowball Earth, there are rapid and extreme negative excursions in the ratio of to . 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 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic. Although, the stratigraphic record of Oman presents a large negative carbon isotope excursion (within the Shuram Formation) away from any glacial evidence.
Banded iron formations 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 hypothesis 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. Being isolated from the oceans, such lakes may have been stagnant and anoxic at depth, much like today's Black Sea; a sufficient input of iron could provide the necessary conditions for BIF formation. A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments. BIFs are also strikingly absent during the Marinoan glaciation.
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 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‰ - values - as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments.
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 8km stratigraphically above the beds of interest. Its dating to 600Ma 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.
Further, tropical continents are subject to more rainfall, which leads to increased river discharge — and erosion. When exposed to air, silicate rocks undergo weathering reactions which remove carbon dioxide from the atmosphere. These reactions proceed in the general form: Rock-forming mineral + CO2 + H2O → cations + bicarbonate + SiO2. An example of such a reaction is the weathering of wollastonite:
The released calcium cations react with the dissolved bicarbonate in the ocean to form calcium carbonate as a chemically precipitated sedimentary rock. This transfers carbon dioxide, a greenhouse gas, from the air into the geosphere, and, in steady-state on geologic time scales, offsets the carbon dioxide emitted from volcanoes into the atmosphere.
A paucity of suitable sediments for analysis makes precise continental distribution during the Neoproterozoic difficult to establish. Some reconstructions point towards polar continents — which have been a feature of all other major glaciations, providing a point upon which ice can nucleate. Changes in ocean circulation patterns may then have provided the trigger of snowball Earth.
Additional factors that may have contributed to the onset of the Neoproterozoic Snowball include the introduction of atmospheric free oxygen, which may have reached sufficient quantities to react with methane in the atmosphere, oxidizing it to carbon dioxide, a much weaker greenhouse gas, and a younger — thus fainter — sun, which would have emitted 6 percent less radiation in the Neoproterozoic.
Normally, as the Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation, the cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and the Earth warms as this greenhouse gas accumulates — this 'negative feedback' process limits the magnitude of cooling. During the Cryogenian period, however, the Earth's continents were all at tropical latitudes, which made this moderating process less effective, as high weathering rates continued on land even as the Earth cooled. This let ice advance beyond the polar regions. Once ice advanced to within 30° of the equator, a positive feedback could ensue such that the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the whole Earth is ice covered.
Polar continents, due to low rates of evaporation, are too dry to allow substantial carbon deposition — restricting the amount of atmospheric carbon dioxide that can be removed from the carbon cycle. A gradual rise of the proportion of the isotope carbon-13 relative to carbon-12 in sediments pre-dating "global" glaciation indicates that CO2 draw-down before snowball Earths was a slow and continuous process.
The start of Snowball Earths are always marked by a sharp downturn in the δ13C value of sediments, a hallmark that may be attributed to a crash in biological productivity as a result of the cold temperatures and ice-covered oceans.
On the continents, the melting of glaciers would release massive amounts of glacial deposit, which would erode and weather. The resulting sediments supplied to the ocean would be high in nutrients such as phosphorus, which combined with the abundance of CO2 would trigger a cyanobacteria population explosion, which would cause a relatively rapid reoxygenation of the atmosphere, which may have contributed to the rise of the Ediacaran biota and the subsequent Cambrian explosion — a higher oxygen concentration allowing large multicellular lifeforms to develop. This positive feedback loop would melt the ice in geological short order, perhaps less than 1,000 years; replenishment of atmospheric oxygen and depletion of the CO2 levels would take further millennia.
It is possible that carbon dioxide levels fell enough for Earth to freeze again; this cycle may have repeated until the continents had drifted to more polar latitudes.
The weightiest argument against the hypothesis is evidence of fluctuation in ice cover and melting during "Snowball Earth" deposits. Such deposits could represent either the beginning or end of a Snowball, thus losing a data point in the support of Snowball Earth, or be contemporaneous with the Snowball, thus disproving any theory of continuous total ice cover. Proof of such melting comes from evidence of glacial dropstones, geochemical evidence of climate cyclicity, and interbedded glacial and shallow marine sediments. A longer record from Oman, well constrained to within 20° of the equator, covers the period from 712 to 545 million years ago - a time span containing the Sturtian and Marinoan glaciations - and shows that this latitude was largely free of ice almost continually throughout the period.
It does not seem mathematically possible to create a scenario in which the entirety of the globe's oceans freeze over; in addition, the levels of necessary to melt a global ice cover have been calculated to be 120,000 ppm, which is considered by some to be unreasonably huge.
Mathematical analysis of other parts of the Snowball Earth hypothesis also produce results at odds to the geological record. There is no sign of there being the 1,000 times increase in weathering necessary to draw down from the atmosphere, nor does data support a prolonged shutdown of the biological pump. Pre-industrial atmospheric levels were 280ppm.
A tremendous glaciation would curtail plant life on Earth, thus letting the atmospheric oxygen be drastically depleted and perhaps even disappear, and thus allow non-oxidized iron-rich rocks to form.
Detractors argue that this kind of glaciation would have made life extinct entirely. However, microfossils such as stromatolites and oncolites prove that in shallow marine environments at least life did not suffer any perturbation. Instead life developed a trophic complexity and survived the cold period unscathed. Proponents counter that it may have been possible for life to survive in these ways:
However, organisms and ecosystems, as far as it can be determined by the fossil record, do not appear to have undergone the significant change that would be expected by a mass extinction. Even if life were to cling on in all the ecological refuges listed above, the post-Snowball biota would have a noticeably different diversity and composition. This change in diversity and composition has not yet been observed. In fact, the organisms which ought to be most susceptible to climatic variation emerge unscathed from the Snowball Earth.
The Neoproterozoic was a time of remarkable diversification of multicellular organisms, including animals. Organism size and complexity increased considerably after the end of the Snowball glaciations. This development of multicellular organisms may have been the result of increased evolutionary pressures resulting from multiple icehouse-hothouse cycles; in this sense, Snowball Earth episodes may have "pumped" evolution. Alternatively, fluctuating nutrient levels and rising oxygen may have played a part. Interestingly, another major glacial episode may have ended just a few million years before the Cambrian explosion.
Mechanistically, the impact of snowball Earth (in particular the later glaciations) on complex life is likely to have occurred through the process of kin selection. Organ-scale differentiation, in particular the terminal (irreversible) differentiation present in animals, requires the individual cell (and the genes contained within it) to "sacrifice" their ability to reproduce, so that the colony is not disrupted. From the short-term perspective of the gene, more offspring will be gained (in the short term) by causing the cell in which it is contained to ignore any signals received from the colony, and to reproduce at the maximum rate, regardless of the implications for the wider group. Today, this incentive explains the formation of tumours in animals and plants.
Such costly, "altruistic" differentiation can be adaptive (maximise the number of surviving offspring) to individual genes if the consequence of altruism (terminal cellular differentiation) benefits other copies of such genes. (Note that "altruism" refers only to the reproductive cost of the trait, and implies no sentience or foresight). Because relatives share genes, genes causing altruism (such as organ scale differentiation) can spread if it occurs between relatives, see kin selection.
It has been argued that because snowball Earth would undoubtedly have decimated the population size of any given species, the extremely small populations that resulted would all have been descended from a small number of individuals (see founder effect), and consequently the average relatedness between any two individuals (in this case individual cells) would have been exceptionally high as a result of glaciations. Altruism is known to increase from rarity when relatedness (R) exceeds the ratio of the cost (C) to the altruist (in this case, the cell giving up its own reproduction by differentiating), to the benefit (B) to the recipient of altruism (the germ line of the colony, that reproduces as a result of the differentiation), i.e. R > C/B (see Hamilton's rule). The evolutionary pressure of the high relatedness caused by the glaciations may have been sufficient to overcome the reproductive cost of forming a complex animal, for the first time in Earth's history.
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, the Sturgis Hooper 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.
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.