Snowball Earth

Snowball Earth is the theory that, at certain times in the Earth's history, the planet was covered by a global glacier that reached from the poles to the equator. Evidence supporting the snowball Earth theory includes the discovery of widespread glacial sediments in equatorial areas. Global glaciation might have facilitated the complex evolution of life by refining the gene pool, as only adaptive organisms were able to survive periods of glacial coverage.

Development of the

The snowball Earth hypothesis holds that several times in Earth's history, the planet has been covered by glaciers stretching from the poles to the equator. Evidence for global glacial periods has been building since the 1940s and became a major subject of debate among geologists following the discovery of new evidence in the 1990s. This evidence provided strong support for the theory and additional evidence to suggest that the Earth may have experienced more than one global glacial period.

Geologist and explorer Douglas Mawson discovered glacial sediments in southern Australia in 1949 that were later linked to similar glacial deposits discovered in Europe and China. In 1964, geologist W. Brian Harland discovered evidence of glaciers near the equator. Harland's findings were initially dismissed, as most geologists believed that glacial deposits near the equator came from rocks that were gradually moved to the equatorial zone by perhaps ocean or river currents.

In the late 1980s, paleomagnetist Joseph Kirschvink discovered sediment samples in Adelaide, Australia, which provide some of the best evidence for the snowball Earth theory. Kirschvink's rock samples clearly had been affected by glacial activity yet bore distinct magnetic signatures that pinpointed their origin as being in the equatorial zone, convincing many geologists that glacial ice at one time covered part of the equator. In a 1992 paper, Kirschvink coined the term “snowball Earth” to refer to these global glacial periods in Earth's history.

Geologist Paul F. Hoffman popularized the snowball Earth theory in a series of works published in the late 1990s. Hoffman discovered distinct carbon caps from geological samples in Namibia, Africa, which indicated a rapid decrease in photosynthetic activity during the period of global glaciation. Hoffman's data bolstered the case for the snowball Earth theory and sparked more research into glacial cycles.

Since the 1980s, scientists have discovered evidence for at least three distinct periods of global glaciation. The oldest is the Makganyene glaciation, which occurred about 2.2 billion years ago. Evidence for this event, reported by Kirschvink, David Evans, and Nik Beukes in 1997, includes sedimentary samples bearing signs of glacial deposition and paleomagnetic traces.

Geologists sometimes refer to the period from 850 to 630 million years ago as the Cryogenian, during which the Earth experienced two or more periods of global glaciation. The first of the Cryogenian ice ages, the Sturtian glaciation, occurred between 720 and 710 million years ago. Geologist Francis Macdonald and colleagues refined the timeline for the Sturtian glaciation by discovering sedimentary deposits indicating that the equator was covered with ice about 716.5 million years ago. Macdonald and colleagues believe that the glacial period lasted 5 million years or more, during which most of the Earth's surface was covered with ice.

The latest glaciation event, the Marinoan glaciation, occurred between 640 and 635 million years ago. Geologists discovered glacial sediment from the Marinoan on every continent, and evidence suggests that the glaciation may have lasted 6 to 12 million years before retreating to the poles.

Evidence for Snowball Earth Events

Glacial tills are samples of rock ground or scarred by the force of glaciers moving across their surface. By dating the sediment within glacial tills, geologists can determine the approximate time of the glacier's formation. In addition, moving glaciers can carry rocks as they move across the Earth's surface. These rocks, called erratics, might differ from the surrounding sediment in which they are deposited. The appearance of erratics has provided further evidence for glacial activity in the past. Magnetic data from glacial deposits can indicate the origin of the sediment, as sediment carries distinct magnetic signatures based on a given locale. Glacial sediment with magnetic signatures indicating equatorial origin is evidence that glaciers reached the temperate and equatorial zones.

Additional evidence comes from measurements of carbon-13, a by-product of photosynthesis, in sediment samples above and below glacial deposits. Hoffman's research indicates that sediment below glacial deposits is rich in carbon-13. This evidence shows that photosynthesizing bacteria were abundant in the environment before the glacial deposits formed. Carbon-13 drops sharply in the sediment approaching the glacial deposits, indicating that changing environmental conditions hindered photosynthesis. Hoffman's carbon-13 data suggests photosynthesis slowed to low levels during the glacial period and rebounded as the glaciers retreated.

Causes of Global Glaciation

Scientists have not determined the exact chain of events that led to increasing glaciation during the three snowball Earth periods. The overall cause seems to have been a reduction in greenhouse gases, most notably methane and carbon dioxide, which resulted in a period of global cooling.

The portion of solar radiation reflected from an object's surface is called the albedo, and different surfaces have different albedo levels depending on their color, chemical makeup, and other factors. The albedo of ice and fresh snow is much higher than that of unfrozen water. The albedo effect is responsible for snow-temperature feedback, in which the presence of snow on the ground further reduces the temperature of the ground by reflecting more sunlight, which in turn leads to more snow accumulation.

In the 1960s, climatologists calculated the theoretical extent of the snow feedback phenomenon. They found that if more than one-half of the Earth was covered in ice, the albedo effect would create a runaway feedback loop, which would rapidly cover the rest of the planet with ice. Runaway feedback occurs when snow and ice accumulate faster than ablation, which is the combined effect of sublimation of evaporation in removing snow from the surface. Geologists believe that global cooling initiated a runaway cooling cycle for each snowball Earth period. Global cooling may have been related to a massive reduction in carbon dioxide and methane immediately preceding the glaciation.

In 2009, geochemist Kent Condie published the results of an extended study of ancient sediment samples suggesting that the Makganyene glaciation, which occurred more than 2 billion years ago, may have been precipitated by a global reduction in tectonic and volcanic activity. According to Condie, evidence indicates that volcanic activity came nearly to a halt and remained at low levels for more than 200 million years. Condie suggests that this would have led to a massive reduction in greenhouse gases and could have precipitated the formation of a global glaciation event.

Some geologists also note that glacial sediment from the Sturtian and Marinoan glacial periods appears close to a layer of igneous rock due to lava flows that hardened over millions of years. Some geoclimatologists suggest that a significant period of widespread volcanic eruptions might have cooled the climate by ejecting smoke and particulate matter into the sky. Alternatively, some scientists suggest that these ancient volcanic deposits may have been caused by a period of intense volcanism that precipitated the end of a snowball Earth period. Intense volcanism would create additional greenhouse gases and lead to global warming.

The Possible End of Global Glaciation

During a snowball Earth period, tectonic activity continues under the Earth's surface, producing volcanic material and large amounts of carbon dioxide. Under the ice-covered earth, chemical mechanisms that remove greenhouse gases from the atmosphere would be greatly reduced in scope. This might lead to a buildup of carbon dioxide, methane, or other greenhouse gases, which would eventually be released into the atmosphere, countering the albedo effect.

Hoffman discovered evidence of thick limestone overlaying glacial sediments in many areas. Limestone is deposited only in areas where the temperature is warm, and conditions are wet. This finding led Hoffman to assume that the Earth's average temperature grew rapidly warmer immediately after the global glaciers began melting. Furthermore, evidence suggests that the Earth remained in a superheated state for some time, allowing the oceans to melt and return to liquid form. Hoffman believes that each glacial period may have been linked to an ensuing period of global heating lasting millions of years.

Geologist Martin Kennedy proposed an alternative theory to explain the end of the Marinoan glaciation. His theory assumes the production of methane, a common greenhouse gas that is more effective in trapping solar radiation than carbon dioxide. Kennedy and colleagues believe that methane slowly accumulated under the ice layers that covered the earth. Tectonic movement eventually caused some of the fragile ice layers to split, releasing trapped methane into the atmosphere. The methane trapped a portion of the solar radiation that would otherwise escape into space, causing the temperature to rise and leading to additional splits in the ice.

Effects on Life

Most climatologists believe that even during the height of a global glaciation, the Earth would have been able to support life. First, geologists believe that the ice and snow covering the Earth would not have been continuous but likely would have been dotted with areas containing liquid water.

In the glaciers of modern Antarctica, for instance, biologists have found rich deposits of algae and microorganisms within gaps in the sea ice. Sedimentary geologist Daniel Le Heron reported in 2011 on discovering sedimentary evidence from the Cryogenian period, indicating that ocean currents continued to flow in some portions of the sea even at the heart of the glacial period. Some climatologists suggest that continued volcanic activity would have littered some areas with blankets of dark dust and debris. These areas would have heated faster because of their lower albedo levels and thus may have become pockets of water, allowing life to exist during the height of the snowball effect.

Second, in 2011, researchers found fossil evidence from the end of the Sturtian glaciation, indicating the presence of the first “shelled” microbes known to science. Like some modern microbes, these ancient single-celled organisms used a process called agglutination to build shells around their body, insulating themselves from environmental hazards. Researchers conducting the study suggested that the harsh environment of the glaciated Earth led to a situation in which only organisms with advanced survival tools, like agglutinated shells, could survive. This paved the way for the evolution of stronger, hardier organisms in subsequent generations and may have been a significant step in the evolution of life.

Third, the end of the Marinoan glaciation came just before another significant turning point in the history of life: the Cambrian explosion, during which the animal fauna of the Earth rapidly evolved into many forms that could colonize the Earth's surface. Research in the twenty-first century has confirmed that the Earth during the Marinoan period was more likely a "slushy" snowball, facilitating this proliferation in life forms. Geologists Daniel Schrag and Hoffman suggested that life forms living during the last glacial period would have been separated into small, isolated populations divided by sheets of thick ice, in which few organisms could survive. These isolated pockets of microorganisms evolved differently, leading to a leap in diversity beyond the marine environment before the glacial period. As the glaciers melted and the Earth rebounded into a period of high average temperatures, these microorganisms proliferated and spread across the globe, exploiting many environments left empty by species that became extinct during the snowball Earth period.

Research into the snowball Earth theory continues in the twenty-first century. Scientists have posited that changes in the Earth’s orbit may have affected the level of glaciation on Earth, allowing some ice-free areas to remain and, therefore, life to continue. Finally, researchers at Yale University have theorized that snowball Earth may have been created by asteroids impacting the Earth’s surface. 

Principal Terms

ablation: the removal of snow through either melting or sublimation

agglutination: the clumping of particles to form a shell around a particular organism or object

albedo: a portion of light from the sun reflected by a certain surface

evaporation: the transformation of a liquid to a gas that occurs at the surface of a liquid

glacial erratic: a rock sample that differs from an area's typical rock size and type and is thought to have been carried with moving glacial ice

glacial till: the ground and deeply scarred rocks believed to originate from the movement of ancient glacial ice across underlying layers of sediment

glacier: an area of solid ice that forms from snow in areas where accumulation of snow exceeds the speed of melting and sublimation

paleomagnetism: the study of the magnetic field in Earth's sediment and the record of Earth as recorded in the magnetic field of rocks

sublimation: a chemical process involving transformation from a solid to a gaseous state

Bibliography

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