Cryogenian

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Cryogenian
c. 720 – c. 635 Ma
Marinoan glaciation[1][2]
←
Transition from cyanobacterial to algal dominated world.[3]
←
Events of the Cryogenian period
Axis Scale: Millions of Years Ago (
PeriodStratigraphic unitSystemTime span formalityFormalLower boundary definitionDefined chronometrically with an interim calibrated age of c. 720 Ma. GSSP is in progress.Lower boundary definition candidatesThe first appearance of widespread glaciation.[4]
Lower boundary GSSP candidate section(s)To be determinedUpper boundary definition
  • Worldwide distinct cap carbonates.
  • Beginning of a distinctive pattern of secular changes in
    carbon isotopes
    .
Upper boundary GSSPEnorama Creek section,
ppm

(5 times pre-industrial)Mean surface temperaturec. 5 °C
(9 °C below modern)

The Cryogenian (from

geologic period that lasted from 720 to 635 million years ago.[6] It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran
.

The Cryogenian was a time of drastic biosphere changes. After the previous

slushball Earth
").

Ratification

The Cryogenian Period was ratified in 1990 by the International Commission on Stratigraphy.[7] 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,[8] but changed in 2015 to 720 million years.[6]

This could cause ambiguity because estimates of rock ages are variable and are subject to laboratory error. For instance, the time scale of the

Treptichnus pedum diagnostic trace fossil assemblages. This means that rocks can be recognized as Cambrian in the field, without extensive lab testing.[citation needed
]

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.[8]

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.[9]

Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.[10]

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

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".[12] Between the Sturtian and Marinoan glaciations was a so-called "Cryogenian interglacial period" marked by relatively warm climate and anoxic oceans,[13] along with marine transgression.[14]

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.[citation needed
]

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

Adelaide Rift Complex.[citation needed
]

Cryogenian biota and fossils

Between the Sturtian and Marinoan glaciations, global biodiversity was very low.[13]

Fossils of

animals, were formed.[16][17][18] However, it is unclear whether these fossils actually belong to sponges, though the authors do not rule out the possibility of such fossils to represent proto-sponges or complex microbial precursors to sponge-grade organisms.[19]
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.[20]

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.[21]

See also

References

  1. ^
    doi:10.1144/M36.1
    .
  2. ^
    PMID 29134193
    .
  3. ISSN 2397-8554
    .
  4. ISSN 0705-3797
    .
  5. doi:10.1080/00241160500409223
    . Retrieved 6 December 2020.
  6. ^ a b "Chart". International Commission on Stratigraphy. Archived from the original on 13 January 2017. Retrieved 14 February 2017.
  7. doi:10.18814/epiiugs/1991/v14i2/005
    . Retrieved 7 September 2013.
  8. ^ a b "GSSP Table - Precambrian". Geologic Timescale Foundation. Retrieved 7 September 2013.
  9. ^
    ISBN 0521548039
    .
  10. ^ Dave Lawrence (2003). "Microfossil lineages support sloshy snowball Earth". Geotimes.
  11. doi:10.1038/ngeo214
    .
  12. ^ Hoffman, P.F. 2001. Snowball Earth theory
  13. ^
    PMID 31636318
    .
  14. S2CID 239629114
    . Retrieved 11 September 2023.
  15. S2CID 54636062
    .
  16. S2CID 4314662. Archived from the original
    (PDF) on 2018-05-08. Retrieved 2009-04-15.
  17. doi:10.1038/ngeo934
    .
  18. ^ "Discovery of possible earliest animal life pushes back fossil record". 2010-08-17.
  19. hdl:11343/52679
    .
  20. ^ "Palaeos Proterozoic: Neoproterozoic: Cryogenian".
  21. ^ Fossil fats reveal how complex life kicked off after Snowball Earth phase

Further reading

External links
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