Editing Cryogenian (section)

==Ratification==
The Cryogenian Period was ratified in 1990 by the [[International Commission on Stratigraphy]].<ref name="Plumb-1991">{{cite journal |last=Plumb |first=Kenneth A. |title=New Precambrian time scale |journal=Episodes |year=1991 |volume=14 |series=2 |issue=2 |pages=134–140 |doi=10.18814/epiiugs/1991/v14i2/005 |access-date=7 September 2013 |url=http://www.stratigraphy.org/bak/Precambrian.pdf |doi-access=free }}</ref> 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&nbsp;million years,<ref name="GSSPTable">{{cite web |title=GSSP Table - Precambrian |url=https://engineering.purdue.edu/Stratigraphy/gssp/index.php?parentid=181 |publisher=Geologic Timescale Foundation |access-date=7 September 2013}}</ref> but changed in 2015 to 720&nbsp;million years.<ref name="ICS">{{cite web |url=http://www.stratigraphy.org/index.php/ics-chart-timescale |title=Chart |publisher=International Commission on Stratigraphy |access-date=2017-02-14 |url-status=dead |archive-url=https://web.archive.org/web/20170113013553/http://www.stratigraphy.org/index.php/ics-chart-timescale |archive-date=2017-01-13 |df=dmy-all}}</ref>

This could cause ambiguity because estimates of rock age are subject to variable interpretation and laboratory error. For instance, the time scale of the [[Cambrian]] Period is not reckoned by rock younger than a given age ({{Period start|Cambrian}} million years), but by the appearance of the worldwide ''[[Treptichnus pedum]]'' diagnostic [[trace fossil]] assemblages, which can be recognized in the field without extensive lab testing.<ref>{{Cite journal |last1=Sanchez |first1=Evelyn A. M. |last2=Uhlein |first2=Alexandre |last3=Fairchild |first3=Thomas R. |date=2021-01-01 |title=Treptichnus pedum in the Três Marias Formation, south-central Brazil, and its implications for the Ediacaran-Cambrian transition in South America |url=https://www.sciencedirect.com/science/article/abs/pii/S0895981120305265 |journal=Journal of South American Earth Sciences |volume=105 |pages=102983 |doi=10.1016/j.jsames.2020.102983 |bibcode=2021JSAES.10502983S |issn=0895-9811|url-access=subscription }}</ref>

Currently, there is no consensus on what global event is a suitable candidate to mark the start of the Cryogenian Period, but a [[ice age|global glaciation]] would be a likely candidate.<ref name="GSSPTable" />