Ediacaran

For other uses, see Ediacara.
Ediacaran Period
635–541 million years ago

Mean atmospheric O
2 content over period duration		 c. 8 vol %[1]
(40 % of modern level)
Mean atmospheric CO
2 content over period duration		 c. 4500 ppm[2]
(16 times pre-industrial level)
Events of the Ediacaran Period
view • discuss • 
-650 —
–
-640 —
–
-630 —
–
-620 —
–
-610 —
–
-600 —
–
-590 —
–
-580 —
–
-570 —
–
-560 —
–
-550 —
–
-540 —
–
-530 —
–
-520 —
N
e
o
p
r
o
t
e
r
o
z
o
i
c
P
Z
Cryogenian
Ediacaran
Cambrian
 
 
Ediacaran
Biota
* * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * *
←
First Trilobites
←
Treptichnus pedum
Large negative peak δ 13Ccarb excursion
←
Baykonur glaciation
←
Gaskiers glaciation
Archaeonassa-type trace fossils
←
Nantuo (Marinoan) glaciation
Stratigraphic scale of the ICS subdivisions and Precambrian/Cambrian boundary.

The Ediacaran Period /iːdiˈækərən/, named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon.

The Ediacaran Period's status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS), making it the first new geological period declared in 120 years.[3][4][5] Although the Period takes its name from the Ediacara Hills where geologist Reg Sprigg first discovered fossils of the eponymous Ediacara biota in 1946, the type section is located in the bed of the Enorama Creek[6] within Brachina Gorge[7] in the Flinders Ranges of South Australia, at 31°19′53.8″S 138°38′0.1″E / 31.331611°S 138.633361°E / -31.331611; 138.633361.

Ediacaran and Vendian

See also: Jotnian

The Ediacaran Period overlaps, but is shorter than the Vendian Period, a name that was earlier, in 1952, proposed by Russian geologist and paleontologist Boris Sokolov. The Vendian concept was formed stratigraphically top-down, and the lower boundary of the Cambrian became the upper boundary of the Vendian.[8][9]

Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern European Platform[10]) and for the carbonate basin (base of the Tommotian Stage of the Siberian Platform).[11] The lower boundary of the Vendian was suggested to be defined at the base of the Varanger (Laplandian) tillites.[9][12]

The Vendian in its type area consists of large subdivisions such as Laplandian, Redkino, Kotlin and Rovno Regional stages with the globally traceable subdivisions and their boundaries, including its lower one.

The Redkino, Kotlin and Rovno regional stages have been substantiated in the type area of the Vendian on the basis of the abundant organic-walled microfossils, megascopic algae, metazoan body fossils and ichnofossils.[9][13]

The lower boundary of the Vendian could have a biostratigraphic substantiation as well taking into consideration the worldwide occurrence of the Pertatataka assemblage of giant acanthomorph acritarchs.[12]

Boundaries of the Ediacaran

The 'golden spike' (bronze disk in the lower section of the image) or 'type section' of the Global Boundary Stratotype Section and Point (GSSP) for the base of Ediacaran period.
The 'golden spike' marking the GSSP
Elatina Fm diamictite below Ediacaran GSSP site in the Flinders Ranges NP, South Australia. A$1 coin for scale.

The Ediacaran Period (ca. 635-542 Mya) represents the time from the end of global Marinoan glaciation to the first appearance worldwide of somewhat complicated trace fossils (Treptichnus pedum (Seilacher, 1955)).[3]

Although the Ediacaran Period does contain soft-bodied fossils, it is unusual in comparison to later periods because its beginning is not defined by a change in the fossil record. Rather, the beginning is defined at the base of a chemically distinctive carbonate layer that is referred to as a "cap carbonate," because it caps glacial deposits.

This bed is characterized by an unusual depletion of 13C that indicates a sudden climatic change at the end of the Marinoan ice age. The lower boundary GSSP of the Ediacaran is at the base of the cap carbonate (Nuccaleena Formation), immediately above the Elatina diamictite in the Enorama Creek section, Brachina Gorge, Flinders Ranges, South Australia.

The GSSP of the upper boundary of the Ediacaran is the lower boundary of the Cambrian on the SE coast of Newfoundland approved by the International Commission on Stratigraphy as a preferred alternative to the base of the Tommotian Stage in Siberia which was selected on the basis of the ichnofossil Treptichnus pedum (Seilacher, 1955). In the history of stratigraphy it was the first case of usage of bioturbations for the System boundary definition.

Nevertheless, the definitions of the lower and upper boundaries of the Ediacaran on the basis of chemostratigraphy and ichnofossils are disputable.[12][14]

Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation) and usually siliciclastic sediments replace laterally the cap carbonates in a rather short distance and cap carbonates do not occur above every tillite elsewhere in the world.

The C-isotope chemostratigraphic characteristics obtained for contemporaneous cap carbonates in different parts of the world may be variable in a wide range owing to different degrees of secondary alteration of carbonates, dissimilar criteria used for selection of the least altered samples, and, as far as the C-isotope data are concerned, due to primary lateral variations of δ l3Ccarb in the upper layer of the ocean.[12][15]

Furthermore, Oman presents in its stratigraphic record a large negative carbon isotope excursion, within the Shuram[16] Formation that is clearly away from any glacial evidence[17] strongly questioning systematic association of negative δ l3Ccarb excursion and glacial events.[18]

As to the Treptichnus pedum, a reference ichnofossil for the lower boundary of the Cambrian, its usage for the stratigraphic detection of this boundary is always risky, because of the occurrence of very similar trace fossils belonging to the Treptichnids group well below the level of T. pedum in Namibia, Spain and Newfoundland, and possibly, in the western United States. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.[12][19]

Absolute dating

No dating has been possible at the type section of the Ediacaran Period in South Australia. Therefore, the age range of 635 to 542 million years before the present is based on correlations to other countries where dating has been possible. The base age of approximately 635 million years ago is based on U-Pb (uranium-lead) isochron dating from Namibia[20] and China.[21]

Applying this age to the base of the Ediacaran assumes that individual cap carbonates are synchronous around the world and that the correct cap carbonate layers have been correlated between Australian and Namibia. This is controversial because an age of about 580 million years has been obtained in association with glacial rocks in Tasmania which some scientists tentatively correlate with those just beneath the Ediacaran rocks of the Flinders Ranges.[22] The age of the top is the same as the widely recognised age for the base of the Cambrian Period[23] 542± 0.3 Mya (million years ago).[24]

Biota

Main article: Ediacara biota

The fossil record from this period is sparse, as more easily fossilized hard-shelled animals had yet to evolve. The Ediacaran biota include the oldest definite multicellular organisms with tissues, and the most common types resemble segmented worms, fronds, disks, or immobile bags.

Ediacara biota bear little resemblance to modern lifeforms, and their relationship even with the later lifeforms of the Cambrian explosion is rather difficult to interpret. More than 100 genera have been described, and well known forms include Arkarua, Charnia, Dickinsonia, Ediacaria, Marywadea, Onega, Pteridinium, and Yorgia.

There is evidence that Earth's first mass extinction happened during this period when early animals changed the environment.[25]

Documentaries

A few English language documentaries have featured the Ediacaran era and biota:

See also

References

  1. ↑ Image:Sauerstoffgehalt-1000mj.svg
  2. ↑ Image:Phanerozoic Carbon Dioxide.png
  3. 1 2 A. Knoll, M. Walter, G. Narbonne, and N. Christie-Blick (2004) "The Ediacaran Period: A New Addition to the Geologic Time Scale." Submitted on Behalf of the Terminal Proterozoic Subcommission of the International Commission on Stratigraphy.
  4. ↑ Knoll, A. H.; Walter, MR; Narbonne, G. M; Christie-Blick, N (2004). "A new period for the geologic time scale" (PDF). Science 305 (5684): 621–622. doi:10.1126/science.1098803. PMID 15286353.
  5. ↑ Knoll, A. H., Walter, M. R., Narbonne, G. M., and Christie-Blick, N. (2006). "The Ediacaran Period: A new addition to the geologic time scale" (PDF). Lethaia 39: 13–30. doi:10.1080/00241160500409223.
  6. ↑ "Geological time gets a new period: Geologists have added a new period to their official calendar of Earth's history—the first in 120 years". London: BBC. 2004-05-17. Accessed 27 December 2010.
  7. ↑ South Australian Museum Newsletter April 2005 Accessed 9 August 2010.
  8. ↑ B. M. Sokolov (1952). "On the age of the old sedimentary cover of the Russian Platform". Izvestiya Akademii Nauk SSSR, Seriya eologicheskaya 5: 21–31.
  9. 1 2 3 Sokolov, B.S. (1997). "Essays on the Advent of the Vendian System." 153 pp. KMK Scientific Press, Moscow. (in Russian)
  10. ↑ Sokolov B. S. (1965) "Abstracts of All-Union Symposium on Paleontology of the Precambrian and Early Cambrian." Nauka, Novosibirsk.
  11. ↑ Rozanov, A.Y., Missarzhevskij, V.V., Volkova, N.A., Voronova, L.G., Krylov, I.N., Keller, B.M., Korolyuk, I.K., Lendzion, K., Michniak, R., Pykhova, N.G., and Sidorov, A.D. (1969). "The Tommotian Stage and the problem of the lower boundary of the Cambrian". Trudy Geologičeskogo Instituta AN SSSR 206: 1–380.
  12. 1 2 3 4 5 M. A. Fedonkin, B. S. Sokolov, M. A. Semikhatov, N. M. Chumakov (2007). "Vendian versus Ediacaran: priorities, contents, prospectives". Archived from the original on May 9, 2013. In: "The Rise and Fall of the Vendian (Ediacaran) Biota" (PDF). Origin of the Modern Biosphere. Transactions of the International Conference on the IGCP Project 493n Moscow: GEOS. August 20–31, 2007. (82mb)
  13. ↑ Khomentovsky, V. V. (2008). "The Yudomian of Siberia, Vendian and Ediacaran systems of the International stratigraphic scale". Stratigraphy and Geological Correlation 16 (6): 581–598. Bibcode:2008SGC....16..581K. doi:10.1134/S0869593808060014.
  14. ↑ Comments By B. S. Sokolov, M. A. Semikhatov, And M. A. Fedonkin. (2004) Appendix 2 in: "The Ediacaran Period: A New Addition to the Geologic Time Scale." Submitted on Behalf of the Terminal Proterozoic Subcommission of the International Commission on Stratigraphy. pp. 32–34
  15. ↑ Bristow, T. F. and Kennedy, M. J. (2008). "Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean" (PDF). Geology 36 (11): 863–866. Bibcode:2008Geo....36..863B. doi:10.1130/G24968A.1. Retrieved 2007-05-05.
  16. ↑ Le Guerroué, E., Allen, P. A., Cozzi, A. (2006). "Chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in Earth history: The Neoproterozoic Shuram Formation (Nafun Group, Oman).". Precambrian Research 146 (1–2): 68–92. doi:10.1016/j.precamres.2006.01.007.
  17. ↑ Le Guerroué, E.; Allen, P. A.; Cozzi, A.; Etienne, J. L.; Fanning, C. M. (2006). "50 Myr recovery from the largest negative δ13C excursion in the Ediacaran ocean". Terra Nova 18 (2): 147–153. doi:10.1111/j.1365-3121.2006.00674.x.
  18. ↑ Le Guerroué, E.; Allen, P. A.; Cozzi, A. (2006). "Parasequence development in the Ediacaran Shuram Formation (Nafun Group, Oman): primary origin stratigraphic test of negative carbon isotopic ratios". Basin Research 18 (2): 205–220. doi:10.1111/j.1365-2117.2006.00292.x.
  19. ↑ A. Ragozina, D. Dorjnamjaa, A. Krayushkin, E. Serezhnikova (2008). "Treptichnus pedum and the Vendian-Cambrian boundary". 33 Intern. Geol. Congr. August 6–14, 2008, Oslo, Norway. Abstracts. Section HPF 07 Rise and fall of the Ediacaran (Vendian) biota. P. 183.
  20. ↑ Hoffmann, K.H.; Condon, D.J.; Bowring, S.A.; Crowley, J.L. (2004-09-01). "U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: Constraints on Marinoan glaciation". Geology 32 (9): 817–820. Bibcode:2004Geo....32..817H. doi:10.1130/G20519.1.
  21. ↑ Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., and Jin, Y. (1 April 2005). "U-Pb Ages from the Neoproterozoic Doushantuo Formation, China" (abstract). Science 308 (5718): 95–98. Bibcode:2005Sci...308...95C. doi:10.1126/science.1107765. PMID 15731406.
  22. ↑ Calver, C.R.; Black, Lance P.; Everard, John L.; Seymour, David B. (2004-10-01). "U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania". Geology 32 (10): 893–896. Bibcode:2004Geo....32..893C. doi:10.1130/G20713.1.
  23. ↑ Ogg, J. G. (2004). "Status of Divisions of the International Geologic Time Scale" (PDF). Lethaia 37 (2): 183–199. doi:10.1080/00241160410006492. Retrieved 2007-05-05.
  24. ↑ Amthor, J. E.; Grotzinger, John P.; Schröder, Stefan; Bowring, Samuel A.; Ramezani, Jahandar; Martin, Mark W.; Matter, Albert (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology 31: 431–434. Bibcode:2003Geo....31..431A. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2.
  25. ↑ http://www.sciencedaily.com/releases/2015/09/150902123456.htm Science Daily

External links

Wikimedia Commons has media related to Ediacaran.
This article is issued from Wikipedia - version of the Sunday, January 03, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.