EASTERN SIBERIA, 251 MILLION YEARS AGO
A gigantic stream of mantle material rose from the bowels in the northeastern part of Pangea, where Siberia was located, and melted the stone shell. The eruptions of basaltic lava and ash lasted several thousand years and coincided with the most massive extinction of animals in the history of the planet. It is the frozen basalt layers that form the characteristic stepped landscape of many uplands of Eastern Siberia, such as the Putorana plateau.
Figure: OLGA OREKHOVA-SOKOLOVA
For a long time, global catastrophes that could affect the evolution of earthly life were of little interest to scientists. It was more important for geologists and paleontologists to understand the progressive and continuous change of species. Only relatively recently, in the middle of the last century, when it was established that mass extinctions coincide in time with catastrophic events, such as outbreaks of volcanism and the fall of meteorites, they began to be studied purposefully.
For the first time, the French naturalist Georges Cuvier spoke about catastrophes that happened on Earth in the past at the beginning of the 19th century. The talented paleontologist understood that the animals of past eras were completely different from the current ones, that, for example, the bones of ichthyosaurs and plesiosaurs are located in certain layers of the Alps and it is useless to look for them in later sediments. And bones of manatees and seals, on the contrary, should not be looked for next to the remains of ichthyosaurs. As an experienced comparative anatomist who studied the vast collections of animal mummies and their stone bas-reliefs, taken by Napoleon's soldiers from Egypt, he saw that animal species did not change for 2000-3000 years. So that in the history of the Earth lasting, as it was believed, no more than 100,000 years, there were frequent changes in the species of the animal world, some destructive short-term events are needed. And Cuvier proposed the idea of periodic catastrophes, the victims of which were countless living beings: "Some, the inhabitants of the land, were absorbed by floods, others, inhabiting the bowels of the waters, ended up on land together with the suddenly raised bottom of the sea …"
NORTH AMERICA, 65.5 MILLION YEARS AGO
Promotional video:
A crater and special rocks found in the area of the Yucatan Peninsula, in the territory of modern Mexico, indicate that an asteroid fell there. Its fall caused catastrophic consequences: the blast wave burned out almost all life in the area, and a monstrous tsunami devastated the coast. The wave carried marine life to the coast, such as ammonite mollusks hidden in a spiral shell and sea lizards - mosasaurs. Their remains were kilometers from the water and served as food for the rare surviving inhabitants of the land. When it hit the ground, the asteroid evaporated and knocked out from the crater a mixture of dust, ash, caustic vapor, which, rising into the atmosphere, poisoned it and eclipsed the Sun. The cold snap and acid rain probably lasted for several years. This event was accompanied by the extinction of 35% of the species of marine life, as well as all large reptiles: sea lizards,dinosaurs and pterosaurs.
Figure: OLGA OREKHOVA-SOKOLOVA
Scientists returned to the idea of geological catastrophes 100 years later, when they noticed that the progressive growth in the diversity of organisms that inhabited the Earth was interrupted by at least two grandiose drops in their numbers. These breaks coincide with the boundaries of geological eras: Paleozoic, Mesozoic and Cenozoic.
The most massive extinction of living beings in the entire history of the Earth occurred 251 million years ago, at the end of the Paleozoic era. Over 90% of marine and 70% of terrestrial species have disappeared forever from the face of the Earth - only the smallest and simplest ones remained. In the oceans, the formation of reefs, which were previously widespread in all seas, stopped, and on land - the accumulation of coal, since the lush forests that covered it from tree-like lymphoids, ferns and various ancient gymnosperms disappeared. Scientists are looking for the reasons for this extinction both in the state of the biosphere itself and outside it. Among the external causes of extinction today, most often called a catastrophe caused by powerful volcanic eruptions in Eastern and partly Western Siberia. It was a short-term geological event that strongly influenced the biosphere. Its traces are captured in the form of a vast stratum of basalts, several kilometers thick, called Siberian traps.
In the cliff of the Putorana plateau, a stratum of basalts is clearly visible, which erupted 251 million years ago in the shortest possible time - thousands of years. The cover of ancient lava in these places reaches a thickness of 1.8 kilometers.
Photo: SERGEY FOMIN / RUSSIAN LOOK
They were formed in the shortest geological time interval - about 160,000 years, and maybe no more than 8,000 years. Basaltic lavas covered a maximum of 7 million square kilometers of land. From 2 to 3 million cubic kilometers of volcanic materials were expelled from the bowels, including millions of gigatons of carbon dioxide. The content of the latter in the atmosphere of that time increased by 7-10 times. (For example, if mankind burns all fossil fuels during the current century, the concentration of carbon dioxide in the atmosphere will rise 2-3 times.) Moreover, the warming up of the atmosphere due to greenhouse gases and peculiar solar panels in the form of clouds, consisting of large particles of volcanic ash and soaring over the entire planet, led to the heating of the upper layers of the ocean and the release of billions of tons of methane,until then chained by ice crystals on the shelf. This gas is one of the most effective greenhouse factors, moreover, it quickly oxidizes, taking oxygen from the atmosphere. The overall effect of atmospheric changes was enhanced by the planet's peculiar paleogeography. In those days, all the continents united into a single supercontinent - Pangea, stretching from pole to pole. (The Siberian platform was in its northeastern part.) Over the central, remote from the coast, a vast area of the supercontinent, there was almost no rain, it was dry and practically lifeless. Terrestrial animals concentrated on the outskirts of Pangea, bordered by mountain ranges, and gradually moved to its southern, Antarctic coast, where it was not too hot. The drop in oxygen content in the atmosphere made life in the mountains impossible,and only small low-lying areas remained inhabited.
In such an environment, most reptiles and fish, as well as calcareous algae, corals, and sponges, found themselves in the most unfavorable conditions. The first suffered from a lack of oxygen and carbon dioxide poisoning, the second was ruined by acidification of sea waters caused by the dissolution of excess carbon dioxide in them. Some terrestrial amphibians survived, as well as reptiles, for example, proterosuchia - the ancestors of dinosaurs, cynodonts - animal-like dinosaurs, ancestors of mammals, and lystrosaurs - their distant relatives. They were small animals that required less energy to stay active, which meant they consumed less oxygen. Lystrosaurs had adaptations that, on the contrary, allowed them to absorb more air, and therefore oxygen, - an enlarged chest and parts of the skull associated with respiration. Among marine animals, the smallest foraminifera, brachiopods, and bivalves also survived the disaster, since they needed less food and oxygen. Their large relatives disappeared. The terrestrial flora suffered no less than the fauna - chlorine-containing volcanic excretions destroyed the ozone layer, harsh ultraviolet radiation crippled spores that had not yet germinated, sulfuric acid rains burned out the foliage, and the last juices from dying trees sucked out the proliferating mushrooms. The past level of biodiversity on Earth was restored only 60 million years later, by the middle of the Jurassic period.harsh ultraviolet radiation crippled spores that had not yet sprouted, sulfuric acid rains burned out the foliage, and the last juices from dying trees sucked out the prolific mushrooms. The past level of biodiversity on Earth was restored only 60 million years later, by the middle of the Jurassic period.harsh ultraviolet radiation crippled spores that had not yet sprouted, sulfuric acid rains burned out the foliage, and the last juices from dying trees sucked out the prolific mushrooms. The past level of biodiversity on Earth was restored only 60 million years later, by the middle of the Jurassic period.
Cosmic factor
In many places on Earth, where ancient seabeds come to the surface, you can see a thin clay layer formed 65 million years ago. It is on it that geologists draw the border between the Mesozoic and Cenozoic eras. A notable example of such a boundary was recently discovered in southeastern Spain, near the town of Caravaca de la Cruz - a thin layer of reddish clay, several millimeters in, among many meters of white marls. Below this layer, there are the remains of various animals that inhabited the seas of the Mesozoic era: the bones of sea lizards, shells of ammonites and the smallest planktonic creatures - foraminifera and coccolithophorids, from which calcareous silts were formed, which eventually turned into calcareous rocks - marls. Fossilized shells of tiny crustaceans are also found here, which swallowed clay particles,They were pressed into lumps - pellets - and in such a more "weighty" form were sent to the bottom, creating a mass of sedimentary material. And “suddenly” all these animals disappeared, and therefore the formation of calcareous sediments completely stopped, while the rate of accumulation of clay particles dropped hundreds of times - this is evidenced by the low thickness of the boundary layer. For several thousand years, the sea remained almost uninhabited, then life took its toll, and the geological chronicle recorded its restoration, which began with the unrestrained reproduction of a few and small surviving species of foraminifera. The shells of their descendants formed new layers of white marl.the rate of accumulation of clay particles dropped hundreds of times - this is evidenced by the low thickness of the boundary layer. For several thousand years, the sea remained almost uninhabited, then life took its toll, and the geological chronicle recorded its recovery, which began with the unrestrained reproduction of a few and small surviving species of foraminifera. The shells of their descendants formed new layers of white marl.the rate of accumulation of clay particles dropped hundreds of times - this is evidenced by the low thickness of the boundary layer. For several thousand years, the sea remained almost uninhabited, then life took its toll, and the geological chronicle recorded its recovery, which began with the unrestrained reproduction of a few and small surviving species of foraminifera. The shells of their descendants formed new layers of white marl.
WESTERN EUROPE, 5.33 MILLION YEARS AGO
While in East Africa the ancestors of the Australopithecus were mastering the savannah, the Mediterranean Sea experienced a severe ecological crisis. The growing mountain ranges blocked its communication with the Indian and Atlantic oceans, due to a lack of water, the sea gradually dried up. A huge basin with a depth of up to 5 kilometers in places remained almost waterless for hundreds of thousands of years. The crisis ended with a sudden breakthrough of Atlantic waters through the Strait of Gibraltar. In this case, three factors simultaneously could play a key role: the rise in the level of the World Ocean, tectonic processes and erosion of the walls of the basin. Sea water plunged into the depression, forming the largest waterfall known today. The drying up and catastrophic filling of the Mediterranean Sea is captured in the many kilometers of salt and gypsum, in numerous caves,buried river canyons; and the stepped bottom of the Strait of Gibraltar.
Figure: OLGA OREKHOVA-SOKOLOVA
One can only guess about what happened on land in these millennia, because there is no boundary clearly traced in the sea layers in the terrestrial sediments. It is precisely established that the hosts of the land - various and numerous dinosaurs and pterosaurs - did not survive this frontier.
What events took place 65 million years ago that separated geological eras and caused such global changes? In search of an answer to this question, scientists turned to cosmic factors. In the middle of the 20th century, astrophysicists discovered the phenomenon of the birth of supernovae associated with huge bursts of energy, and paleontologists had the assumption that this suddenly released energy, reaching the Earth, could tear off its magnetosphere, which protected the planet from hard radiation, and thereby doom its inhabitants to death. However, it was not possible to substantiate this hypothesis by studying the geological record.
In addition, another idea of a space catastrophe was soon born, which quickly gained popularity. American geophysicist Walter Alvarez discovered near the Italian town of Gubbio in a thin layer of clay, marking the border of the Mesozoic and Cenozoic eras, an unusually high content of iridium, hundreds of times higher than the average concentration in ordinary rocks. Walter's father, atomic physicist Luis Alvarez, helped to find an explanation for this phenomenon. It turned out that the fragments of meteorites that fell to Earth also contain a lot of iridium and other platinum group metals. This means that these elements could get into the clay layer as a result of the impact of a space body. The Alvareses became interested in the coincidence of the time of the supposed fall of the meteorite and the death of the dinosaurs and linked these two events into a single hypothesis of "asteroid winter" by analogy with "nuclear winter":if an asteroid over 10 kilometers across hit the Earth at a speed of 20 kilometers per second, then a monstrous release of energy occurred, 10,000 times higher than all the present nuclear reserves of earthlings. Some of the Mesozoic inhabitants immediately died from shock and heat waves, the rest died out from the consequences of the fall - the dust that flew into the atmosphere reflected the sun's rays. Cold snap began, and vegetation, left without light and heat, began to die. This was followed by the mass extinction of animals deprived of food.the rest died out from the consequences of the fall - the dust that flew into the atmosphere reflected the sun's rays. Cold snap began, and vegetation, left without light and heat, began to die. This was followed by the mass extinction of animals deprived of food.the rest died out from the consequences of the fall - the dust that flew into the atmosphere reflected the sun's rays. Cold snap began, and vegetation, left without light and heat, began to die. This was followed by the mass extinction of animals deprived of food.
After the appearance of a small note by Alvaresov in 1980 in the journal Science, the number of scientific articles on global catastrophes has grown hundreds of times. First of all, they were looking for evidence of the correctness of physicists who touched on the interests of paleontology. And they found a lot: in almost all sections, where the boundary layer aged 65 million years is quite well pronounced, an iridium anomaly was noted, as well as quartz crystals with a shock structure - small parallel cracks that appear at very high pressure, tektites (cooled drops of the melt formed during impact of a large meteorite on rocks), layers of soot. The Chicxulub crater was also discovered in Mexico, the time of its appearance approximately coincides with this border.
Mediterranean flood
Georges Cuvier considered marine floods to be the primary cause of global extinctions, but this hypothesis was not confirmed by modern geologists. In order to flood the entire land, except for high mountains, and destroy most of the terrestrial inhabitants, at least two conditions are necessary: an increase in the level of the World Ocean by hundreds of meters and the transience of the event. Nowadays, when the average height of the continents above sea level is 670 meters, the volume of the World Ocean should almost double. Such an event has never happened in earth's history and, due to the lack of such reserves of free water, is hardly possible in the future. Two main phenomena regulate the level of the World Ocean - changes in the area of continental glaciers and mid-ocean ridges (high mountain ranges located in the central parts of all oceans). Sprawling glaciersabsorbing huge masses of water, and the compression of the mid-oceanic ridges lead to the fact that the oceans and seas recede, draining vast areas of the shelf. Reverse processes - the melting of glaciers and the growth of a network of seamounts pushing water masses out of the ocean bowl, contribute to the rise in the level of the World Ocean. The coincidence of these two factors can cause a very significant rise in the sea, as happened in the middle of the Cretaceous, about 90 million years ago. According to the latest estimates, then the level of the World Ocean rose by 250 meters, flooding vast areas of land. However, to consider this increase as a catastrophic flood is hampered by the long duration of the event - millions of years, this is how long the growth of underwater mountain ranges goes. The ocean comes slowly, reclaiming from land by a millimeter a year. The animal world manages to adapt to this pace,and there is no rapid mass extinction.
The seabed of the Cretaceous period before it was empty 65 million years ago. On the left - shells of mollusks, “dinosaurs of the seas”, so named for their large size and simultaneous extinction. The diorama was created from authentic paleontological objects by Santiago Lafargue.
Photo: ANDREY ZHURAVLEV
Local floods, that is, rapid flooding of the land, occurred in the past several times, but in a very limited space - they never covered the entire planet and did not cause much damage to the inhabitants of land. The largest reliably confirmed flood occurred just 6 million years ago in the Mediterranean. By that time, due to isolation from the Indian and Atlantic oceans, the Mediterranean Sea dried up, turning into a vast basin, in places almost 5 kilometers deep relative to the level of the World Ocean. Its bottom was gradually filled with a three-kilometer layer of gypsum and salt formed during the evaporation of sea water, and in the warm brines of shallow lakes preserved in some places, only special bacteria - haloarchaea - could survive. This stage in the history of the region is called the Messina Crisis - after the name of the province in Sicily, where salt was mined since ancient times. Geologists have very accurately established the time when the Messinian crisis came to an end - this happened 5.33 million years ago, when the waters of the Atlantic Ocean began to penetrate along tectonic cracks through the western side of the basin. At some point, the water cut through a fairly wide channel in the rocks - the current Strait of Gibraltar - and gushed like a waterfall into the dry, saline lowland. The filling of the Mediterranean Sea took place very quickly - only 15,000-20,000 years, during which ordinary marine communities settled in it. The hypothesis of a catastrophically rapid flood in the Mediterranean basin was one of the first to be expressed by the Soviet geologist Ivan Chumakov,who worked on the construction of the Aswan Dam in Egypt in the 1970s and discovered, according to drilling data, an ancient Nile bed filled with marine sediments. The Messina flood significantly influenced the climate in its region, but did not lead to significant changes in the biosphere.
In neocatastrophism - this is the name of the updated Cuvier theory - there are a lot of assumptions that do not yet have actual proof. If the existence of epochs of powerful volcanism is beyond doubt, because they left unambiguous traces in the earth's crust, then it is not easy to prove the fall of the asteroid and especially the exact time of this fall. In addition, in both cases, it is extremely difficult to establish exactly how the consequences of the disaster led to the extinction of the species. There is still no explanation for the fact that some major catastrophes (for example, basalt outpourings in South America and Africa 130 million years ago) did not lead to the mass death of living organisms. Not for all major extinctions in the history of the Earth (there are six of them) it was possible to find catastrophic causes - volcanic, cosmic, or some other. Due to the lack of facts, it is still difficult to judgehow strong the impact of catastrophes on the evolution of life is, however, scientists, even those who support neo-catastrophism, agree on one thing: and the most destructive of the ancient catastrophes could not completely destroy earthly life. There was always someone who gave rise to new inhabitants of the planet.
Andrey Zhuravlev