Precambrian

Etymology

The term of “Precambrian” is rather old, since it was used to indicate in a more or less vague way the ignored period preceding the era Primaire. The word is based on the term Cambrien, which comes itself from the Latin name of the Wales, Cambria .

Today, it is always used by the Géologue S and the Paléontologue S to indicate the three first éon S under only one expression. There are some time spoken about éon Cryptozoïque , then the triptych “Protérozoïque”, “Archéen” and “Hadéen” put this term in disuse.

Planetary environment with the Precambrian one

Although the Precambrian one covers 88% of the Histoire of the Earth, there remains still much less known than the periods which succeed to him.

Formation of the ground

The solar system was formed there is 4 567 My. The Proto-ground would have been formed by Accrétion residual matter within a primitive Nébuleuse, there is approximately 4 560 My.

According to any probability, to have taken shortly after its form coarsely spherical, it would have been struck by a Planet of the size of Mars called Théia. This “ Great collision ” would have occurred towards −4 533 My. The ejected remains would then have formed a ring of matter around the Earth, which would have given birth, by the same process of accretion, with the only natural satellite of the Earth, the the Moon.

Certain specialists consider that one can speak about Hadéen only starting from this event.

A stable earth's crust was apparently places from there towards 4 400 My, as the crystals of Zircon indicate it found in the west of the Australia (dated towards 4 404 My ± 8).

Reducing atmosphere

The primitive atmosphere is due to the Dégazage of the Earth. An important volcanic activity contributes to it. The volcanic gases consist of vapor of Eau to more de 80%, of Carbon dioxide entre 6 et 12%, of H₂S or SO₂ à 3%… After cooling, a great part of this water will condense to form the oceans. The resulting atmosphere, rich in Diazotizes, Carbon dioxide, water, Sulfur dioxide and perhaps Chlorure of hydrogen, Ammoniaque and Méthane, would be hostile with the current life.

Between −3.4 and −2.2 Ga, the paléosols show an iron impoverishment. This one, solubilized out of ferro-iron (Fe²⁺) by the reducing atmosphere moderately , accumulates in the oceans and precipitates in the form of Pyrite or of Oxyde S in the ribboned Gisements of iron (−3.8 à −1.8 Ga). Taking into account the elements present in the air, the ground and water, the dominant colors were to be the pale red for the ground and the green olive for the oceans.

The constancy of the δ¹³C_org since -3.5 even -3.8 Ga, indicates a fixing of carbon by the enzyme RubisCO.

Oxidizing atmosphere

The atmospheric production of Dioxygène started with the appearance of the Photosynthèse at the Cyanobactérie S, as of -3.5 Ga. It will result from it, in -2.4 Ga, an ecological crisis called the “ Great oxidation ” or “ Catastrophe of the oxygène ”. Oxygen was indeed toxic for the organizations Anaérobie S of the time. Oxygen reacted with oxydable large surfaces present at the surface of the ground (mainly, the Fer).

With −2.2 Ga, the increase in the δ¹³C_org in the Carbonate S is explained by a more important organic matter fossilization, in particular in the Stromatolite S, of the structures in Carbonate of calcium built by cyanobacteries. This causes to enrich the atmosphere in dioxygene. It is on this date that appear the Formations of iron or red Couches, proof of an oxidizing atmosphere. Starting from −1.8 Ga, the paléosols grow rich out of iron. The Pression partial of Dioxygène is about 15% of the current one. After a glacial episode in −700 My and its new ribboned iron layers, the oceans cease being Anoxique S to become rich in Sulfate S. the Métazoaire S develop.

Climate

At the beginning of Archaean, the weaker solar activity was compensated by an atmosphere very charged in Gaz with greenhouse effect. The temperature was high. The isotopic analysis of the Silicium watch which the temperature of the oceans decreased of 70°C there is 3 500 My, with 20°C there is 800 My. If this atmosphere had been maintained, the ground would not be livable any more today.

Fortunately, an atmospheric significant amount of carbon could be gradually collected in the form of Carbonate of calcium, which precipitates at the bottom of the oceans. Carbon is found thus trapped in the rocks Sédiment surfaces. The plate tectonics restores of it a part with the atmosphere through the volcanic activity (slow cycle of carbon). The slow impoverishment of the atmosphere out of carbon dioxide leads to the Glaciation huronienne towards −2.2 Ga.

Four periods of glaciation will mark the era Néoprotérozoïque, between −1 000 and −542 My. The best known one is the Glaciation Varanger. The Earth, completely frozen during several million years, evokes a “ then; swell of neige ”.

Plate tectonics

The Plate tectonics of Precambrian is far from known. It is generally allowed that the main part of the terrestrial masses formed only one and single Supercontinent, Rodinia, there is 1 billion years. It is divided into several pieces towards −600 My. The corresponding volcanic activity will increase the concentrations of atmospheric CO₂ and will put a term at the cycle of glaciations.

Life before Cambrien

Subdivisions

The radiometric Datation makes it possible to delimit the various formations from the Precambrian one. Archean (older than butt 2500 My), Proterozoic (2500-600 My), and Neoproterozoic (600-542 My) appear to cuts general currency. Summon additional terms are included in the geological time line. See Timetable off the Precambrian. -->

Protérozoïque : this éon extends from −2 500 My at the beginning of the Cambrien in −542 My.

Néoprotérozoïque : it is the the most recent era of Protérozoïque; its beginning was fixed at −900 My but a modern tendency would bring back it to −600 My, roughly. It corresponds to the old “Précambrien Z” of North-American geology.

Édiacarain : this geological Period was officially defined in March 2004 by the International union of geological sciences. This period starts with a characteristic layer gone back to approximately −620 My. She knew the appearance of the Faune of Édiacarain starting from −565 My and leads to the period of Cambrien in −542 My.
Cryogénien a proposal for a subdivision of Néoprotérozoïque.
Tonien a proposal for a subdivision of Néoprotérozoïque.

Mésoprotérozoïque : it is the central era of Protérozoïque, roughly of -1 600 with −900 My. It corresponds to the old “Précambrien Y” of North-American geology.
Paléoprotérozoïque : it is about old Protérozoïque, roughly of −2 500 with −1 600 My. It corresponds to the old “Précambrien X” of North-American geology.

Archaean: roughly between -3 800 and −2 500 My.
Hadéen : éon former to −3 800 Ma. In the beginning this éon was conceived like former to the first rock deposits. However, some crystals of Zircon gone back to approximately −4 400 My the presence of an earth's crust shows during this éon.

It was proposed to divide Précambrian five let us éons, characterized as follows;:

Accretion and differentiation, before the Great collision.
Hadéen, corresponding to the Great late bombardment (supposed between -4.1 and -3.8 Ga).
Archaean, corresponding to the first formations of the earth's crust (girdles green stones of Isua) until the ribboned Gisements of iron.
Transition, until the red first continental Layers.
Protérozoïque, characterized by the Plate tectonics and extending until Cambrien.

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