The Role of Carbon in a Snowball Earth

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 surface-to-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.