   #copyright

Snowball Earth

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

                                                           Snowball Period
                                                   (millions of years ago)

                                                        [USEMAP:44002.png]

   The Snowball Earth hypothesis is a controversial theory (Sankaran,
   2003) that attempts to explain a number of phenomena noted in the
   geological record by proposing that an ice age that took place in the
   Neoproterozoic was so severe that the Earth's oceans froze over
   completely, with only heat from the Earth's planetary core causing some
   liquid water to persist under ice more than two kilometers thick.

   This theory is contested by various scientists. Climate modelling by
   Dick Peltier at the University of Toronto explains that a low latitude
   glaciation on the snowball earth scale is not plausible in terms of
   energy balance and general circulation models. As well, recent work has
   shown that the oceans were unfrozen during the supposed Snowball Earth
   time period (Arnaud, 2004; Arnaud and Eyles, 2004) and that the theory
   completely ignores a host of geological evidence regarding the true
   origin of the supposed Snowball Earth glacial deposits, which in
   further study have been interpreted as debris flow deposits (Eyles and
   Januszczak, 2004).

Origins

   The general hypothesis has been around for several decades. Joseph Lynn
   Kirschvink, professor of geology at the California Institute of
   Technology coined the term "Snowball Earth" in 1992 although J.D.
   Roberts, some 20 years earlier (1971, 1976), had referred to this same
   interval as an "Anti-Greenhouse Earth". The hypothesis has since been
   reformulated and championed by Paul F. Hoffman, the Sturgis Hooper
   professor of geology at Harvard University, and his colleague Daniel P.
   Schrag. The hypothesis is not without criticism however and a number of
   challenges have been forwarded regarding the synchroneity of the
   supposed "Snowball" events.

Overview

   Since the 1960s, it has been hypothesized that the Earth's continents
   were subjected to severe glacial action between about 750 million and
   580 million years ago, so much so that the period is named the
   Cryogenian Period. Paleontologist W. Brian Harland pointed out that
   glacial till deposits of this period can be found on all continents,
   and first proposed that the Earth must have been in an ice age at this
   time; his views were widely publicized by an article in Scientific
   American in 1964. The problem was that the evidence-bearing deposits
   are found on all continents; but even during the worst of the ice age
   just past, no evidence of ice has been found in equatorial continents
   except on the higher parts of the highest mountain ranges. The then-new
   theory of plate tectonics made the oddly placed glacial discontinuities
   and deposits of glacial till even more enigmatic: studies of the
   magnetic orientations of the rocks of the late Proterozoic period
   showed that the continents were clustered around the (magnetic) equator
   during at least the start of the corresponding time around 750 Ma— in
   one of the earliest of the configurations known as supercontinents.
   This equatorial clustering and collision of continents about 750 Ma ago
   has been named Rodinia; it being near the equator, rather than near the
   poles as might have been expected, taken together with thermal evidence
   of a severe ice age 750 to 635 Ma ago (the dating suggested by the
   widespread geologic deposits) is what has led to the Snowball Earth
   hypothesis.

   The Snowball Earth hypothesis argues from the documented locations of
   glacial till dropped by glaciers to suggest that the Earth must have
   completely frozen over. The mechanism by which it did so is still
   mysterious.

   One suggestion is that, normally, as the ice spread, it covered some of
   the land and thus slowed the carbon dioxide absorption, increasing the
   greenhouse effect, and the ice spread would eventually stop; but this
   time the continents were clustered along the equator and thus this
   control mechanism would not work until the freezing process had run
   away and the whole Earth iced over.

   Once frozen, the condition would tend to stabilize: a frozen Earth has
   a high albedo (reflecting more of the Sun's radiation), and a frozen
   Earth, with reduced evaporation, has a very dry atmosphere (water vapor
   being one of the greenhouse gases). A "Snowball Earth" would have a
   bright clear blue sky above its reflective surface.

   The mechanism by which the Earth would thaw after such a frozen period
   would leave distinctive traces, which are the subject of ongoing
   research.

   White Earth is a name given to a theoretical equilibrium found in
   computer climate simulations whereby the model Earth undergoes complete
   glaciation. While this seems to have originally been considered a
   degenerate case, by the time James Gleick wrote his history of chaos
   theory Chaos: Making A New Science, it was not dismissed in his book
   but simply restated as something that probably just had not happened
   yet. The current evidence for the Snowball Earth would seem to back
   that theory and its computer models.

Evidence

   Geological formations which "Snowball Earth" proponents point to as
   evidence of the hypothesis are iron-rich rocks like taconite deposits
   and carbonate cap rocks. The association of the Snowball Earth event
   with the Cambrian explosion (the sudden appearance of multicellular
   lifeforms between 570 and 530 million years ago) is also of great
   interest.

Lack of photosynthesizers

   There are two stable isotopes of carbon in sea water: carbon-12 (C-12)
   and carbon-13 (C-13).

   Because biochemical processes tend to preferentially incorporate the
   lighter C-12 isotope, there is a tendency for ocean-dwelling
   photosynthesizers, both protists and algae, to be very slightly
   depleted in the rare heavier C-13, 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 C-12/C-13 ratio within
   organic remains, and a lower ratio in corresponding ocean water.

   During the proposed period of Snowball Earth, there are variations in
   the concentration of C-13 that are rapid and extreme compared to normal
   modern variations. This is consistent with a deep freeze that killed
   off most or nearly all photosynthetic life in the water. The main
   problem with this idea is that the variations in carbon isotope ratio
   are inferred to be synchronous, but geochronologic confirmation of this
   synchroneity is lacking.

Banded iron formation (BIF)

   In the Earth's oxygen rich (now nearly 21% by volume) atmosphere, iron
   naturally rusts, forming banded sediments known as banded iron
   formations.

   Since non-oxidized iron-rich rock deposits can only form in the absence
   of that ubiquitous atmospheric oxygen, and since these subject deposits
   are seen at the supposed time of the worst glaciations, presence of
   non-oxidized iron deposits laid down in the Cryogenian period lends
   strength to the Snowball Earth theory.

   The total amount of oxygen locked up in the banded iron beds is
   estimated to be perhaps 20 times the volume of oxygen present in the
   modern atmosphere, and virtually all of it results from iron dissolved
   in water then subjected to oxygen, which precipitates out of the
   solution. Banded iron beds significantly are considered to be
   Precambrian sedimentary rocks and are rare in Phanerozoic strata.

   Proponents of the theory point out that oxygen in the Earth's
   atmosphere is not naturally stable, and must receive continuous
   maintenance (replenishment) from the biosphere as it is constantly
   leached out of the atmosphere in a wide variety of chemical reactions,
   particularly those involving iron and silicon.

   Carbon dioxide is an important greenhouse gas, and the biggest remover
   of it from the atmosphere is atmospheric weathering wherein silicate
   rocks are broken down into sand and dust which are blown or washed
   away, exposing new rock surface to further attack by water or the
   atmosphere; much of their component calcium and magnesium dissolves out
   and combines with CO[2] to form dissolved bicarbonates. The speed of
   this process can be seen by observing the dates on older eroded
   headstones in a cemetery. In a Snowball Earth, essentially all rock
   would eventually become locked up and covered by ice and snow leading
   to a long-term gradual steady carbon dioxide buildup.

Survival of life through the frozen periods

   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, which did not happen. Proponents counter that it may
   have been possible for life to survive in these ways:-
     * 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.
     * Deep ocean regions far from the supercontinent Rodinia or its
       remnants as it broke apart and drifted on the tectonic plates may
       have allowed for some small regions of open water preserving small
       quantities of life with access to light and CO[2] for plants to use
       during photosynthesis generating traces of oxygen enough to sustain
       some oxygen-dependent organisms. This would still happen even if
       the sea froze over completely provided that the small portions 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 and made small temporary melt pools
       which would freeze over at sunset.
     * As eggs and dormant cells and spores deep-frozen into ice right
       through 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.
     * 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.

Carbonate cap rocks; how the Earth thawed

   The carbon dioxide levels necessary to unfreeze the Earth have been
   estimated as being 350 times what they are today, but would be able to
   accumulate due to the opposite of the effect mentioned earlier as a
   possible mechanism triggering the freeze in the first place; if the
   Earth was completely covered with ice, silicate rocks would not be
   exposed during erosion, and carbon dioxide would not then be removed
   from the atmosphere. Eventually enough CO[2] emitted by volcanoes would
   accumulate, perhaps after an era of increased volcanic activity (a
   prodigious producer of this greenhouse gas), that the oceans around the
   equator would finally melt, which would produce a band of open ice-free
   water, much darker than the highly reflective ice, which would absorb
   more energy from the sun. This would in turn heat the Earth more,
   melting more water to absorb more light, and so on. Concurrently, the
   abundance of CO[2] would provide plenty of food to feed a
   cyanobacterial population explosion, resulting in a relatively rapid
   reoxygenation of the atmosphere to feed the following Cambrian
   Explosion with the new multicellular lifeforms. This positive feedback
   loop would melt the ice in geological short order, perhaps less than
   1000 years; replenishment of atmospheric oxygen and depletion of the
   CO[2] levels would take more thousands of years.

   However, the carbon dioxide levels would still be two orders of
   magnitude higher than usual. Rain would wash CO[2] out of the
   atmosphere as a weak solution of carbonic acid, which would turn
   exposed silicate rock to carbonate rock, which would then erode easily,
   wash into the ocean, and form deep layers of carbonate sedimentary
   rock. Thick layers of exactly this abiotic carbonate sediment can be
   found on top of the glacial till that first suggested the Snowball
   Earth.

   Eventually the carbon dioxide level would get so low that the Earth
   would freeze over again. This cycle went on until Rodinia had dispersed
   so much that the Earth's land was no longer strung out along the
   equator and the primary cause of Snowball Earth would no longer
   operate.

Evolution of life

   The Neoproterozoic was a time of remarkable diversification of
   multicellar biota, especially animals. Animal size and complexity
   increased considerably with time, sufficiently so that soft-bodied
   fossils allowed the Ediacaran Period to be distinguished by the IUGS
   (International Union of Geological Sciences). This development of
   multicellular animals 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. Some proponents of the Snowball Earth theory also point out
   that the last important glacial episode may have ended only a few
   million years before the beginning of the Cambrian Explosion.

Other Snowball Earths

   Another Snowball Earth has also been proposed for the first known ice
   age, 2.3 billion years ago. There the proposed mechanism is the first
   appearance of atmospheric oxygen, which would have absorbed any methane
   in the air. As methane is a powerful greenhouse gas, and as the Sun was
   notably weaker at the time, temperatures plunged. The evidence here is
   weaker, but a layer of iron-rich rock can also be found from this time.

   One competing theory 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". An even less severe 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 (there is
   some evidence to believe that this is the case). In either of these two
   situations, the freeze over would be limited to relatively small areas,
   as is the case today, and severe changes to the Earth's climate are not
   necessary.

   Alternatively, the glacial evidence for Snowball Earth may be
   reinterpreted by the concept of inertial interchange true polar wander
   (IITPW). This was proposed by Kirschvink and others of Caltech in July
   1997 and holds that the contintental land masses may have moved far
   more quickly than has previously been supposed under the influence of
   physical laws affecting the distribution of mass for the planet as a
   whole. If the continents move too far from the equator, the entire
   lithosphere may slide to bring them back to the equator at speeds
   hundreds of times that of ordinary tectonic plate movements. This has
   the effect of making it look like the magnetic north pole has wandered
   when in fact the continents have re-aligned in respect to it. This idea
   has been challenged by Torsvik et al. (1998), Meert (1999) and Torsvik
   and Rehnstrom (2001) showed that the amount of polar wander proposed by
   Kirschvink et al. (1997) was insufficient to support the hypothesis.
   Thus, while the physical mechanism of IITPW is geophysically sound, the
   idea that an event occurred in the Cambrian is without merit.

   If such rapid movement did take place it would account for the
   existence of such features of glaciation in close temporal proximity to
   the presence of the continents near the equator. Inertial Interchange
   true polar wander has also been linked to the Cambrian Explosion of
   evolutionary forms as animals were forced to evolve to adapt to rapidly
   changing habitats and environments. The problem with linking IITPW to
   the Cambrian explosion is that newer data no longer support such rapid
   motion during the Cambrian.
   Retrieved from " http://en.wikipedia.org/wiki/Snowball_Earth"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
