The largest extinction in Earth's history has been associated with sea waves, bacteria, warming, asteroids and volcanoes. However, all these versions contradicted each other and the newly discovered facts. The last solution to the great riddle, as usual, refutes all the previous ones. Will it put an end to a detective story called "Perm mass extinction"?
Anyone who reads for the first time about the disaster on the border of the Permian and Triassic periods of the geological history of our planet is in awe. More than 95 percent of all marine species, 73 percent of terrestrial vertebrate species and even 83 percent of all insect species became extinct in a matter of tens of thousands of years. Even today it is difficult for a man with all his resources, weapons and impact on the environment to destroy even a small share of such biological diversity. What should have happened for such an apocalypse to occur - never repeated either before or after the massive Permian extinction?
Killers - volcanoes and warming
When geologists first discovered a large gap in the remains of any living organism in layers of geologic rocks - and even deposits of fossil fuels - about 252 million years ago, it was hard not to understand that something had happened. One of the main tools in the study of such ancient times is the search for deviations by isotopes. Plants prefer to assimilate carbon dioxide molecules from the atmosphere, which contain atoms of light carbon-12, and oxygen, which contain lighter oxygen-16. If life is hit, the heavier carbon-13 and oxygen-18 begin to occur more frequently in the sediment. By analyzing the ratios of these isotopes, you can find out, for example, what was the content of carbon dioxide in the air - and indirectly understand what the climate was like. So,during the formation of the mineral calcite in the soil under conditions of a high concentration of CO2, it increases the content of carbon-13, which usually gets into fertile soil (with decaying plant remains) with difficulty.
The study of the boundaries of the Permian and Triassic deposits by such methods showed a very strange picture. 252 million years ago, the concentration of carbon dioxide for some reason soared to 2000 parts per million (now 400 per million). Accordingly, the average annual temperature on the planet has probably risen by 8 degrees Celsius. Even at the equator, where the warming effect is weakest, it has become an average of 6 degrees warmer. Seas in shallow waters in places warmed up to 40 degrees.
Paleontologists quickly established that most of the extinct marine organisms must have had low resistance to carbon dioxide poisoning. And their death fits very well with its sudden excess in the atmosphere. But the species with the lowest metabolic rate turned out to be the most resistant to extinction (it is more difficult to quickly poison themselves). Calcareous sponges, some corals, brachiopods and many other organisms with an external carbonate skeleton have shown good survival. Everything seems to be correct: there are opportunities for "storage" of excess carbon dioxide and proven mechanisms for spending it on the construction of outer shells.
A logical picture emerged from this. The main supplier of carbon dioxide for the atmosphere is volcanic eruptions. The sudden rise in CO2 content must be associated with them. If you really want to find something, then everything will work out. Indeed, it turned out that the Siberian traps - a huge lava province in millions of square kilometers - poured out just at the border of the Permian and Triassic. Here it is, the reason for the pernicious saturation of the atmosphere with greenhouse gases and general extinction. For some reason, too many volcanoes suddenly started working.
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Flickr / Sarah Bodri / AA / ABACA / EAST NEWS
Something does not grow together
In all respects, a clear explanation immediately raised many questions from skeptics. First, why was there no such mass death either before or after? Why did the Siberian traps form at that time? We don't know much about the interior of the planet, but that's enough to understand: lava spills regularly and in any era.
The first objection was quickly followed by a second. In the spores of plants of that time, traces of damage were found, showing that they were influenced by harsh ultraviolet light. As long as the planet has an ozone layer, this cannot happen. In other words, it sharply weakened 252 million years ago. But together with carbon dioxide, volcanoes emit a lot of sulfur dioxide. And its maximum in the absorption spectrum lies in the ultraviolet region (190-220 nm). This coincides with the maximum in the ozone absorption spectrum, that is, if there is a lot of this gas in the atmosphere, the ultraviolet light cannot reach the planet's surface, even if there is much less oxygen on it. This means that the extinction in time could not coincide with the "volcanic warming".
Complicated matters and the fact that volcanic eruptions were and are still going on in the era of human existence. And from this experience it is known: they do not bring warming. On the contrary, after them comes a volcanic winter. 70 thousand years ago, one supervolcano killed nine out of ten living people in a volcanic winter. Only those who had not yet managed to leave Africa survived. It is clear that if volcanoes carried heat, then they would be the worst.
Finally, a quarter of a century ago, Mount Pinatubo showed that even a relatively moderate eruption cools the entire planet by a noticeable amount. But it cannot cause warming. The fact is that the carbon dioxide emitted by the volcano is vigorously absorbed by the biosphere and rocks, which the same volcano brings to the surface. Moreover, the higher the temperature, the faster the metabolism of plants and the rate of binding of the greenhouse gas to rocks.
Asteroid trail
There was an attempt to tie extinction to the traditional heroes of such tragedies - asteroids. At the beginning of the century, reliable data were obtained on the presence of a huge 480-km impact crater of Wilkes Land under the ice of Antarctica. It is 2.5 times larger in size than the Chiksulubsky, left by the asteroid that killed the dinosaurs. Obviously, a crater of half a thousand kilometers remains only after a really dangerous shell. True, it is not clear how the asteroid could cause warming and the ensuing extinction. 66 million years ago, everything was the other way around: the cold of the asteroid winter overtook the area where there was an eternal plus before.
And here the old theory came forward: that the impact of a really large asteroid could cause a series of powerful volcanic eruptions at a point exactly opposite to the impact area - on the other side of the Earth. The authors of its Permian version suggested that millions of square kilometers of Siberian traps are a natural consequence of a body impact, which left a nightmarish trail under the ice of Wilkes Land.
Whether it is true or not is very difficult to establish. The surface of our planet is moving all the time due to plate tectonics, and it is not so easy to understand what was opposite what 252 million years ago. It is even more difficult to calculate such a process - we do not have enough knowledge about what is happening below the upper layers of the mantle, where it is impossible to look with the available sounding means. The hypothesis is supported by the fact that the Chiksulubian "dinosaur killer" was indeed almost the antipode of the Deccan traps in India. Then the former rulers of the planet were destroyed by a double blow - first an asteroid winter, and then a volcanic one caused by the impact of the same body. But whether it was the same in the Permian period is a more complicated question. Both types of winters do not bring warming, and it is assumed that life was almost killed by them.
Flickr / NASA Goddard Space Flight Center / Wikimedia Commons
Extinct from greed
Against the background of all these discrepancies, other theories naturally sprouted. It turned out that about 252 million years ago, archaea (Methanosarcina), through horizontal gene transfer from other organisms, learned to process acetic acid salts into methane. By that time, a lot of dead organic matter had accumulated on the seabed, in which there was a lot of unprocessed acetic acid salt - there was no one to absorb it there. Archaea can live there, and having learned to produce methane from the buried remains, they had to begin rapid reproduction. At the same time, no one could manage to absorb their methane - on the surface of the planet, among oxygen-containing organisms, there is a shortage of methane consumers. And this gas, although short-lived, but over a 100-year cycle, gives 34 times more greenhouse effect than carbon dioxide. This is the cause of the fatal warming - and without any problems with the volcanic winter.
And everything would be fine with this version, if not for one "but". Archaea need an enzyme that includes nickel to generate methane along this route. And this metal is quite scarce for living beings. It could have been brought about by volcanic eruptions, especially underwater ones. But in the remaining layers, such nickel emissions are observed after sharp fluctuations in the ratio of carbon isotopes. That is, at first almost everyone died out, and only then nickel appeared on the scene - without which the scenario “the greed of the archaea is to blame for everything” should not have worked.
Was there a warming?
Excavations in sub-Saharan Africa turned out to be another problem - in other places there are almost no remaining layers from the land surface for the time of the Permian extinction. It turned out that 252 million years ago, the rivers there from full-flowing plains began to turn into meandering and rich islets. This is a sign of aridization, drier climate. But strong global warming cannot be combined with an arid climate. Depending on wind conditions, each degree of temperature increase results in an increase in precipitation of 2-7 percent. A global warming of 8 degrees should result in a more humid climate.
Another question immediately arose. Of course, the "average annual" 8 degrees is a very sharp shift up the thermometer. The same gap between the climate of Murmansk and Voronezh or Voronezh and Costa Rica. But the problem is that a similar in scale warming on the planet happened, and even later than 252 million years ago. 56 million years ago on the planet in a matter of thousands, if not hundreds of years, the temperature soared by the notorious 8 degrees. The content of carbon dioxide in the atmosphere struck not only 2000, but also 3000 parts per million. Who died out? There is really nothing to say about it. In some equatorial regions, there are fewer photosynthetic protists, for example, some dinoflagellates. But they immediately moved into the ocean, which is now called the Arctic (then, of course, there was no ice there).
In parallel, on land, instead of extinction, there was a real heyday. There were no noticeable extinctions of species, but artiodactyls and equids spread literally all over the world - and exactly at the time of the peak of warming. Our primate ancestors made the same expansion. If it were not for the warming, which is quite comparable with the border of the Permian and Triassic, perhaps, there would be no evolutionary success of people. How did it happen that the non-warm-blooded fauna almost completely died out from the increase in temperatures at the end of the Permian, and the warm-blooded and non-warm-blooded inhabitants of a closer era did not suffer?
EAST NEWS / Wikimedia Commons
Turn upside down
And now, in 2017, completely different data appear: it turns out that due to the huge distance in time, the previous dating of the Permian extinction was slightly inaccurate, literally for a few hundred thousand years. For a quarter of a billion years, the accuracy is excellent, but it was this small error that prevented the main thing from being seen.
A group of Swiss researchers using the uranium-lead method has more accurately dated the peak of the Permian extinction than ever before. And then it turned out that it lasted only 89 ± 38 thousand years, and it did not coincide with the onset of the seas due to warming, as was previously thought, but with their previous retreat. The seas, as you know, quickly become shallow only during a cold snap, when their water is bound in the polar caps.
Unfortunately, there are problems here - with such a short cooling period, very little undisturbed rocks should have remained from it. Actually, this is why this period of short-term cooling has not yet been studied in detail. At that time there was no land at the poles, and it is very difficult to find traces of ice caps that once were on the surface of the ocean. In addition, the oceanic crust is renewed on average once every 200 million years, that is, even traces from the seabed of that time are already buried deep in the bowels of the planet. So finding out exactly how cold it got will be very difficult.
And yet, so far, this hypothesis is in the least contradictory to everything that scientists have managed to learn about the greatest extinction of all time. Finally it became quite clear what volcanoes and possibly asteroids had to do with it. Both those and others can really cause powerful cold snaps. Ash eruptions and sulfur dioxide emissions from their vents are capable of reaching the stratosphere and blocking sunlight. Siberian traps are unprecedentedly large in the history of the planet, there are simply no other such huge ones. The magnitude of the volcanic winter they caused could have been the same.
The uniqueness of the Perm catastrophe is also explainable in this scenario. If its causes were associated only with planetary processes, then it had to be repeated more than once. But an external threat - such as a huge asteroid - is a probabilistic factor. Craters of 500 kilometers in the entire history of complex (post-Cambrian) life have not been repeated. This means that other such disasters should not have happened. The only question here is: are we looking at the sky carefully enough not to miss another such stone in the future?
Alexander Berezin