PROTEROZOIC EON: CHARACTERISTICS, GEOLOGY, FLORA AND FAUNA - GEOGRAFÍA - 2025

The Proterozoic Aeon is one of the geological scales that make up the Precambrian. It goes from 2.5 billion years ago to 542 million years ago. It was a time of many momentous changes, important for the evolution of the planet.

Among these we can mention: appearance of the first photosynthetic organisms and increase in atmospheric oxygen. In short, in this eon, the planet underwent the first changes that prepared it to become a habitable place.

Stromatolites, characteristic of this eon. Source: C Eeckhout, via Wikimedia Commons

From a geological point of view, during this eon certain structures were formed that were the starting point to give rise to what later became known as the supercontinent Pangea.

This eon was a time of transition, from a planet that, due to its conditions, can be considered hostile, to one in which, gradually, it was possible for life to settle and develop.

characteristics

Presence of cratons

Scholars in the area have established that cratons are the "cores" of the continents. This means that cratons are the first structures from which the continental shelves were established.

They are made up of archaic rocks, with an antiquity dating from 570 million years to 3.5 giga years.

The main characteristic of cratons is that for thousands of years they have not suffered any type of fracture or deformation, so they are the most stable places in the earth's crust.

Some of the best known cratons on the planet are: Guiana Shield in South America, the Siberian Shield, Australian Shield and the Scandinavian Shield.

Stromatolites appeared

Stromatolites are structures that are formed by microorganisms, specifically cyanobacteria, in addition to precipitated calcium carbonate (CaCO ₃). Likewise, it has been discovered that in stromatolites there are not only cyanobacteria, but there may also be other organisms such as fungi, insects, red algae, among others.

Stromatolites are geological records of great importance for the study of life on the planet. This is because, in the first place, they constitute the first record of life on Earth (the oldest are 3,500 million years old).

Likewise, stromatolites provide evidence that already in that ancient era, so-called biogeochemical cycles were carried out, at least that of carbon.

In the same way, stromatolites have been of great help in the area of ​​paleontology as indicators. This means that, according to the studies carried out, these are developed under specific environmental conditions.

For this reason, it has been possible to predict the characteristics that a region had during a certain time, only with the analysis of the stromatolites found there.

These structures produce a mucilaginous matrix, in which sediments and calcium carbonate are fixed. They have some photosynthetic activity, so they release oxygen into the atmosphere

Increased oxygen concentration

One of the most important and representative characteristics of the Proterozoic era is that there was a significant increase in the concentration of atmospheric oxygen.

During the Proterozoic era, there was great biological activity, which resulted in a greater availability of atmospheric oxygen. Now, with respect to the element oxygen, various events occurred that were milestones in this era.

It is important to mention that atmospheric oxygen did not reach a significant level until the so-called chemical sinks were satisfied, among which the most important was iron.

As atmospheric oxygen increased, banded iron deposition also increased. This in turn helped to remove free oxygen, as it reacted with iron to form ferric oxide (Fe ₂ O ₃), precipitating as hematite on the seabed.

Once these chemical sinks were filled, biological activity continued, including photosynthesis, so atmospheric oxygen continued to increase. This is because it was not used by the chemical sinks, since they were completely full.

The great oxidation

This was an event of great importance and significance. It comprises a series of events that are related to the increase in atmospheric oxygen discussed in the previous point.

When the amount of oxygen exceeded that which was absorbed by the various chemical reactions, anaerobic organisms (which were in the majority) were directly affected, for which oxygen was very toxic.

This also had consequences at the climatic level, since the various chemical reactions that involved free oxygen, methane and ultraviolet radiation, resulted in a considerable reduction in environmental temperature, which, in the long run, led to the so-called glaciations.

geology

The archaeological records of this era are among the best that exist, in terms of the amount of information they have provided.

The primary change that occurred during the Proterozoic Eon was at the tectonic level. In this era, tectonic plates grew larger and only underwent deformations as a result of multiple collisions at their edges.

According to specialists, a total of five supercontinents were formed in this era:

• Ancient Siberia: made up of much of Mongolia and the Siberian shields.
• Gondwana: perhaps one of the largest, as it was made up of territories in what is now known as South America, Africa, Antarctica, Central America, and much of Asia.
• Old Continent of North America: also another of great size, including the Canadian Shield, the island of Greenland and part of Siberia.
• Ancient China: includes China, part of Mongolia, Japan, Korea, Pakistan and some territories of India.
• Former Europe: encompasses much of what is now the European continent, in addition to part of the Canadian coast.

Likewise, according to geological evidence, at that time, the Earth rotated much faster on its axis, the days having an approximate duration of 20 hours. On the contrary, the translation movement occurred slower than now, since the years had an average duration of 450 days.

Similarly, the rocks that have been recovered and studied, coming from the Proterozoic Era, have shown that they suffered little effect of erosion. Rocks that have remained totally unaltered have even been rescued, which has been of great help to those who study these phenomena.

Flora and fauna

The first forms of organic life began to appear in the previous era, the Archaic. However, it was thanks to the atmospheric transformation that occurred in the Proterozoic Era that living beings began to diversify.

Since the archaic, the simplest forms of life that are still known had begun to appear: prokaryotic organisms. These include blue-green algae (cyanobacteria) and bacteria themselves.

Later, eukaryotic organisms (with a defined nucleus) began to appear. Likewise, in this period the green algae (Clorophytas) and the red algae (Rodhophytas) also appeared. Both are multicellular and photosynthetic, so they contributed to the expulsion of oxygen into the atmosphere.

It is important to note that all living beings that originated in this era were found in aquatic environments, since these were the ones that provided them with the minimum conditions necessary to be able to survive.

Among the members of the fauna of this period we can mention organisms that today are considered little evolved, such as sponges. It is known that they existed because certain chemical analyzes detected a particular form of cholesterol that is only produced by these organisms.

Similarly, fossils of animals representing coelenterates have also been recovered from this period. This is a large group in which mainly jellyfish, corals, polyps and anemones are found. The main feature of them is radial symmetry

Ediacara fauna

In the Ediacara Mountains (Australia), in 1946 the paleontologist Reginald Sprigg made one of the greatest discoveries in paleontology. He discovered a site with fossil records of the first known living beings.

Here fossils of sponges and anemones were observed, as well as other species that still baffle paleontologists today, as some classify them as soft organisms (of the animal kingdom) and others as lichens.

Among the characteristics of these beings we can mention: absence of hard parts such as a shell or some bone structure, without intestine or mouth, in addition to being vermiform without a specific symmetry pattern.

Recreation of the Ediacara Fauna. Source: Ryan Somma, via Wikimedia Commons

This discovery was very important, because the fossils found do not present similarities with those that correspond to more recent eras. In the Ediacaran fauna there are flat organisms that can have radial or spiral symmetry.

There are also a few that have bilateral symmetry (which is abundant today), but they are a tiny percentage compared to the others.

At the end of the period, this fauna practically disappeared in its entirety. Today no organisms have been found that represent an evolutionary continuity of these species.

Weather

At the beginning of the period the climate could be considered stable, with a large amount of what is known as greenhouse gases.

However, thanks to the emergence of cyanobacteria and their metabolic processes that resulted in the release of oxygen into the atmosphere, this rare balance was destabilized.

Glaciations

During this period the first ice ages that the Earth experienced occurred. Among these, the best known and perhaps the most devastating was the Huronian Ice Age.

This glaciation occurred specifically two billion years ago and resulted in the disappearance of anaerobic living beings that populated the Earth at that time.

Another great glaciation that took place in this period was the so-called superglaciation, explained in the theory of the "Snowball Earth". According to this theory, there was a time, during the cryogenic period of the Proterozoic Era, in which the planet was entirely covered by ice, which from space gave it the appearance of a snowball.

According to various studies and the evidence gathered by scientists, the main cause of this glaciation was a significant decrease in some greenhouse gases such as carbon dioxide (CO2) and methane (CH4).

This occurred through various processes, such as the combination of CO2 with silicates to form calcium carbonate (CaCO3) and the elimination of CH4 by oxidation, thanks to the increase in atmospheric oxygen (O2).

Due to this, the Earth entered a progressive cooling spiral, in which its entire surface was covered with ice. This resulted in the Earth's surface reflecting sunlight heavily, causing the planet to continue cooling.

Subdivisions

The Proterozoic Aeon is divided into three eras: Paleoproterozoic, Mesoproterozoic and Neoproterozoic.

Paleoproterozoic era

It spans from 2.5 billion years ago to 1.8 billion years ago. During this era, two great events of great importance took place: the great oxidation, a product of photosynthesis that cyanobacteria began to carry out, and one of the first lasting stabilizations of the continents. The latter was thanks to the great expansion of cratons, which contributed to the development of large continental-type platforms.

Likewise, it is believed, according to various evidences, that it was in this era that the first mitochondria appeared, a product of the endosymbiosis of a eukaryotic cell and a proteobacterium.

This was a momentous event, since the mitochondria use oxygen as an electron acceptor during the process of cellular respiration, with which aerobic organisms would have had their origin.

This era is subdivided into four periods: Sidérico, Riácico, Orosírico and Estaérico.

Mesoproterozoic era

This era spans from 1600 to 1200 million years ago. It is the middle age of the Proterozoic Aeon.

Characteristic events of this era include the development of the supercontinent known as Rodinia, as well as the fragmentation of another supercontinent, Columbia.

From this era there are some fossil records of some organisms that bear certain similarities with the current rhodhophytes. Likewise, it has been concluded that during this era stromatolites are particularly abundant.

The Mesoproterozoic Era is subdivided into three periods: Callimic, Ectatic and Esthetic.

Neoproterozoic era

It is the last era of the Proterozoic Aeon. It covers from 1000 to 635 million years ago.

The most representative event of this era was the superglaciation in which the Earth was covered with ice almost entirely, which is explained in the Snowball Earth Theory. During this period it is believed that the ice could even reach tropical areas close to the Equator.

Similarly, this era was also important from the evolutionary point of view, since the first fossils of multicellular organisms came from it.

The periods that make up this era are: the Tonic, Cryogenic and Ediacaran.

References

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4. Kearey, P., Klepeis, K., Vine, F., Precambrian Tectonics and the Supercontinent Cycle, Global Tectonics, Third Edition, pp. 361–377, 2008.
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