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Cryogenian period

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Cryogenian period

Cryogenian Period
850–635 million years ago
Template:Infobox time scale



Events of the Cryogenian Period
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First sponge-like animal.[2]
Kaigas glaciation?[3]

The Cryogenian (from Ediacaran.

The Sturtian and Marinoan glaciations,[5] which are the greatest ice ages known to have occurred on Earth, occurred during this period. These events are the subject of much scientific controversy.

The main debate contests whether these glaciations covered the entire planet (the so-called 'Snowball Earth') or if a band of open sea survived near the equator (termed 'slushball Earth').

Ratification

The Cryogenian period was ratified in 1990 by the International Commission on Stratigraphy.[6] In contrast to most other time periods the beginning of the Cryogenian is not linked to a globally observable and documented event. Instead the base of the period is defined by a fixed rock age, that is set at 850 million years.[7]

This is problematic as estimates of rock ages are variable and are subject to laboratory error. For instance, the Cambrian Period is marked not by rock younger than a given age (Template:Geologic Ages Inline million years), but by the appearance of the worldwide Treptichnus pedum diagnostic trace fossil assemblage. This means that rocks can be recognized as Cambrian when examined in the field and do not require extensive testing to be performed in a lab to find a date.

Currently, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, but a global glaciation would be a likely candidate.[7]

Climate

The name of the geologic period refers to the very cold global climate of the Cryogenian: characteristic glacial deposits indicate that Earth suffered the most severe ice ages in its history during this period (Sturtian and Marinoan).

Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[8]

The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 750 million years ago to 700 Ma, and the Marinoan glaciation which ended approximately 635 Ma.[9] The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply frozen planetary oceans called "Snowball Earth".[10]

Paleogeography

During the Cryogenian, the supercontinent Rodinia broke up, and the supercontinent Pannotia began to form.

Cryogenian biota and fossils

Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian period.[11] During the Cryogenian period, the oldest known fossils of sponges (and therefore animals) make an appearance.[12][13][14]

In popular culture

The Australian TV series A Time Travellers Guide to Australia known in the US as Australia the First 4 Billion Years covers the cryogenian era and its remnant evidence in Australian rocks.

There are at least two BBC Horizon documentaries that cover the Snowball earth cryogenian era.

References

Further reading

  • The Cryogenian Period
Preceded by Archean Eon 2.5 Ga - Proterozoic Eon - 542 Ma Followed by Phanerozoic Eon
2.5 Ga - Paleoproterozoic Era -1.6 Ga 1.6 Ga - Mesoproterozoic Era -1.0 Ga 1.0 Ga - Neoproterozoic Era -542 Ma
Siderian Rhyacian Orosirian Statherian Calymmian Ectasian Stenian Tonian Cryogenian Ediacaran