Dinosaurs were killed by darkness: volcanoes have nothing to do with it

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Dinosaurs were killed by darkness: volcanoes have nothing to do with it
Dinosaurs were killed by darkness: volcanoes have nothing to do with it
Anonim

The explosion of an asteroid with a capacity of one hundred million megatons for years plunged the Earth into pitch darkness, destroying all large animal species without the help of volcanoes.

A dinosaur reacting to a flash from a falling celestial body, as seen by the artist. In fact, the hero of the image could be covered in feathers. / © Dinosaurs in the Wild

Sixty-six million years ago, all terrestrial animal species living outside their burrows became extinct. They tried to blame for this both the colossal eruptions of the Deccan volcanoes and the explosion of the asteroid. As it became clear now, volcanoes have nothing to do with it, and below we will tell you why.

Even 15-20 years ago, many researchers (and there are such in Russia today) argued that dinosaurs became extinct because they were less competitive than mammals. They say that they ate the eggs of clumsy lizards and in other ways prevented them from living, which is why the dinosaurs gradually became extinct.

Dinosaurs: not inferior to mammals, but still extinct

In recent years, it has become increasingly clear that many - if not the vast majority - of dinosaurs were warm-blooded, often covered in feathers, and quite fast. But on the cold-blooded lizards from "Jurassic Park" they were not very similar. Thanks to a peculiar skeleton with air cavities and an unusual respiratory system, they could grow much larger than any land mammals.

The first moments after the explosion of the Chickslube asteroid 66 million years ago / © Wikimedia Commons

Unlike modern birds, taxonomically related to dinosaurs, some of the ancient "terrible lizards" had not smooth, but toothy beaks: something like an ostrich, only weighing up to tens of tons and with teeth. From this picture, it is easy to understand why mammals remained nocturnal burrowing animals until the extinction of terrestrial dinosaurs. It is difficult to run away from an ostrich, and it is difficult to fight off a toothy, multi-ton predator. Bigger rivals simply did not leave them a chance to dominate during the day.

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In addition, paleontologists have convincingly proved that the number of species of terrestrial dinosaurs (excluding birds) remained large until 66.04 million years ago, and then almost instantly dropped to zero. Moreover, 93% of mammalian species were ordered to live long at the same time - and we, for example, are descended from a few surviving species. From this it is clear that mammals drove out land dinosaurs not because they were something better, but because they died due to some external events.

Search for the killer. Volcanoes, or five billion Hiroshima

However, around that time, two catastrophes happened at once, which could explain the Great Extinction. In Hindustan, one and a half million square kilometers was flooded with a thick layer of lava (today these are the Deccan traps). At the same time, a lot of sulfur dioxide, a powerful anti-greenhouse gas, was released into the atmosphere.

It effectively blocks sunlight, creating a volcanic winter on the planet's surface. The eruption that created the Deccan Traps was exceptionally violent. Therefore, a number of scientists suggested that the volcanic winter after it was just as powerful. Cold can kill species: most major extinctions on Earth are accompanied by a sharp drop in temperatures.

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But about 66 million years ago, another disaster struck the planet, capable of "freezing" large species of animals. The Chicxulub asteroid with a diameter of at least ten kilometers fell near the Yucatan Peninsula. When a large body with a speed of many kilometers per second collided with solid rocks, an explosion occurred, whose power is estimated at 100 million megatons (approximately five billion Hiroshima). Like any major asteroid impact, it lifted a huge amount of dust into the stratosphere, causing an asteroid winter.

The asteroid has a number of "bonuses": it throws up not only dust, but also large debris, which gain speed up to kilometers per second and make a suborbital space flight.Returning from there at speeds up to 6 km / s, the wreckage gets very hot. Some of them - the largest and least dense - give secondary explosions in the atmosphere, some - smaller and denser - fall red-hot to the ground. Along the way, 100 million megatons gave the strongest tsunami. But all these effects are, in fact, secondary: according to calculations, an asteroid winter is much more dangerous for the biosphere than a global fiery rain or a high wave that has bypassed all the oceans of the planet.

So the extinction could have been caused by both volcanic and asteroid winters. But how do you know which one? It would be nice to do this by finding out what happened first. One problem: the eruption of the Deccan Traps, it seems, began a little earlier than the extinction of the dinosaurs, tens of thousands of years. Although in full force went only after him. That is, dates alone cannot understand the situation: some other indicators are needed.

Someone acidified all the seas at once - and instantly

An international team of scientists decided to answer this question by looking at the sediments of the former seabed. They found a number of clay samples 66.04 million years old, including one extra thick (and informative), in what is now Holland. These sediments contain shells of marine unicellular organisms with a small shell - foraminifera. The authors of the new work studied how the level of boron changed in these deposits, an element whose accumulation in shells of mollusks depends on the acidity of the seas. Acidity is quantitatively expressed as the pH value.

This pH has been extremely stable over the last 100,000 years of the Cretaceous, according to new work. This, by the way, is not particularly expected: in the last 20 thousand years of the Earth's history, the acidity of its oceans jumped back and forth at a fairly rapid pace. This is because the acidity of the ocean quickly reacts to the CO2 content in the atmosphere. In a warm climate, there is more CO2 in the air, in a cold climate - less. It turns out that the end of the era of the dinosaurs was distinguished by a surprisingly stable climate, without such sharp fluctuations as in our time.

Scientists believe that the invariability of the acidity of the seas at this moment removes all suspicions from the Deccan traps: they cannot be dinosaur killers. If their eruptions really caused a global volcanic winter, then the pH could not remain stable in any way. Sulfur dioxide falls out of the atmosphere in the form of acid rain. Approximately 30 thousand years of the eruption of the Deccan Traps before the extinction of the dinosaurs did not give a sharp surge in such rains. This means that the scale of the eruptions then was not so great as to affect the climate and arrange extinction.

But 66, 04 million years ago, not a trace remained of the former stability. For a period of no more than a thousand years, the pH of the oceans, judging by the boron in sedimentary rocks, dropped immediately by 0.25. Let us emphasize: a thousand years here is precisely the upper limit. Perhaps it happened faster, just one sedimentary rocks cannot give a more accurate dating of the event.

What acidified oceans are talking about

It may seem that 0.25 is not that much. Undoubtedly, this level of acidification is rarely capable of killing marine life on its own. Yes, even 20 years ago, many sounded the alarm that ocean acidification would lead to the mass death of organisms with shells and calcareous skeletons (from mollusks to corals). It was believed that acidified water would begin to dissolve their shells and skeletal structures.

Today we know that this is, to put it mildly, an oversimplification. In fact, many organisms that use carbonates to build their bodies benefit from ocean acidification. And the rate of their growth in general, and the rate of calcium uptake by them to build their skeletal structures in acidified water increases markedly. From 1751 to 1996, an increase in acidity lowered the pH of the oceans from 8, 25 to 8, 14. At the same time, contrary to fashionable forecasts about the "dissolution" of organisms with carbonate skeletons in acidified seas, the biomass of coccolithophorids increased by 40%.

Some corals, which were also predicted to be extinct due to acidification of the oceans, grow better in highly acidic (up to pH 7, 3) water. Despite such conditions, they reach three times the size of what they have today in non-acidified seas.

However, the increasing acidity of the ocean is an important indicator of what is happening on the planet. The authors of the new work calculated: acidification in the first thousand years after the fall of the Chiksulub asteroid indicates a sharp change in the composition of the atmosphere. Before the most powerful asteroid explosion, CO2 in the atmosphere was 900 parts per million, and immediately after it became 1600 parts per million. This is a huge leap forward.

But an increase in CO2 content alone cannot lead to extinction. There has been much greater growth in the history of the Earth. Only 10-15 thousand years ago, jerks of CO2 concentration back and forth happened repeatedly. Fifty-odd million years ago, during the Paleocene-Eocene thermal maximum, the level of CO2 in the atmosphere rose by about half, to a level higher than what it was at the time of the extinction of the dinosaurs. The pH of the oceans dropped by 0.28, more than after Chicxulub.

Despite this, the number of species on the planet not only did not decrease, but also increased rapidly. This means that the extinction of the dinosaurs was not caused by the "rush" of carbon dioxide. Rather, the extinction itself triggered this tide.

What the soaring CO2 told about

Carbon dioxide in the atmosphere can be not only a cause of global changes such as the current warming, but also a sign of them. What is it about?

According to modern data, after the fall of the Chicxulub asteroid on the planet for 80 years, rather low temperatures were established. According to a 2016 study, 325 billion tons of sulfur were released into the air, which could produce up to a trillion tons of sulfur dioxide (SO2).

At first glance, this is not so much: there are more than three trillion tons of CO2 in the atmosphere today. But the sulfur compounds ejected high into the atmosphere by the asteroid's explosion actually block the sun's rays effectively. Calculations show that even if only 100 billion tons of sulfur would fall into the atmosphere from the explosion of an asteroid, the planet would cool down strongly.

Three years after the explosion of the asteroid, the average temperature should have dropped by 27 ° C. That is, from plus 22 ° C, normal for the Cretaceous period, it dropped to minus 5 ° C. Obviously, with three times more sulfur emissions, the global cooling should have been much more serious than 27 degrees, but how much is difficult to say yet. Therefore, we restrict ourselves to the calculation for 100 billion tons. Is a 27 ° C drop in average temperature really scary and could cause extinction?

The average annual temperature in Moscow is now about plus 6 ° C. Its decrease by 27 degrees means the transformation of the Moscow climate into a climate that has no analogues in the Northern Hemisphere today. On the coast of Antarctica - from minus 10 ° C to slightly lower levels. Minus 20 ° C is the climate of the interior of Antarctica, where even penguins are not found. There are no large animals at all, and, frankly, there is an acute shortage of small multicellular animals. That is, in the event of a repeat of the “Chiksulub winter”, the Moscow area would become uninhabited today.

One could argue that the Russian capital is not the best place to live and that the average on Earth is now + 15 ° C. This is 7 ° C lower than at the end of the dinosaur era. A drop of 27 ° C would make the average temperature of the Earth equal to minus 12 ° C. It is easy to find an analogue for such a climate: Franz Josef Land, where the average annual temperature is exactly minus 12 ° C.

That is, on average, the planet would become noticeably colder than the current Wrangel Island, but not colder than the northernmost Russian islands. On Franz Josef Land, fewer than three dozen species of birds and mammals are found: for the rest it is too cold.

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The warmest place on Earth today has an average annual temperature of + 34 ° C. Its fall by 27 ° C is an average annual plus 7 ° C, about the same in Saratov. At first glance, dinosaurs might have survived in tropical oases.

Not really. According to the calculations of the researchers, the warmest after the asteroid winter was over the seas. Because of their huge heat capacity, it simply could not get too cold there, so the average temperature over the open ocean was plus 5, 9 ° C.

Over land, however, the situation was worse: the average annual temperature dropped to minus 32 ° C in the coldest year of an asteroid winter. Over the continental parts of the tropics at the end of the dinosaur era, the average annual temperature after Chicxulub dropped from plus 27 ° C to minus 22 ° C, that is, again, to what is now observed only in the interior of Antarctica. Full recovery of pre-Xulub temperatures was only possible after at least three decades of asteroid winter.

The average temperature of the Earth after Chicxulub could not rise above zero degrees in the first five years after the impact. From this it is obvious: large animals simply could not survive there. In the Mesozoic it was warm, very warm, even the present Antarctica and the Arctic had a climate that we would define as temperate (approximately modern Moscow).

To arrange a five-year winter in such places, in which only tundra vegetation can survive, means to destroy almost all adult plants. Seeds in the soil may remain - and this is what happened: angiosperms recovered relatively quickly, but in five years of winter without plant food, almost all herbivores will die. And the larger they are, the faster they will die. And after them the carnivores will not keep themselves waiting: they will not last five years without herbivorous prey.

The masses of dead animals and plants had to gradually rot, supplying the atmosphere with CO2. However, the vegetation of the same range could not have been. Another scientific group showed that soot and sulfur dioxide, thrown into the stratosphere by an asteroid impact, dropped the level of sunlight by 100 times for a period of up to two years. Terrestrial plants cannot efficiently photosynthesize under one-hundredth of normal sunlight. That is, there was no one to take CO2 from the air in order to get oxygen from it for a couple of years.

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After the sunlight slowly returned, there was no instant return to normal climate either. Dozens of years of asteroid winter have led to a sharp expansion of exotic sea ice during the time of the dinosaurs. Ice reflects the sun's rays well, additionally cooling the Earth. The pre-shock richness of the flora simply could not return quickly.

An even more important source of CO2 could be the asteroid impact itself. It inevitably had to emit a lot of carbonaceous material into the atmosphere. In addition, global fires from the planetary rain of fire also introduced a lot of carbon dioxide into the atmosphere. According to some estimates, the total contribution of the Chicxulub asteroid to the Earth's atmosphere could be hundreds of billions of tons.

Normally, CO2 can raise the temperature of the planet because it traps infrared radiation. However, during an asteroid winter, this does not work: the sulfur dioxide in the atmosphere prevents the sun's rays from reaching the dense troposphere. CO2 cannot keep enough heat near the surface if too little of this heat gets there.

So, a sharp surge in CO2 after the impact of an asteroid indicates a key one: it was he who gave rise to a powerful surge in the content of greenhouse gas in the atmosphere, and the planet owes it to the extinction of dinosaurs. Volcanic activity in the area of ​​the Deccan Traps could aggravate the situation, but it was not she who delivered the first and most powerful blow to the dinosaurs.

Why terrestrial dinosaurs are extinct, but birds and we are not

On the whole, the situation is clear. An explosion of 100 million megatons plunged the planet into a continuous night for a couple of years, and into an extremely cold climate for many years. It is understandable why three quarters of plant species and 93% of mammalian species have died. But part survived. Of the dinosaurs, birds also survived. Why did the terrestrial dinosaurs die?

For example, it is difficult for large animals to feed themselves in difficult conditions.But among the dinosaurs there were many small ones - the size of a person. Why didn't they survive and then give rise to larger species?

The most likely answer to this question boils down to one thing: eggs. Dinosaurs, as it is clear today, hatched them (unlike cold-blooded lizards). The Mesozoic era did not know such a period as after the impact of the Chicxulub asteroid, so it was possible to hatch eggs for a long time. While birds usually sit on eggs for 11 to 85 days (the largest of them, the ostrich, takes 42 days), terrestrial dinosaurs incubated for three to six months.

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While all was well, long incubation was a plus for non-avian dinosaurs. Scientists believe that the main reason for prolonged incubation was the time required for the development of teeth in the embryo. Modern birds do not have them, which allows them to hatch eggs quickly. But it is difficult to occupy many ecological niches without teeth. It is more difficult to tear large prey or grind coarse plant food with a toothless bird beak. In general, teeth are clearly needed for most flightless species.

But after the Chiksulub catastrophe, long-term incubation of eggs from a plus became a fat minus: it was difficult with food, there are no living plants, herbivores die, carrion spoils, and only those who constantly move can survive, taking rare food from the inhabitants of all new lands. After laying eggs, it is very difficult to transfer them to another place, and it is difficult to warm them on the way (asteroid winter is not a joke). It is impossible to feed in one place even for three months: there is a catastrophe around apocalyptic proportions, you cannot sit in one place.

Birds, especially small ones, can hatch eggs quickly, so they survived, and the number of their species today is almost double that of mammals. To be honest, the thesis "mammals began to dominate the planet" actually sounds like "mammals began to dominate among flightless animals."

The mammals themselves are not extinct because their pregnant females carry their young with them, that is, they are not tied to one place. Oviparous mammals, such as modern echidnas and platypuses, hatch their eggs underground, where temperature fluctuations are less. This also reduced their chances of extinction in a cold snap..

What does it all give us

At first glance, the conclusions from the new work are rather commonplace. It turns out that a strike of five billion Hiroshima, followed by a rain of fire, megatsunami and tens of years of asteroid winter can destroy all large species of land animals. Is it really necessary to prove something so obvious?

Oddly enough, yes. We have already noted: in Russia there are still scientists who are ready to deny that dinosaurs became extinct precisely after Chicxulub. In the West, many researchers have tried to attribute the catastrophe that happened 66 million years ago to volcanoes. Finding out who is really to blame is not just a matter of abstract curiosity. Volcanic eruptions cannot be prevented.

But the fall of large asteroids and comets to Earth is quite capable of stopping mankind - if it wants to create the necessary missiles with thermonuclear warheads of the required power. But without clear scientific evidence that asteroids are the most dangerous factor for life on earth, no one will invest in protection against them.

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