Since has been divided over the extent of

Since the first discovery of
glacial deposits in low latitude Neoproterozoic rocks, the scientific community
has been divided over the extent of this glaciation. During the two main
glaciation periods, the Sturtian and Marinoan, was the earth completely covered
in ice or did an equatorial oasis exist? 
Evidence for these glaciations is very scarce the UK, with only the
Dalradian Supergroup providing any sort of indication of the mass trauma
affecting the planet, why is this?  These
issues and evidence for both sides of the story will be presented in this literature
report; analysing evidence and looking at data collected from all over the
globe, discovering what features such as palaeomagnetism, dropstones,
striations and other structures can tell us about these possible icehouse
conditions.

 

Introduction –

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Snowball Earth and the Cryogenian
period are marked by a period of massive glaciations originating from 720 to
635 Ma ago.  These events marked the
first large scale low-latitude glaciation periods, with two worldwide events;
the Sturtian (720 Ma) and the Marinoan (660 Ma), being generally considered the
centre pieces of this glaciation period (Allen and Etienne, 2008).  These events occurred as the supercontinent
Rodinia started to break apart, the Mirovia ocean started to close and the
Panthalassa began to form.  Much of the
Neoproterozoic glacial deposits settled as glacially influenced marine strata,
these deposited along rifted continental margins (Deynoux, 2003). Tillites have
been found all over the world in areas thought to be at low latitudes during
the Neoproterozoic; particularly on opposing sides of the Atlantic; these are
glacial deposits that contain very large, coarse sediment, often seen as large
blocks of glacial till and consolidated masses of unweathered rock bodies in a
coarse matrix, very much like a breccia (Encyclopaedia Britannica).

 

Figure 1. Diagram showing the “snowball” theory, worldwide
glaciation occurs as the temperature drops enough for the ice-albedo
feedback to overcome the melting/sublimation; the tropics are completely
covered in ice.

The first Neoproterozoic tillite was
identified as “Reusch’s Moraine”, a unit that was covered and buried by
overlying sediment and discovered by Hans Reusch in 1891, this site was among
one of the first to bear evidence for the Cryogenian Glaciations (Edwards, 2004).  The idea of snowball earth re-emerged in 1964
when Brian Harland presented a paper that suggested that glacial tillites found
in Svalbard and Greenland were deposited at tropical latitudes, having analysed
the palaeomagnetic data for the area (Harland, 1964).  These were further ratified by the Russian
climatologist Mikhail Budyko, who developed a simple energy -balance climate
model; this investigated the effect of ice cover on a global climate.  In summary it suggested that as the ice cap
advances out of polar regions the albedo level increases to a point where it
cannot melt, and further cooling ensues (Budyko, 1969); although this suggests
a snowball earth is possible it does not show how to escape the icy
environment.  This model is shown within figure 1 on a simple scale. The
temperature is cold enough that the water freezing rate overtakes that of
melting and causes worldwide glaciation, increasing albedo and lowering
temperatures further.  The salinity
massively increased from modern levels as water is stored within the ice.

 

Figure 2. Diagram showing the “slushball” theory, there is no
worldwide glaciations as melting and sublimation of the ice occurs, an
equatorial oasis exists (Pollard and Keating, 2005)

When the event is modelled through data
collected recently, an ice-albedo feedback would cause the ice to rapidly
advance towards the equator once the glaciers spread to within 25/30 degrees of
the equator; with the presence of glacial deposits around this area it suggests
global ice cover (Meert, Van der Voo and Payne, 1994). Figure 2 shows the effect of ice on a “slushball” earth, the ice
advances to a low latitude but cannot advance any further as the rate of
melting/sublimation overtakes that of freezing. 
The high ice content increases the salinity levels of the water, but not
to the extent of the “snowball” earth hypothesis.

 

The first use of the term
“snowball earth” was introduced by Joseph Kirschvink in 1992.  The major points in the paper primarily being
that the presence of banded iron formations in the Neoproterozoic sequences are
consistent with a global glacial episode and secondly showing a mechanism that
allows an escape method from the cold temperatures; this being the accumulation
of CO2 from volcanic eruptions leading to incredibly high levels of greenhouse
gases (Kirschvink,
1992).  Later evidence supported these
theories with a succession of Neoproterozoic rocks in Namibia being discovered,
incorporating evidence from cap carbonates (Hoffman, 1998). Cap carbonates are layers
of textured carbonate rocks that form at the uppermost layer of sedimentary
sequences; these reflecting major glaciations in the geological record (Le
Heron, 2015).  In the following sections
the different methods in which the evidence has been found will be
evaluated.  Much of this information has
been obtained from looking at features such as palaeomagnetism, glaciogenic
structures like striations and dropstones and diamictites.  Recent studies have found even more evidence
that promotes either a global icehouse period or a period with an equatorial
oasis free of ice; banded iron formations have been found in Neoproterozoic
sediments, these are some of the only BIFs found earlier than the Great
Oxidation Event.  These will be discussed
in the findings section along with why there is little evidence in UK
stratigraphy.

 

Methods –

Some of the key findings that
support the snowball earth theory must be critically evaluated; the evidence
provided to support the theory must meet two different hypotheses (Eyles and
Januszczak, 2004).

 

1.    A bed must contain sedimentary features that could only have been
deposited by glacial activity

2.    By looking at the most accurate paleomagnetic data available, the bed
must have lain between the tropical latitudes.

 

Figure.3
Map and table showing the global distribution of Neoproterozoic glacial
deposits with estimated palaeolatitudes. 
Includes the reliability of the data through various statistical
tests.  Considers not only the
reliability but the confidence that the deposits are significant, low
latitude ice sheets. (Hoffman and Schrag, 2002)

Evidence should also show that glaciers were
active around the whole globe at this time which can be particularly difficult
to prove.  After the Ediacaran,
biostratigraphic markers could be used to date rocks, but before it is much
more challenging.  Without the
involvement of a similar fossil species each rock type must be taken back to a
lab and dated; which can often be inaccurate up to around a million years even
without the involvement of metamorphism (Eyles and Januszczak, 2004).  The Elatina deposit in Australia is currently
the only rock that is undisputedly of glacial origin and deposited at glacial
latitudes (Le Heron, 2015).  Prior tests
showed the sediment to have originated as a glaciogenic sediment with a
paleolatitude below 10 degrees.  This
result was scrutinised, and a fold test was conducted on the sediment. The
result came back positive (Williams, 1996); this was a surprising result and
proved that the Natural Remnant Magnetisation (NRM) is primary and has not been
reactivated.  Multiple tests were later
recompleted to further confirm the formation of the rock.  This new information led to the re-emergence
of the topic and in 2000 a paper with many worldwide paleolatitude data points
published and the reliability of this data tested (Evans, 2000), much of these
data can be seen in figure 3.  The
addition of this data and the result of the fold tests confirms the legitimacy of the
Elatina deposit as viable Snowball Earth evidence.

 

Palaeomagnetism of these rocks
could be massively inaccurate due to a possible swing in the earth’s magnetic
field.  750 Ma ago the core of the planet
would have been much hotter.  Studies
have suggested this warmer core could lead to a non-dipolar distribution, the
thermal currents circling around the core would do so at a higher speed,
possibly giving rise to up to 8 poles. 
The palaeomagnetic data would then be completely incorrect and much of
it would have to be recalculated (Abrajevitch and Van der Voo, 2010).  This has often been suggested as the poles
implied by the data during the Neoproterozoic often involve much faster plate
movements than seen in accurate data; this could just be the presence of many
more poles than in modern times (Abrajevitch and Van der Voo, 2010).

 

Early studies of the Cryogenian
Glaciation and Snowball Earth relied on the continental drift theory; this was
under much scrutiny during the late 1800s so much of the research never saw the
light of day.  Sir Douglas Mawson
identified extensive and thick glacial sediments and towards the end of his
career veered towards the possibility of global glaciation (Alderman and
Tilley, 1960).  Although this was under
the false pretension that the continents have remained constant throughout
time.

 

In finding this evidence there are
some problems the scientists must overcome. 
There is no suitable event currently to mark the start of the Cryogenian
period, commonly these events are marked by the appearance of a new fossil, this
making it easy to recognise the age of the rock in the field (Plumb, n.d).  As there are no fossils to see in the
Cryogenian Period the process of dating rocks in the field becomes almost
impossible, therefore each rock must be taken back to a lab for testing.  This has caused the base of the Cryogenian
period to move around; it was set at 850Ma ago until 2015, when it was changed
to 720Ma ago on the basis that the start of the period is synchronous with the
start of the Glaciation periods (J. G. Ogg, 2016).

 

Key Findings –

Palaeomagnetism is the study of
the record of the Earth’s magnetic field within rocks and sediment. As a rock
forms magnetic minerals within the rock align towards the North Pole, looking
at the angle the minerals are aligned gives us into us an insight into the
latitude the lithology was formed at and multiple sets of paleomagnetic data
can give us the movement of the tectonic plate it sits on. Palaeomagnetism
provides a tremendous amount of evidence for the Cryogenian glaciation.  Some of these measurements have been taken
from Neoproterozoic rocks that suggest the rocks were deposited within 10
degrees of the equator (Evans, 2000).  The accuracy of
this data is widely disputed, some theories suggest the existence of localised
glacial regimes due to the shape of Rodinia and how it was breaking up to
explain the very low latitude palaeomagnetism data (Young, 1995).  Other studies have suggested that the data is
inaccurate and there is no palaeomagnetism data to show any glacial deposits to
within 25 degrees of the equator (Meert and van der Voo, 1994).

 

There are many different glacial
deposits features that can be seen in the rocks; dropstones, glacial
striations, diamictites and banded iron formations can all be seen throughout
the Neoproterozoic rock records. Dropstones are very common throughout low
latitude Neoproterozoic marine sediments; although these are evidence of
widespread glaciation these can disprove the snowball earth theory.  If the earth was covered in ice these
dropstones would not form; sea glaciers do not tend to melt after calving from
the glacier, they tend to accumulate more ice on the bottom of the
iceberg.  The presence of these
dropstones indicates that the icebergs were melting in warmer waters, this
giving more evidence for the “slushball earth” (McMechan, 2000).  Hoffman and Schrag suggested that the
presence of numerous dropstones indicates the glaciers depositing the stones
were either continental glaciers or that the glaciers were regularly melting
with seasonal ice in the area.  A study
undertaken by McMechan is one of the only studies that attempts to find the
origin of the rocks; looking at Neoproterozoic rocks found in British
Columbia.  The origin of these rocks was
found to be continental rather than sea ice; as the material was dropped some
way from the shore (at least 1000m depth) there could not have been any
constraining sea ice and must have been in water warm enough to melt the ice.
This would not have been possible on the current snowball earth model.

 

Glacial striations form when rocks
embedded within a glacier scrape along the bed rock causing very distinctive
marks or scratches along the rock.  One
of the most famous examples for the Neoproterozoic is the Oaibac?c?annjar’ga
striated pavement in Norway.  The main
features within the pavement include a 2.5mm thick zone of brecciation
underneath rare polished striations; this indicates a hard substrate being the
main component within the pavement and is the first indicator in suggesting a
glacial origin rather than some of the other possible striation forming
mechanisms such as mud flows and other soft sediment methods (Hoffman,
1998). 

 

Diamictites are a lithified
sedimentary rock that contain poorly sorted sub-aerial sediment; with clasts
ranging in size from clay to boulders, supported in a matrix made up of
mudstone or sandstone (Flint, Sanders and Rodgers, 1960).  This term is often used to describe glacial
tillites, being formed in meltwater deposits, moraine glacial till, ice-rafted
sediments.  Neoproterozoic Diamictites
have been found all over the world at low latitude locations, this is another
suggestion that during the Neoproterozoic there were some periods of mass ice cover,
but this does not prove snowball earth. 
Diamictites can also be caused by a multitude of different formation
events; they can be of volcanic, tectonic, marine, erosional and impact
material.  Although these can quite
easily be distinguished through looking at the clast material it can often
cause mistakes within the field and misidentification.

 

The UK contains three main
Neoproterozoic units, the Torridonian (~950Ma), the Moine (~840Ma) and the
Dalradian (~600Ma); the Dalradian is the only rock to show evidence for the
glaciations.  This could be due to several
different reasons, the UK has experienced many orogenies since this period; the
Knoydartian, Caledonian and Variscan to name the largest that influence the
area.  These would all destroy many
sedimentary deposits and structure within the rocks through metamorphism and
mountain building events; possibly burying other sediment which could contain
evidence for snowball earth. This is slightly contradicted in some lightly
affected areas of the Moine supergroup in which the unit has not undergone much
deformation, and still shows no evidence for any glaciation; although possibly
due to the much earlier deposition date of the Moine and Torridian supergroups (Vance,
Strachan and Jones, 1998).  The only unit
with evidence for the event is the Dalradian supergroup (Prave and Fallick,
2011).  This is a siliciclastic dominated unit and three distinct glacial events
can be seen within this supergroup.  The
oldest unit in this group is the Port Askaig Formation, this bed is seen
throughout abundant localities throughout Scotland and Northern Ireland.  The second glacial (Sturtian) tells us much
more about the period, this section is seen throughout the middle section of
the Argyll group, and contains pelites and schists that have dropstones located
within them; this seen to be ice rafted debris and suggests a largely glacial
influence within the area (Prave and Fallick, 2011).  These rocks are overlain by a 1-7m thick
layer of dolomitic limestone which can be interpreted as cap carbonate; this is
seen in outcrops in Donegal, Ireland.  The
last glaciation event (Marinoan) recorded in the Dalradian supergroups is seen
the lower Southern Highland group; this includes evidence such as dropstones
and diamictite beds (Prave and Fallick, 2011).

 

Banded Iron Formations (BIF) are
layer of sedimentary rocks that contain layers of iron oxide and iron poor
chert.  If this iron oxide is presented
to oxygen the iron rusts and thus becomes insoluble within water.  This is currently not possible today due to the
relatively high levels of oxygen in the atmosphere as the amount of iron oxide
needed to accumulate and form the rock is not possible.   Much of the BIF formations found around the
planet today are associated with the great oxidation event in the Palaeoproterozoic
around 2400 Ma ago (Sosa Torres, Saucedo-Vázquez et al, 2014).  The only other period we see this occurring
is during the Cryogenian glaciation; for the rocks to be so rich in iron to
form BIF there must have been oceanwide anoxia, so the ferrous oxide could
accumulate.  This suggest the ocean had
limited gas exchange with the atmosphere. 
Some studies suggest this is due to a large-scale advance of sea ice
blocking the ocean beneath (Kirschvink, 1992). 
There is speculation as to whether this hypothesis is correct in that
there is no evidence of BIFs during the Marinoan glaciation, and that in
stagnant inland sea the iron oxide can accumulate to such a level to form BIFs;
especially in a prehistoric atmosphere with less oxygen (Eyles and Januszczak,
2004).

 

Crowley, Hyde and Peltier, 2001)  suggested that the earth escaped from this
period through a period of intense volcanism; with carbon dioxide levels
estimated to be 350 times what they are today With other greenhouse gases such
as methane contributing; this would inevitably lead to a global hothouse period
and as a dark band of water started to appear around the equator the albedo
level changed and started to absorb radiation from the sun rather than reflect
and initiate a “positive feedback”; in which the rate of melting exponentially increases
as more ice is melted (Crowley, Hyde and Peltier, 2001).  This massive deglaciation is thought to have
helped with the rise in Ediacaran biota and the Cambrian Explosion; as the
glaciers retreated they leave large amounts of glacial deposits that erode and
weather.  These sediments would be high
in nutrients like phosphorus, and when combined with the abundance of CO2 in
the atmosphere would cause a population explosion within cyanobacteria; this in
turn causing a massive reoxygenation event and the development in life (Liang
et al., 2006). 

 

Conclusion –

The presence of a mass glaciation
is not disputed, but deciding on the extent of the glaciation is very
difficult.  Computer models have not been
able to accurately replicate the glaciation and the process of escaping it;
maybe with advances in technology in the future this will become easier to
process and see.  Evidence for the
“slushball” and “snowball” hypothesis is all over the planet and often
contradict each other.  With more
analysis they may become more obvious but currently they there is not enough to
justify proving either of these, what can be proven is that there were multiple
massive low latitude glaciation events, we just do not know the extent of these
icehouse periods.

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