Cryogenian

Cryogenian
c. 720 – c. 635 Ma Pha. Proterozoic Archean Had.
A map of the world as it appeared at the start of the Cryogenian, c. 720 Ma
Chronology
−720 — – −710 — – −700 — – −690 — – −680 — – −670 — – −660 — – −650 — – −640 — – −630 — N e o p r o t e r o z o i c Tonian Cryogenian Ediacaran           ← Marinoan glaciation[1][2] ← Transition from cyanobacterial to algal dominated world.[3] ← Sturtian[1][2]   Major Glacial period Events of the Cryogenian period Axis Scale: Millions of Years Ago (Ma).
Etymology
Name formality	Formal
Name ratified	1990
Usage information
Celestial body	Earth
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Period
Stratigraphic unit	System
Time span formality	Formal
Lower boundary definition	Defined chronometrically with an interim calibrated age of c. 720 Ma. GSSP is in progress.
Lower boundary definition candidates	The first appearance of widespread glaciation.[4]
Lower boundary GSSP candidate section(s)	To be determined
Upper boundary definition	Worldwide distinct cap carbonates. Beginning of a distinctive pattern of secular changes in carbon isotopes.
Upper boundary GSSP	Enorama Creek section, Flinders Ranges, South Australia 31°19′53″S 138°38′00″E / 31.3314°S 138.6334°E / -31.3314; 138.6334
Upper GSSP ratified	March 2004[5]
Atmospheric and climatic data
Mean atmospheric O2 content	c. 12 vol % (60 % of modern)
Mean atmospheric CO2 content	c. 1300 ppm (5 times pre-industrial)
Mean surface temperature	c. 5 °C (9 °C below modern)

The Cryogenian (from Ancient Greek: κρύος, romanized: krýos, meaning "cold" and γένεσις, romanized: génesis, meaning "birth") is a 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 Boring Billion years of stability, at the beginning of Cryogenian the severe Sturtian glaciation began, freezing the entire planet in a state known as a snowball Earth. After 70 million years it ended, but was quickly followed by the global Marinoan glaciation. There is controversy over whether these glaciations covered the entire planet, or if a band of open sea survived near the equator (slushball Earth).