From the origin of the earth to 570 million years ago.
The Precambrian era lasted from the formation of the Earth until the appearance of the first multicellular organisms about 570 million years ago. The age of the oldest rocks known to us is only 3.9 billion years, so we know negligibly little about the youth of our planet. Moreover, even these rocks have undergone such great transformations over billions of years that they can tell us little about anything.
About 2.5 billion years ago, the entire terrestrial land was, apparently, united into one huge supercontinent, which subsequently split into several.
By the end of the Precambrian era, the continents merged again, forming a new supercontinent. All these perturbations on land and at sea were accompanied by grandiose climatic changes. During the Precambrian, the world experienced at least three ice ages. The most ancient began about 2.3 billion years ago. The most grandiose glaciation in the entire history of our planet occurred between 1 billion and 600 million years ago.
Earth's early atmosphere did not contain oxygen. It consisted mainly of methane and ammonia gases, smaller amounts of hydrogen sulfide, water vapor, nitrogen and hydrogen, as well as carbon monoxide and dioxide. However, with the emergence of life on Earth, the picture changed dramatically.
First cells. Methane and other gases contained in the primitive atmosphere of the Earth, dissolved in the water of the seas, lakes and puddles, forming a complex chemical "broth" (1). Laboratory experiments have shown that under the influence of a lightning discharge in such a "broth" chemical reactions begin to occur and more complex chemical compounds are formed, very similar to those found in living cells (2). Ultimately, some of the organic compounds acquired the ability to reproduce themselves, that is, they began to create copies of themselves (3). Fat globules were also contained in the same "broth" (4). When the wind strongly stirred the "broth", some complex compounds could get inside these fat globules (5) and remain there "locked up". Over time, these hybrid structures evolved into living cells surrounded by a fatty membrane.
The matter of life.
All living things contain a certain set of special chemical compounds.
The cell mainly consists of proteins or of the substances synthesized by them. All proteins found in living matter are formed by strands of special chemicals - amino acids. Cells also contain another chemical, ATP, which is used to store energy.
The program to create new cells - and even new animals or plants - exists as a special chemical code contained in a long molecule called DNA. Each species of living organisms has its own special type of DNA. All these substances - proteins, ATP and DNA - contain carbon, that is, they are organic compounds. But how did the first organic substances appear?
Life puts experiments
The gases that formed the Earth's early atmosphere gradually dissolved into the World Ocean, and a kind of "warm soup" of chemical compounds arose in it. Since there was no oxygen in the atmosphere, there was no ozone layer (ozone is a type of oxygen) that could protect the earth's surface from harmful ultraviolet solar radiation.
In the 20s. 20th century Russian scientist Alexander Oparin and English scientist John Haldane put forward a hypothesis according to which for many millions of years this radiation,
together with lightning discharges, created more and more complex chemical compounds in the chemical "broth", until finally one organic compound arose - DNA, capable of reproducing itself.
In the 50s. 20th century American chemist Stanley Miller decided to test this hypothesis. He mixed methane and ammonia over the surface of warm water and passed an electric current through them, creating something like lightning. Miller repeated this experiment many times, changing the composition of the gas mixture and temperature regime. In several cases, he found that after only 24 hours, about half of the carbon contained in the methane had been converted into organic compounds such as amino acids. Hence, we can conclude that with sufficient time and the appropriate composition of the gas mixture, more complex chemical compounds could also be formed in the same way, perhaps even those that are part of DNA.
The first living cells
The chemical "broth" in the primitive ocean became thicker, and more and more new compounds were formed in it. Some of them formed thin continuous films on the surface of the water - like a film of oil spilled on the sea. The water was mixed, for example, during a storm, and the film was torn into separate spherical formations, similar to oil balls. Inside them were separate chemical compounds that began to resemble real living cells. As soon as the DNA molecules were formed in the "broth" and found themselves, together with other substances, inside such a shell, it marked the beginning of life on Earth.
The first cells in many ways resembled modern bacteria. They produced the necessary energy by splitting inorganic compounds. Cells could extract carbon from methane, as well as from carbon monoxide and carbon dioxide dissolved in water.
From hydrogen sulfide and other compounds containing it, they extracted hydrogen. All these elements of the cell were used to reproduce new living matter. Similar bacteria are now found around hot mineral springs and active volcanoes.
Primitive forms of bacteria and cyanides (blue-green algae) are still found in abundance in hot mineral springs. Some of them use minerals from these sources as "raw materials" for photosynthesis.
Scientists believe that life could have originated in a similar environment. At the bottom of the picture, if you look closely, you can distinguish two people on the path near the source.
Taming the energy of the sun.
The next most important stage in the evolutionary process is the taming of solar energy by living matter. Instead of extracting energy from inorganic compounds, cells began to use directly the energy of sunlight.
This marked the beginning of photosynthesis, a special process in which plants synthesize nutrients using the energy of sunlight. And instead of extracting the hydrogen they need from substances such as hydrogen sulfide, they learned to extract it from a much more common substance - water.
Photosynthesis: a giant leap in evolution
Plants, algae and some types of bacteria "capture" sunlight with the help of colored chemical compounds contained in cells - the so-called pigments. They use this light energy to synthesize all the organic compounds they need for growth and reproduction. This process is called photosynthesis, which means "creation with the help of light." In order to create complex compounds, such as sucrose or proteins found in living cells, from simple chemicals, such as water or carbon dioxide, you need to expend a certain amount of energy. It's a lot like building a wall: to lift bricks to the top of the wall and fix them in place, you need energy. In photosynthesis, this energy comes from sunlight. Carbon dioxide (containing carbon and oxygen) and water (comprising hydrogen and oxygen) give carbon, oxygen, and hydrogen. Sucrose and other organic compounds produced during photosynthesis are synthesized from them. In this case, not all oxygen is consumed; some of it is released into the atmosphere.
To capture the sun's rays, these new photosynthetic cells produced pigments, colored substances capable of absorbing light. Until that time, life on Earth was dull and colorless. Now she began to play with the multicolor of new colors. From now on, living organisms are no longer tied to places with particularly energy-intensive substances: water and sunlight have become much more affordable sources of energy.
The new photosynthesizers lived mainly in mineral springs and warm coastal waters of the seas, where it was shallow enough for sunlight to reach them, but at the same time deep enough to protect them from the harmful effects of ultraviolet radiation. Some of the cells continued to release hydrogen from hydrogen sulfide; their descendants are still found near hot mineral springs.
Live stromatolites in Shark Bay, Australia. Since photosynthesis occurs in stromatolites, they extract carbon dioxide dissolved in it from water. In this case, calcium carbonate (lime) is released from the solution. The sticky slime produced by stromatolites traps tiny particles of lime, and eventually layers of limestone are formed.
Sectional image of a fossil stromatolite showing layers of limestone and cyanobacteria.
The era of stromatolites.
Some of the earliest photosynthetic organisms that have come down to us in fossil form are stromatolites (see also p. 34). These strange structures appear at first glance to be composed of many limestone rings separated by thin conical layers. In fact, they were formed by primitive organisms similar to the simplest cyanoacteria, which are sometimes called blue-green algae. Stromatolites were distinguished by an incredible variety of shapes and sizes. Some were round like potatoes, others were cone-shaped, others were tall and thin or even branched.
Fossilized stromatolites are found all over the world. In many places they form huge reefs, often rising from the sea floor hundreds of meters through clear water, like modern coral reefs in the tropics. The oldest fossil stromatolites were found in Western Australia, in rocks 2.8 billion years old. However, unidentified structures, which, according to scientists, could also be fossilized stromatolites, are found even in rocks that are at least 3.5 billion years old. Living stromatolites live on Earth today. They, like their distant ancestors, prefer warm shallow water. However, the current range of stromatolites is limited only to those places where there are few animals that feed on them.
red deposits
Some of the oldest fossils, including many stromatolites, occur in rocks called shales, which is not typical of sedimentary rocks of later eras. This baffled geologists for a long time, until they finally realized that the formation of such layers is associated with the vital activity of stromatolites. Gradually, the concentration of oxygen in the oceans increased, and he began to enter into chemical reactions with iron dissolved in water. formed
con" with its own shell - the so-called organelles. Each compartment had a special internal environment, so various processes now took place in different parts of the cell. Now chemical reactions in cells began to proceed much more efficiently. DNA - a substance containing the genetic code - was ordered into special Chromosome Structures Scientists believe these new cells formed when aerobic cells invaded other cells, perhaps to protect themselves from new "predatory" cells, while the new cells shared energy and produced chemicals with each other.
compounds of iron and oxygen - the so-called iron oxides. They could not dissolve in water and settled to the bottom along with other sediments.
Approximately 2.2 billion years ago, sedimentary rocks of a new type, the so-called red-colored deposits, also began to form on land. These rocks contained a large amount of iron oxides, which gave them a reddish hue of rust color. "So, by that time, oxygen appeared in the atmosphere. All the iron in the ocean was already bound, and excess oxygen entered the atmosphere in the form of gas.
poisoned by oxygen
Throughout the Precambrian, the concentration of oxygen in the Earth's atmosphere constantly increased. However, this did not bring anything good to many living organisms of that time. For them, it was tantamount to massive atmospheric pollution. After all, the first living organisms arose in an oxygen-free environment, and oxygen turned out to be a deadly poison for them. Many species disappeared from the face of the Earth - this was the first great extinction in its history. The paths of evolution are truly inscrutable: today we cannot imagine life without oxygen, and for the first living organisms, oxygen in the atmosphere was deadly.
Ultimately, evolution produced cells capable of not only
live in an oxygen environment, but also turn it to your advantage. After all, some compounds formed during photosynthesis can be split with the help of oxygen, and the energy released in this case can be used to create a number of new compounds. In most living cells, the process of respiration still proceeds in this way. Scientists call it aerobic breathing ("aerobic" means "using air"). During this process, much more energy is released than in any other biodegradation processes that occur without the participation of oxygen. Some "breathing" cells have even acquired the ability to absorb other cells, using them as food.
The very first cells, the so-called prokaryotes (left), were extremely primitive. All the chemicals they contained, including the DNA with the genetic code, were mixed and scattered throughout the cell. Later eukaryotic cells (right) had small internal compartments with their own membrane. They contained chemicals for certain reactions, and each of them had exactly the environment that is necessary for the most rapid course of this reaction. DNA was concentrated in chromosomes located inside the cell nucleus, surrounded by a nuclear envelope. The nucleus controlled all the life of the cell.
Setting the stage for evolution.
Oxygen accumulated in the atmosphere, and the ozone layer began to form there, which absorbed the harmful ultraviolet radiation of the Sun. Now life has been able to move closer to the surface of the oceans and even penetrate into the wet coastal regions of the land. Cyanobacteria also became more and more complex. They began to group into clods and thin threads. Yet the new aerobic oxygen-breathing cells gradually took over.
Variability is the catalyst of life
More importantly, new cells began to multiply in a completely different way. Instead of simply dividing in half and forming two other cells - exact copies of the previous one, these new cells began to do something strange. Two cells merged into one, exchanged some of their DNA, and then divided again into two or more new cells. This is called sexual reproduction. The new cells now had mixed DNA from both of their parents. Sexual reproduction led to a sharp increase in variability among cells, which, in turn, gave a powerful impetus to the evolutionary process.
First great extinction
The Late Precambrian was marked by grandiose natural disasters. They were accompanied by numerous volcanic eruptions, earthquakes and mountain building processes. A huge amount of volcanic ash thrown into the atmosphere led to a cooling of the climate; huge land masses moved towards the pole, and giant ice sheets spread across the globe.
During this period, many species of ancient organisms became extinct. In the end, the ice began to melt, the ocean level gradually rose, and the water flooded the coastal regions of the continents. For creatures that lived in shallow water, new, not yet occupied lands opened up with unlimited possibilities for leading a specialized lifestyle. By this time, far less of the dangerous ultraviolet radiation from the Sun was reaching the Earth's surface than before, because it could not overcome the thickened ozone layer. In addition, there was now more oxygen in the atmosphere, which suited the new generation of living organisms quite well.
Today, a great variety of the most diverse single-celled organisms lives in the upper layers of the oceans. Many of them must be very similar to those that inhabited the seas of the Precambrian era. Above: Here is the microscopic, vitreous skeleton of radiolarians, single-celled animals with long thin appendages covered with sticky mucus, with which they caught prey - tiny organisms. Bottom: Calcareous multi-chamber foraminiferal shells are important guide fossils. These shells form the basis of some types of limestone. Like radiolarians, single-celled foraminifera had long sticky appendages for catching prey.
Mystery of multicellular organisms.
No one really knows exactly how the first multicellular animals arose. Perhaps, at some point, the divided cells stopped completely separating from each other. Or, on the contrary, various cells began to unite and organize themselves. At first glance, this seems incredible, but do not rush to conclusions. In 1907, the biologist H. J. Wilson conducted a series of experiments with sponges. He cut the red sponge into small pieces and began to pass them through a special device in order to separate the cells from each other - until he finally got a red precipitate in a carafe of water. Much to his surprise, in a matter of hours, the cells were again grouped into a single whole. Then they gradually began to self-organize into a new sponge, forming chambers, channels and branched tubules. A week later, the sponge was as good as new. Perhaps this is how the first multicellular animals formed.
Now there are such strange creatures as slime molds, or myxomycetes. They look like brightly colored lumps of slime crawling along the ground or along the bark of trees. One type of slime mold, the cellular slime mold, spends most of its life as individual cells, swarming in the soil, where they feed on bacteria. But when the supply of food dries up, each cell produces a special substance that attracts other slime mold cells. Millions of these cells come together and form a huge cellular mass that looks a lot like a multicellular organism. This mass moves and reacts to light and chemicals like a single animal. Ultimately, the slime mold appears in the form of a fruiting body, in many ways similar to the sporangium of a fungus. It has a high leg with a protective outer shell, and a spore sac is located on top.
Marks in the silt
These early soft-bodied animals had little chance of being preserved as a fossil. However, they left their traces or, more precisely, imprints in the rocks. The pits from which soft-bodied animals obtained food, body prints and marks in the thickness of the silt, where they rested, were found in rocks that are 700 million years old or more. However, in deposits, up to those whose age is 640 million years, such traces are extremely rare. By this period, the glaciation of the Late Precambrian had just come to an end and the conditions for a new grandiose evolutionary explosion had formed.
One being or many organisms? In response to a chemical "signal," millions of amoeba-like slime mold cells come together to form a moving film that eventually releases long-stalked spore capsules, much like protozoan fungi.
Animals Ediacaran.
In a remote part of South Australia, in the Ediacaran Mountains, there are ancient shallow-water and coastal sedimentary rocks, which are 640 million years old. Many fossil remains of animals of the Precambrian era have been preserved here. At least 30 different genera of multicellular organisms have been found in these rocks; it should be noted that similar assemblages of fossils are found in rocks of the same age in many places around the globe.
Ediacaran animals lived mainly on the seabed. They fed in a layer of organic matter (detritus) that covered the bottom silt formed by the remains of many single-celled organisms that inhabited the water column above them. Flatworms and annelids swam above the very bottom or crawled among the sediments. They had nowhere to hurry, because there were very few predators (animals that feed on other animals).
Sea feathers rose from the sea floor like some kind of feather-like flowers, carefully filtering the water in search of food. The tubeworms lay among the bottom sediments, moving their tentacles in the detritus-laden water. Primitive echinoderms, relatives of modern starfish and sea urchins, spent their entire lives in a thick layer of silt. There were also many large, flat pancake-shaped animals; these jellyfish-like creatures also seem to have lived on the muddy bottom. And above them in the sea water slowly floated real jellyfish.
Harbingers of the future
In the Ediacaran deposits, there are numerous fossilized imprints of soft-bodied animals that once crawled along the seabed. In some places, paired V-shaped marks were imprinted in the mud, similar to scratches left by pairs of tiny legs. Perhaps these are traces of primitive arthropods, or arthropods, the distant ancestors of fossil trilobites, as well as modern insects - spiders and scorpions. True, the solid remains of these animals have not yet been found: apparently, they have not yet acquired a hard shell.
All Ediacaran animals were soft-bodied. Many varieties of jellyfish lived there (1). Dixonia (2) and spriggins (3) were flat, worm-like creatures. Spriggin had many tiny swimming plates along the sides, like modern marine worms. Perhaps this animal is the ancestor of trilobites. Charniodisk (4), rank" (5) and pteridinium, leaf-shaped sea feathers were colonies of tiny hydra-like animals that filtered food particles from the water. But tribrachidium (7) is a complete mystery to us. It had a Y-shaped central a mouth with bristle-like processes, possibly the ancestor of modern echinoderms.
The Precambrian accounts for most of the geological history of the Earth - about 3.8 billion years. At the same time, its chronology is developed much worse than the Phanerozoic that followed it. The reason for this is that organic remains in Precambrian deposits are extremely rare, which is one of the distinguishing features of these ancient geological formations. Therefore, the paleontological method of study is not applicable to the Precambrian strata.
Intensive study of the geological history of the Precambrian began at the end of the 20th century, due to the advent of powerful methods of isotope geochronology.
The stratigraphic division of the Precambrian has been the subject of much debate. It is generally divided into Vendian, Proterozoic, and Archean. In the 90s, the Stratigraphic Commission adopted a unified Precambrian time scale, but it causes a lot of controversy.
Precambrian rocks come to the earth's surface on crystalline shields and form the foundation of the platforms. Very often they have undergone several stages of severe deformation, metamorphism, melt intrusion and partial melting. Deciphering such events is a daunting task, and Precambrian geology is considered by specialists to be one of the most complex areas of geology.
Organic world of the Archean era
Organic remains are almost non-existent in Archean deposits, but it does not follow from this that animals and plants did not exist at all in the Archean era. It is believed that in the Archaean, at least at the end, unicellular, and perhaps multicellular organisms lived on the globe that did not have a mineral skeleton that could be preserved in a fossil state to this day.
The organic world of the Proterozean era
Organic remains are much more common in Proterozoic deposits than in Archean ones. They are represented by calcareous secretions of blue-green algae, siliceous and calcareous skeletons of radiolarians and foraminifers, sponge spicules, worm passages, remains of coelenterates and arthropods, and primitive brachiopod shells. In addition to calcareous algae, accumulations of graphite-carbonaceous matter formed as a result of decomposition are among the oldest plant remains. Corycium enigmaticum. Filamentous algae, fungal filaments and forms similar to modern coccolithophores have been found in the siliceous slates of the iron ore formation of Canada. Ferrous bacteria have been found in the ferruginous quartzites of North America and Siberia.
The structure and composition of the Precambrian
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App. 1. ratio with noun. Precambrian associated with it 2. Peculiar to the Precambrian, characteristic of it. 3. Formed during the Precambrian period. Explanatory Dictionary of Efremova. T. F. Efremova. 2000... Modern explanatory dictionary of the Russian language Efremova
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As we have already pointed out, the system of living beings is inextricably linked with their phylogeny. These are two sides of the same phenomenon: the system is a static state of the modern animal world, phylogenetics gives an idea of the process ... ... Biological Encyclopedia
Despite the fact that very little is known about those distant times, many of the amazing creatures that inhabited the Earth in that era come to life in the capable hands of paleontologists and biologists.
Of the creatures, no skeletons, of course, have been preserved. Mostly because, in fact, animals did not have any skeletons at that time. In the Cambrian, however, they nevertheless acquired a bone shell and the beginnings of a notochord, but due to the prescription of times, one should not count on their safety. All information about the animals of the Vendian period (Precambrian, or, as it is also called, the Ediacaran, which lasted from about 635 to 541 ± 1 million years ago) and the Cambrian (which began approximately 541.0 ± 1 million years ago and ended 485.4 ± 1 .9 million years ago) scientists receive by fingerprints.
One of the main sources of these prints today is the Burgess Shale, located in Canada.
This soft-bodied animal of the Vendian period had a solid crescent-shaped head, similar to a trilobite shield, as well as a long body, which consisted of identical segments and resembles the body of polychaete worms.
Another Ediacaran animal, quite strongly reminiscent of the aforementioned spriggin. A characteristic feature of many Vendian organisms is that the segments of their bodies are, as it were, shifted relative to each other (dickinsonia, charnia, etc.) contrary to all the laws of bilateral symmetry (mirror reflection symmetry, in which the object has one plane of symmetry, relative to which its two halves are mirrored symmetrical; bilateral symmetry includes the bodies of humans and most modern animals - NS). This fact baffles scientists, since it was previously believed that Vendian animals were the ancestors of annelids. Today, this idea is being questioned, which is very puzzling for researchers trying to trace the origin of one species from another.
Another "inhabitant" of the Vendian period is dikinsonia
Ediacaran animals - iranians (shown in blue), below - three-beam albumares
But this creature of the Cambrian period seemed so amazing to paleontologists that for a second it seemed to them that they were seeing hallucinations. Hence the name. After all, judging by the surviving prints of this animal, it is logical to assume that instead of legs it had spikes (and two or three in one segment), and there were a number of some kind of soft processes on its back! This is hardly possible from the point of view of biological science. Fortunately, clearer prints were later found, which show that the hallucigenia was simply turned upside down, and the second row of its soft legs was not reflected in the print. So the hallucination looked like this:
A worm-like animal of the Cambrian. Possibly fed on sponges, since her remains are often found along with the remains of sponges.
A representative of a new generation of multicellular organisms, a genus of fossil soft-bodied scaly animals. It is assumed that Viwaxia lived from the end of the Lower Cambrian to the Middle Cambrian.
Primitive chordates, only about 5 cm long, possessed, perhaps, one of the first spines in history. Over millions of years, this simple structure will turn into a spine, without which we would not be able to stand or walk. By the way, the appearance of the skeleton as such, as well as more perfect eyes, are one of the most important factors characterizing the Cambrian explosion.
Another important representative of the Cambrian and subsequent geological epochs. This is an extinct class of marine arthropods. Perhaps one of the most numerous and most tenacious species of creatures that have ever lived on Earth. Trilobites were not very pretty and resembled modern woodlice, only much harder and larger - their body length could reach 90 cm. To date, more than 10 thousand fossil species of the trilobite class are known.
From the ancient Greek class Dinocarida (Dinocarida), to which the anomalocaris belongs, is translated as "unusual" or "terrible" shrimp. Probably the most amazing animal of the Cambrian seas. Anomalocaris, a predator of the genus of fossil arthropods, was not immediately found - first they discovered its parts and shrugged their hands over such an amazing animal for a long time. So, the imprint of the toothy mouth of anomalocaris was considered a strange jellyfish with a hole in the middle. The limbs with which he grabbed the victim were shrimp. The picture became clearer when a complete imprint of the animal was found.
Anomalocaris lived in the seas, swam with the help of flexible side blades. They are among the largest organisms known from Cambrian deposits. The length of their body could reach 60 cm, and sometimes 2 m.
No less amazing creatures similar to anomalocaris. Like Anomalocaris, they are all representatives of the extinct class of dinocarids. But instead of grasping processes - “shrimp”, opabinia has a folding proboscis and five eyes.
Marella does look like a monster from horror films, and Hurdia victoria was one of the largest predators of the Cambrian period, reaching a length of 20 cm. The mouth of these creatures was framed by 32 plates carrying two or three teeth each.
In general, as already written somewhere, the Precambrian would be perfect for beer lovers because of the abundance of snacks for it. As always, not everyone understood the joke and began to demand fresh trilobites in bars
In the Ediacaran (635-541 million years ago), life on Earth consisted mainly of unicellular bacteria and algae, but after the Cambrian period, multicellular organisms and animals began to dominate. The Cambrian was the first period (542-252 million years ago), which lasted about 57 million years, and then was replaced by , and periods. During these periods, as well as in subsequent eras, vertebrates predominated, which initially developed during the Cambrian.
Climate and geography
Not much is known about the global climate during the Cambrian period, but unusually high levels of carbon dioxide in the atmosphere (about 15 times higher than at present) meant that the average temperature could exceed 50°C. About 85% of the Earth was covered with water ( compared to 70% today), most of this area was occupied by the vast oceans of Panthalassa and Iapetus; the average temperature of these vast seas could range from 38 to 43 ° C. By the end of the Cambrian, 485 million years ago, the main part of the planet's land was concentrated on southern continent Gondwana, which had only recently broken away from even greater Pannotia in the preceding Proterozoic eon.
Sea life
Invertebrates
The main evolutionary event of the Cambrian was the "Cambrian explosion" - a phenomenon that entailed a dramatic change in the bodies of invertebrate organisms. This process took tens of millions of years.
Opabinia
For some reason, the Cambrian saw some truly bizarre creatures appear, including five-eyed opabinii, spiny hallucigenia, and large anomalocaris (which were some of the largest animals of the time).
Vivaxia
Most of these did not leave a single living descendant. This prompted speculation about what might have happened in subsequent geological epochs if, say, an "alien" wivaxia had evolved.
However, such bright representatives of invertebrates were far from the only life forms in the ocean. The Cambrian period marked the worldwide distribution of early plankton, as well as trilobites, worms, tiny mollusks, and small protozoa. In fact, the abundance of these organisms allowed Anomalocaris and other animals to flourish; these larger invertebrates were at the top and spent all their time feeding on the smaller invertebrates that were in close proximity to them.
Vertebrates
The Cambrian period marked the appearance of the earliest identified proto-vertebrate organisms, including Pikaya and slightly more advanced ones. Myllokunmingia and Haikouichthys. These three genera are considered to be the very first prehistoric fishes, although there is still a possibility that earlier candidates from the late Proterozoic will be found.
Vegetable world
There is still some controversy as to whether any true plants existed during the Cambrian period. If so, they were composed of microscopic algae and lichens (which have no tendency to fossilize). It is known that macroscopic plants such as seaweeds had not yet evolved during the Cambrian period, as evidenced by a notable gap in the fossil record.
The term "Precambrian" is very convenient in that it covers the entire period of the geological history of the Earth from the time when geological processes began to occur on it, and until the beginning of the Cambrian. This period of time is estimated differently in different sources, but the discrepancies are small. The beginning of the Precambrian is approximately 4.0 billion years, the end is 570 million years. Sometimes the Precambrian was called azoe ("lifeless"), cryptozoe ("hidden life"), emphasizing with these names the absence of life or the development of only the simplest forms of organisms in the Precambrian era. It has now been established that both of these names turn out to be incorrect, since the lower biological forms appeared almost simultaneously with the most ancient manifestations of sedimentation, and in the Late Precambrian, in addition to the lower ones, relatively highly organized forms existed. In principle, the biostratigraphic method can be applied to the Riphean and Vendian deposits (with its more advanced development). This was facilitated by numerous finds of stromatolites in the Riphean deposits and non-skeletal fauna of the Ediacaran type in the Vendian. Thus, the Late Proterozoic can no longer be attributed to the Cryptozoic, since life there existed in an explicit, and not hidden, microscopic form.
The Precambrian period of time is 7/8 of the history of the Earth. At this time, life was born, the earth's crust was radically transformed and its main structures were laid, most of the mineral resources (over 60%) were formed. However, the Precambrian has been relatively poorly studied, and there are objective reasons for this. The point is primarily in the strong dislocation of Precambrian rocks and the high degree of their metamorphism.
The main type of metamorphism in the Precambrian is regional, occurring at high temperatures and pressures. In most cases, the following pattern is observed: the older the rocks, the more strongly they are metamorphosed. The oldest rocks are so strongly metamorphosed that it is very difficult, and sometimes impossible, to determine from which rocks - sedimentary or igneous - they arose. The processes of metasomatism and granitization, widespread in Precambrian formations, led to the formation of migmatites - peculiar rocks of a banded texture, and even to complete metasomatic processing of the original rocks and their transformation into granites. These processes proceeded, as a rule, with intensive inflow and removal of elements and compounds by hot steam-water solutions. Migmatites and granites compose vast granite-gneiss fields.
Other distinguishing feature Precambrian rocks - their strong dislocation, the presence of complex folds of many orders. Among the Precambrian formations, according to the nature of tectonics, a number of structural stages can be distinguished, indicating the manifestation in the Precambrian of a number of epochs of folding. Researchers have to put up with the approximate and inaccurate division and correlation of Precambrian formations according to the degree of metamorphism, the depth of magmatic and tectonic processing, and the petrographic features of rocks, since it is impossible to fully apply the biostratigraphic method to the Precambrian. Radiological methods also have great limitations associated with a strong distortion of dating under the influence of the secondary changes mentioned above, which “rejuvenate” the most ancient rocks. The most suitable for dismembering the Precambrian is the geohistorical method used in conjunction with the radiogeochronological method.
The peculiarity of the conditions in the Precambrian led to the formation of rocks characteristic only for this time. An example is jaspilites - ferruginous quartzites, consisting of stripes, composed mainly of quartzite and hematite (or magnetite). Jaspilites are mainly confined to the Proterozoic strata; they were formed with the participation of microorganisms.
The Precambrian is divided into large stratigraphic units, the boundaries between which coincide with manifestations of diastrophism. The most general division of the Precambrian was carried out at the end of the 19th century. In 1872, the American geologist J. Dan called the most ancient formations Archean (Greek archeos - ancient). But with the findings of the remains of bacteria and cyanobionts, archaea can also be called archaeosa (Greek archeos - ancient, zoe - life). The last term also belongs to J. Dan. E. Emmons and D. Walcott in 1888 singled out the upper part of these most ancient strata, which contained the remains of: the vital activity of organisms, under the name of Proterozoic (Greek: proteros - primary, zoe - life).
These subdivisions existed for a long time in the rank of eras (groups), but after their significantly longer duration (about 2 billion years each) was revealed compared to the eras of the Phanerozoic, it was necessary to introduce new, larger geochrons (stratons) . In the current stratigraphic code (1992), the Archean and Proterozoic have the rank of acrons (akrotems), each divided into two zones - early and late, which in the stratigraphic scale correspond to eonotems - lower and upper. The Lower and Upper Archean eothemes do not have more subdivisions in the international stratigraphic scale, and the Lower Proterozoic is divided into two erathems - lower and upper. In Russia they are called Lower Karelia and Upper Karelia, since the most representative and well-studied sections of the Proterozoic are located in Karelia. The Upper Proterozoic is subdivided into Riphean and Vendian. The rank of the Riphean is not entirely clear, and the Vendian is a period (system). Riphean is divided into three eras (eras): Lower Riphean, Middle Riphean and Upper Riphean.
Life originated in the early Archean and was originally represented by prokaryotes - single-celled organisms that do not have a nucleus. These include bacteria and cyanobionts (blue-green algae). The latter played a decisive role in the formation of an oxygen atmosphere. As M. Rutten (1973) points out, the content of oxygen produced by an inorganic way cannot rise above 0.001 pG of the current content in the atmosphere. This is the so-called "Yuuri level". Photosynthesis of chlorophyll-containing cyanobionts about 3 billion years ago became so widespread that the oxygen content in the atmosphere grew faster than its loss during the oxidation of minerals in the earth's crust. Thus, Yuri's level was overcome. But such an atmosphere is still considered oxygen-free. The oxygen atmosphere must contain at least 0.01 of the current oxygen level, taken as 1 (or at least 1%). This is the "Pasteur level". By the Middle Riphean (1.3 billion years ago), the first fungi and algae appeared. At the beginning of the Late Riphean (about 1 billion years ago), eukaryotes appeared in a very noticeable amount - unicellular and multicellular organisms, the cells of which contain a nucleus. The Vendian period is the time of the mass appearance of skeletal animals - a peculiar fauna of the Ediacaran type.
Throughout the Archean Akron, the earth's crust was everywhere very mobile and permeable. There was no differentiation into platforms and geosynclines. Only at the end of the Early Archean did the regime approach the geosynclinal one. Archean rocks older than 2.8 Ga are characterized by basic and ultrabasic volcanism and granitization. At that time, the earth's crust was everywhere in eugeosynclinal conditions (pangeosynclinal stage, according to V.V. Belousov). The Archean strata often form granite-gneiss domes - rounded or elongated structures, composed of granites in the core, and granite-gneisses, migmatites and crystalline schists along the periphery. The formation of such structures is associated with the plastic flow of matter.
In the Precambrian, several major stages of geological development are distinguished, separated by global diastrophic cycles (epochs of folding, tectogenesis) of the first order, which took place 3750-3500 (Sami), 2800-2600 (Kenoran, or White Sea), 2000-1900 (Karelian), - 1000 (Grenville) and 680-650 (Katangian, or Baikal) million years ago. In addition, diastrophic cycles of the second and lower orders are distinguished, which will be discussed below.
As a result of the Saami tectogenesis, extensive folded ovals were formed, composed of complexes of "gray gneisses", i.e. mostly plagiogneisses of tonalitic, trondhjemite and granodioritic composition, underlying the rocks of greenstone Archean belts.
Kenoran folding, which manifested itself 2.8 billion years ago in South Africa, led to the formation here of the oldest relatively rigid area on the planet - the protoplatform. The Belomorian folding, which manifested itself at about the same time, also caused the extinction of the protogeosynclinal regime in some areas and their transformation into protoplatforms (Anabar massif, Aldan shield, etc.). Later epochs of tectogenesis led to an increase in the area of protoplatforms. Thus, starting from the end of the Archean (2.8 billion years ago), one can speak of a protoplatform stage in the development of the earth's crust. Protogeosynclines (preceding geosynclines) existed between protoplatforms, where the same conditions prevailed as in pangeosynclines.
The Karelian folding at the end of the Early Proterozoic completed a new cycle of geosynclinal sedimentation. One of its consequences was the withering away of the geosynclinal regime over vast areas, the formation of the first large stable blocks - epikarelian platforms, which resulted from the merger of protoplatforms after the consolidation of the protogeosynclines located between them. Within these territories, the formation of a typical platform cover began.
Thus, by the end of the Early Proterozoic (completion of the Karelian folding), the East European Platform formed in a significant part of Eastern and Northern Europe, the Siberian Platform in most of Central Siberia, and the Sino-Korean and Tarim Platforms in northern China and the Korean Peninsula. platforms, in southern China - the South China platform, in most of the Hindustan peninsula - the Indian platform, in the central and western parts of Australia - the Australian platform. In Africa and on the Arabian Peninsula, the North African, South African and Arabian platforms stand out, in most of North America - the North American platform. Two platforms are slated for most of South America. Almost all of Antarctica, with the exception of its western part, is occupied by the Antarctic platform. Along with the platforms, there were geosynclines and geosynclinal belts that separated the epikarelian platforms from each other and differed from the protogeosinklinals in linear structures.
The Baikal folding that occurred at the end of the Riphean and in the Vendian led to the final consolidation of the ancient platforms. Since the Precambrian, there have been the North American, East European, Siberian, Chinese, South American, African-Arabian, Indian, Australian, and Antarctic platforms. It is assumed that the last five southern platforms in the Paleozoic constituted the Gondwana superplatform.
All the time after the Baikal folding can be called the time of platforms and geosynclines. Geosynclinal conditions prevailed within the following areas. Between the East European, Siberian and Chinese platforms was the Ural-Mongolian mobile (geosynclinal) belt. Between the North American and East European platforms, the Grampian geosynclinal region of the North Atlantic mobile belt can be traced, the North American platform was bordered from the north by the Innuit geosynclinal region, from the southeast by the Appalachian geosyncline of the same belt. Around the entire coastal part of the Pacific Ocean there was a huge Pacific mobile belt with two branches - the West and East Pacific geosynclinal regions. Between Gondwana and the platforms of the Northern Hemisphere there was a sublatitudinal Mediterranean mobile belt.
Among the Precambrian formations, lithological-stratigraphic complexes stand out - associations of rocks that are distinguished by lithological originality, corresponding to a major stage in the geological development of the territory and occupying a certain stratigraphic position, separated from adjacent complexes along the section by structural or significant stratigraphic unconformity. The complex is the largest unit of the local stratigraphic scale; it combines a number of series or suites and has its own name, derived from the name of the stratotype locality, or the most typical series included in it. With the help of complexes, large stratigraphic units of the Precambrian in the rank of eonotems and erathems receive a more fractional subdivision.
In the Lower Archaean, according to the data of L.I. Salop (1982), the following lithological-stratigraphic complexes are distinguished (from bottom to top): Iengrsky, Ungrinsky, Fedorovsky, Sutamsky, Slyudyansky, Isuansky (Isua series). In the Upper Archaean, complexes are distinguished: komatiite, Kivatinsky, Timiskamingsky, Modis.
The Lower Karelian erathem includes six lithostratigraphic complexes (from bottom to top): Dominion-Reef (Tungud-Nadvoitsky), Witwatersrand, Lower Yatulian, Upper Yatulian (Animikian), Ladoga (Transvaalian), Vepsian. In the Upper Karelian erathem, as in younger formations, lithostratigraphic complexes are not distinguished.
Precambrian formations are extremely rich in minerals. More than 70% of the reserves of iron and chromium are concentrated in the Precambrian; 70% gold, uranium, nickel; over 60% copper and manganese; 100% extraction of muscovite and phlogopite. This circumstance determines the important practical significance of studying the Precambrian.
ARCHEAN AKRON (ARCHEAN AKROTEMA) - AR
The Archean Akron lasted over 1.5 billion years, although its exact duration is unknown and the lower limit has not been established. It is conditionally determined by the age of the most ancient rocks and may decrease as new data are obtained, although this age, now approaching 4.2 billion years, is unlikely to change significantly. Archean rocks are traced on the shields of ancient platforms. The age of rocks of the Isua series in Greenland is estimated at 3.760-4.000 Ma (magnetite quartzites, tonalites). The granulite-gneisses and charnockites of the Kan Complex of the South Yenisei uplift of the Siberian Platform are 4100 Ma old. According to Australian geologists at the International Geological Congress in Moscow in 1984, gneisses of the Yilgarn shield. The Australian platform has an age of 4.100-4.200 billion years. The upper age limit of the Archean Acron is at the level of 2.500-2.600 Ma.
According to the Precambrian stratigraphic scale adopted in Russia (Table 1, color incl.), divide the Archean into two parts in the rank of eonotems - lower and upper Archean, which correspond to the Early and Late Archean zones.
EARLY ARCHEAN EON (LOWER ARCHEAN EONOTEM) - AR,
general characteristics
The age boundary between the Early and Late Archean zones is drawn at the level of 3.150 Ma. The most ancient formations are sometimes called "katarhei" (from the Greek kata - below, the term of J. Sederholm, 1893), although the volume of this straton is not defined and is understood in different ways.
The Lower Archean formations, which make up significant areas of the basement of ancient platforms, are the beginnings of the continental crust and are represented by various deeply metamorphosed para- and ortho-rocks. The most ancient of them are the so-called "gray gneiss". These are mainly gneisses of andesidacitic composition, as well as amphibolites, ferruginous quartzites and other products of metamorphism of both sedimentary and igneous rocks. Facies of metamorphism - granulite, amphibolite.
These most ancient formations, with an age usually exceeding 3.5 billion years, are developed on all continents. In Europe, these are the Kola series of the Kola Peninsula, the White Sea series of Karelia, etc.; in Asia - the Aldan series of the Aldan shield, the Anabar series of the Anabar massif, the Kan complex of the Kan ledge, the Zerenda series of Kazakhstan, the Hindustansky complex of India, etc.; in Africa - the "ancient gneisses" of Swaziland, the gneiss-granulite complex of Zimbabwe, etc.; in North America - gneiss-granulite complexes of the Canadian Shield; in South America, Australia, Antarctica - the oldest shield complexes.
organic world
The origin of life and the earliest stages of its development were discussed in detail in Chapter 5. Apparently, already earlier than 3,500 million years, in the early Archean, real living organisms - prokaryotes (bacteria and cyanobionts) appeared. The identification of organic remains in the most ancient rocks is very difficult. Microorganisms Eobacterium isolatum were found in the siliceous schists of the Onverwacht Group of the Swaziland Superseries (South Africa) and the overlying Fig-Three Series with an age of 3.500-3.100 Ma.
In the rocks of the Isua series in southwestern Greenland with an age of about 3.800 million years, isolated sticks 0.45-0.7 µm long and 0.18-0.32 µm in diameter with two-layer shells, filament-like formations, microscopic spherical, discoid and polygonal shells of unicellular prokaryotes (cyanobionts). These are the most ancient paleontological remains. In the first half of the Archean, prokaryotes went through a difficult path of development, since already in the middle of the Archean there were two independent kingdoms organic world bacteria and cyanobionts (blue-green algae). These first inhabitants of the Earth lived in an almost anoxic environment, inhabiting shallow water bodies at depths, most likely from 10 to 50-60 m, since a layer of water at least 10 m thick was required to protect against the destructive ultraviolet radiation of the Sun.
According to the scheme of L.I. Salop (1982), six diastrophisms are distinguished in the Archean Akron: Gotthobian of the second order (-4000 million years), Saami of the first order (3750-3500 million years), Belinguian, Swaziland, Barbertonian of the second of the third order (in the Late Archean) and Keno-Ransky (White Sea) of the first order (2800-2600 Ma). All these cycles of diastrophism included folded deformations, intense and diverse magmatism, migmatization, granitization, and other processes.
According to the nature of the tectonic regime, L.I. Salop proposed calling the early Archean a permobile eon (lat. per - entirely, mobilis - mobility). According to other authors, this is a nuclear or pangeosynclinal stage in the history of the Earth.
The most characteristic elements of the Early Archean structure are extensive "folded ovals" up to 600-800 km in diameter and "interoval fields" located between them - a combination of domes and troughs. In creating these structural forms, vertical movements were of primary importance. Extensive fields of granitoids are confined to the central parts of the ovals. The centripetal vergence of the folds on the wings of the ovals is characteristic. The latter are not arranged in an orderly manner, which indicates the absence of a guide frame - cratonic blocks, platforms. Not smaller structural forms are granite-gneiss domes.
The tectonic regime of the Early Archean is characterized by the following features:
The lack of differentiation of the earth's crust into platforms and geosynclines;
- lack of contrasting relief and coarse clastic deposits;
The monotony of supracrustal rocks (lat. supra - top, top, crusta - bark) on all
continents - a sign of "Pantalassa", a planetary ocean;
The wide distribution of anorthosites is a sign of a calm tectonic setting;
only at the end of the Early Archean did the regime somewhat approach the geosynclinal one;
Thin and rather plastic primary crust, due to which arched uplifts and deep faults could not occur;
The introduction of huge masses of granitoids as a result of Saami diastrophism, which led to a thickening of the earth's crust up to 25-30 km (Salop, 1982).
The most common supracrustal rocks are melanocratic amphibole, amphibole-pyroxene and pyroxene plagiogneisses, crystalline schists, and amphibolites. These are highly metamorphosed basic or ultrabasic lavas, possibly tuffs. In Western Greenland, on the Kola Peninsula, on the Aldan Shield, in South Africa, komatiites, high-magnesian volcanic ultramafic-mafic rocks, have been established. Metabasites are often granitized, transformed into plagiogneisses (migmatites), enderbites (sodium charnockites), charnockites. Metavolcanites are associated with biotite, garnet-biotite, sillimanite- and cordierite-bearing gneisses.
Undoubtedly, marbles of calcite and dolomite composition, graphite-bearing gneisses and crystalline schists are considered sedimentary. Characteristic is the transformation of rocks under conditions of granulite and amphibolite facies. The granulite facies of regional metamorphism is an exceptional feature of the Lower Archean.
The Lower Archean formations on the Aldan shield are best studied. The supracrustal complex of the shield, the Aldan Series, is the most complete representation of all known subdivisions of the Lower Archean. The age of the series is 3.800-4.000 Ma. The rocks of the Aldan series are represented by quartzites, pyroxene and amphibole schists, amphibolites, and gneisses of the Iyengrian subseries with a thickness of more than 3 km. Above lies the Timptonian subseries - gneisses, amphibolites with members of marbles and lime-silicate rocks. Thickness is about 8 km. Even higher is the Dzheltulinskaya subseries, composed of garnet-biotite, pyroxene gneisses, granulites and marbles. Power is more than 4 km. The total thickness of the Aldan series is about 15 km. Among the Precambrian deposits, various lithostratigraphic complexes are distinguished, composed of rock associations, reflecting the specifics of their formation environment. In the Lower Archean formations, six lithostratigraphic complexes are distinguished (Salop, 1982):
1. Iengry metabasite-quartzite: basic schists, amphibolites
(metabasites), horizons of quartzites and high-alumina gneisses (Zverevskaya sequence of Stanovoy
ridge, the Daldynskaya series of the Anabar uplift, the Ranomena series of Madagascar).
2. Ungra metabasite: melanocratic two-pyroxene and amphibole crystalline schists, amphibolites after basic and ultrabasic volcanics, interlayers of gneisses and silicate-magnetite rocks (Ungrin Formation of the Aldan Shield, Upper Anabar Subseries
Anabar massif, Kan series of the Yenisei uplift, in North America - the lower part of the Grenville complex; the lower part of the Lower Archean complexes of Equatorial, Western, and Northwestern Africa; in Australia - the lower parts of gneiss-granulite complexes).
3. Fedorovsky metabasite-carbonate: basic pyroxene crystalline schists, amphibolites (metabasites) with subordinate interlayers of carbonate rocks (marbles, lime-silicate schists). Interlayers of gneisses, quartzites, magnetite rocks. The most ancient evaporites in the history of the Earth (anhydrite-containing marbles, calcareous crystalline schists of the Aldan Shield, Canada, Brazil), as well as rocks rich in phosphorus, are confined to this complex. Distributed in the Anabar massif, in the Yenisei ridge, in the Sayan region (upper part of the Sharyzhalgai series), on the Ukrainian shield (black grouse-Bug series, Belotserkovskaya suite), in North America (upper part of the Grenville complex), in Africa, etc.
4. Sutam complex: thinly bedded garnet-biotite gneisses, coarsely bedded or massive leucocratic garnet granulites, interlayers of various gneisses, metabasites, marble ditches, high alumina gneisses. Known in the Anabar massif, in the Eastern Sayan, the Stanovoy Range, the Kola Peninsula, in Africa.
5. Slyudyansky complex: carbonate and silicate-carbonate rocks and various crystalline paraschists (garnet-biotite, sillimanite-cordierite, etc.). carbonates
here at least 30%, and metabasites are of subordinate importance. The Slyudyanskaya series of the Southern Baikal region, the Biryusa and Derba series of the Eastern Sayan, the Vakhanskaya series of the Pamirs, etc. The age of 3.7 billion years was obtained from marbles from the Biryusa series.
6. Isua Series: amphibolites, para- and orthoschists, jaspilites, felsic metavolcanics, metaconglomerates. The power of the series is 2 km. In Greenland, the rocks of the series occur in the form of an arcuate band among the vast field of Amitsok gneisses - tonalite spectacled rocks with dark-colored minerals of granulite and amphibolite facies. The rocks of the Isua series date back to 3.760 Ma; gneisses Amitsok - 3.980 Ma, granite-gneisses Gotkhob - 4.065 Ma.
The Isua Series probably formed between two periods of tectono-magmatic activity. Prior to the deposition of this series, the Gotkhobian diastrophism (folding phase) of the II order (4 Ga) took place, which is associated with the formation of the Amitsok gneiss (granulite facies). At the end of the formation of the Isua Series, the Saami diastrophism of the first order (3.750-3.500 Ma) occurred, which completed the Saami era of tectogenesis.
Consistency of the composition of the supracrustal strata of the Lower Archean over vast areas suggests uniform conditions for their formation.
The absence of any signs of erosion areas indicates the deposition of sediments and the outpouring of lavas in the vast shallow ocean - "Pantalassa". The absence of coarse clastic rocks indicates the absence of a dissected relief.
For the first time, carbonate rocks appear in the Fedorov Complex, which marks an important stratigraphic milestone associated with a decrease in the content of CO 2 and strong acids in the composition of the atmosphere and hydrosphere. After the deposition of the fourth - Sutam - complex, the CO 2 content dropped even more, so that the fifth - Slyudyansky - complex turned out to be significantly carbonate.
Iron ore sequences could have occurred due to the removal of iron during volcanic eruptions, and silica (SiO 2) was in excess in solution. Graphite rocks of the Lower Archean (Fedorovka, Slyudyanka complexes, etc.) are most likely of abiogenic origin, since at that time there was not enough biomass to form such a large amount of graphite-bearing rocks. The same consideration applies to phosphate-bearing. breeds.
A significant part of the Lower Archean supracrustal strata is composed of deeply metamorphosed volcanic rocks of basic and partly ultrabasic composition. The presence of acidic lavas has not been proven. The metabasites of the Iengry and Ungrin complexes correspond to tholeiite basalts, the Fedorov complex corresponds to alkaline basalts, and the younger parts of the Aldan series correspond to volcanic rocks of the tholeiitic and alkaline-basalt series, with the participation of basalts and nephelinites. Thus, over time, an increase in the alkalinity of rocks is observed.
The metabasites of the Slyudyanka complex are similar to the basaltic andesite formation of island arcs and, to some extent, basalts of geosynclinal formations.
Some Lower Archean complexes are characterized by the presence of deeply metamorphosed ultramafic high-alumina rocks - komatiites (more widely developed in the Upper Archean).
Plutonic formations are most developed in the Saami cycle of tectogenesis in the interval of 3.750-3.500 Ma. The possibility of applying the actualistic method in the geological interpretation of Lower Archean rocks is severely limited, since the genesis of many rocks is unclear. For example, there are no psephytes in the Lower Archaean (except for the upper parts of the Isua series). Quartzites are associated with mafic and ultrabasic rocks, which is not observed in the Phanerozoic. Peculiar tectonic structures - gneiss ovals have no analogues in younger strata.
Physical and geographical conditions
Features of the Lower Archean metasedimentary rocks indicate the existence of a hot hydrosphere. The study of the isotopic composition of siliceous rocks, in particular, the ratios of deuterium to hydrogen and 18 O/ 16 O isotopes, which depend on temperature, showed the following distribution of the mean annual temperature (Salop, 1982).
In the early Archean, the temperature of the Earth's surface was probably above 70°C or even above 100°C. Such a surface temperature could only be due to the greenhouse effect created by a powerful atmosphere. An analogy arises with the modern atmosphere of Venus, the surface temperature of which is 480 ° C, the pressure of the carbon dioxide atmosphere is about 90 bar.
The atmosphere and hydrosphere are mainly products of degassing and separation of liquid and gaseous constituents from the mantle. The formation of the primary earth's crust was accompanied by the formation of a primary, essentially hydrogen atmosphere, which later dissipated into outer space. The secondary primitive (primary in the geological sense) atmosphere arose only after the temperature dropped, when the gases could no longer overcome the force of attraction. Subsequently, the atmosphere changed depending on the processes of volcanism, sedimentation, and then on the photosynthesis of plants.
The composition of the primitive atmosphere corresponded to the composition of the gaseous products of volcanic eruptions (water vapor, carbon dioxide, nitrogen, "sour smoke" - HC1, HF, H 2 S , ammonia, methane).
The water content of the Earth's mantle is three times the mass of water in modern oceans. The source of this water was the formation of lavas of basaltic and andesitic composition. Carbon dioxide in geological history has been deposited in carbonates 10 thousand times more than it is now contained in the atmosphere (and assimilated by plants and buried 1000 times more than in the atmosphere).
The primary atmosphere contained about 99% CO 2 (excluding water). The pressure should have been about 70 bar, and taking into account the dissolution of CO 2 in the hydrosphere, 50-60 bar. At this pressure, the boiling point of water should be 260-285°C.
Free oxygen was practically absent in the secondary (primitive) atmosphere. Its main source is biogenic photosynthesis. Oxygen, as L.I. Salop points out, was absent in this atmosphere, judging by the isotopic composition of sulfur in sedimentary rocks, until the boundary of about 2.3-2.4 billion years (PR |). According to M. Rutten (1973), about 3 billion years ago, the Urey point was exceeded, when the oxygen content was 0.001 of the modern one, and by the end of the Archean (2.5 billion years), the Pasteur point was reached, in which the oxygen content is 0.01 from modern. Up to this level, the atmosphere is still considered oxygen-free. Analysis of gas inclusions in chemogenic quartzites of the Iengry series gave the following results: CO 2 - 60%, H 2 S , SO , NH 3 , HCI , HF about 35%, N 2 + rare gases 1-8%. In younger chemogenic siliceous sediments, the oxygen content naturally increases: AR 2 - 5.5%, PR -PZ - 12%, PZ 2 -KZ - 18%. Simultaneously, there is a decrease in CO2 content from 42% in AR2 to the current one in the Cenozoic.
Thus, the early Archean atmosphere was very dense, anoxic, hot, and consisted mainly of water vapor, carbon dioxide, and a number of other components ("acid smoke" is typical). This atmosphere caused a strong greenhouse effect.
The hydrosphere in the early Archean was sharply carbonic, containing strong acids, i.e. was aggressive, markedly mineralized and salty. This is also evidenced by ancient evaporites (the Fedorov complex on the Siberian platform, in Canada, Brazil). As a result of interaction with a large amount of alkalis and alkaline earths, the composition of water approached neutral (pH about 7).
LATE ARCHEAN EON (UPPER ARCHEAN EONOTEM) - AR
general characteristics
The Late Archean eon covers the time of 3.150-2.600 (according to other sources 2500) million years. The formations of the Upper Archean eonoteme sharply differ from the Lower Archean, marking the beginning of a new major stage in the history of the Earth - platform-geosynclinal. The stratotype of the Upper Archean is the superserie Swaziland (South Africa, Swaziland). The supracrustal complex is characterized by sedimentary-volcanogenic strata close to the eugeosynclinal type. Miogeosynclinal and platform formations are still insignificantly distributed. The rocks are metamorphosed under conditions of amphibolite and greenschist facies, so the primary nature is recognized quite well. Often there are conglomerates, jaspilites are typical, and granitization is locally developed.
Upper Archean supracrustal rocks and intrusions that cut through them are widespread on all continents. These are, for example, the Lop complex of Karelia, the leptite formation of Sweden, the Teterevskaya, Konka-Verkhovtsevskaya series of Ukraine, the Swaziland supergroup of South Africa, the Sherry Creek formation of the USA, the Pilbara complex of Australia, etc.
organic world
By the late Archean, conditions were created that were more favorable for the existence and reproduction of organisms: the temperature of the water decreased, its acidity and chemical aggressiveness decreased. The first definable organic remains were found in the Upper Archean rocks: phytoliths (stromatolites, oncoliths) and microfossils. Stromatolites are represented by small scalloped and dome-shaped forms and bedded formations. These, as already mentioned above, are the waste products of cyanobionts. Microfossils are also cyanobionts and bacteria. The siliceous rocks of the Fig Tree series (South Africa) contain microscopic formations resembling unicellular algae and bacteria. The amount of biomass in comparison with the early Archaean increased significantly, but it was represented exclusively by prokaryotes, since eukaryotes had not yet arisen. Late Archean prokaryotes differ from younger analogous fossils in their smaller cell size.
The activity of cyanobionts gradually led to an increase in the amount of oxygen in the atmosphere and hydrosphere. About 3 billion years ago, the Juri point was exceeded; the oxygen content in the atmosphere has risen above 0.001 from the modern one. Subsequently, activation of the development and complication of other groups of organisms, as well as a change in the processes of sedimentation and accumulation, will subsequently be associated with this.
Structures of the earth's crust and rock formation
In all areas, the greenstone rocks of the Upper Archean are developed in the form of narrow, often irregularly shaped areas, representing structures of the geosynclinal type, separated by vast fields of deeply metamorphosed rocks of the Lower Archean. A pronounced unconformity is observed almost everywhere between the Upper Archean and Lower Proterozoic strata.
The Upper Archean is characterized by various volcanics with a predominance of mafic ones: tholeiitic basalts, komatiites, diabases, and basaltic andesites. Often there is a spherical separation. The clastic rocks are dominated by greywackes, arkoses, siltstones, pelites, and conglomerates. The most common tectonic structures are gneiss and granite-gneiss domes, with a diameter of 10-40 (no more than 100) km. The domes are bordered by greenschist rocks and form entire groups that make up extended "granite-greenstone belts" located between relatively stable massifs - protoplatforms.
Greenstone belts are, most likely, extensive troughs complicated by faults and resulting from the global stretching of the earth's crust. According to LI Salop, the systems of troughs and uplifts separating them should be considered as ancient geosynclinal areas - protogeosynclines.
Greenstone belts are unevenly distributed. The areas of development of the Lower Archean strata, devoid of greenstone belts, are probably the oldest more stable elements of the earth's crust, which can be called protoplatforms.
The most complete and best studied sections of the Upper Archean are found in South Africa, Canada, and Western Australia.
The development field of the Swaziland superseries (South Africa, Swaziland) - the Upper Archean stratotype - is located in the Barberton mountainous region and is structurally the Swaziland synclinorium.
According to D. Hunter, the lower part of the section is represented by an ancient gneissic complex consisting of rocks of amphibolite and granulite facies of metamorphism. They were formed long before the accumulation of the Swaziland supergroup and occur in this latter in the form of conglomerate pebbles.
The rocks of the Swaziland Superseries are characterized, in contrast to the rocks of the basement, by low stages of metamorphism (greenschist facies) with clearly distinguishable primary structures. From bottom to top, three series stand out in this "super-series": Onverwacht, Fig-Three and Modis.
The Onverwacht series is divided into three formations:
Lower Onverwacht: basic pillow lavas and ultramafic lenses, thin interlayers of black siliceous rocks, felsic tuffs. The ultramafic and basic rocks are rich in Mg and poor in Al and K, and are identified as a special group of komatiites with a thickness of more than 2 km.
Middle Onverwacht (Comati River Formation): cushion basalts and ultramafic lavas, porphyry feldspar intrusions (3-4 km).
Upper Onverwacht - cyclic repetition of pillow basalts or andesites, acid lavas and siliceous rocks (5 km).
The Fig-Three series (fig tree) includes (from bottom to top):
Chemogenic sediments (banded siliceous, talc-carbonate, quartz-sericite rocks);
Graywackes, shales, banded siliceous rocks;
Graywackes, shales, ferruginous quartzites, tuffs.
The total thickness of the Fig-Three series is over 2 km. The Modis series lies with unconformity and is represented by polymictic conglomerates, feldspar sandstones, siltstones, and shale (thickness 3.1 km).
The total thickness of the Swaziland superseries is up to 16 km. After the deposition of rocks of the Modis series, all sequences of the Swaziland superserie were folded into folds, broken by steep thrusts into scaly and fan-shaped plates and intruded by numerous bodies of granitoids, the oldest of which are 3-3.4 billion years old.
The Swaziland superserie belongs to the oldest formations of greenstone synclinories.
On the Canadian Shield, the sedimentary-volcanogenic strata of the Superior Province (Lake Superior) are considered as the Upper Archean parastratotype.
They compose greenstone belts - elongated isolated sections of the synclinor structure, in which linear, often isoclinal folds alternate with domed structures. Greenstone belts are separated by fields of granitoids, granite-gneisses and gneisses.
Greenstone strata usually have a three-membered structure: clastic rocks, sometimes volcanics, are at the bottom and at the top, volcanics predominate in the middle part.
All greenstone strata are intruded by large massifs of biotite and amphibole granites and granodiorites with an age of 2.600-2.800 Ma. These intrusions are associated with the White Sea (Kenoran) diastrophism.
On the Baltic Shield, Upper Archean formations are best studied in Karelia, the Kola Peninsula, and eastern Finland. The Gimolskaya Series developed in Karelia near the border with Finland is accepted as a regional stratotype (in Finland, this is the Ilomanti Series). This series is characterized by a two-member structure: at the bottom there are basic effusives, above sedimentary rocks and felsic volcanic rocks.
All Upper Archean sequences of the Baltic Shield overlie transgressively, sometimes with conglomerates at the base, on Lower Archean rocks, mainly on gray gneisses, and are overlain with sharp unconformity by Lower Proterozoic rocks.
The Upper Archean sequences are intruded by a large number of granodiorite and microcline-plagioclase granite massifs with an age of 2.600-2.800 Ma.
Correlation of the Upper Archean of the Baltic Shield with the stratotype (South Africa, Swaziland): Finnish komatiites correspond to the lower part of the Onverwacht Series. The lower volcanogenic sequences are compared with the upper part of the Onverwacht Series. The upper volcanogenic-terrigenous sequences correspond to the Fig-Three series. The uppermost formations of the Gimolskaya series (Okunevskaya, Keivskaya) approximately correspond to the Modis series.
The total thickness of the Upper Archaean of the Baltic Shield is 4-8 km (2-4 times less than in the strato-type - in South Africa). On the Ukrainian Shield, the upper Archean is most fully represented in the basin of the middle reaches of the Dnieper, where the Konk-Verkhovtsevskaya series is developed, unconformably occurring on the gneisses of the lower Archean. At the base of the series there are high-alumina or pure quartzites. Metabasites, intermediate and felsic volcanics, less often metasedimentary rocks (up to 5 km thick) occur above. Jaspilites are found in the middle part of the series.
The rocks of the series occur in narrow synclines, bent in plan, located between the domes of Lower Archean gneissic granites. The age of granitoids intruding greenstone rocks is 2.600-2.800 Ma.
The upper part of the Konksko-Verkhovtsevskaya series approximately corresponds to the Fig-Three series. The composition of the Upper Archean deposits in different parts of the world is very similar to each other. Among them, according to L.I. Salop, there are four globally expressed lithostratigraphic complexes.
