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 drastic climate changes. After the long environmental stability/stagnation during the Boring Billion, the Sturtian glaciation began at the beginning of the Cryogenian, freezing the entire planet in a state of severe icehouse climate known as a snowball Earth. After 70 million years it ended, but was quickly followed by another global ice age, the Marinoan glaciation. The extreme climates with massive expanses of ice sheets blocking off sunlight would have significantly hindered life in the shallow seas and caused major changes in the world's living things.

Ratification

The Cryogenian Period was officially recognized in 1990 by the International Commission on Stratigraphy. Unlike many other time periods, it does not start with a special event that scientists can see everywhere in the rock. Instead, scientists chose a specific age of rocks to mark the beginning — first set at 850 million years, and later changed in 2015 to 720 million years.

Because figuring out the exact age of rocks can be tricky, some people worry this might cause confusion. For example, the Cambrian Period starts when a special trace fossil called Treptichnus pedum first appears, which is easier to see in the field. Right now, scientists do not agree on a single global event to mark the start of the Cryogenian, though a big global glaciation would be a good choice.

Climate

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

Between these icy times, the climate warmed up a bit, and the oceans changed in ways that affected sea life. These changes show how Earth’s climate has shifted dramatically 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, the large landmasses known as cratons began to drift apart from the supercontinent Rodinia. This caused the ocean called Mirovia to shrink, while another vast ocean named Panthalassa started to form. These landmasses may have later come back together to form a new supercontinent called Pannotia during the Ediacaran period.

During the Cryogenian, large glaciers covered parts of the continents. Important glacial deposits formed along the edges of continents or in their interiors. The breaking apart of continents, such as Laurentia, happened around the same time as major glacial events in places like North America and Australia.

Cryogenian biota and fossils

During the Cryogenian Period, life on Earth was quite simple. Fossils of tiny organisms called testate amoeba first appeared, along with possible early signs of simple animals like sponges. Scientists are still studying these fossils to learn more about them.

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, marking an important change in ocean life.