Carnien

Article

August 14, 2022

The Carnian is the first stratigraphic stage of the Upper Triassic. It is located between 237 Ma and 227 Ma approximately. It is preceded by the Ladinian (~242 – ~237 Ma), last stage of the Middle Triassic and followed by the Norian (~227 - ~208.5 Ma), second stratigraphic stage of the Upper Triassic.

Stratigraphy

The Carnian was first defined in 1869 by geologist Edmund Moïssissovics. There is a doubt about the origin of the name which comes either from the Austrian land of Carinthia (in German Kärnten), or from the Carnic Alps. The beginning of the Carnian is defined as related to the appearance of the ammonite species Daxatina canadensis. The World Reference Profile is located at Stuores-Wiesen near Badia in Val Badia in the South Tyrol region of Italy (46° 31′ 37″ N, 11° 55′ 49″ E). The end of the Carnian (the beginning of the Norian) is the appearance of the ammonite biozones Klamathites macrolobatus or Stikinoceras Kerri and of the conodonts Metapolygnathus communisti or Metapolygnathus primitius.

Divisions

The Carnian stage is subdivided into two or three regional sub-stages according to the authors. Edmund Moïssissovics subdivided the Carnian into three substages, from oldest to newest: Cordevolian, Julian and Tuvalian. These subdivisions are still in use. Other authors subdivide the Carnian into two: the Julian (Lower Carnian) and the Tuvalian (Upper Carnian).

Palaeogeography

The Carnian paleogeography was basically the same as that of the rest of the Triassic. Most of the continents were merged to form the supercontinent Pangea, and there was only one world ocean, Panthalassa. This world ocean had a western arm, called Paleotethys, at tropical latitudes. Paleotethys sediments are found in southeastern Europe, the Middle East, the Himalayas, and even on the island of Timor. This unique land-sea partition has led to a “mega-monsoon” climate, a greater monsoon regime than currently found.

Climate

As during most of the Mesozoic, there was no ice cap on Earth at that time. The climate was mainly arid in the tropics, but an episode of humid tropical climate is found at least in the Paleotethys. This still poorly known climatic event, called the “Carnian pluvial episode”, is dated 232 million years ago. It lasted a million years. The nature of this event has been debated, but it has gained greater credibility since geochemical and stratigraphic evidence of a link between this climate change and the basalt eruptions of Wrangellia (a large igneous province in present-day Canada have been provided (in 2012). Some scientists previously considered it to be only an artifact, due to the migration of the continents from the Tethyan zone to the equatorial climatic belt. According to this hypothesis, the apparent change of climate from arid to humid, then back to an arid climate simply testifies that the continent has moved up from the tropical regions of the southern hemisphere towards the equator and then towards the northern tropical latitudes.

Life

In the marine realm, the Carnian saw the first abundant occurrences of calcareous nanoplankton, a morphological group containing the Coccosphaerales.

Invertebrates

Only a few typical and characteristic Carnian invertebrates are known. Among the molluscs, ammonites of the genus Trachyceras are found only in the early Carnian (ie, in the Julian in the subdivision into two substages of the Carnian). The family Tropitidae and the genus Tropites appear in the early Tuvalian, the second Carnian substage. Bivalves of the genus Halobia, a bottom dweller, are differentiated from the genus Daonella at the beginning of this stage