Archives par mot-clé : Precambrian

Le Précambrien : les bactéries, la tectonique des plaques et l’oxygène (2/2)

by Alain Préat, 25 septembre 2019 in Science-Climat-Energie


Résumé : L’oxygène n’est pas apparu aussi brutalement qu’on le pensait sur notre planète (nb: première partie 1/2, ici).

Malgré un apport en oxygène lié aux cyanobactéries dès l’Archéen, ce ne se sont pas ces microorganismes qui sont à la base de la première grande ‘révolution’ de l’oxygène qui a eu lieu à la limite Archéen/Paléoprotérozoïque (il y a 2,5 milliards d’années) dans l’atmosphère, lors du Grand Evénement d’Oxydation. Ce sont les processus liés au cycle de la tectonique des plaques (activité mantellique et périodes intenses d’érosion/altération) qui ont contribué de manière déterminante à l’augmentation de la concentration de l’oxygène atmosphérique vers 2,5 milliards d’années. Les deux principaux processus responsables de cette augmentation sont liés à l’enfouissement de la matière organique et de la pyrite (= FeS2). L’altération des séries riches en ces deux composants conditionnera ensuite pendant près d’un milliard d’années la composition chimique des océans en oxygène, soufre et fer. Au cours du temps, l’oxygène proviendra de l’activité des cyanobactéries et l’atmosphère réductrice du début de l’Archéen sera remplacée par une atmosphère oxydante à la fin du Précambrien.

Abstract : Oxygen did not appear as abruptly as we thought on our planet.

Despite an oxygen supply related to cyanobacteria, since the Archean, it is not these microorganisms that are at the base of the first great oxygen revolution that took place at the Archean/Paleoproterozoic boundary (2.5 billion years) in the atmosphere during the Great Oxidation Event. Two processes related to the cycle of plate tectonics (mantle activity and intense periods of erosion/weathering) were mostly involved in the increase of the of atmospheric oxygen concentration 2.5 billion years ago. These two main processes are related to the burial of organic matter and those of pyrite(= FeS2) The alteration of series with high contents of the two elements will then condition for nearly a billion of years the oxygen, sulfur and iron chemical composition of the oceans. The oxygen will finally come from the activity of cyanobacteria and the early Archean reducing atmosphere will be replaced by an oxidizing atmosphere at the end of the Precambrian.

Le Précambrien : les bactéries, la tectonique des plaques et l’oxygène (1/2)

by Alain Préat, 20 septembre 2019, in ScienceClimatEnergie


Résumé : L’oxygène n’est pas apparu aussi brutalement qu’on le pensait sur notre planète.

Malgré un apport en oxygène lié aux cyanobactéries dès l’Archéen, ce ne se sont pas ces micro-organismes qui sont à la base de la première grande ‘révolution’ de l’oxygène qui a eu lieu à la limite Archéen/Paléoprotérozoïque (il y a 2,5 milliards d’années) dans l’atmosphère, lors du Grand Evénement d’Oxydation. Ce sont les processus liés au cycle de la tectonique des plaques (activité mantellique et périodes intenses d’érosion/altération) qui ont contribué de manière déterminante à l’augmentation de la concentration de l’oxygène atmosphérique vers 2,5 milliards d’années. Les deux principaux processus responsables de cette augmentation sont liés à l’enfouissement de la matière organique et de la pyrite (= FeS2). L’altération des séries riches en ces deux composants conditionnera ensuite pendant près d’un milliard d’années la composition chimique des océans en oxygène, soufre et fer. Au cours du temps, l’oxygène proviendra de l’activité des cyanobactéries et l’atmosphère réductrice du début de l’Archéen sera remplacée par une atmosphère oxydante à la fin du Précambrien.

Abstract : Oxygen did not appear as abruptly as we thought on our planet.

Despite an oxygen supply related to cyanobacteria, since the Archean, it is not these microorganisms that are at the base of the first great oxygen revolution that took place at the Archean/Paleoproterozoic boundary (2.5 billion years) in the atmosphere during the Great Oxidation Event. Two processes related to the cycle of plate tectonics (mantle activity and intense periods of erosion/weathering) were mostly involved in the increase of the of atmospheric oxygen concentration 2.5 billion years ago. These two main processes are related to the burial of organic matter and those of pyrite (= FeS2). The alteration of series with high contents of the two elements will then condition for nearly a billion of years the oxygen, sulfur and iron chemical composition of the oceans. The oxygen will finally come from the activity of cyanobacteria and the early Archean reducing atmosphere will be replaced by an oxidizing atmosphere at the end of the Precambrian.

OLYMPUS DIGITAL CAMERA

Figure 2a( en haut). Stromatolithe columnaire, Néoprotérozoïque, (Formation SC1c in Préat et al. 2018), Bassin du Niari, République du Congo (Brazzaville), photo A. Préat, 2016.

Oxygen could have been available to life as early as 3.5 billion years ago

by Imperial College, November 27, 2018 in ScienceDaily


The levels of oxygen dramatically rose in the atmosphere around 2.4 billion years ago, but why it happened then has been debated. Some scientists think that 2.4 billion years ago is when organisms called cyanobacteria first evolved, which could perform oxygen-producing (oxygenic) photosynthesis.

Other scientist think that cyanobacteria evolved long before 2.4 billion years ago but something prevented oxygen from accumulating in the air.

Cyanobacteria perform a relatively sophisticated form of oxygenic photosynthesis — the same type of photosynthesis that all plants do today. It has therefore been suggested that simpler forms of oxygenic photosynthesis could have existed earlier, before cyanobacteria, leading to low levels of oxygen being available to life.

Now, a research team led by Imperial College London have found that oxygenic photosynthesis arose at least one billion years before cyanobacteria evolved. Their results, published in the journal Geobiology, show that oxygenic photosynthesis could have evolved very early in Earth’s 4.5-billion-year history.

See also here

Plate tectonics may have been active on Earth since the very beginning

by University of Tennessee at Knoxville, September 26, 2018 in ScienceDaily


A new study suggests that plate tectonics — a scientific theory that divides the earth into large chunks of crust that move slowly over hot viscous mantle rock — could have been active from the planet’s very beginning. The new findings defy previous beliefs that tectonic plates were developed over the course of billions of years.

The paper, published in Earth and Planetary Science Letters, has important implications in the fields of geochemistry and geophysics. For example, a better understanding of plate tectonics could help predict whether planets beyond our solar system could be hospitable to life.

“Plate tectonics set up the conditions for life,” said Nick Dygert, assistant professor of petrology and geochemistry in UT’s Department of Earth and Planetary Sciences and coauthor of the study. “The more we know about ancient plate tectonics, the better we can understand how Earth got to be the way it is now.”

Billion-year-old lake deposit yields clues to Earth’s ancient biosphere

by McGill University, July 18, 2018 in ScienceDaily


The findings, published in the journal Nature, represent the oldest measurement of atmospheric oxygen isotopes by nearly a billion years. The results support previous research suggesting that oxygen levels in the air during this time in Earth history were a tiny fraction of what they are today due to a much less productive biosphere.

“It has been suggested for many decades now that the composition of the atmosphere has significantly varied through time,” says Peter Crockford, who led the study as a PhD student at McGill University. “We provide unambiguous evidence that it was indeed much different 1.4 billion years ago.”

The study provides the oldest gauge yet of what earth scientists refer to as “primary production,” in which micro-organisms at the base of the food chain — algae, cyanobacteria, and the like — produce organic matter from carbon dioxide and pour oxygen into the air.

Le Précambrien de l’Afrique de l’Ouest : que d’événements globaux riches d’enseignements

by Alain Préat, 31 mai 2018, Académie Royale des Sciences d’Outre- Mer


Le Précambrien représente 88% de l’histoire de la Terre âgée de 4,567 milliards d’années (Ga).

C’est au cours de cette période peu connue, peu enseignée que se sont déroulés ou mis en place des événements physico-chimiques et biologiques déterminants: différenciation des enveloppes terrestres, tectonique des plaques et premières ‘pangées’ ou supercontinents, champ magnétique, chaînes de montagnes, glaciations, anoxies des bassins, remplacement du CO2-CH4par l’oxygène atmosphérique, formation de gisements (uranium, manganèse, nickel …. et même pétrole), émergence dès 3,8 Ga des procaryotes puis des eucaryotes …

Vu l’absence de fossiles stratigraphiques, et donc de biozones, la stratigraphie du Précambrien est encore très difficile, elle  est intialement basée sur la lithostratigraphie. De grands progrès ont récemment été réalisés grâce à la chimiostratigraphie istotopique (C, O, Sr….) en plus de la radiométrie absolue.

L’exposé se consacrera aux événements sédimentaires liés au Grand Evénement de l’Oxygène il y a environ 2,5-2,1 Ga (Paléoprotérozoïque) et à ceux liés à la ‘Terre Boule de Neige’ (Snowball Earth) avec la glaciation marinoenne il y a 0,635 Ga (Néoprotérozoïque), à partir des séries de l’Afrique de l’Ouest.

 

Did the transition to plate tectonics cause Neoproterozoic Snowball Earth?

by R.J. Stern and N.M. Miller, December 20, 2017 in TerraNova


When Earth’s tectonic style transitioned from stagnant lid (single plate) to the modern episode of plate tectonics is important but unresolved, and all lines of evidence should be considered, including the climate record. The transition should have disturbed the oceans and atmosphere by redistributing continents, increasing explosive arc volcanism, stimulating mantle plumes and disrupting climate equilibrium established by the previous balance of silicate‐weathering greenhouse gas feedbacks. Formation of subduction zones would redistribute mass sufficiently to cause true polar wander if the subducted slabs were added in the upper mantle at intermediate to high latitudes. The Neoproterozoic Snowball Earth climate crisis may reflect this transition. The transition to plate tectonics is compatible with nearly all proposed geodynamic and oceanographic triggers for Neoproterozoic Snowball Earth events, and could also have contributed to biological triggers. Only extraterrestrial triggers cannot be reconciled with the hypothesis that the Neoproterozoic climate crisis was caused by a prolonged (200–250 m.y.) transition to plate tectonics.