A in tropical paleo-latitudes supports this extreme theory

A
white planet with a surface almost entirely frozen solid traveling through
space may sound like a world far from our own. At least three times in Earth’s
history, according to modern evidence, this was the case of our own planet,
with the most recent global glaciation event occurring around 700 million years
ago (mya). Evidence found in tropical paleo-latitudes supports this extreme theory
and in 1992 Joseph Kirschvink published an article which was included in a book
entitled “The Proterozoic Biosphere: A multidisciplinary study”. His article
coined the term “Snowball Earth” and described the severe climatic conditions
observed at this time. The theory has been around for over 50 years with
geologists such as Douglas Mawson finding glacial sediments in Australia that
first aroused suspicion. Since then the theory has been discussed and debated
with new lines of evidence appearing constantly. At first the idea was deemed
too drastic for reality but today there is enough evidence for an almost
completely global surface freeze to be a legitimate theory which has brought
geologists, climatologists, geochemists and many more together to investigate
the theory.

The
thesis of this essay is that the earth reached a near complete surface freeze
during severe glaciation events during the Paleoproterozoic and the Neoproterozoic
eras with glaciers extending close to the equatorial line. It is an important
claim to make because it describes an unprecedented change in climate that
would drastically change life on earth as we know it, if it were to happen
again. Understanding the evidence which I will use to support my thesis
requires some background information regarding geological proxies and paleo
indicators. Carbon-13 (C13) is an isotope of carbon where the carbon atom has
an extra neutron which geologists can use as a proxy for photosynthetic
activity (Hoffman et al 1998). The more organic, photosynthesising life present
at any given time corresponds to an increase in the ratio of C13 compared to
C12. These ratios can be measured off a standard to give a result that is
either higher (?13C = +) showing us that
biological productivity was higher at this time than the productivity
associated with the standard, or lower (?13C = -) showing us
that productivity was lower than that of the standard. Typically ice ages are
associated with net decreases in biological activity so using carbon 13/12
ratios can allow us to make predictions regarding the event and severity of an
ice age. Glacial tillites are sedimentary deposits formed when masses of ice
containing assorted sediment melt and release the sediment to form a distinct
deposit known as glacial tillite or dropstones. Another sedimentary rock
formation useful for paleo environment indication are carbonate rocks such as
limestone or dolostone. These are formed in typically warm, shallow marine
environments (Armstrong et al 2008) similar
to that of the Bahamas today and so, indicate a drastic change in
climatic conditions, from widespread cooling to sudden warming. Banded iron
formations (Bifs) are composed from alternating layers of oxidised iron or
“rust” and chert. They form from the precipitation of iron from the ocean, an
occurrence only observed in anoxic marine conditions. The lack of free oxygen
available in anoxic conditions prevents iron ions present in the ocean from
bonding with oxygen and so the ions are present in solution. Once free oxygen is
introduced into the environment iron ions bond with the oxygen and are oxidised
to form an iron oxide (rust) which comes out of solution to form as a
precipitate.   They were common in the
paleoprotozoic era (Holland 2006) during “the great oxygenation event” where
photosynthesising cyanobacteria produced free oxygen into a previously anoxic
environment producing Bifs. I will be focusing on three discrete glacial periods:
the Paleoproterozoic “Makganyene” glaciation and the Neoproterozoic “Sturtian”
and “Marinoan” glaciations.

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

The first evidence
to spark this theory was that of a physical nature and so it is where I shall
start my defence of the Snowball earth hypothesis. Glacial tillites have been
found in areas of tropical latitudes such as the Otavi group of northern
Namibia (Hoffman et al. 1998), the Elatina formation of Australia and the
Nantuo formation in southern China (Macdonald et al 2010). The origins of these
deposits have been radiometrically dated to investigate possible linkage
between the deposits. The Otavi group’s Ghaub formation has been dated to 635.5±0.6 Ma (Hoffmann et al. 2004) and
the Nantuo formation has been dated to 636.3±4.9 Ma (Zhang et al. 2008) proving
the lithostratigraphic connection between these deposits which correlate to the
terminus of the Marinoan glaciation (Macdonald et al. 2010). Sceptics doubted
the importance of these dates by claiming that continental drift would have
placed these regions of now low latitudes to higher paleo latitudes. However,
paleo magnetic reconstructions have reinforced the snowball earth hypothesis
suggesting that on at least two occasions in the neo-proterozoic (sturtian and
marinoan glaciations) the ice line stretched to close proximity with the
equatorial line (Hoffman et al 1998). A study into the Franklin Igneous province
(FIP) of Canada which contains ancient tillite depostions dated the glacial
till to 716.47±0.24 mya. At this time the FIP was located in the north west of
an ancient continent called Laurentia (Macdonald et al 2010).  Paleo magnetic reconstructions places the
north west of Laurentia to within 10 degrees of the equatorial line (Park 1994)
providing proof that glacial deposits occurred in tropical paleo latitudes.

 

A geological
proxy that has proven to be extremely useful in the study and correlation of
snowball earth has been the carbon 13/12 ratios found in glacial and post glacial
formations. Successions of Neoproterozoic rock studied by Kaufman et al. in North-western
Canada, Namibia and Spitsbergen have shown variations in carbon ratios with the
most notable variation viewed across the different rock successions being two
distinct drops in ?13C which provides evidence that during two discrete periods of
time during the Neoproterozoic biological productivity dropped in various parts
of the world. Returning to the Otavi group of Namibia, the two negative ?13C
excursions are overlain by negative ?13C carbonates and
underlain by positive carbonates (Kaufman et
al 1997). This points towards rapid climate change from glacial conditions to
carbonate forming conditions with little lag time, geologically speaking. This
fits the snowball earth hypothesis in terms of the earth returning from the
widespread glaciation. With snowball earth conditions present two main
mechanisms of carbon recycling would have been inhibited: silicate weathering and
photosynthetic activity (Kirschvink 1992) which we know decreased due to – ?13C
ratios. This would lead to a build-up of atmospheric carbon. It is also
believed that widespread volcanic activity would have added gasses associated
with a greenhouse effect (Hoffman et al 1998) to the atmosphere coupling with
the high carbon levels resulting in a level of carbon dioxide roughly 350 times
that of modern day levels. The effect of carbon dioxide concentrations this
high would include large scale warming of the earth with ice melting and
carbonates being precipitated again thus explaining the cap carbonates found today
overlying glacial deposits (Hoffman et al 1998).

 

?13C ratios have been
used to theorise the genesis of a snowball earth which, considering the global
nature and the severity of this event has been a contentious issue regarding
the snowball earth hypothesis. For this we can look at the evolution of the
first eukaryotes particularly the photosynthesising cyanobacteria which evolved
no later than 2.78 gya (Kopp et al. 2005) and may have led to a catastrophic
climate even known as “the great oxygenation”. It is thought that cyanobacteria
thrived for millions of years and produced oxygen continually into the
atmosphere upsetting the predominantly methane based climate. This then led to
a reverse greenhouse effect inducing global cooling events known as the
makaganyene and huronian glaciations, dated to c. 2.3-2.2 gya (Kopp et al
2005). Evidence for this glaciation again includes glacial deposits correlated
to the time and a paleo latitude close to the equator (Tajika, 2003). Cyanobacterial
colonies produce formations known as stromatolite domes which are preserved in
the fossil record dating back to this time (Des Marais, 2000) and
were thought to be a line of evidence supporting the theory that free oxygen in
the atmosphere would have been extremely high. Unfortunately other organisms
have been found to be capable of creating stromatolite dome structures and so
this line of evidence is encouraging but not damning. Banded Iron formations have
been correlated with the time frame of this oxygenation event (Holland 2006). These
form when an anoxic environment is exposed to free oxygen, providing evidence
of an oxygenation event. An example is the Ongeluk formations (Kopp et al 2005)
which contains an abundance of BIF formations dated to this period of earth’s
history. While almost all BIF formations have been dated to paleo Proterozoic
times there is a reoccurrence of these formations around 700 mya (Holland 2006)
which is expected to be in connection to the Sturtian and Marinoan glaciations.
When considering the genesis of Neoproterozoic ice ages it is important to note
that they correspond with the breakup of the supercontinent Rodinia (Li et al
2008) which is believed to have produced an increase in photosynthetic activity
as reflected in positive ?13C values
of sediments deposited just before the glacial till (Kauffman et al 1996). However,
once a snowball earth is established and photosynthetic activity drops due to
ice coverage, it is thought that marine environments could have become anoxic once
more (Holland 2006). Then as the ice age dissipates and oxygen re-enters the
ocean banded iron formations would be deposited once again, the widespread
nature of these deposits as found in Canada, Namibia, south America and
Australia (Holland 2006) indicate the global nature of a the glaciation.

 

While I have
discussed geological evidence regarding the genesis of a global freeze
climatologists have consistently been sceptical regarding the parameters of
snowball earth from a sustainable climate perspective. Climate models have been
created to simulate snowball conditions but doing so has proven difficult because
parameters such as the albedo effect believed to have been imperative to
snowball earth, (which describes the reflective nature of ice coverage creating
a positive feedback loop where growing ice reflects solar radiation causing
temperature to drop and thus creating more ice), and cloud cover come with many
uncertainties in terms of recreation and the level of importance associated
with these conditions  (Voigt et al 2011).
One consensus that modellers such as Hyde et al (2000) and Peltier et al (2004)
reached was that the climatic forcings necessary for a snowball earth to occur
would not be achievable by earth’s climate and so in reality a situation
involving glaciers co existing with large bodies of tropical water would occur,
given the name “slushball earth”. However the model ECHAM5/MPI-OM created
by Voigt et al which is one of the most recent and in depth models created with
the focus being on the marinoan glaciation concluded that forcings required to
initiate a snowball earth are lower than previously thought (Voigt et al 2011).
Results include a temperature drop of 4.6K when compared to modern day
conditions and a total solar irradiance (TSI) bifurcation point higher than the
TSI expected during the marinoan period meaning a snowball earth is plausible.
However the model could not find a steady state existing with global sea ice
coverage above 55% and could not account for land glaciers forming. While this
sounds like evidence against the snowball earth hypothesis it is important to
note that the sources of error associated with modelling the earth’s climate
are so high that these results do not disagree with the snowball earth
hypothesis and the model did in fact show that snowball earth initiation is
easier than previous models had before it. Climate models improve with
technology and gathered evidence so while the conclusion is unsatisfying, newer
climate models with reduced sources of error are perhaps required to fully
understand the climatic conditions associated with a snowball earth. Constructing
an accurate climate model for this scenario would be of great benefit to
climate studies today because it would involve understanding how the planet
reacts to an extreme change in climate and how it could possibly return to
equilibrium. This has relevance when we consider that anthropogenic activity
has the possibility of pushing the earth’s climate to an extreme warming event
according to the more pragmatic predictions of the coupled model
intercomparison project phase 5, included in the reference list. Having an in
depth knowledge of extreme cold conditions could also prove worthy for
expanding to other environments such as Mars and understanding how to go from a
desolate cold environment to a green and blue planet.

 

In conclusion there
is physical and isotropic evidence preserved in the earth that supports the
claim, grandiose as it is, that at least three times in our planets history a
snowball earth occurred with glaciers extending down to low paleo
latitudes.  It is important to remember
that this hypothesis is quite new to the field of science and has some way to
go, especially with the precise dating of discrete glacial events, before it is
universally accepted as a truism. The young age of this theory calls for a
critical mind-set to be adopted by all investigating the theory so as to
thoroughly examine evidence until undisputed fact remains. It is an area of
exciting research and it should follow that the evidence I have included in
this argument is not final or representative of all the abundant literature
published on this topic, it is however hopefully a comprehensive introduction
to the hypothesis. It is also worth addressing that climate models for the
hypothesis, particularly their general lack of agreement with snowball earth,
do not act to disprove the argument on the merit that a mechanism for sustaining
global glaciers is not present. In his seminal article regarding snowball earth,
Joe Kirschvink references continental drift as an example of something that for
many years was accepted as a theory before a complete mechanism for the process
was understood, showing that lack of mechanism alone is not always enough to
disprove an argument which is critical for increasing knowledge in the field of
science.