Tremadocium

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system series step ≈ age ( mya )
higher higher higher younger
Ordovician Upper Ordovician Brain antium 443.4
⬍
445.2
Katium 445.2
⬍
453
Sandbium 453
⬍
458.4
Middle Ordovician Darriwilium 458.4
⬍
467.3
Dapingium 467.3
⬍
470
Lower Ordovician Floium 470
⬍
477.7
Tremadocium 477.7
⬍
485.4
deeper deeper deeper older

The tremadocium is the lowest chronostratigraphic level of the Lower Ordovician series or Ordovician in the history of the earth . Expressed in absolute numbers ( geochronologically ), the stage covers approximately the period from approximately 485.4 to approximately 477.7 million years. The tremadocium follows the uppermost, not yet formally named level ("level 10") of the furongium series of the Cambrian . It is followed by the floium , the second stage of the subordovician.

Naming and history

The stage is named after the town of Tremadoc in Wales . The stage and name were proposed by Adam Sedgwick in 1846 (as the "Tremadoc Group").

Definition and GSSP

The basis of the tremadocium (and thus of the ordovician ) is defined by the first appearance of the conodont species Iapetognathus fluctivagus . This limit lies somewhat above the Cordylodus Lindstromi conodont zone and somewhat below the first occurrence of the first planktonic graptolites. The upper limit is set with the first appearance of the graptolite species Tetragraptus approximatus . The reference profile (GSSP = Global Boundary Stratotype Section and Point) of the International Commission for Stratigraphy is the so-called "Green Point Profile" in Gros Morne National Park , about 70 km from Deer Lake airport and about 10 km north of Rocky Harbor , in western Newfoundland ( Canada ).

Environmental parameters

The δ 18 O values are the lowest in the entire Ordovician during the Tremadocium at −9 ‰ PDB and are a proxy for very warm climatic conditions. In the course of the further Ordovician, the global climate should then gradually cool down. At the turn of the Silurian, the δ 18 O values ​​are only −5 ‰ PDB. The δ 13 C values show a comparable development, which have their lowest value at −1.5 ‰ PDB and then rise to 1.5 ‰ PDB by the beginning of the Silurian. The opposite trend runs from 87 Sr / 86 Sr , which has its maximum in the tremadocium at 0.7090 and then falls to 0.7078 by the end of the ordovician.

The sea ​​level shows transgressive behavior at the beginning of the tremadocium , peaking at 486 million years BP and then falling again to a very clear low at 484 million years BP. The rest of the level is then under a renewed rise, which should only reach its maximum in the middle floium.

Occurrence

During the Tremadocium, the following groups and formations were deposited:

evolution

The lower limit of the tremadocium is characterized by the mass extinction at the Cambrian / Ordovician border ( Cambrian-Ordovician species extinction ). Many genera of the brachiopods and conodonts disappeared, and the number of trilobite taxa also fell sharply. Nevertheless, the degree of biodiversity of the Cambrian could be preserved. The rapid spread of species ( Ordovician radiation ) that began a little later had its roots in the end of the Tremadocium.

During the Tremadocium, the planktonically living graptolites appeared for the first time . In the case of the jawless (Agnatha), the taxon Anatolepis is found in the fossil record . The cephalopods (cephalopods) the Nautiloide occurs Proterovaginoceras on.

literature

  • Roger A. Cooper, Godfrey S. Nowlan and S. Henry Williams: Global Stratotype Section and Point for base of the Ordovician System. Episodes, 24 (1): 19-28, Beijing 2001 ISSN  0705-3797
  • Felix Gradstein, Jim Ogg and Alan Smith: A Geologic timescale. Cambridge University Press 2004 ISBN 978-0-521-78673-7 .
  • Hans Murawski & Wilhelm Meyer: Geological dictionary . 10., rework. u. exp. Ed., 278 p., Enke Verlag, Stuttgart 1998 ISBN 3-432-84100-0 .

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