The Hirnantian stage and its GSSP: A record of rapid global climate change

The Hirnantian was originally defined as a regional British stage at the top of the British Ashgill Series and the Ordovician System. It was characterized by a distinctive shelly fauna of trilobites and brachiopods, the Hirnantia-Dalmanitina fauna, which was described in the mid 1800s from the Hirnant Limestone and associated strata of North Wales. The Hirnantia-Dalmanitina fauna is now considered to be a cool water fauna. It occurs in strata worldwide that record a prominent, but short-lived, sea-level lowstand that is attributed to a major glacial episode in the latest Ordovician, and its widespread appearance follows upon a mass extinction event - the second greatest of the Phanerozoic - that decimated warm-water benthic faunas.

In spite of its short duration (1 my or less), the ICS Subcommission on Ordovician Stratigraphy, in its development of a global chronostratigraphic classification for the Ordovician System, recognized the Hirnantian as a standard global stage because of its varied, yet distinctive, stratigraphy. Hirnantian strata on most continents record an abrupt, rapid, extensive sea-level fall and a protracted lowstand with most epeiric seas drained and vast continental surfaces exposed. Sections representing high paleo-latitudes contain glacial and glacio-marine strata. A subsequent rapid, extensive sea-level rise re-established epeiric seas with widespread anoxia and led to deposition of black graptolite shale of the Silurian. The mass extinction record of benthic organisms is lost at an abrupt facies change and hiatus in shallow-marine sections. The fossil records of stratigraphically continuous sections from shelf basin and continental margin settings are dominated by planktic graptolites, which provide high resolution correlation of uppermost Ordovician and Silurian strata.

Hirnantian strata include several species of the genus Normalograptus, and the stage is composed of the N. extraordinarius and N. persculptus zones. Diversity and abundance of normalograptids are very low in underlying strata that, in constrast, contain a diverse DDO graptolite fauna with as many as 20 genera. Oceanographic changes associated with glaciation and sea-level fall that initiated the Hirnantian led to the deterioration of the habitat favored by DDO graptolites - the margin of oxygen minimum zones underlying upwelling zones. Meanwhile, normalograptids living in the photic zone flourished and diversified. The Hirnantian sea-level fall and subsequent rise are tracked closely by a prominent positive carbon isotope excursion. Accordingly, Hirnantian strata and the base of the Hirnantian Stage are distinct in stratigraphic sections worldwide and can be correlated with high resolution and great precision. The GSSP for the Hirnantian Stage is located in the Wangjiawan section in China at the FAD of N. extraordinarius.