Editing Ediacaran (section)

==Upper and lower boundaries==
[[File:Ediacaran-GSSP-IMGP6652-.jpg|thumb|250px|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 the Ediacaran System]]
[[File:Ediacaran GSSP - closeup.JPG|thumb|The 'golden spike' marking the GSSP]]
The Ediacaran Period (c. 635–538.8 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)).<ref name=Knoll2004a>A. Knoll, M. Walter, G. Narbonne, and N. Christie-Blick (2004) "[http://www.stratigraphy.org/bak/ediacaran/Knoll_et_al_2004a.pdf 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.</ref>

Although the Ediacaran Period does contain soft-bodied [[fossil]]s, 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 <sup>13</sup>[[Carbon|C]] that indicates a sudden climatic change at the end of the [[Marinoan]] [[ice age]]. The lower [[GSSP|global boundary stratotype section (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 [[ichnofossil]]s are disputable.<ref name=Fedonkin2007/><ref name=Comments2004>Comments By B. S. Sokolov, M. A. Semikhatov, And M. A. Fedonkin. (2004) Appendix 2 in: "[http://www.stratigraphy.org/bak/ediacaran/Knoll_et_al_2004a.pdf 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</ref>

Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation){{vague|date=June 2013}} and usually siliciclastic sediments laterally replace the cap carbonates in a rather short distance but cap carbonates do not occur above every tillite elsewhere{{clarify|reason=location not specified|date=May 2013}} 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 <sup>l3</sup>C<sub>carb</sub> in the upper layer of the ocean.<ref name=Fedonkin2007/><ref name= Bristow2008>{{cite journal
 |author1      = Bristow, T. F.
 |author2      = Kennedy, M. J.
 |year         = 2008
 |title        = Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean
 |journal      = Geology
 |volume       = 36
 |issue        = 11
 |pages        = 863–866
 |url          = http://taylorandfrancis.metapress.com/index/PUNM6BQLNW1BJMFN.pdf
 |access-date  = 2007-05-05
 |doi          = 10.1130/G24968A.1
 |bibcode      = 2008Geo....36..863B
 |archive-date = 7 May 2020
 |archive-url  = https://web.archive.org/web/20200507214320/http://taylorandfrancis.metapress.com/index/PUNM6BQLNW1BJMFN.pdf
 |url-status   = dead
}}</ref>

Furthermore, [[Oman]] presents in its stratigraphic record a large negative carbon isotope excursion, within the Shuram<ref name="LeGuerroue2006a">{{cite journal
|author1=Le Guerroué, E. |author2=Allen, P. A. |author3=Cozzi, A. | year = 2006
| title =Chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in Earth history: The Neoproterozoic Shuram Formation (Nafun Group, Oman).
| journal = Precambrian Research
| volume = 146
| issue = 1–2
| pages = 68–92
| doi = 10.1016/j.precamres.2006.01.007
|bibcode=2006PreR..146...68L}}</ref> Formation that is clearly away from any glacial evidence<ref name=LeGuerroue2006b>{{cite journal
| author = Le Guerroué, E. |author2=Allen, P. A. |author3=Cozzi, A. |author4=Etienne, J. L. |author5=Fanning, C. M.
| year = 2006
| title =50 Myr recovery from the largest negative ''δ''<sup>13</sup>C excursion in the Ediacaran ocean
| journal = Terra Nova
| volume = 18
| issue = 2
| pages = 147–153
| doi = 10.1111/j.1365-3121.2006.00674.x
| url = http://www3.interscience.wiley.com/journal/118624047/abstract
| archive-url = https://archive.today/20130105074949/http://www3.interscience.wiley.com/journal/118624047/abstract
| url-status = dead
| archive-date = 2013-01-05
|bibcode=2006TeNov..18..147L|s2cid=140710102 }}</ref> strongly questioning systematic association of negative δ <sup>l3</sup>C<sub>carb</sub> excursion and glacial events.<ref name=LeGuerroue2006c>{{cite journal
| author = Le Guerroué, E.
|author2=Allen, P. A. |author3=Cozzi, A. 
| year = 2006
| title =Parasequence development in the Ediacaran Shuram Formation (Nafun Group, Oman): primary origin stratigraphic test of negative carbon isotopic ratios
| journal = Basin Research
| volume = 18
| pages = 205–220
| doi = 10.1111/j.1365-2117.2006.00292.x 
| url = http://www3.interscience.wiley.com/journal/118629748/abstract
| archive-url = https://archive.today/20130105102240/http://www3.interscience.wiley.com/journal/118629748/abstract
| url-status = dead
| archive-date = 2013-01-05
| issue = 2
| bibcode =2006BasR...18..205L|s2cid=128910191 }}</ref> Also, the [[Shuram excursion]] is prolonged and is estimated to last for ~9.0 Myrs.<ref>{{Cite journal|title=Rock magnetic cyclostratigraphy of the Doushantuo Formation, South China and its implications for the duration of the Shuram carbon isotope excursion|last1=Gong|first1=Zheng|last2=Kodama|first2=Kenneth|year=2017|last3=Li|first3=Yong-Xiang|doi=10.1016/j.precamres.2016.12.002|volume=289|journal=Precambrian Research|pages=62–74|bibcode=2017PreR..289...62G}}</ref>

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 (island)|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.<ref name=Fedonkin2007/><ref name= Ragozina2007>A. Ragozina, D. Dorjnamjaa, A. Krayushkin, E. Serezhnikova (2008). "[http://vendian.net76.net/Treptichnus_pedum.htm ''Treptichnus pedum'' and the Vendian-Cambrian boundary] {{Webarchive|url=https://web.archive.org/web/20111004184532/http://vendian.net76.net/Treptichnus_pedum.htm |date=4 October 2011 }}". 33 Intern. Geol. Congr. August 6–14, 2008, Oslo, Norway. Abstracts. Section HPF 07 Rise and fall of the Ediacaran (Vendian) biota. P. 183.</ref>