Eocene – Wikipedia

L’ Eocene is the second era of paleogenic and also the second in the Cenozoic era. He follows the Paleocene and precedes the oligocene. It extends from 56.0 to 33.9 million years ^{[ first ]} . The beginning of the Eocene is marked by the emergence of the first modern mammals, its end with a massive extinction, called “large cut”, perhaps linked to the meteoritic impacts of Popigaï in Siberia and Chesapeake, in North America.

His name comes from the Greek “ἠώς” ( eos , aube) et ‘new’ ( prices , new) which is a reference to new species of mammals appearing during this time.

As with all ancient geological eras, reference stratigraphic layers are precisely known but their exact dates are subject to variations. The first works to establish this paleontological period were carried out in XIX ^{It is} century, in England by Barbara Yelverton.

This period is often divided into Upper Eocene , AVERAGE And lower . The divisions given for the Paris Basin are drawn, among other things, from Pomerol and Firegueur ^{[ 2 ]} .

Superior [ modifier | Modifier and code ]

Priaonian (37.0 to 33.7 And ^{[ 3 ]} ) or (37.2 ± 0.1 to 33.9 ± 0.1 mA). In the Paris basin, it consists of the Ludien (stratotype in Ludes) with gypsum, the “marls to Pholadomya “And” Champigny limestone “.

AVERAGE [ modifier | Modifier and code ]

• Bartonian (41.3 to 37.0 And ^{[ 3 ]} ) or (40.4 ± 0.2 to 37.2 ± 0.1 mA). It is divided, in the Paris basin into two sub-steps:
  • The Marinel at the top (Stratotype in Marines) with the sands of Marines, the limestones of Saint-Ouen, the limestones of Ducy, the sands of Mortefontaine, the Sables of Argenteuil, Les Sables de Monceau les Sables de Cresnes…;
  • The Autaversian at the base, with more or less greedy paleosols, the sands of flowery, the sands of Ermenonville, the sands of Beauchamp, the sands of Auvers (stratotype in Auvers-sur-Oise), Les Sables du Guépelle , Limestone of Jaignes, the limestone of Nogent-l’Artaud, the clays of Villeneuve-sur-Verberie and the sands of Mont-Saint-Martin…
• Lutetien (49.0 to 41.3 And ^{[ 3 ]} ) or (48.6 ± 0.2 to 40.4 ± 0.2 And ). In the Stratotypical Paris basin, it is divided into three sub-steps:
  • The superior lutetian with limestone pebbles and limestone marine sands from biozone to Discorinopsis Kerfornei ;
  • the average, limestone, end, to milioles, Orbitolites complanatus And NUMMULITES laevigatus ^{[ 4 ]} ;
  • the lower lutetian, silico-limestone, rude, to NUMMULITES laevigatus .

Lower [ modifier | Modifier and code ]

• YPRESIEN (54.8 to 49.0 And ^{[ 3 ]} ) or (55.8 ± 0.2 to 48.6 ± 0.2 And ).

This is divided, in the Paris basin in two sub-steps:

In South America, the Age of terrestrial mammals in South America ( SALMA ):

At the start of the Eocene occurred a global warming, one of the most extreme identified today. This event occurs fairly quickly and lasts less than 100,000 years. The average temperature increases by at least 7 °C In high latitudes ^{[ 5 ]} , or even until 15 °C For the surface temperature of the oceans in the low latitudes of the southern hemisphere. Models explain the high temperature by heat transport via ocean currents between high and low latitudes. The temperature gradient between these latitudes is much less than today ^{[ 6 ]} . The climate remains globally hot throughout this period although cooling slowly. Two main causes are mentioned to explain the hot climate: the increase in the greenhouse gas level, such as methane and a circulation of ocean currents different from that of previous periods. The end of the eocene is marked by rapid cooling ^{[ 7 ]} .

Reconstituted geography of the Earth at the start of the Eocene. The large continental masses are connected by terrestrial bridges allowing many animal and plant migrations during eustatic lowering consecutive to large glaciations: Europe is an archipelago linked to North America by the Corridor of Geer (in) , Asia is connected to North America by Beringie.

The continents have continued their drift: Andes and rocky mountains continue their overcrowding.

At the start of this period, South America to the west, Antarctica in the south and Australia in the east are still connected, with similar flora and fauna (marsupial, araucariacea …). Circum-antarctic ocean current is not yet in place and the climate of this continental mass is temperate hot. When these three continents separated, about 44 million years ago, the circum-antarctic current is set up, the hot equatorial currents are deféchis and the transport of heat between the equator and the southern pole decreases: isolated, Antarctica cools and begins to cover with ice. Other causes are advanced to explain this cooling, for example the decrease in greenhouse gases in the atmosphere.

In Europe, the Tethys sea finishes fragmenting, while the rise of the Atlas, Alps, Apennines, Carpathians, Balkans, Pinde, Taurus, Pont and Caucasus isolates its last basins in the form Mediterranean, Adriatic and black seas. A shallow sea covers northern Europe. Although the North Atlantic continues to open up, a connection between Europe and North America still remains between Scandinavia and the future Greenland, their remaining very similar biodiversity.

India continues to move away from Africa and its contact with Asia causes Himalayas to overcrowd.

The Eocene-Oligocene limit, there are approximately 34 And , is marked on the lands emerged by a very important wildlife reshuffle, which earned it, first for the grounds of Europe, the name of “large cut”. Today, we see that it is, for Mammalian fauna in particular, a global upheaval.

Flora [ modifier | Modifier and code ]

At the start of the Eocene, high temperatures and the hot oceans generate a wet and soft environment, with forests stretching from one pole to another, except in some arid deserts, the forests are dominant on the land.

Arctic forests are extended. Fossils and preserved remains of trees such as Metasequoia and Glyptostrobus are common on the island of themselves, located only a few degrees further south than in the Canadian Arctic; These are not fossils but leftover preserved in poor oxygen water. There are also subtropical, even tropical fossils, dating from Eocene to Greenland, Alaska; Tropical forests grow in Europe; Palm fossils are discovered in Alaska and Northern Europe during the start of the Eocene; They become less abundant when the climate refreshes.

The climate cools from the average eocene, the continental climate becomes drier and the forests clear up clearly in certain regions. THE Poaceae are confined to the banks of rivers and lakes and have not yet extended to the plains and savannas. Cooling leads to the expansion of deciduous trees, more resistant to temperature change, which take the place of sempervirent plants. At the end of the Eocene, the forests made up of deciduous covers large parts of the continents. The Antarctica towards the end of this period is much colder; The tropical flora has disappeared and at the start of the oligocene large expanses of tundra are present.

Terrestrial fauna [ modifier | Modifier and code ]

The oldest modern mammal fossils appear during a brief period at the start of the Eocene. At the same time, several new groups of mammals arrive in North America, such as artiodactyls, perissodactyls and primates, with fine limbs, feet, hands capable of catching and teeth capable of chewing.
All these new mammals’ orders are small, below 10 kg : Based on the size of the teeth, the mammals of the Eocene are 60% smaller than those of the Paleocene and are also smaller than those who followed them during the Oligocene. These size differences are probably connected to the warm climate and the heat elimination problem for large mammals.

Due to their influence between Europe and North America, the two groups of ungulates (artiodactyls and perissodactyls) became dominant at that time. Other forms of mammals also appeared during the Eocene: bat, proboscidian, rodents and primates.

The older forms of mammals have declined in number and variety. There are representatives of this fauna in North America, Europe, Patagonia, Egypt and Southeast Asia. Marine fauna is better represented in South Asia and southeast of the United States.

During the Eocene, plants and marine fauna evolved towards more current forms. Many modern forms of birds and marine mammals have also appeared.

Marine life [ modifier | Modifier and code ]

The eocene oceans are hot, the fish abound there.

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