Snowball Earth Draft

8/3/2019 Snowball Earth Draft

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Snowball Earth

Jonna Reamer

Fall Seminar 2010


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Abstract-

During the Neoproterozoic there were two periods of intense glaciation. These glaciations

are believed to have encompassed the entire globe in ice, creating a Snowball Earth. Cap

carbonates have been analyzed to indicate that every continent shows evidence of this event

through paleomagnetism, pH levels, iridium anomalies, and 13C signatures. The extent of the

glaciation was perpetuated by an ice albedo feedback loop, and it is also proposed that

cyanobacteria disrupted the CO 2 levels in the atmosphere, cooling the temperature and allowing

for the onset of glaciation.

Introduction-

There have been many glaciations throughout the course of Earths history ; however, the

evidence provided by paleomagnetism and cap carbonates suggests an extreme scenario during

the Neoproterozoic. The explanation for the extent of the glaciations is somewhat controversial.

In 1992, Joseph L. Kirschvink proposed the Snowball Earth Hypothesis, which was later revised

by Paul Hoffman and Daniel Schrag (Fairchild and Kennedy, 2007). This hypothesis has been

one of the leading concepts considered since . Kirschvinks belief is that after glacial conditions

had been established, the increased albedo from the ice caused a positive feedback loop that

failed to be counteracted until the entire Earth was encased in ice and snow (Hoffman et al.,

1998).

One of the main lines of evidence supporting Kirschvinks ideas is the staggering

amount of cap carbonates which all date to approximately the same time (Kasemann et al.,

2010). The time frame being examined is roughly 710 Ma previously to 635 Ma, and has been

broken up into two separate periods of glaciations, both thought to encompass the entire Earth,

called the Sturtian and Marinoan glacial epochs (Bodiselitsch et al., 2005).


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Though there have been a variety of models constructed to depict what might have

occurred based on the evidence present today, the one at the forefront of the debate is the

Snowball Earth. CO 2 levels, iridium anomalies, paleomagnetism and ocean acidification all

support this hypothesis.

Carbonate Rocks-

The carbonate rocks associated with these glaciat ions were analyzed for 13C content and

it was determined that during the time period of 710Ma to 635Ma are negative values with just a

slight variation to positive in the middle of the time interval (Kasemann et al., 2010). As is

characteristic of glacial periods, these cap carbonates contain layers of microcrystalline dolomite(Fairchild and Kennedy, 2007 ). The negative 13C values are interpreted to represent periods of

glaciations resulting from seawater variations. The carbonates below the glacial layers

demonstrate positive carbon-13 values, as do the carbonates above the glacial layer (Hoffman et

al., 1998). The glacial deposits and debris flows seen on the continental slope show that the ice

grinding line did not stray far from the edge of the platform, which indicates grounded ice in a

tropical region (Hoffman et al., 1998). The previously mentioned data from cap carbonates

supports a period of extensive glaciations which covered the equatorial regions. The global cap

carbonate distribution may be noted in figure 1.


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Fig. 1: a) Distribution of glacial deposits during Sturtian (740-660Ma) b) distribution of glacial

deposits during Marinoan (


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Although the ice was cutting off the atmosphere from the water on the planet, and

shutting down the hydrological cycle, there were a whole new set of conditions developing under

the ice. The glaciations did not shut down volcanic processes. Volcanism and hydrothermal

activities at oceanic ridges continued. This activity combined with a decrease in oxygenic

photosynthesis and the removal of atmospheric influence led to reducing conditions in the

seawater (Kirschvink et al., 2000). The formation of reducing conditions in the ocean is further

supported by the existence of Banded Iron Formations dated to this period (Kirschvink et. Al.,

2000).

This evidence all implies that during the glaciations, the seawater was cut off from theatmosphere while the CO 2 levels were building. Snowball Earth, would have resulted in ice

separating the atmosphere from the oceans. The ocean acidification event occurred at the same

time as deglaciation leading to a build up of CO 2 in the atmosphere. The greenhouse effect

caused deglaciation, during which the seawater was rapidly exposed to the CO 2 causing the pH

to become more acidic.

Duration/Intensity-

The Snowball Earth hypothesis states that the duration of each of the two glaciations

lasted more than a million years. Cap carbonates have been examined for paleomagnetism and

show that throughout the glacial periods multiple pole reversals occurred (Bodiselitsch et al.,

2005). Specifically, the Elatina formation from Southern Australia provides a duration for the

Marinoan Glacial Epoch of one million years (Bodiselitsch et al., 2005). By examining the rate

of subaerial volcanic CO 2 outgassing today, a similar time frame is shown. Without an exchange

from seawater to atmosphere, and the luminosity of the Neoproterozoic, as well as differences in

the amount of carbonate deposition this time interval remains valid. While an exact time frame


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has not been established, the Marinoan glacial episode is believed to have been a minimum of

three million years long (Bodiselitsch et al., 2005).

It can logically be implied that while the Earth was covered with ice, any extraterrestrial

material that may have made its way to the surface would have been on top of the ice. When

deglaciation occurred any of this material would be deposited at the base of the cap carbonates.

Examination of these cap carbonates showed Iridium and Platinum group anomalies for both the

Marinoan and Sturtian Glacial Epochs (Bodiselitsch et al., 2005). The anomalies were

significant, at approximately 2 ppb for the iridium (Bodiselitsch et al., 2005). The extraterrestrial

matter would have been composed of interplanetary dust particles as well as any remnants of asteroids or comets that collided with the Earth between 710Ma and 635Ma (Bodiselitsch et al.,

2005). The Kipushi Petit Conglomerate/cap carbonate transition was yielded a chromium/iridium

ratio of 2.9 x 10 3, which closely resembles the ratio found in carbonaceous chondrites at 5.5 x

103 (Bodiselitsch et al., 2005). A demonstration of the iridium anomalies in comparison with

other elements can be seen below in figure 2.


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Fig. 2: Iridium anomalies from cap carbonates from the Congo for Marinoan and Strutian Glacial

Epochs (Bodiselitsch et al., 2005).

The intensity of the period is implied by the extent of glaciation described in the

hypothesis. The duration and intensity of the Snowball Earth epochs was aided greatly by the

feedback of ice albedo. As the planet became more severely covered with ice, albedo increased,

leading to the formation of more ice. Climate models representing the time indicate that there


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would have been between 500 and 1500m of pack ice (Kirschvink et al., 2000). If extended to

predict surface temperatures, the same models would produce temperstures ranging from -20C

to -50C (Kirschvink et al., 2000).

Paleogeography-

When considering how glaciations began during this period, it is important to note the

position of the continents. During the Neoproterozoic, the supercontinent Rodinia had been

established, amassing the continents on one side of the Earth and greatly affecting the climate.

Through paleomagnetism the orientation of the continents has been established but an anomaly

has popped up. Fairchild and Kennedy (2007) speculate that true polar wander occurredtriggering the onset of glaciations. The process would have been quicker than any known

Phanerozoic rates of plate motion that have been calculated. The event would have rotated the

entire crust and mantle without moving the outer core. In the case of the Neoproterozoic, Rodinia

would have rotated to leave the supercontinent centered on the equator (Fairchild and Kennedy,

2007). From this position with the supercontinent being above a mantle plume, breakup would

have occurred as shown in Figure 3.


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Fig. 3: Left- Laurentia 780-723Ma, Right- 750Ma Rodinia, beginning to fragment (Fairchild and

Kennedy, 2007).

Contrasting Evidence-

The Snowball Earth Hypothesis depicts a certain scenario of tectonics happening across

the Earth, and does not leave much room for variation. However, there are a series of basins in

India, the Purana basins, which have been dated by fossil evidence at 500-700Ma (University of

Florida, 2010). The Purana Basins indicate that India was rifting and thinning out at the time of

formation, which does not agree with the Snowball Earth Theory.

More recently, a kimberlite from one of the Purana Basins has been dated using zircon

crystals, and the basin age has been revised to more than 1billion years old, which removes the

basins from consideration as their formation was well before the glaciations in the

Neoproterozoic (University of Florida, 2010). Further, the kimberlites paleomagnetic orientation

was compared to rocks from other Purana Basins, and the orientations found to be nearly

indistinguishable (Universoty of Florida, 2010). This evidence revises the dates of all the Purana

Basins, therefore supporting the Snowball Earth Hypothesis.


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There is also significant debate about the Slushball Earth Theory. Instead of having the

oceans in tropical regions be completely covered by ice, Slushball Earth dictates that there was

some open ocean in tropical latitudes (Fairchild and Kennedy, 2007). While this would provide

for some sea-air exchange, it is uncertain what amount of open ocean would be enough to really

affect the processes happening. The oceanic pH at the time and CO 2 levels suggest that there was

no sea-air exchange. However, it is unknown what the influence of small amounts of seawater-

air exchange would have on these figures (Fairchild and Kennedy, 2007).

Biological Influence-

As is expected with rapid and extreme adjustments to the climate, biological productivitywas affected negatively. With the entire planet covered in ice, it would seem that no species

would thrive however, there is evidence of a cyanobacterial bloom. It is proposed that this bloom

occurred during deglaciation and created an oxygen spike (Kopp et al., 2005). This increase in

oxygen combined with the reducing environment developed under the ice, caused oxidative

precipitation rich in iron and manganese (Kopp et al., 2005). Which is seen in the Banded Iron

Formations. The oceans were rich in these elements from the hydrothermal activity, which

continued despite the glaciations.

For the onset of glaciations to occur, there must have been a disturbance to the system at

hand. Kopp (2005) proposes that cyanobacteria were a significant factor through oxygen

disturbance. There had been smaller glaciations previous to the Marioan and Sturtian glaciations

and during this time the methane greenhouse had been in effect due to the large amounts of CO 2

in the atmosphere, warming the temperature of the atmosphere. Banded Iron Formations

deposited after the Makganyene glaciation (~2.3Ga) show increased oxygen levels. Specifically

the Ongeluk and Hotazel Formations from South Africa demonstrate these increased levels


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(Kopp et al., 2005). Figure four shows these formations as well as the onset and end of

glaciations. The Ongeluk and Hotazel are directly between the onset and end of glaciations

(Kopp, et al., 2005). The onset of the cyanobacterial growth during a partial glaciation

perpetuated the glaciation to the extent seen during the Snowball Events.

Fig. 4: Indicating the age of Rock formations from South Africa and their relationship to the

Snowball Earth Glaciations (Kopp et al., 2005).

Deglaciation-

After the establishment of a Snowball Earth state, in order to deglaciate the Earth again

there must be a trigger for the climate change. A major inconsistency with climate models for the

Snowball Earth Glaciations is in the amount of CO 2 needed for the deglaciation to occur. If the

model cannot account for deglaciation, then as Earth is currently not under Snowball Earth

conditions, the model would be rendered implausible. Generally, climate models dictate that

0.3bars is needed for the partial pressure of CO 2 in order for deglaciation to occur (Abbot and


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Halevy, 2010). Evidence from oxygen isotopic anomalies shows that at 635 Ma, CO 2 levels had

reached a maximum of 0.08bars (Abbot and Halevy, 2010). Naturally, this has raised significant

doubt as to the plausibility of the Snowball Earth Hypothesis.

If the ocean in tropical regions was not completely iced over, this open water may

significantly effect albedo. With some open seawater, the hydrological cycle would still be

functioning and the layer of dust on the ice surface would decrease albedo (Abbot and Halevy,

2010). With this factored into the climate model, it can now be expected that deglaciation would

be possible with only 0.01-0.1bars of CO 2 (Abbot and Halevy, 2010). Formation of a planetary

net ablation zone would occur in this scenario as there is no heat flow during Snowball Earth(Abbot and Halevy, 2010). As a result, any dust or aerosols would stay within 25 of the equator.

Even with sublimation occurring on the ice surface at an estimate of 0.01m/yr, the climate model

consistently produces deglaciation. As ice is sublimated, it must be replaced by freezing at the

bottom of the ice sheet or ice sheet flow from higher latitudes, either of which would explain the

occurrence of dust on the surface in tropical regions (Abbot and Halevy, 2010).

Conclusion-

There have been significant findings all over the Earth such as the banded iron formations

and cap carbonates, which support the Snowball Earth Hypothesis. While many aspects of this

hypothesis, such as the true polar wander and deglaciation, are still under debate, it appears that

Snowball Earth is the best explanation for the paleomagnetic, iridium anomalies, 13C signatures,

and pH levels during the Neoproterozoic.


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References Cited-

Abbot, D. S., Halevy, I., 2010, Dust Aerosol Important for Snowball Earth Deglaciation, Journal

of Climate, vol. 23, p. 4121-4132.

Bodiselitsch, B., Koeberl, C., Master, S., Reimold, W., 2005, Estimating Duration and Intensity

of Neoproterozoic Snowball Glaciations from Ir Anomalies, Science, vol. 308, p. 239-242.

Fairchild, I.J., Kennedy, M.J., 2007, Neoproterozoic Glaciation in the Earth System, Journal of

the Geological Society, vol. 164, p. 895-921.

Hoffman, P., Kaufman, A., Halverson, G., Schrag, D., 1998, A Neoproterozoic Snowball Earth,

Science, vol. 281, p. 1342-1346.

Kasemann, S., Prave, A., Fallick, A., Hawkesworth,C., and Hoffmann, K., 2010, Neoproterozoic

Ice Ages, Boron Isotopes, and Ocean Acidification: Implications for a Snowball

Earth, Geology, vol.38, p. 775-778.

Kirschvink, J., Gaidos, E., Bertani, L., Beukes, N., Gutzmer, J., Maepa, L., Steinberger, R., 2000,

Paleoproterozoic Snowball Earth: Extreme Climatic and Geochemical Global Change and Its

Biological Consequences, Proceeding of the National Academy of Sciences of the United

States of America, vol. 97, p. 1400-1405.

Kopp, R., Kirschvink, J., Hilburn, I., Nash, C., Hoffman, P., 2005, The Paleoproterozoic

Snowball earth: A Climate Disaster Triggered by the Evolution of Oxygenic Photosynthesis,

Proceedings of the National Academyof Sciences of the united States of America, vol. 102,

p. 11131-11136.


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University of Florida; Geologists push back date basins formed, supporting frozen Earth

theory. NewsRx Science, 14 Jul 2008: Research Library, ProQuest. Web. 25 Oct. 2010.

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