Unit 1: Many Planets, One Earth // Section 5: Testing the Thermostat: Snowball Earth
We can see how durable Earth's silicate weathering "thermostat" is by looking at some of the most extreme climate episodes on our planet's history: severe glaciations that occurred during the Proterozoic era. The first "Snowball Earth" phase is estimated to have occurred about 2.3 billion years ago, followed by several more between about 750 and 580 million years ago. Proponents of the Snowball Earth theory believe that Earth became so cold during several glacial cycles in this period that it essentially froze over from the equator to the poles for spans of ten million years or more. But ultimately, they contend, the carbon-silicate cycle freed Earth from this deep-freeze state.
If Earth has a natural thermostat, how could it become cold enough for the entire planet to freeze over? One possible cause is continental drift. Researchers believe that around 750 million years ago, most of the continents may have been clustered in the tropics following the breakup of the supercontinent Rodinia (Fig. 9), and that such a configuration would have had pronounced effects on Earth's climate (footnote 4).
Figure 9. The breakup of Rodinia
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Source: © Snowball Earth.org.
Had the continents been located closer to the poles as they are today, ice sheets would have developed at high latitudes as the planet cooled. Ice cover would prevent the rocks beneath from weathering, thus slowing the rate at which carbon was removed from the atmosphere and allowing CO2 from volcanic eruptions to build up in the atmosphere. As a result, Earth's surface temperature would warm.
But if continents were clustered at low latitudes, Earth's land masses would have remained ice-free for a long time even as ice sheets built up in the polar oceans and reflected a growing fraction of solar energy back to space. Because most continental area was in the tropics, the weathering reactions would have continued even as the Earth became colder and colder. Once sea ice reached past about 30 degrees latitude, Snowball Earth scholars believe that a runaway ice-albedo effect occurred: ice reflected so much incoming solar energy back to space, cooling Earth's surface and causing still more ice to form, that the effect became unstoppable. Ice quickly engulfed the planet and oceans froze to an average depth of more than one kilometer.
The first scientists who imagined a Snowball Earth believed that such a sequence must have been impossible, because it would have cooled Earth so much that the planet would never have warmed up. Now, however, scientists believe that Earth's carbon cycle saved the planet from permanent deep-freeze. How would a Snowball Earth thaw? The answer stems from the carbon-cycle thermostat discussed earlier.
Even if the surface of the Earth was completely frozen, volcanoes powered by heat from the planet's interior would continue to vent CO2. However, very little water would evaporate from the surface of a frozen Earth, so there would be no rainfall to wash CO2 out of the atmosphere. Over roughly 10 million years, normal volcanic activity would raise atmospheric CO2 concentrations by a factor of 1,000, triggering an extreme warming cycle (Fig. 10). As global ice cover melted, rising surface temperatures would generate intense evaporation and rainfall. This process would once again accelerate rock weathering, ultimately drawing atmospheric CO2 levels back down to normal ranges.
Figure 10. The geochemical carbon cycle on a Snowball Earth
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Source: © Snowball Earth.org.
Many geologic indicators support the snowball glaciation scenario. Glacial deposits (special types of sediments known to be deposited only by glaciers or icebergs) are found all around the world at two separate times in Earth's history: once around 700 million years ago and then again around 2,200 million years ago. In both cases some of these glacial deposits have magnetic signatures that show that they were formed very close to the equator, supporting an extreme glacial episode.
Another important line of evidence is the existence of special iron-rich rocks, called iron formations, that otherwise are seen only very early in Earth's history when scientists believe that atmospheric oxygen was much lower. In the presence of oxygen, iron exists as "ferric" iron (Fe3+), a form that is very insoluble in water (there is less than one part per billion of iron dissolved in seawater today). However, before oxygen accumulated in the atmosphere, iron would have existed in a reduced state, called "ferrous" iron (Fe2+), which is readily dissolved in seawater. Geologists believe that iron formations were produced when iron concentrations in the deep ocean were very high but some oxygen existed in the surface ocean and atmosphere. Mixing of iron up from the deep ocean into the more oxidized ocean would cause the chemical precipitation of iron, producing iron formations.
Geologists do not find iron formations after about 1.8 billion years ago, once oxygen levels in the atmosphere and ocean were high enough to remove almost all of the dissolved iron. However, iron formations are found once again around 700 million years ago within snowball glacial deposits. The explanation appears to be that during a Snowball Earth episode sea ice formed over most of the ocean's surface, making it difficult for oxygen to mix into the water. Over millions of years iron then built up in seawater until the ice started to melt. Then atmospheric oxygen could mix with the ocean once again, and all the iron was deposited in these unusual iron formations (Fig. 11).
Figure 11. Banded iron formation from Ontario, Canada
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Source: © Denis Finnin, American Museum of Natural History.
The Snowball Earth is still a controversial hypothesis. Some scientists argue that the evidence is not sufficient to prove that Earth really did freeze over down to the equator. But the hypothesis is supported by more and more unusual geological observations from this time, and also carries some interesting implications for the evolution of life.
How could life survive a snowball episode? Paradoxically, scientists theorize that these deep freezes may have indirectly spurred the development of complex life forms. The most complex life forms on Earth at the time of the Neoproterozoic glaciations were primitive algae and protozoa. Most of these existing organisms were undoubtedly wiped out by glacial episodes. But recent findings have shown that some microscopic organisms can flourish in extremely challenging conditions—for example, within the channels inside floating sea ice and around vents on the ocean floor where superheated water fountains up from Earth's mantle. These environments may have been the last reservoirs of life during Snowball Earth phases. Even a small amount of geothermal heat near any of the tens of thousands of natural hot springs that exist on Earth would have been sufficient to create small holes in the ice. And those holes would have been wonderful refuges where life could persist.
Organisms adaptable enough to survive in isolated environments would have been capable of rapid genetic evolution in a short time. The last hypothesized Snowball Earth episode ended just a few million years before the Cambrian explosion, an extraordinary diversification of live that took place from 575 to 525 million years ago (discussed in section 8, "Multi-Celled Organisms and the Cambrian Explosion"). It is possible, although not proven, that the intense selective pressures of snowball glaciations may have fostered life forms that were highly adaptable and ready to expand quickly once conditions on Earth's surface moderated.