Cryogenian

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. It is the second of the three periods of the Neoproterozoic era, preceded by the Tonian and followed by the Ediacaran.

The Cryogenian was a time of big climate changes. After a long time of little change during the Boring Billion, the Sturtian glaciation began at the start of the Cryogenian. This froze the whole planet in a very cold icehouse climate called a snowball Earth. After 70 million years, it ended, but then another global ice age called the Marinoan glaciation began. These extreme climates with huge ice sheets blocked sunlight and made life harder in the shallow seas.

Ratification

The Cryogenian Period was named an official time period in 1990 by the International Commission on Stratigraphy. It is different from many other time periods because it does not start with a special event that can be seen in rocks everywhere. Instead, scientists picked a certain age of rocks to mark the beginning. First, they chose 850 million years ago, but in 2015, they changed it to 720 million years ago.

Finding the exact age of rocks can be hard, so some people think this may cause confusion. For example, the Cambrian Period starts when a special trace fossil called Treptichnus pedum first appears, which is easier to find in the field. Right now, scientists do not agree on one big event to mark the start of the Cryogenian, though a large global glaciation would be a good choice.

Climate

During the Cryogenian period, Earth went through some of its coldest times. There were huge glaciers that covered large parts of the planet during two big ice ages called the Sturtian and Marinoan glaciations. These glaciers may have even reached areas near the equator, leading scientists to think Earth might have been a "Snowball Earth" with frozen oceans.

Between these icy times, the climate warmed up a little, and the oceans changed in ways that affected sea life. These changes show how Earth’s climate has shifted a lot over millions of years.

Glaciers Sturtian glaciation Marinoan glaciation tillite Snowball Earth anoxic oceans

Paleogeography

Main article: Rodinia § Geodynamics

Before the Cryogenian period, around 750 million years ago, big land areas called cratons started to move away from the supercontinent Rodinia. This made the ocean called Mirovia get smaller, while a new huge ocean named Panthalassa began to form. These land areas might have joined together again to make a new supercontinent called Pannotia during the Ediacaran period.

During the Cryogenian, big glaciers covered parts of the continents. Important glacial deposits formed along the edges of continents or inside them. The continents, such as Laurentia, began to break apart around the same time as big glacial events in places like North America and Australia.

Cryogenian biota and fossils

During the Cryogenian Period, life on Earth was very simple. Fossils of tiny organisms called testate amoeba first appeared. There were also possible early signs of simple animals like sponges. Scientists are still studying these fossils to learn more.

By the end of this period, new types of tiny plants and animals called heterotrophic plankton began to appear in the oceans. These new organisms changed the oceans by feeding on other tiny plants and bacteria. This was an important change in ocean life.