Cryogenian – Wikipedia

The Cryogenian or Glaciation Varanger is the second period of neoproterozoic. It covers 720 to 635 And ^{[ first ]} . She follows the Tonien and precedes the publisher.

The earth has in fact undergone two episodes of glaciation separated by an interglacial period of some 10 million years old, the Stuttienne glaciation, from around 720 to 660 million years, and the Marinone glaciation, from around 650 to 635 million years ^{[ 2 ] , [ 3 ] , [ 4 ]} .

Almost all of the surface of the earth would have been covered with ice during this glaciation. This theory developed by the geologist Paul F. Hoffman is known as Snowball earth (phenomenon that it would be more fair to call Earth ice ball ).

Its name is made up of the terms κρύος, cryos (“Fridge») And Iruένεις («óves meat).

The first traces of multicellular life discovered date from approximately −2,1 For (fossils of the Franceville sedimentary basin) ^{[ 5 ]} , but it took a while to see the first animals, more complex.

Recent work ^{[ 6 ]} suggest that this era, despite apparently difficult living conditions, coincided with an increase in algae populations in the cryogenic oceans and with the emergence of first forms of animal life (still very primitive, with the first sponges in particular )) ^{[ 7 ]} . They will be locally so abundant and during a period so long that their Spicules is the main constituent of certain rocks (Spiculites ^{[ 8 ] , [ 9 ] , [ ten ]} , Gaizes et spongolites ^{[ 11 ]} ).
It is therefore a period of transition between a living world bacterial, then algo-bacterial in part photosynthetic ^{[ twelfth ] , [ 13 ]} Towards a world also colonized by eucaryotes ^{[ 6 ]} . The period from 800 to 717 mA is indeed characterized by growth in the diversity of eukaryotic microfossils.

This corresponds to one of the first great ecological upheavals, one of the deepest that life on earth has known: with total reorganization of the distribution of the carbon cycle and nutrients (phosphorus in particular ^{[ 14 ] , [ 15 ]} ) in the water column and the increase in energy flow to more and complex trophic levels ^{[ 6 ]} .

The progress of biogeochemistry has enabled the retrospective study of certain eukaryotic biomarkers such as colastane, ergostane, stigmastane, dinostane, isopropylcholesaine, n-propylcholesaine and cryostane ^{[ 16 ]} . It confirmed the importance of this geological period for evolutionary cancellation ^{[ 16 ]} . And it provides clarifications on the evolutionary geography of the first eurcaryotes as well as on the taxonomic enrichment of this group during the great glaciation of the earth and to the ediacarian (635-541 ma) ^{[ 16 ]} .
The 26-Methylsterol could have protected sponges, but also other eukaryotes, against their own membranylic toxins (various protisists secrete lytic toxins (that is to say capable of dissolving (lyser) of the cellular walls), which allows them to escape predation and to parasitize or kill eucaryotic prey ^{[ 16 ]} .
As the cell membranes built with sterols can be the target of such attacks, the abundance of cryostane pleads in favor of a predation in the Chuar group and pleads for the hypothesis of an eukaryophagy which would have become widespread in Cryogenian ^{[ 16 ]} .

The molecular fossil assessment of eukaryotic steroids ^{[ 17 ]} encourages to think that bacterial biomass was initially still the only important primary source in the cryogenian oceans. These steroids then diversified and became very abundant, which is a sign of the rapid increase in marine planktonic algae biomass ( Archaeplastida ), in an interval of time geologically very short, between the Glaciations Sturtian and Marinoan “La Terre Boule de Snow ^{[ 6 ]} . It also provides clues to the life and appearance of sponges that seem to be among the first important colonial animal organizations ^{[ 18 ]} .

Some geobiologists believe that it is the time for the development of cyanobacteria ^{[ 6 ]} .

The explosion of Algual and Cyanobacterial Plankton has given birth to new food chains, and especially to more efficient and complex nutrient and energy transfers ^{[ 19 ]} , allowing the development of increasingly complex and stable ecosystems as shown in the joint appearance of biomarkers for sponges (who seem to have had to adapt to toxic protists ^{[ 16 ]} ) and predatory Rhizarians, and the subsequent radiation of the Eumetazoa in the ediacarian period ^{[ 7 ]} .

The precise geological times of this transition still remain to be refined, as well as the possible links with on the one hand the increase in the rate of atmospheric oxygen ^{[ 20 ] , [ 6 ]} (and therefore the appearance of a layer of protective ozone), and on the other hand with the beginning of animal evolution ^{[ 6 ]} .

At the time of the cryogenian, the supercontinent of Rodinia begins to fragment, a rift separating the continent into two large masses. It is this fragmentation that would be at the origin of the glaciation ^{[ 21 ]} .
Indeed, the heart of a supercontinent is generally far from the oceanic and therefore desert influence. Its fragmentation makes new surfaces accessible to erosion and transport: the erosion of exposed silicates leads to the capture of CO _{2 pour former des bicarbonates solubles, et leur arrivée dans l’océan les fait précipiter sous forme de carbonates, fixant ainsi le carbone. La fragmentation d’un supercontinent est donc toujours suivie d’une chute dans l’effet de serre du gaz carbonique, entraînant un refroidissement global.}

At the same time, the huge flows of lavas produced by the fracture of Rodinia formed basalt surfaces on the surface of the continents. However, these surfaces, altered under the effect of humidity, consume eight times more carbon than the same granite surface. In addition, the sun was younger and radiated 6% less heat.

All these factors were able to lead to a particularly intense ice age having covered the earthly surface of glaciers up to 30 ° of latitude. Once this threshold has been reached, the global Albédo would then have become as it would have produced a self-employed loop which would have covered the entire planet with ice. Some scientists believe that this very severe glaciation concerned the whole planet; Others believe that the equatorial strip has been spared from ice.

At the end of the Ice episode, despite the ice down to the equator, volcanic activity continued to emit CO ₂ and methane (ch ₄ ) in the air. When the concentration of carbon dioxide has reached 350 times that of today, a sufficient greenhouse has occurred to start the debacle.

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