Mindi Purdy
“The Role of Carbon in a Snowball Earth”
Individual Research Paper
GLG 401
Due: 4/28/2003
Introduction
Imagine a world covered in ice for millions of years, void of activity save for
tectonics and a tiny fraction of organisms that managed to survive the extreme
environment. Although this description seems to originate from a science fiction movie
on a planet far, far away from our solar system, many lines of evidence suggest that Earth
experienced this extreme glaciation with ice covering a kilometer deep in the oceans
(Hoffman and Schrag 2000) during the Paleoproterozoic (2,400 to 1,600 million years
ago) and Neoproterozoic (1,000 to 570 million years ago) Eras.
The theory of Snowball Earth first originated in 1964, when Brian Harland of
Cambridge University noticed that Neoproterozoic glacial deposits were widely
distributed on almost every continent. The idea of plate tectonics was just beginning to be
accepted in the scientific community, and Harland thought that continents were clustered
together near the equator in the Neoproterozoic Era. This idea was based on the magnetic
orientation of mineral grains in these glacial deposits. When the rocks hardened, the
grains aligned themselves to the magnetic field, and since the deposits were clustered
about the equator, the grains were almost horizontal. If the rocks had formed near the
poles, the orientation should have been more vertical (Hoffman and Schrag 2000).
Harland published “The Great Infra-Cambrian Glaciation” in Scientific American in
1964, announcing his ideas about a great Neoproterozoic ice age.
Around the same time that Harland publicized his theory, Mikhail Budyko of the
Leningrad Geophysical Observatory developed a mathematical energy-climate model that
explained how tropical glaciers could form. He used equations that described how solar
radiation interacts with the Earth’s surface and atmosphere to control climate (Hoffman
and Schrag 2000). “Budyko showed that if Earth’s climate were to cool, and ice were to
form at lower and lower latitudes, the planetary albedo would rise at a faster and faster
rate because there is more surface area per degree of latitude as one approaches the
Equator” (Hoffman and Schrag 1999). Through the model, it was found that at a critical
latitude of 30° north or south, the positive feedback became overwhelming, and
essentially, the freeze became irreversible, yielding an entirely frozen earth.
Budyko’s simulation was surrounded with controversy, because it was believed
that such a catastrophe would extinguish all life, and also that once the Earth entered the
frozen state, it would have been permanent. In the late 1970s, organisms, coined
extremophiles, were discovered that could survive extreme environments, such as
psycrophiles (cold-loving), thermophiles and hyperthermophiles (heat-loving), and
acidophiles (acid-loving). The second problem was addressed by Joseph Kirschvink in
the late 1980s. He pointed out that “during a global glaciation, an event he termed a
snowball earth, shifting tectonic plates would continue to build volcanoes and to supply
the atmosphere with carbon dioxide” (Hoffman and Schrag 2000). If the Earth were
completely frozen over, the processes that remove carbon dioxide from the atmosphere
would essentially cease, allowing carbon dioxide to build up in the atmosphere to
extreme levels (around 350 times current values, or 0.12 bar). This was the key to
reversing a snowball Earth.
The Role of Carbon
Throughout my research on the Snowball Earth theory, the most convincing
evidence I have found for the existence of a snowball earth is carbon data. Its presence in
records explains the instigation and cessation of a snowball earth, as well as levels of
biological productivity and oceanic circulation. Schrag et. al. states, “Although the flux of
carbon from volcanic sources is insignificant on the timescale of human civilization, even
a slight imbalance between source and sink over millions of years would strip all the
carbon out of the ocean and atmosphere, or lead to an extreme greenhouse climate”
(2002).
Carbon Dioxide
One of the most important greenhouse gases because of its abundance and
relatively short residence time in the atmosphere is carbon dioxide. The short residence
time of carbon dioxide makes it susceptible to small imbalances that can create rapid and
large impacts on the ocean-atmosphere system (Schrag, et. al. 2002). This molecule is
important in regulating Earth’s temperatures and appears to be especially important in the
instigation and cessation of a snowball earth event.
Climate and carbon dioxide have an intimate relationship. For example, an
increase in volcanic outgassing of carbon dioxide would raise temperatures (greenhouse
effect), which would create more precipitation, which in turn would intensify weathering
rates and draw down atmospheric carbon dioxide levels, keeping the whole system in
check. A snowball earth event would require that carbon dioxide levels decrease
dramatically so that temperatures could drop. The problem with this theory is that as
temperatures fall and ice sheets cover land, weathering and other sinks for carbon dioxide
should decrease so that the system is balanced.
One theory regarding the carbon dioxide drop is that it coincided with the breakup
of Rodinia and the formation of Gondwana in the Neoproterozoic Era. The breakup
created new continental margins, forming excellent carbon dioxide and carbon sinks. The
Pan-African and Himalayan-Tibetan orogenesis may have contributed to high erosion
rates and the drawdown of carbon. These factors combined with low latitude continents
may have allowed the carbon dioxide values to fall low enough to initiate a snowball
earth event. “If there were less continental area at high latitudes, the strength of the
feedback would diminish; with a drop in the source, presumably volcanic activity, a
lower level of CO2 would be required to achieve a reduction in the weathering sink to
keep the carbon cycle in balance (See Figure 1)” (Schrag et. al. 2002).
Figure 1 (Schrag, et. al. 2002): A schematic representation of the fraction of land area available for silicate
weathering as a function of the partial pressure of atmospheric carbon dioxide for high- and low-latitude continental
distributions. As carbon dioxide drops, glaciation commences on high-latitude continents, reducing the rate of silicate
weathering in those areas and stabilizing the atmospheric CO2. If most of the continents were in the tropics, this effect
would not commence until CO2 levels were substantially lower.
Carbon Isotopes
As Hoffman and Schrag (1999) have explained, carbon during the Neoproterozoic
was supplied to the ocean and atmosphere by the outgassing of carbon dioxide by
volcanoes. This carbon dioxide contained about 1% carbon-13 and 99% carbon-12.
Carbon is removed by the burial of calcium carbonate in the oceans, in addition to
terrestrial removal by silicate weathering. If removal by burial of calcium carbonate were
the only process in effect, calcium carbonate would have the same ratio of carbon-13 and
carbon-12 as the volcanic output, but carbon is also removed from the ocean in the form
of organic matter, and organic carbon is depleted in carbon-13 (2.5% less than in calcium
carbonate).
Due to extreme climatic conditions, snowball events should drastically decrease
levels of biological productivity. This drop in biological productivity should trigger
decreased levels of C13 in the sediments (See Figure 2).
Figure 2 (Hoffman and Schrag 1999)
Another theory behind the extreme drop in C13 levels deals with oceanic
circulation. During nonglacial periods, it is typical for oceans to exhibit a large surfaceto-deep C13 gradient, with surface waters having unusually high values and deep waters
having very low values. Halverson, et. al. states, “Drawdown of atmospheric CO2
eventually leads to glaciation, which in turn enhances thermohaline overturning. Overturn
causes the C13 depleted, DIC-charged, deep water to invade the surface, releasing CO2 to
the atmosphere (initiating glaciation) and precipitating carbonates with low C13 values”
(2002).
The large C13 values seen before the glacial events are thought be connected the
low latitude continental distribution. “The high C13 values in preglacial Neoproterozoic
carbonates are consistent with a concentration of continents in the topics; … in addition,
the high fractional organic carbon burial may also be important for setting the stage for a
global glaciation by lowering atmospheric carbon dioxide” (Schrag, et. al. 2002). In other
words, “if the fraction of organic burial goes up, the burial of carbon will momentarily
exceed the volcanic source, reducing atmospheric CO2 until a new steady state is
achieved with lower atmospheric CO2 and lower silicate weathering” (Schrag, et. al.
2002). This relationship can be illustrated by the following formula, where Fvolc, Fsil, and
Forg are the rates of carbon dioxide release from volcanic outgassing, carbon dioxide
uptake from silicate weathering, and organic carbon burial, ksil is the slope of the
weathering-CO2 feedback, and qorg is the rate of organic burial (Schrag, et. al. 2002).
Kaufman, Jacobsen, and Knoll (1993) measured various isotope levels during the
Varanger Glaciation around 610 Ma to 585 Ma, and our group analyzed this data to
determine whether or not the data showed a significant difference between glacial,
hothouse, and normal periods. We used ANOVA to analyze the data and found that the
carbon isotopic records did indeed show a significant difference between the different
climate periods (See Figure 3). More recent measurements by Jacobsen and Kaufman
(1999) include carbon isotopic variations in carbonates from an extended time period
(See Figure 4).
Figure 3: ANOVA analysis of C13 levels during hothouse, normal, and snowball periods
Figure 4 (Jacobsen and Kaufman): C13 levels of carbonates in the Neoproterozoic
Methane
Methane is an extremely potent greenhouse gas, although its abundance is
nowhere near that of carbon dioxide’s. An interesting theory concerning C13 values is that
the preglacial drop in C13 could be driven by the release of methane to the carbon cycle.
“Because methane formed in deep-sea sediments has very low C13 values (approximately
-70‰), a relatively small amount of methane can have a very large effect on the average
C13 value of dissolved inorganic carbon in seawater” (Schrag, et. al. 2002). Some of the
release mechanisms for methane include slope failure, sea level fall, uplift of a hydrate
reservoir to a shallower depth, an increase in either bottom water temperatures or the
geothermal gradient (Halverson, et. al. 2002), and a rapid destabilization of methane
hydrates on the seafloor (Schrag, et. al. 2002). The last release mechanism is especially
appealing, because “substantial amounts of methane hydrate [could have] formed
following tens of millions of years with a high fraction of organic carbon burial and
significant regions of anoxia” (Schrag, et. al. 2002).
The GEOCARB model was used to explore the relationship between sustained
methane levels of 36 ppm (produced by the addition of a flux of methane to the
atmosphere) and carbon dioxide and carbon-13 levels. The initial carbon dioxide
concentration of 300 ppm and carbon-13 values (5‰) were assumed in all calculations
with the C13 value based on measurements of carbonates immediately underlying the
glacial deposits (Schrag, et. al. 2002). Figure 5 shows the results of the simplified
GEOCARB model using a linear methane decay model.
The addition of methane into the carbon cycle seems to be the key to lowering
carbon dioxide levels and C13 levels concurrently. “If the carbon cycle existed in this
delicate state of balance with both CO2 and methane sharing the greenhouse forcing, the
Earth would be vulnerable to a catastrophic destabilization of climate” (Schrag, et. al.
2002). In addition, Schrag, et. al. states that “all other hypotheses for lowering the C13
values of marine carbonates so precipitously imply an increase in atmospheric CO2 and a
warmer climate” (2002 ).
Figure 5 (Schrag, et. al. 2002, Initiation)
Conclusion
Although the Snowball Earth Theory is fairly radical, there seems to be no other
way to explain the biological, sedimentary, and isotopic data. The question then becomes,
is it possible that another snowball earth could occur in the near future? The Earth has
been in its coldest state since the Neoproterozoic, and we are approximately 80,000 years
from the next glacial maximum (Hoffman and Schrag 1999). “Some evidence suggests
that each successive glaciation over the last several cycles has been getting stronger and
stronger, [and] during the most recent glacial event, 20,000 years ago, the deep ocean
cooled to near its freezing point, and sea ice reached latitudes as low as 40° to 45° north
and south” (Hoffman and Schrag 1999). It’s quite possible that the next ice age could
reach the critical latitude and plunge the Earth into another deep freeze. With the delicate
balance of methane, carbon, and carbon dioxide levels, will the human input of CO2
trigger these climate forcers and initiate another snowball earth? My guess is that we’ll
not live to see such a spectacular event.
Works Cited
Halverson, et. al. “A Major Perturbation of the Carbon Cycle Before the Ghaub
Glaciation (Neoproterozoic) in Namibia: Prelude to Snowball Earth?”.
Geochemistry, Geophysics, and Geosystems. Vol. 3, No. 6, 2002.
Hoffman, Paul and Daniel Schrag. “Snowball Earth.” Scientific American. January 2000,
Issue 100.
Hoffman, Paul and Daniel Schrag. The Snowball Earth. 2000. Accessed April 26, 2003.
http://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.html
Jacobsen, Stein and Alan Kaufman. "The Sr, C, and O Isotopic Evolution of
Neoproterozoic Seawater." Chemical Geology. Volume 161: 37-57.
Kaufman, Alan, Stein Jacobsen, and Andrew Knoll. "The Vendian Record of Sr and C
Isotopes in Seawater: Implications for Tectonics and Paleoclimate." Earth and
Planetary Letters. Volume 120: 409-430.
Schrag, et. al. “On the Initiation of a Snowball Earth.” Geochemistry, Geophysics, and
Geosystems. Vol. 3, No. 6, 2002.