Stenian

The Stenian Period (/ˈstiːni.ən/ STEE-nee-ən, from Ancient Greek: στενός, romanized: stenós, meaning "narrow") is the final geologic period in the Mesoproterozoic Era. It lasted from 1200 Mya to 1000 Mya (million years ago). Because there are very few fossils from this ancient time, scientists use special dating methods to figure out when this period happened instead of looking at rock layers.

During the Stenian Period, a giant landmass called a supercontinent, named Rodinia, came together. This process began in the earlier Ectasian Period and finished during the Stenian. Rodinia stayed together into the next period, called the Tonian, before eventually breaking apart later in Earth's history. At this time, Rodinia was surrounded by a large ocean known as the Mirovian. Many smaller seas that had formed when an earlier supercontinent called Columbia broke apart disappeared during this period.

History

Before the Stenian period was officially recognized in 1991, a different time period called the Riphean age was used. This age was thought to span from 1600 to 600 million years ago and was mainly used in Russia and textbooks around the world.

In 1991, a scientist named K.A. Plumb introduced the Stenian period as part of a new way to measure Precambrian time. He defined it as lasting from 1200 to 1000 million years ago. The name Stenian comes from the Ancient Greek word στενός, meaning "narrow," referring to the narrow belts of rocks formed during this time.

Geography

Most ideas about how the landmass named Rodinia looked place Laurentia in the middle, with other large land areas around it. These surrounding areas include the East European Craton to the southeast, Amazonia to the south, and several others such as Siberia, North, and South China cratons, whose positions can vary in different maps. The whole landmass was encircled by a huge ocean called Mirovia.

Geology

This period saw important changes on Earth, including the formation of the Keweenawan Rift, which created the Keweenawan Supergroup. The Musgrave orogeny also occurred, forming the Musgrave Block along with the Warakurna Large igneous province.

Many mountain-building events, known as orogenies, happened during this time. The Grenville orogeny was one of the major events, and several others like the Kibaran orogeny, the Dalslandian orogeny, and the Sunsás orogeny also took place. These events helped shape the supercontinent Rodinia.

Biology

Fossils of the oldest known sexually reproducing organism, Bangiomorpha pubescens, appeared in the Stenian Period at the Hunting Formation in Somerset Island around 1.047 Bya. This period also marks the first known preserved case of multicellularity in green algae, with Proterocladus antiquus dating back roughly 1 billion years ago.

Acritarchs became more abundant and developed spiny shapes during this time, likely due to increased competition among tiny, complex life forms. Stromatolites, which are layered structures made by ancient bacteria, reached their highest variety in this period but quickly decreased near its end around 1 billion years ago. By this time, complex cells called eukaryotes had become the main life forms in non-ocean environments.

Climate

During the Stenian Period, the Sun shone with about 5 to 18 percent less brightness than it does today. Carbon dioxide levels in the air were not more than ten times higher than they were before the industrial era. Oxygen levels were still quite low, only about half a percent to five percent of what we have now. There were no big ice ages during this time, and each day lasted around 19 hours.