The Cryogenian period, part of the Proterozoic Era, had at least two major ice ages and may have had a “snowball Earth” scenario. Glaciers may have extended from pole to pole, and the average global temperature fell by at least 20°C. Life existed before and during the period, but diversity decreased.
The Cryogenian period is a geological period from 850 million to 630 million years ago. It occurred before the Ediacaran Period and after the Tonic Period and is part of the much longer Proterozoic Era, meaning “early life era”. In Greek, Cryogenian means “origin of ice”. Although ice did not originate with the Cryogenian period, it was everywhere and glaciers may have extended from pole to pole. A human transported at that moment could have skied around the earth. The Cryogenian Period is one of the few geological periods in the last billion years to be named after a substance or concept (in this case cold) rather than a modern area where fossils of the period are found (e.g., the Jurassic is named after the Jura).
During the Cryogenian period there were at least two major ice ages, and possibly as many as four. Glacial deposits in the Cryogenian strata at the equatorial paleolatitiudias (in particular, the Congo and Kalahari cratons) have led many scientists to consider the possibility of a “snowball Earth” – a planet so cold that the oceans have frozen. This has led to a great deal of controversy in the scientific community. Many scientists doubt the geophysical feasibility of a completely frozen ocean. Simulations have been performed, but such calculations push the limits of available computing power and radical simplification assumptions are sometimes required. One study ignores the existence of continents, for example. Some scientists take a middle ground and argue for a “Slushball Earth” scenario, in which the oceans are covered in large amounts of sea ice, but not frozen all the way through.
The two major glaciations confirmed in the Cryogenian period were the Sturtian glaciation (760 million years ago at 700 mya) and the Marinoan/Varanger glaciation (710/650 at 635 Ma). During these glaciations, the average global temperature would have fallen by at least 20°C, from 22°C (71°F) to about 2°C (37°F), and perhaps much lower, to below -30°C ( -22°F). The temperature at the poles may have been so low that the carbon dioxide could have frozen into dry ice, which has a freezing point of -78.5°C (-109.3°F). Dry ice is the main constituent of the polar ice caps of Mars.
Life existed long before the Cryogen period and obviously survived through it. Plants, animals and fungi all existed, albeit almost exclusively in unicellular form. There is evidence of small burrows under microbial mats, perhaps made by simple multicellular organisms. Small organic fossils called acritarchs were found in large numbers and diversity before the Cryogenian period, but both crashed during the period. Acritarchs have been isolated from Cryogen sediments, they just aren’t very numerous or diverse.
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