Cryogenian

Cryogenian Period 850–635 million years ago PreЄ Є O S D C P T J K Pg N

Events of the Cryogenian Period view • discuss •  -860 — – -840 — – -820 — – -800 — – -780 — – -760 — – -740 — – -720 — – -700 — – -680 — – -660 — – -640 — – -620 — – Cryogenian Ediacaran Tonian   N e o p r o t e r o z o i c         Marinoan Sturtian ← First sponge-like animal. ← Kaigas glaciation? ← First demosponges.  Neoproterozoic  Cryogenian  Major Glacial period

The Cryogenian (from Greek cryos "cold" and genesis "birth") is a geologic period that lasted from 850 to 635 million years ago. It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran. The Sturtian and Marinoan glaciations, 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 involves whether these glaciations covered the entire planet (the 'Snowball Earth') or a band of open sea survived near the equator (the 'slushball Earth').

Ratification

The period has not received the international ratification that most geological time periods undergo (the most recent being the Ediacaran Period, which was ratified in 2004). The start of the period is defined only on the ages of the rocks and not on any observable and documented global event. 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 (541.0 ± 1.0 million years), but by the appearance of the worldwide Treptichnus pedum diagnostic trace fossil assemblage. This means that rocks can be recognised as Cambrian when examined in the field and do not require extensive testing to be performed in a lab to find a date. As yet, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, and its base is only loosely set to 850 million years.

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. It 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. 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".

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. During the Cryogenian period, the oldest known fossils of sponges (and therefore animals) make an appearance.

Further reading

• "Cryogenian Period". GeoWhen Database. Archived from the original on December 2, 2005. Retrieved January 5, 2006. 
• James G. Ogg (2004). "Status on Divisions of the International Geologic Time Scale". Lethaia 37 (2): 183–199. doi:10.1080/00241160410006492. 
• Brain, C. K.; Prave, A. R.; Hoffmann, K. H.; Fallick, A. E.; Herd, D. A. Sturrock, C.; Young, I.; Condon, D. J. Allison, S. G. (2012). "The first animals: ca. 760-million-year-old sponge-like fossils from Namibia". South African Journal of Science S. Afr. J. Sci. 108: 1–8. doi:10.4102/sajs.v108i1/2.658. 
• 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