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The Snowball Earth hypothesis proposes that during one or more of Earth's icehouse climates, Earth's surface became entirely or nearly entirely frozen, sometime earlier than Mya million years ago during the Cryogenian period. Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice - or slush -covered ocean   and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open or seasonally open water. The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity.
Skeptics suggest that the palaeomagnetic data could be corrupted if Earth's ancient magnetic field was substantially different from today's. Depending on the rate of cooling of Earth's coreit is possible that during the Proterozoic, the magnetic field did not approximate a simple dipolar distribution, with north and south magnetic poles 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.
Palaeomagnetic data would then have to be re-interpreted, as the sedimentary minerals could have aligned pointing to a 'West Pole' rather than the North Pole. Alternatively, Earth's dipolar field could have been oriented such that the poles were close to the equator. This hypothesis has been posited to explain the extraordinarily rapid motion of the magnetic poles implied by the Ediacaran palaeomagnetic record; the alleged motion of the north pole would occur around the same time as the Gaskiers glaciation.
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 orogeny releases hot water as a by-product of metamorphic reactions; this water can circulate to rocks thousands of kilometers 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.
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.
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.
However, evidence of sea-level glaciation in the tropics during the Sturtian is accumulating.
It appears that some deposits formed during the snowball period could only have formed in the presence of an active hydrological cycle. Bands of glacial deposits up to 5, meters thick, separated by small meters bands of non-glacial sediments, demonstrate that glaciers melted and re-formed repeatedly for tens of millions of years; solid oceans would not permit this scale of deposition. Further, sedimentary features that could only form in open water for example: wave-formed ripplesfar-traveled ice-rafted debris and indicators of photosynthetic activity can be found throughout sediments dating from the snowball-Earth periods.
While these may represent "oases" of meltwater on a completely frozen Earth,  computer modelling suggests that large areas of the ocean must have remained ice-free; arguing that a "hard" snowball is not plausible in terms of energy balance and general circulation models.
There are two stable isotopes of carbon in sea water : carbon 12 C and the rare carbon 13 Cwhich makes up about 1. Biochemical processes, of which photosynthesis is one, tend to preferentially incorporate the lighter 12 C isotope. Thus ocean-dwelling photosynthesizers, both protists and algaetend to be very slightly depleted in 13 C, relative to the abundance found in the primary volcanic sources of Earth's carbon.
The organic component of the lithified sediments will remain very slightly, but measurably, depleted in 13 C. During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of 13 C to 12 C. Banded iron formations BIF are sedimentary rocks of layered iron oxide and iron-poor chert.
The Snowball Earth hypothesis proposes that during one or more of Earth's icehouse climates, Earth's surface became entirely or nearly entirely frozen, sometime earlier than Mya (million years ago) during the Cryogenian countryconnectionsqatar.coments of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and . Dating Glacial Deposits, speed dating barcelona gay district, free email dating service, urban slang for dating/ Dating glacial sediments is important, and there are a variety of methods we can use, such as radiocarbon and cosmogenic nuclide exposure dating. Dating glacial sediments is important, and there are a variety of methods we can use, such as radiocarbon and cosmogenic nuclide exposure dating.
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 Palaeoproterozoic 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. The only extensive iron formations that were deposited after the Palaeoproterozoic after 1.
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 could 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. Around the top of Neoproterozoic glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary limestone or dolomite metres to tens of metres thick. These cap carbonates have unusual chemical composition, as well as strange sedimentary structures that are often interpreted as large ripples.
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 CO 2 from the atmosphere to form carbonic aci which would fall as acid rain. This would weather exposed silicate and carbonate rock including readily attacked glacial debrisreleasing large amounts of calciumwhich when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock.
Dating glacial deposits
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.
Isotopes of the element boron suggest that the pH of the oceans dropped dramatically before and after the Marinoan glaciation.
Although the boron variations may be evidence of extreme climate changethey need not imply a global glaciation. Earth's surface is very depleted in the element iridiumwhich 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 meteorite.
Using the ratio of mobile cations to those that remain in soils during chemical weathering the chemical index of alterationit 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. In addition, glacial sediments of the Port Askaig Tillite Formation in Scotland clearly show interbedded cycles of glacial and shallow marine sediments.
The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase Earth's albedowhich would result in positive feedback for cooling.
A GOOD EXAMPLE OF GLACIAL TILL
If enough snow and ice accumulates, run-away cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where solar radiation is most direct.
Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more Solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow.
This positive feedback loop could eventually produce a frozen equator as cold as modern Antarctica.
Global warming associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium. A tropical distribution of the continents is, perhaps counter-intuitively, necessary to allow the initiation of a snowball Earth.
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.
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 dioxidea greenhouse gas, from the air into the geosphereand, in steady-state on geologic time scales, offsets the carbon dioxide emitted from volcanoes into the atmosphere.
As ofa precise continental distribution during the Neoproterozoic was difficult to establish because there were too few suitable sediments for analysis. 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 atmosphereoxidizing 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 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 Earth warms as this greenhouse gas accumulates-this ' negative feedback ' process limits the magnitude of cooling. During the Cryogenian period, however, Earth's continents were all at tropical latitudes, which made this moderating process less effective, as high weathering rates continued on land even as Earth cooled. This let ice advance beyond the polar regions. Polar continents, due to low rates of evaporationare 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 relative to carbon in sediments pre-dating "global" glaciation indicates that CO 2 draw-down before snowball Earths was a slow and continuous process.
In JanuaryGernon et al. Gernon et al. Global temperature fell so low that the equator was as cold as modern-day Antarctica.
A lack of heat-retaining clouds, caused by water vapor freezing out of the atmosphere, amplified this effect. Over 4 to 30 million years, enough CO 2 and methanemainly emitted by volcanoes but also produced by microbes converting organic carbon trapped under the ice into the gas,  would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of permanently ice-free land and water developed;  this would be darker than the ice, and thus absorb more energy from the Sun-initiating a " positive feedback ".
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 phosphoruswhich combined with the abundance of CO 2 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.
Although the positive feedback loop would melt the ice in geological short order, perhaps less than 1, years, replenishment of atmospheric oxygen and depletion of the CO 2 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. More recent evidence suggests that with colder oceanic temperatures, the resulting higher ability of the oceans to dissolve gases led to the carbon content of sea water being more quickly oxidized to carbon dioxide.
This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of Earth's surface, and the prevention of a total snowball state. During millions of years, cryoconite would have accumulated on and inside the ice.
Psychrophilic microorganisms, volcanic ash and dust from ice-free locations would settle on ice covering several million square kilometers.
Once the ice started to melt, these layers would become visible and color the icy surfaces dark, helping to accelerate the process. Ultraviolet light from the Sun would also produce hydrogen peroxide H 2 O 2 when it hits water molecules.
Normally hydrogen peroxide is broken down by sunlight, but some would have been trapped inside the ice. When the glaciers started to melt, it would have been released in both the ocean and the atmosphere, where it was split into water and oxygen molecules, leading to an increase in atmospheric oxygen.
While the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to posit a "slushball Earth", in which a band of ice-free, or ice-thin, waters remains around the equatorallowing for a continued hydrologic cycle. This hypothesis appeals to scientists who observe certain features of the sedimentary record that can only be formed under open water, or rapidly moving ice which would require somewhere ice-free to move to.
Recent research observed geochemical cyclicity in clastic rocksshowing that the "snowball" periods were punctuated by warm spells, similar to ice age cycles in recent Earth history. 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.
A less extreme snowball Earth hypothesis involves continually evolving continental configurations and changes in ocean circulation. The snowball Earth hypothesis does not explain the alternation of glacial and interglacial events, nor the oscillation of glacial sheet margins. The argument against the hypothesis is evidence of fluctuation in ice cover and melting during "snowball Earth" deposits.
Evidence for such melting comes from evidence of glacial dropstones,  geochemical evidence of climate cyclicity,  and interbedded glacial and shallow marine sediments. There have been difficulties in recreating a snowball Earth with global climate models. Simple GCMs with mixed-layer oceans can be made to freeze to the equator; a more sophisticated model with a full dynamic ocean though only a primitive sea ice model failed to form sea ice to the equator.
Strontium isotopic data have been found to be at odds with proposed snowball Earth models of silicate weathering shutdown during glaciation and rapid rates immediately post-glaciation.
Therefore, methane release from permafrost during marine transgression was proposed to be the source of the large measured carbon excursion in the time immediately after glaciation. Nick Eyles suggests that the Neoproterozoic Snowball Earth was in fact no different from any other glaciation in Earth's history, and that efforts to find a single cause are likely to end in failure.
The associated tectonic uplift would form high plateaus, just as the East African Rift is responsible for high topography; this high ground could then host glaciers. Banded iron formations have been taken as unavoidable evidence for global ice cover, since they require dissolved iron ions and anoxic waters to form; however, the limited extent of the Neoproterozoic banded iron deposits means that they may not have formed in frozen oceans, but instead in inland seas.
Such seas can experience a wide range of chemistries; high rates of evaporation could concentrate iron ions, and a periodic lack of circulation could allow anoxic bottom water to form. Continental rifting, with associated subsidence, tends to produce such landlocked water bodies. This rifting, and associated subsidence, would produce the space for the fast deposition of sediments, negating the need for an immense and rapid melting to raise the global sea levels. In either of these two situations, the freeze would be limited to relatively small areas, as is the case today; severe changes to Earth's climate are not necessary.
The evidence for low-latitude glacial deposits during the supposed snowball Earth episodes has been reinterpreted via the concept of inertial interchange true polar wander IITPW. This could feasibly produce the same distribution of glacial deposits without requiring any of them to have been deposited at equatorial latitude. A tremendous glaciation would curtail photosynthetic life on Earth, thus depleting atmospheric oxygen, and thereby allowing 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.
aries of dating glacial landforms and dating glacial sedi-ments are blurred. For example, moraines comprise glacial sediments, and dating sediments associated with a landform can constrain landform age. However, focus on dating landforms inherently results in omitting certain landforms from this entry whose age in absolute time. Numerical dating of glacial deposits is important for understanding Quaternary glacial evolution. Optically stimulated luminescence (OSL) dating is one of Cited by: Glacial deposits underlie many notable landforms, of which drumlins and eskers are among the most distinctive. Drumlins are streamlined hills ideally having the shape of a teardrop or inverted spoon. They occur in fields containing dozens or hundreds to thousands of individual drumlins.
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. This change in diversity and composition has not yet been observed  -in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth.
A snowball Earth has profound implications in the history of life on Earth. While many refugia have been postulated, global ice cover would certainly have ravaged ecosystems dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials. Because about half of the oceans' water was frozen solid as ice, the remaining water would be twice as salty as it is today, lowering its freezing point.
When the ice sheet melted, it would cover the oceans with a layer of hot freshwater up to 2 kilometres thick. Only after the hot surface water mixed with the colder and deeper saltwater did the sea return to a warmer and less salty state. 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. 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.
Introduction to dating glacial sediments
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. Another major glacial episode may have ended just a few million years before the Cambrian explosion. One hypothesis which has been gaining currency in recent years: that early snowball Earths did not so much affect the evolution of life on Earth as result from it.
In fact the two hypotheses are not mutually exclusive. The idea is that Earth's life forms affect the global carbon cycle and so major evolutionary events alter the carbon cycle, redistributing carbon within various reservoirs within the biosphere system and in the process temporarily lowering the atmospheric greenhouse carbon reservoir until the revised biosphere system settled into a new state.
The Snowball I episode of the Huronian glaciation 2. Global ice cover, if it existed, may-in concert with geothermal heating-have led to a lively, well mixed ocean with great vertical convective circulation. There were three or four significant ice ages during the late Neoproterozoic. Of these, the Marinoan was the most significant, and the Sturtian glaciations were also truly widespread.
The status of the Kaigas "glaciation" or "cooling event" is currently unclear; some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe it may indeed be a third ice age. Emerging evidence suggests that the Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis. The snowball Earth hypothesis has been invoked to explain glacial deposits in the Huronian Supergroup of Canada, though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested.
As the Sun was notably weaker at the time, Earth's climate may have relied on methane, a powerful greenhouse gas, to maintain surface temperatures above freezing. In the absence of this methane greenhouse, temperatures plunged and a snowball event could have occurred. Before the theory of continental drift, glacial deposits in Carboniferous strata in tropical continental areas such as India and South America led to speculation that the Karoo Ice Age glaciation reached into the tropics.
However, a continental reconstruction shows that ice was in fact constrained to the polar parts of the supercontinent Gondwana. From Wikipedia, the free encyclopedia. Redirected from Snowball Earth hypothesis. Worldwide glaciation episodes during the Neoproterozoic Era. Proterozoic snowball periods. Sturtian . Marinoan .
Neoproterozoic era. Estimate of Proterozoic glacial periods. As for the Kaigas, its very existence is doubted by some. An earlier and longer possible snowball phase, the Huronian glaciationis not shown. Life timeline. This box: view talk edit. Single-celled life. Multicellular life. Earliest water. Earliest life. Earliest oxygen. Atmospheric oxygen.
Oxygen crisis. Sexual reproduction. Earliest plants. Earliest animals. Ediacara biota. Uranium-series uses the decay of uranium and thorium isotopes U, U and Th in calcites in particular, such as stalactites and stalagmites in caves.
Potassium-argon and argon-argon dating can be used to date the formation of volcanic rocks. Older marine sediments can be dated using palaeo-magnetism. This is caused by a number of factors, including variations in solar radiation, magnetic storms, and internal geophysical factors. Unconsolidated sediments contain magnetic minerals, such as those on the continental shelf and slope. These minerals are magnetised during formation.
The sediments can be compared to palaeo magnetostratigraphic data, and this can be used as a proxy age determination. Save my name, email, and website in this browser for the next time I comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Other methods of dating glacial sediments There are so many other methods of dating Quaternary sediments and organic material that it is impractical to cover them all here in detail.
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