Snowball Earth

Introduction

The snowball Earth theory says that Earth was completely covered with ice during parts of the Neoproterozoic period (1 billion to 540 million years ago). There has been much scientific debate over whether Earth ever really did exist as a “snowball.” Debate continues, although recent scientific opinion has tended to favor the idea based on many converging lines of evidence. If snowball Earth did once exist, it may have played a role

in stimulating the evolution of multicellular life—all life-forms consisting of more than one cell.

Historical Background and Scientific Foundations

The idea that Earth was once in a “snowball” condition was first advanced by British geologist W. Brian Harland (1917-2003). In 1964, Harland noted the layered deposits of glacial till close to the equator near sea level. (Glacial till is mixed-up pieces of broken rock and finer particles gathered by moving glaciers, then deposited when the glacier melts.) Harland proposed that the equatorial low-altitude glaciers, along with the glaciers that had left their marks in Neoproterozoic rocks on every other continent, could be explained only if the whole Earth was frozen, including the oceans. This would have happened from about 600 million years ago to about 580 million years ago. In fact, there may have been two to or three previous snowball-Earth glaciations during the lengthy Neoproterozoic period.

Several other geological clues point, according to many scientists, to the existence of a snowball Earth. One is the unusual mineral deposits called banded iron formations. These were probably deposited by anaerobic bacteria—bacteria that thrive in the absence of oxygen— in an ocean containing little oxygen. If the ocean were largely sealed off from the oxygen-rich atmosphere by ice, it might have become low in oxygen, allowing banded iron formations to be made. Another clue is that the layer of glacial till that seems to signal a global freeze is topped or capped, like icing in a layer cake being topped by another layer of cake, with thick deposits of carbonate minerals. These would have formed during a hothouse Earth or runaway greenhouse condition that would naturally follow a snowball Earth. Since they came later, they would be deposited on top.

According to the snowball Earth theory, continental drift was the underlying cause of the big freeze. Earlier in the Neoproterozoic, all the continents happened to drift to scattered positions around the equator. A greater portion of the world's land was then exposed to erosion of silicate rock. Such erosion, by encouraging carbon dioxide (CO₂) in the atmosphere to combine chemically with calcium to form calcium carbonate (CaCO₃), could have greatly reduced the amount of CO₂ in the atmosphere. This would have cooled Earth. As ice formed, it would have cooled the planet even faster by reflecting sunlight into space. Eventually the whole planet would freeze over, except for large areas of dry, cold, light-colored sand dunes in the interiors of the continents.

WORDS TO KNOW

BANDED IRON FORMATION: Layered (banded) sedimentary rocks containing alternating layers of stone and iron. Most banded iron formations formed several billion years ago as a result of metabolic activity by bacteria in Earth's early, low-oxygen oceans. Some scientists have hypothesized that younger banded-iron formations may have formed during a snowball Earth period or periods about 600 million years ago. During such a period, the seas would have frozen over and become depleted of oxygen.

CAMBRIAN EXPLOSION: Relatively sudden evolution of a wide variety of multicellular forms of life at the beginning of the Cambrian period, about 530 million years ago, after billions of years during which Earth was inhabited almost entirely by single-celled organisms. A few forms of multicellular life did appear shortly before the Cambrian Explosion. The explosion may have been triggered by the ending of a major snowball Earth period or by the achievement of sufficiently high oxygen levels thanks to billions of years of oxygen production by algae (single-celled aquatic plants).

CONTINENTAL DRIFT: A theory that explains the relative positions and shapes of the continents, and other geologic phenomena,by lateral movement of the continents. This was the precursor to plate tectonic theory.

GEOLOGICAL RECORD: Evidence of Earth's history left in rocks and sediments over thousands to billions of years. Events that can be inferred from the geological record include climate changes, biological evolution, continental drift, and asteroid impacts.

GLACIAL TILL: Rock and soil scoured from Earth and transported by a glacier, then deposited along the glacier's sides or at its end.

HOTHOUSE EARTH: Hypothetical very hot global climate period occurring as rebound from a possible snowball Earth condition many millions of years ago. The term is sometimes used, loosely, to refer to the hotter Earth now developing because of anthropogenic climate change.

NEOPROTEROZOIC PERIOD: Unit of geological time from 1 billion to 540 million years ago. The middle portion of the Neoproterozoic, from 850 to 630 million years ago, is named the Cryogenian period (from the Greek kruos, for frost) because during it the most severe ice age in Earth's history occurred, possibly freezing the entire planet during a snowball Earth episode.

But such a situation could not last forever, because volcanoes cannot be sealed off by ice and are always adding CO₂ to the atmosphere. With the atmosphere sealed off from the seas by ice, this volcanic CO₂ would accumulate in the air. Eventually there would be so much CO₂ that an intense greenhouse effect would warm the planet. Then, the runaway freezing would happen in reverse: as ice melted, it would expose dark water, which would absorb sunlight, heating Earth faster. Temperatures would have risen by 122°F (50°). Evaporation from the newly exposed, hot oceans would have fallen as torrential rains. Combining with the high levels of CO₂, these rains would have formed carbonic acid, washing bicarbonate ions to the oceans in large quantities, where they would form carbonate cap deposits on top of the glacial deposits laid down by the just-ending ice period. This sequence of events would explain the geological record. Eventually, Earth's climate would stabilize.

Impacts and Issues

The snowball-Earth idea was initially met with scientific disbelief because it seemed that such a total freeze must have extinguished all life and because there were other ways of explaining much of the physical evidence. Evidence has slowly mounted in favor of snowball Earth, although the idea is still disputed. The objection that no life could have survived a totally frozen world was answered in the 1970s, when communities of living organisms were discovered clustered around deep-sea hot water vents supplied by Earth's interior heat. During a snowball Earth period, such communities could have survived even if the oceans were completely sealed off by ice from all sunlight. (Land-dwelling life had not yet evolved.)

In 2004, geochemists studying iridium in Neoproterozoic rocks found evidence favoring the snowball Earth theory. The geochemists found that the top few centimeters of rock above the Neoproterozoic glacial deposits were unusually rich in iridium, an element that is more abundant in meteorites than in Earth rocks. The iridium layer could be explained if meteor dust accumulated on top of a world-covering ice layer for about 12 million years, then was released quickly as the ice melted, sinking to become part of the geological record.

Some biologists believe that the snowball Earth phase of the Neoproterozoic, followed by intense greenhouse warming, may have triggered the Cambrian explosion. The Cambrian explosion, which is the geologically sudden (over mere tens of millions of years) appearance of multicellular life that occurred about 600 million years ago, starting roughly with the beginning of the Cambrian period (542-488 million years ago). Before the Cambrian, for about three billion years—more than three-quarters of the history of life on the planet—Earth was populated only by single-celled organisms. Prolonged isolation of vent-dwelling organisms and intense selective pressure from changing conditions—first a very cold Earth, then a very hot one—may have encouraged the development of multicellular life.

See Also Continental Drift; Paleoclimatology.