Transition from cyanobacterial to algal dominated world. 
Major Glacial period
Axis Scale: Millions of Years Ago (
- Worldwide distinct cap carbonates.
- Beginning of a distinctive pattern of secular changes in carbon isotopes.
(5 times pre-industrial)
(9 °C below modern)
The Cryogenian (from
Cryogenian was the 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 Earth in a planetary state known as a Snowball Earth. After 70 million years it ended, but was quickly followed by the Marinoan glaciation, which was also a global event. These events are the subject of much scientific controversy specifically over whether these glaciations covered the entire planet or a band of open sea survived near the equator (termed "slushball Earth").
The Cryogenian Period was ratified in 1990 by the International Commission on Stratigraphy. 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, but changed in 2015 to 720 million years.
This could cause ambiguity because estimates of rock ages are variable and are subject to laboratory error. For instance, the time scale of the
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.
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.
Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.
The Cryogenian is generally considered to be divisible into at least two major worldwide glaciations. The
Before the start of the Cryogenian, around 750 Ma, the
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
Cryogenian biota and fossils
Fossils ofred algae and green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba.
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.
- Timeline of glaciation – Chronology of the major ice ages of the Earth
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