I have often thought about how Earth could have gone into a snowball stage while a human type of civilization was on it which likely would end that civilization and reduce people back to living like Native Alaskans did and some still do as hunter gatherers once again, especially of this happened over the whole earth. It wouldn't kill all species even humans totally, but it would reduce humans a lot in numbers. Some of the things that could cause this would be a Maunder minimum that lasted a long time combined with a lot of volcanoes going off around the same time which would cloud the atmosphere with pyroclastic dust(glass dust made from exploding rocks which causes them to powder in the air from quick exits from volcanoes).
For example, one of the main causes of the French Revolution(in the late 1700s) was Icelandic volcanoes going off which prevented normal crops in France. This caused starvation but because the aristocracy was preoccupied the people guillotined them all because they were starving.
Snowball Earth
From Wikipedia, the free encyclopedia
(Redirected from Snowball earth)
Proterozoic snowball periods
-1000 —
–
-950 —
–
-900 —
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-850 —
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-800 —
–
-750 —
–
-700 —
–
-650 —
–
-600 —
–
-550 —
A
recent estimate of the timing and duration of Proterozoic glacial
periods. Note that great uncertainty surrounds the dating of pre-
Gaskiers
glaciations. The status of the Kaigas is not clear; its dating is very
insecure and many workers do not recognise it as a glaciation. From
Smith 2009.
[1] An earlier and longer possible snowball phase, the
Huronian glaciation, is not shown here.
The
Snowball Earth hypothesis posits that the
Earth's surface became entirely or nearly entirely frozen at least once, sometime earlier than 650
Mya (million years ago). Proponents of the hypothesis argue that it best explains
sedimentary deposits generally regarded as of
glacial origin at
tropical paleolatitudes, and other otherwise enigmatic features in the
geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation, the
geophysical feasibility of an
ice- or
slush-covered ocean,
[2][3]
and the difficulty of escaping an all-frozen condition. A number of
unanswered questions exist, including whether the Earth was a full
snowball, or a "slushball" with a thin equatorial band of open (or
seasonally open) water.
The geological time frames under consideration come before the sudden
appearance of multicellular life forms on Earth known as the
Cambrian explosion,
and the most recent snowball episode may have triggered the evolution
of multi-cellular life on Earth. Another, much earlier and longer,
snowball episode, the
Huronian glaciation, which occurred 2400 to 2100 Mya may have been triggered by the first appearance of oxygen in the atmosphere, the "
Great Oxygenation Event."
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 about the possibility of
global glaciation.
[4]
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.
[5]
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 loop 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.
[6] 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. In 1971, Aron
Faegre, an American physicist, showed that a similar simple energy
balance model predicted three stable global climates, one of which was
snowball earth.
[7] This model introduced
Edward Norton Lorenz concept of intransitivity indicating that there could be a major jump from one climate to another, including to snowball earth.
The term "snowball Earth" was coined by
Joseph Kirschvink in a short paper published in 1992 within a lengthy volume concerning the biology of the
Proterozoic eon.
[8] 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 CO
2 from volcanic outgassing leading to an ultra-greenhouse effect.
Franklyn Van Houten's
discovery of a consistent geological pattern in which lake levels rose
and fell is now known as the "Van Houten cycle." His studies of
phosphorus deposits and
banded iron formations
in sedimentary rocks made him an early adherent of the "snowball Earth"
hypothesis postulating that the planet's surface froze more than 650
million years ago.
[9]
Interest in the snowball Earth increased dramatically after
Paul F. Hoffman applied Kirschvink's ideas to a succession of Neoproterozoic sedimentary rocks in
Namibia and elaborated upon the hypothesis by incorporating such observations as the occurrence of
cap carbonates in the journal
Science in 1998.
[10]
In 2010, Francis MacDonald reported evidence that
Pangaea was at equatorial latitude during the
Cryogenian with glacial ice at or below sea level and the associated
Sturtian glaciation was global.
[11][12]
Evidence
The snowball Earth hypothesis was originally devised to explain the apparent presence of glaciers at tropical latitudes.
[13] An
ice-albedo feedback would result in the ice rapidly advancing to the equator once glaciers spread to within 25°
[14] to 30°
[15] of the equator, according to modelling. Therefore, the presence of glacial deposits seemingly within the
tropics suggests 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:
- that a bed contains sedimentary structures that could have been created only by glacial activity;
- that the bed lay within the tropics when it was deposited.
During a period of global glaciation, it must also be demonstrated
that glaciers were active at different global locations at the same
time, and that no other deposits of the same age are in existence.
This last 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 that
can be done is to estimate the age of the rocks using radiometric
methods, which are rarely accurate to better than a million years or so.
[16]
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.
[17]
Palaeomagnetism
The snowball Earth hypothesis was first posited in order to explain
what were then considered to be glacial deposits near the equator. Since
tectonic plates move in time, ascertaining their position at a given
point in history is not easy. In addition to considerations of how the
recognisable landmasses could have fit 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,
[18] although the accuracy of this reconstruction is in question.
[16] 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.
[19] Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator.
[20]
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.
Alternatively, the Earth's dipolar field could have 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.
[21]
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.
[14]
Moreover, further evidence is accumulating that large-scale
remagnetization events have taken place, that may require revision of
the position of the paleomagnetic poles.
[22][23]
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.
[24]
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.
[25] Thus the glacial origin of many of the key occurrences for snowball Earth has been contested.
[16] As of 2007, there was only one "very reliable" – still challenged
[16] – datum point identifying tropical
tillites,
[18]
which makes statements of equatorial ice cover somewhat presumptuous.
However evidence of sea-level glaciation in the tropics during the
Sturtian is accumulating.
[26] Evidence of possible glacial origin of sediment includes:
- Dropstones (stones dropped into marine sediments), which can be deposited by glaciers or other phenomena.[27]
- Varves (annual sediment layers in periglacial lakes), which can form at higher temperatures.[28]
- Glacial striations (formed by embedded rocks scraped against bedrock): similar striations are from time to time formed by mudflows or tectonic movements.[29]
- Diamictites (poorly sorted conglomerates). Originally described as glacial till, most were in fact formed by debris flows.[16]
Open-water deposits
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 5,500 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.
[30] It is considered
[by whom?] possible that
ice streams such as seen in
Antarctica today could have caused 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. While these may
represent "oases" of
meltwater on a completely frozen Earth,
[31]
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.
[32]
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 carbon atoms.
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 lower
13C/
12C
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.
[33]
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.
[34]
Banded iron formations
2.1 billion year old rock 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 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,
[8] 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.
[16]
A further difficulty in suggesting that BIFs marked the end of the
glaciation is that they are found interbedded with glacial sediments.
[19] BIFs are also strikingly absent during the
Marinoan glaciation.
[citation needed]
Cap carbonate rocks
Volcanoes may have had a role in replenishing CO
2, possibly ending the global ice age that was the snowball Earth during the
Cryogenian Period.
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.
[35]
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.
[36]
These cap carbonates have unusual chemical composition, as well as
strange sedimentary structures that are often interpreted as large
ripples.
[37] 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 CO
2 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.
[16] 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.
[38]
Changing acidity
Isotopes of the element
boron suggest that the
pH of the oceans dropped dramatically before and after the
Marinoan glaciation.
[39] This may indicate a buildup 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,
[40] 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.
[41]
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.
[42] 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.
[43]
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.
Mechanisms
One computer simulation of conditions during a snowball Earth period
[44]
The initiation of a snowball Earth event would involve some initial
cooling mechanism, which would result in an increase in the Earth's
coverage of snow and ice. The increase in Earth's coverage of snow and
ice would in turn increase the Earth's
albedo, which would result in
positive feedback
for cooling. If enough snow and ice accumulates, runaway 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.
Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a
supervolcano, a reduction in the atmospheric concentration of
greenhouse gases such as
methane and/or
carbon dioxide, changes in
solar energy output, or perturbations of the
Earth's orbit.
Regardless of the trigger, initial cooling results in an increase in
the area of the Earth's surface covered by ice and snow, and the
additional ice and snow reflects more solar energy back to space,
further cooling the Earth and further increasing the area of the Earth's
surface covered by ice and snow. This positive feedback loop could
eventually produce a frozen
equator as cold as modern-day
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
the Earth's surface would require as little as 1,000 years.
Continental distribution
A tropical distribution of the continents is, perhaps
counter-intuitively, necessary to allow the initiation of a snowball
Earth.
[45]
Firstly, tropical continents are more reflective than open ocean, and
so absorb less of the Sun's heat: most absorption of solar energy on
Earth today occurs in tropical oceans.
[46]
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 + CO
2 + H
2O → cations + bicarbonate + SiO
2. An example of such a reaction is the weathering of
wollastonite:
- CaSiO3 + 2CO2 + H2O → Ca2+ + SiO2 + 2HCO3−
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.
[47]
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.
[48]
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,
[49] and a younger — thus fainter — Sun, which would have emitted 6 percent less radiation in the Neoproterozoic.
[16]
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,
[50] 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 CO
2 draw-down before snowball Earths was a slow and continuous process.
[51]
The start of snowball Earths are always marked by a sharp downturn in the δ
13C value of sediments,
[52]
a hallmark that may be attributed to a crash in biological productivity
as a result of the cold temperatures and ice-covered oceans.
During the frozen period
Global ice sheets may have delayed or prevented the establishment of multicellular life on Earth.
[citation needed]
Global temperature fell so low that the equator was as cold as modern-day
Antarctica.
[53]
This low temperature was maintained by the high albedo of the ice
sheets, which reflected most incoming solar energy into space. A lack of
heat-retaining clouds, caused by water vapor freezing out of the
atmosphere, amplified this effect.
Breaking out of global glaciation
The
carbon dioxide levels necessary to unfreeze the Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere.
[54]
Since the Earth was almost completely covered with ice, carbon dioxide
could not be withdrawn from the atmosphere by release of alkaline metal
ions weathering out of
siliceous rocks. Over 4 to 30 million years, enough CO
2 and
methane, mainly emitted by
volcanoes,
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;
[55] 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
phosphorus, which 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. 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 CO
2 levels would take further
millennia.
Destabilization of substantial deposits of
methane hydrates locked up in low-latitude
permafrost may also have acted as a trigger and/or strong positive feedback for deglaciation and warming.
[56]
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.
[57]
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 the surface of the Earth, and
the prevention of a total snowball state.
[58]
Slushball Earth hypothesis
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
equator, allowing 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 rocks, showing 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.
[59] Synthesised evidence has produced models indicating a "slushball Earth",
[60] where the stratigraphic record does not permit postulating complete global glaciations.
[59] Kirschivink's original hypothesis
[8] had recognised that warm tropical puddles would be expected to exist in a snowball earth.
The snowball Earth hypothesis does not explain the alternation of
glacial and interglacial events, nor the oscillation of glacial sheet
margins.
[61]
Scientific dispute
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,
[30] geochemical evidence of climate cyclicity,
[42] and interbedded glacial and shallow marine sediments.
[43]
A longer record from Oman, constrained to 13°N, covers the period from
712 to 545 million years ago — a time span containing the
Sturtian and Marinoan glaciations — and shows both glacial and ice-free deposition.
[62]
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.
[63] In addition, the levels of CO
2 necessary to melt a global ice cover have been calculated to be 130,000 ppm,
[54] which is considered by some
[who?] to be unreasonably large.
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.
[64]
"Zipper rift" hypothesis
Nick Eyles suggest 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.
[16] The "Zipper rift" hypothesis proposes two pulses of continental "unzipping" — first, the breakup of the supercontinent
Rodinia, forming the proto-Pacific ocean; then the splitting of the continent
Baltica from
Laurentia,
forming the proto-Atlantic — coincided with the glaciated periods. 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.
High-obliquity hypothesis
A competing hypothesis to explain the presence of ice on the equatorial continents was that the Earth's
axial tilt
was quite high, in the vicinity of 60°, which would place the Earth's
land in high "latitudes", although supporting evidence is scarce.
[65] A less extreme possibility would be that it was merely the Earth's
magnetic pole
that wandered to this inclination, as the magnetic readings which
suggested ice-filled continents depends on the magnetic and rotational
poles being relatively similar. In either of these two situations, the
freeze would be limited to relatively small areas, as is the case today;
severe changes to the Earth's climate are not necessary.
Inertial interchange true polar wander
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).
[66][67]
This hypothesis, created to explain palaeomagnetic data, suggests that
the Earth's axis of rotation shifted one or more times during the
general timeframe attributed to snowball Earth. This could feasibly
produce the same distribution of glacial deposits without requiring any
of them to have been deposited at equatorial latitude.
[68]
While the physics behind the proposition is sound, the removal of one
flawed data point from the original study rendered the application of
the concept in these circumstances unwarranted.
[69]
Several alternative explanations for the evidence have been proposed.
Survival of life through frozen periods
A tremendous glaciation would curtail photosynthetic 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.
[70] Proponents counter that it may have been possible for life to survive in these ways:
- In reservoirs of anaerobic and low-oxygen life powered by chemicals in deep oceanic hydrothermal vents surviving in Earth's deep oceans and crust; but photosynthesis would not have been possible there.
- As eggs and dormant cells and spores deep-frozen into ice right through the most severe phases of the frozen period.
- Under the ice layer, in chemolithotrophic (mineral-metabolizing) ecosystems
theoretically resembling those in existence in modern glacier beds,
high-alpine and Arctic talus permafrost, and basal glacial ice. This is
especially plausible in areas of volcanism or geothermal activity.[71]
- In deep ocean regions far from the supercontinent Rodinia or its remnants as it broke apart and drifted on the tectonic plates, which may have allowed for some small regions of open water preserving small quantities of life with access to light and CO2
for photosynthesizers (not multicellular plants, which did not yet
exist) to generate traces of oxygen that were enough to sustain some
oxygen-dependent organisms. This would happen even if the sea froze over
completely, if small parts of the ice were thin enough to admit light.
- In nunatak areas in the tropics,
where daytime tropical sun or volcanic heat heated bare rock sheltered
from cold wind and made small temporary melt pools, which would freeze
at sunset.
- In pockets of liquid water within and under the ice caps, similar to Lake Vostok in Antarctica. In theory, this system may resemble microbial
communities living in the perennially frozen lakes of the Antarctic dry
valleys. Photosynthesis can occur under ice up to 100 m thick, and at
the temperatures predicted by models equatorial sublimation would prevent equatorial ice thickness from exceeding 10 m.[72]
- In small oases of liquid water, as would be found near geothermal hotspots resembling Iceland today.[73]
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.
With the advent of more precise dating, a phytoplankton extinction
event which had been associated with snowball Earth was shown to precede
glaciations by 16 million years.
[74]
Even if life were to cling on in all the ecological refuges listed
above, a whole-Earth glaciation would result in a biota with a
noticeably different diversity and composition. This change in diversity
and composition has not yet been observed
[75]– in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth.
[41]
Implications
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.
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.
Effect on early evolution
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 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.
It has been argued
[76]
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 in the context of a
post-glaciation population boom may have been sufficient to overcome the
reproductive cost of forming a complex animal, for the first time in
Earth's history.
There is also a rival 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
2.4 to 2.1 billion years) and Snowball II (of the Precambrian's
Cryogenian between 580 – 850 million years and which itself had a number
of distinct episodes) are respectively thought to be caused by the
evolution of
oxygenic photosynthesis and then the rise of more advanced multicellular animal life and life's colonization of the land.
[77][78]
Effects on ocean circulation
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.
[79]
Occurrence and timing of snowball Earths
Neoproterozoic
There are 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.
[80]
Even the leading snowball proponent Hoffman agrees that the
~million-year-long Gaskiers glaciation did not lead to global
glaciation,
[45] although it was probably as intense as the
late Ordovician glaciation.
The status of the Kaigas "glaciation" or "cooling event" is currently
unclear; some workers 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.
[81] It was certainly less significant than the
Sturtian or
Marinoan
glaciations, and probably not global in extent. Emerging evidence
suggests that the Earth underwent a number of glaciations during the
Neoproterozoic, which would stand strongly at odds with the snowball
hypothesis.
[3]
Paleoproterozoic
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.
[82][83]
The glacial sediments of the Makganyene formation of South Africa are
slightly younger than the Huronian glacial deposits (~2.25 billion years
old) and were deposited at tropical latitudes.
[84] It has been proposed that rise of free oxygen that occurred during the
Great Oxygenation Event removed methane in the atmosphere through oxidation. As the
Sun
was notably weaker at the time, the 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.
[83]
Karoo Ice Age
Before the theory of continental drift, glacial deposits in
Carboniferous strata in tropical continents 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.
See also
References
Smith, A.G. (2009). "Neoproterozoic timescales and stratigraphy". Geological Society, London, Special Publications (Geological Society, London, Special Publications) 326: 27–54. Bibcode:2009GSLSP.326...27S. doi:10.1144/SP326.2.
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Further reading
- Tziperman, E.; Halevy, I.; Johnston,
D. T.; Knoll, A. H.; Schrag, D. P. (2011). "Biologically induced
initiation of Neoproterozoic snowball-Earth events". Proceedings of the National Academy of Sciences 108 (37): 15091. doi:10.1073/pnas.1016361108.
- Etienne, J.L., Allen, P.A., Rieu, R.
and Le Guerroué, E. (2007). "Neoproterozoic glaciated basins: A critical
review of the Snowball Earth hypothesis by comparison with Phanerozoic
glaciations". In: Glacial Sedimentary Processes and Products. Edited
by: Michael Hambrey, Poul Christoffersen, Neil Glasser and Bryn Hubbard.
IAS Special Publication. v. 39 (Malden, MA: IAS/Blackwell Pub.) 39: 343–399. doi:10.1002/9781444304435.ch19. ISBN 978-1-4051-8300-0.
- Gabrielle Walker (2003). Snowball Earth. Bloomsbury Publishing. ISBN 0-7475-6433-7.
- Micheels, A., Montenari, M. (2008).
"A snowball Earth versus a slushball Earth: Results from Neoproterozoic
climate modeling sensitivity experiments". Geosphere 4 (2): 401–10. doi:10.1130/GES00098.1. (Geol. Soc. America).
- Roberts, J.D. (1971). "Late Precambrian glaciation: an anti-greenhouse effect?". Nature 234 (5326): 216–7. Bibcode:1971Natur.234..216R. doi:10.1038/234216a0.
- Roberts, J.D. (1976). "Late Precambrian dolomites, Vendian glaciation, and the synchroneity of Vendian glaciation". J. Geology 84: 47–63. Bibcode:1976JG.....84...47R. doi:10.1086/628173.
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- Torsvik, T.H., Rehnström, E.F.
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External links
- The Snowball Earth 1999 overview by Paul F. Hoffman and Daniel P. Schrag, 8 August 1999
- Snowball Earth web site Exhaustive on-line resource for snowball Earth by pro-snowball scientists Hoffman and Schrag.
- New Evidence Puts 'Snowball Earth' Theory Out In The Cold
sciencedaily.com. 2007. Analyses in Oman produce evidence of hot-cold
cycles in the Cryogenian period, roughly 850-544 million years ago. The
UK-Swiss team claims that this evidence undermines hypotheses of an ice
age so severe that Earth's oceans completely froze over.
- Channel 4 (UK) documentary, Catastrophe: Snowball Earth episode 2 of 5, first screened Dec 2008, documentary narrated by Tony Robinson, advocates snowball Earth and contains interviews with proponents.
- First breath: Earth's billion-year struggle for oxygen New Scientist, #2746, 5 February 2010 by Nick Lane. Posits an earlier much longer snowball period, c2.4 - c2.0 Gya, triggered by the Great Oxygenation Event.
- 'Snowball Earth' theory melted BBC News online (2002-03-06) report on findings by geoscientists at the University of St Andrews,
Scotland that casts doubt on the snowball Earth hypothesis due to
evidence of sedimentary material, which could only have been derived
from floating ice on open oceanic waters.
- Life may have survived 'snowball Earth' in ocean pockets BBC News online (2010-12-14) report on research presented in the journal Geology by Dr Dan Le Heron (et al.) of Royal Holloway, University of London
who studied rock formations in Flinders Ranges in South Australia,
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Kirschvink, Joseph (1992). "Late Proterozoic low-latitude global glaciation: the Snowball Earth". In J. W. Schopf; C. Klein. The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press.
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