Hirnantian glaciation

The Hirnantian glaciation was a major Late Ordovician episode of ice-sheet expansion on Gondwana. It lowered global sea levels and coincided with the Late Ordovician mass extinction.

Gondwana: A southern supercontinent that assembled during the Neoproterozoic and existed through much of the Paleozoic. Its high-latitude landmass hosted the ice sheets that drove this glaciation.
Hirnantian: The final age of the Ordovician Period, lasting roughly from 445 to 444 million years ago. This age gives the glaciation its name and contains its main interval.
Hirnantia fauna: A distinctive Late Ordovician marine brachiopod assemblage associated with cool-water conditions. Its distribution records ecological shifts during the glacial interval.
Ordovician greenhouse climate: The generally warm global climate of the Ordovician, when atmospheric carbon dioxide was high and polar ice was limited. The glaciation interrupted the warm conditions that prevailed through much of the period.
Ordovician paleogeography: The distribution of continents and oceans during the Ordovician Period. Gondwana’s polar position made extensive glaciation possible.
Hirnantian glacial deposits: Sedimentary deposits formed by or associated with Hirnantian glaciers, including glacially influenced strata in North Africa. These deposits provide direct geological evidence of ice on Gondwana.
Gondwanan glacial sediments: Late Ordovician sediments in Gondwana that record glacial erosion, transport, or deposition. Their facies reveal where ice and meltwater affected the continent.
Late Paleozoic ice age: A prolonged interval of Gondwanan glaciation spanning much of the Carboniferous and Permian periods. It was a later, longer-lived icehouse episode on Gondwana.
Eustatic sea-level change: Global change in sea level caused by changes in ocean-water volume or the capacity of ocean basins. Water stored in expanding ice sheets produced a major global sea-level fall.
Carbon dioxide drawdown: A reduction in atmospheric carbon dioxide through processes such as silicate weathering and organic-carbon burial. Carbon dioxide decline is a leading explanation for Ordovician cooling and ice growth.

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Late Ordovician mass extinctionRelated: Its growth lowered sea levels and cooled oceans during the extinction’s first phase.