Hirnantian

The Hirnantian is the final internationally recognized stage of the Ordovician Period of the Paleozoic Era. It lasted about 2.1 million years, from 445.2 to 443.1 Ma (million years ago). During this time, the Earth went through big changes in climate.

Early in the Hirnantian, the world became very cold because of major glaciation. This caused sea levels to drop a lot. Later, the temperatures rose, the glaciers melted, and the seas came back to where they had been before.

These climate changes are thought to have caused a major extinction event. It was the second largest extinction in geologic history, with about 85% of sea-dwelling species disappearing. Only the End-Permian mass extinction was bigger. After the climate settled back down, the surviving species evolved into new forms that were very similar to the ones that had existed before the extinction.

Naming and history

The Hirnantian was named after Cwm Hirnant south of Bala, in northern Wales. Cwm Hirnant means the "valley of the long stream" in Welsh.

The stage was introduced in 1933 by B.B. Bancroft. In 2003, the International Commission on Stratigraphy officially added the Hirnantian to its international time scale. In 2006, the ICS chose the Wangjiawan section as the official Global Boundary Stratotype Section and Point for the Hirnantian Stage.

GSSP

The reference point for the start of the Hirnantian is located at the Wangjiawan section near the village of Wangjiawan, about 42 km north of Yichang in Hubei, China. This spot marks the beginning of the Hirnantian and is found in rock layers made of shale and chert. The exact beginning is identified by the first appearance of a specific type of graptolite called Normalograptus extraordinarius.

Major events

The Hirnantian was a time of big climate changes that led to a major mass extinction event. It started with a hot climate and high sea levels, but then the Earth cooled, glaciers grew, and sea levels dropped. This drop in water level caused many species that needed shallow water to die out.

Later, the climate warmed again, glaciers melted, and sea levels rose. This change caused more extinctions among the species that had survived the first drop in sea level.

Dating

Scientists use special methods to figure out how old rocks and fossils are. For the Hirnantian time period, they looked at rocks from the Dob's Linn area in the United Kingdom. They studied tiny crystals called zircon found in ash deposits to get two important dates.

The older date, from rocks called Hartfell shale, shows that the Hirnantian began around 445.7 million years ago. The younger date, from rocks called Birkhill shale, shows that it ended around 438.7 million years ago. These dates help scientists understand when events happened during this time.

Subdivisions

The Hirnantian Stage has two main parts, each marked by a special type of graptolite, which are tiny fossils found in rocks. The first part starts when the graptolite Normalograptus extraordinarius first appears, and the second part begins when Normalograptus persulptus appears. These markers help scientists date events that happened during this time.

These graptolite zones help us learn more about what happened during the Hirnantian Stage and the end of the Ordovician Period.

Paleogeography

During the Hirnantian, most of Earth’s land was part of a huge supercontinent named Gondwana. This landmass covered the south pole and included places like South America, Africa, most of Australia, India, and Antarctica. To the north of Gondwana was a vast ocean called the Panthalassic ocean.

Other land areas included parts of what are now the southeastern United States, such as Florida, Mississippi, Alabama, and South Carolina, which were near the South pole. The rest of North America, known as Laurentia, sat to the north with warmer climates. Farther north was Baltica, made up of parts of modern Norway, Sweden, Finland, Denmark, Germany, and Russia. An island group called Avalonia stretched west and included parts of Britain, Ireland, and eastern areas of Newfoundland, Nova Scotia, New Brunswick, and New England.

Correlations to regional stages

The Hirnantian is an internationally recognized stage of Earth's history, but it began as a regional stage in the United Kingdom. Different parts of the world have their own ways of dividing geological time, so here are some matches between regional stages and the Hirnantian:

• Australasia – The Hirnantian matches the top 20% of the Bolindian epoch.
• Baltica – The Hirnantian equals the upper 50% of the Porkuni stage.
• China – The Hirnantian is about the upper 25% of the Wufeng stage.
• North America – The Hirnantian matches all of the Gamachian stage.
• United Kingdom – The Hirnantian is nearly the same as the regional Hirnantian stage, starting a little earlier and making up about the upper 20% of the Ashgill epoch.