The Cryogenian (from Ancient Greek: κρύος, meaning "cold" and γένεσις, meaning "birth") was a time period that lasted from 720 to 635 million years ago. It was the second of three periods in the Neoproterozoic era, coming after the Tonian period and before the Ediacaran period.

This time was marked by major climate changes. After a long period of little change called the Boring Billion, the Sturtian glaciation began at the start of the Cryogenian. This event caused the entire planet to freeze, creating a very cold climate known as a snowball Earth. This freezing period lasted about 70 million years and ended, but it was quickly followed by another global ice age called the Marinoan glaciation. Scientists disagree about whether these ice ages covered the entire planet or if some open water near the equator remained (called "slushball Earth"). However, the extreme cold and large ice sheets would have blocked sunlight, greatly reducing life in shallow seas and causing major extinctions and changes in Earth's ecosystems.

Ratification

The Cryogenian Period was officially named in 1990 by the International Commission on Stratigraphy. Unlike most other time periods, the start of the Cryogenian is not linked to a clearly observable event. Instead, the beginning of the period is marked by a specific age of rocks, which was first set at 850 million years but later changed to 720 million years in 2015.

This could create uncertainty because estimates of rock ages may vary due to differences in how scientists interpret data and possible errors in laboratory testing. For example, the start of the Cambrian Period is not based on rocks older than a certain age (538.8 million years), but instead on the first appearance of specific types of fossils called Treptichnus pedum. These fossils can be identified in the field without needing complex lab tests.

At present, scientists do not agree on which global event best marks the start of the Cryogenian Period. However, a widespread ice age is considered a possible candidate.

Climate

Special kinds of glacial deposits show that Earth experienced its most extreme ice ages during the Sturtian and Marinoan periods. Eyles and Young noted that glaciogenic deposits from this time can be found on every continent. These deposits provide evidence of the most extensive and long-lasting glaciation in Earth's history. Multiple glacial periods occurred, alternating with times when the climate was relatively warmer, and glaciers reached sea level in regions that were once near the equator.

Glaciers expanded and shrank in repeated cycles, possibly extending as far as the equator. The Cryogenian period is typically divided into at least two major global glaciations. The Sturtian glaciation lasted from 720 to 660 million years ago, and the Marinoan glaciation ended around 635 million years ago, at the end of the Cryogenian. Glacial tillite deposits are found in areas that were located at low latitudes during the Cryogenian, leading scientists to propose the idea of Earth being completely frozen, known as "Snowball Earth." Between the Sturtian and Marinoan glaciations was a time called the "Cryogenian interglacial period," marked by warmer temperatures, oxygen-poor oceans, and rising sea levels.

Paleogeography

Before the Cryogenian period, about 750 million years ago, the large landmasses that formed the supercontinent Rodinia began to separate. During this time, the superocean Mirovia started to shrink, while the superocean Panthalassa began to develop. Later, these landmasses may have come together again to form another supercontinent called Pannotia during the Ediacaran period.

Eyles and Young noted that most glacial deposits from the Neoproterozoic era were found in marine areas along the edges or centers of continents that were breaking apart. Worldwide deposits of dolomite may have helped lower the amount of carbon dioxide in Earth's atmosphere. The splitting of the edges of Laurentia around 750 million years ago happened at the same time as the formation of the Rapitan Group in North America and the Sturtian in Australia. A similar event of rifting occurred around 650 million years ago, with the Ice Brook Formation in North America forming alongside the Marinoan in Australia. The Sturtian and Marinoan are parts of the larger Adelaide Rift Complex.

Cryogenian biota and fossils

Between the Sturtian and Marinoan glaciations, the variety of life on Earth was very low.

Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian Period. Since 2009, some scientists have suggested that the Cryogenian Period may contain the oldest known fossils of sponges, which could mean early animal life existed at that time. However, it is not certain if these fossils are truly from sponges. Researchers have not ruled out the possibility that they might be early sponge-like organisms or complex microbes that could lead to sponge-like life. The question of whether this event affected life on Earth remains unresolved. For example, Porter (2000) proposed that new groups of life evolved during this time, including red algae, green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba.

At the end of the Cryogenian Period, heterotrophic plankton appeared. These plankton fed on unicellular algae and prokaryotes, reducing the dominance of bacteria in the oceans. Unicellular algae (Archaeplastida) experienced a rapid increase in diversity, and their population grew by up to 100 to 1,000 times.