Cryogenian

The Cryogenian ( /kraɪoʊˈdʒɛniən/, from Greek κρύος (krýos), meaning "cold" and γένεσις (génesis), meaning "birth") is a geologic period that lasted from 720 to 635 million years ago.[4] It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran.

Cryogenian Period
720–635 million years ago

A map of the world as it appeared during the mid-Cryogenian. (690 ma)

Mean atmospheric O
2 content over period duration		 c. 12 vol %
(60 % of modern level)
Mean atmospheric CO
2 content over period duration		 c. 1300 ppm
(5 times pre-industrial level)
Mean surface temperature over period duration c. 5 °C
(-9 °C above modern level)
Events of the Cryogenian Period
-720 —
–
-710 —
–
-700 —
–
-690 —
–
-680 —
–
-670 —
–
-660 —
–
-650 —
–
-640 —
–
-630 —
←
Events of the Cryogenian period
Axis Scale: Millions of Years Ago (Ma).

The Sturtian and Marinoan glaciations occurred during the Cryogenian period,[5] which are the greatest ice ages known to have occurred on Earth. 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 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 was originally set at 850 million years,[7] but changed in 2015 to 720 million years.[4]

This is problematic because estimates of rock ages are variable and are subject to laboratory error. For instance, the time scale of the Cambrian Period is not reckoned by rock younger than a given age (541 million years), but by the appearance of the worldwide Treptichnus pedum diagnostic trace fossil assemblages. 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). According to Eyles and Young, "Late Proterozoic glaciogenic deposits are known from all the continents. They provide evidence of the most widespread and long-ranging glaciation on Earth." Several glacial periods are evident, interspersed with periods of relatively warm climate, with glaciers reaching sea level in low paleolatitudes.[8]

Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[9]

Diamictite of the Elatina Formation in South Australia, formed during the Marinoan glaciation of the late Cryogenian

The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 720 to 660 million years ago, and the Marinoan glaciation which ended approximately 635 Ma, at the end of the Cryogenian.[10] 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".[11]

Paleogeography
Main article: Rodinia § Geodynamics

Before the start of the Cryogenian, around 750 Ma, the cratons that made up the supercontinent Rodinia started to rift apart. The superocean Mirovia began to close while the superocean Panthalassa began to form. The cratons (possibly) later assembled into another supercontinent called Pannotia, in the Ediacaran.

Eyles and Young state, "Most Neoproterozoic glacial deposits accumulated as glacially influenced marine strata along rifted continental margins or interiors." Worldwide deposition of dolomite might have reduced atmospheric carbon dioxide. The break up along the margins of Laurentia at about 750 Ma occurs at about the same time as the deposition of the Rapitan Group in North America, contemporaneously with the Sturtian in Australia. A similar period of rifting at about 650 Ma occurred with the deposition of the Ice Brook Formation in North America, contemporaneously with the Marinoan in Australia.[8] The Sturtian and Marinoan are local divisions within the Adelaide Rift Complex.

Cryogenian biota and fossils

Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian period.[12] During the Cryogenian period, the oldest known fossils of sponges (and therefore animals) make an appearance.[13][14][15] The issue of whether or not biology was impacted by this event has not been settled, for example Porter (2000) suggests that new groups of life evolved during this period, including the red algae and green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba.[16] The end of the period also saw the origin of heterotrophic plankton, which would feed on unicellular algae and prokaryotes, ending the bacterial dominance of the oceans.[17]

gollark: I feel like they could store it as not YAML.
gollark: https://github.com/fish-shell/fish-shell/issues/2674
gollark: I think it has a size limit?
gollark: It saddens me that it doesn't keep shell history forever, though.
gollark: fish has that built-in. It's quite good.

See also

References
  1. Arnaud, Emmanuelle; Halverson, Galen P.; Shields-Zhou, Graham Anthony (30 November 2011). "Chapter 1 The geological record of Neoproterozoic ice ages". Memoirs. Geological Society of London. 36 (1): 1–16. doi:10.1144/M36.1.
  2. Pu, Judy P.; Bowring, Samuel A.; Ramezani, Jahandar; Myrow, Paul; Raub, Timothy D.; Landing, Ed; Mills, Andrea; Hodgin, Eben; MacDonald, Francis A. (2016). "Dodging snowballs: Geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota". Geology. 44 (11): 955. doi:10.1130/G38284.1.
  3. "Press release: Discovery of Possible Earliest Animal Life Pushes Back Fossil Record". National Science Foundation. August 17, 2010.
  4. "Chart". International Commission on Stratigraphy. Archived from the original on 13 January 2017. Retrieved 14 February 2017.
  5. These events were formerly considered together as the Varanger glaciations, from their first detection in Norway's Varanger Peninsula.
  6. Plumb, Kenneth A. (1991). "New Precambrian time scale" (PDF). Episode. 2. 14: 134–140. Retrieved 7 September 2013.
  7. "GSSP Table - Precambrian". Geologic Timescale Foundation. Retrieved 7 September 2013.
  8. Eyles, Nicholas; Young, Grant (1994). Deynoux, M.; Miller, J.M.G.; Domack, E.W.; Eyles, N.; Fairchild, I.J.; Young, G.M. (eds.). Geodynamic controls on glaciation in Earth history, in Earth's Glacial Record. Cambridge: Cambridge University Press. pp. 5–10. ISBN 0521548039.
  9. Dave Lawrence (2003). "Microfossil lineages support sloshy snowball Earth". Geotimes.
  10. Shields, G. A. (2008). "Palaeoclimate: Marinoan meltdown". Nature Geoscience. 1 (6): 351–353. Bibcode:2008NatGe...1..351S. doi:10.1038/ngeo214.
  11. Hoffman, P.F. 2001. Snowball Earth theory
  12. Porter, S.A. & Knoll, A.H. (2000). "Testate amoeba in the Neoproterozoic Era: evidence from vase-shaped microfossils in the Chuar Group, Grand Canyon". Paleobiology. 26 (3): 360–385. doi:10.1666/0094-8373(2000)026<0360:TAITNE>2.0.CO;2. ISSN 0094-8373.
  13. Love; Grosjean, Emmanuelle; Stalvies, Charlotte; Fike, David A.; Grotzinger, John P.; Bradley, Alexander S.; Kelly, Amy E.; Bhatia, Maya; Meredith, William; et al. (2009). "Fossil steroids record the appearance of Demospongiae during the Cryogenian period" (PDF). Nature. 457 (7230): 718–721. Bibcode:2009Natur.457..718L. doi:10.1038/nature07673. PMID 19194449.
  14. Maloof, Adam C.; Rose, Catherine V.; Beach, Robert; Samuels, Bradley M.; Calmet, Claire C.; Erwin, Douglas H.; Poirier, Gerald R.; Yao, Nan; Simons, Frederik J. (17 August 2010). "Possible animal-body fossils in pre-Marinoan limestones from South Australia". Nature Geoscience. 3 (9): 653–659. Bibcode:2010NatGe...3..653M. doi:10.1038/ngeo934.
  15. "Discovery of possible earliest animal life pushes back fossil record". 2010-08-17.
  16. http://palaeos.com/proterozoic/neoproterozoic/cryogenian/cryogenian2.html
  17. Fossil fats reveal how complex life kicked off after Snowball Earth phase

Further reading
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