Almost 40 years ago, Walter Alvarez, then young geologist fresh out of the University of Berkeley, started a revolution in the sciences of Earth discovering that a strange stratum dark clay marked the sudden disappearance of the plankton sailor at the end of Cretaceous and at the beginning of the Tertiary era. Now, it was at this time that not only the large marine reptilesthem ammonites and the belemnites, but above all the dinosaurs.
With his father, the Nobel Prize physical Luis Alvarez, and especially with chemists Frank Asaro and Helen Michel, all from UC Berkeley, he undertook to make the layer speak by dating it and analyzing it precisely. The stratum was found to contain an abnormally high amount of a rare element on the Earth’s surface,iridium. This metal being abundant in comets and the asteroidsWalter Alvarez deduced that the biological crisis occurred 66 million years ago, the famous Cretaceous-Tertiary crisis (or KT, from German Kreide-Tertiär), was due to the fall of a small celestial body on Earth.
This would have resulted in a mega-tsunamibut also ejecta of matter burning that will put the fire to a good part of the planet’s forests, and a volume considerable amount of dust andaerosols who would have risen to the high atmospherespreading across the globe and changing the climate and thesunstroke by causing a global cooling causing the collapse of the food chain.
An artist’s impression of the KT crisis, the great extinction caused by the fall of an asteroid on Earth 66 million years ago. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Mark A. Garlick
A second large impact crater at the end of the Cretaceous?
The non-avian dinosaurs did not survive this catastrophe and a good part of the biosphere either, already undoubtedly tested by the giant eruptions at the origin of the formation basaltic plateaus (the traps) of the Deccan, west of India. This thesis has been broadly verified since a large impact crater was found precisely correlated to the deposition of the layer ofclay black, theastrobleme of Chicxulub.
A team of researchers in geosciences and planetology has just made a new contribution to Walter Alvarez’s thesis by publishing an article in the prestigious journal Science Advancesarticle accompanied by another intended for the general public in another periodical: The Conversation.
Geologists, geophysicists and planetary scientists announce that they have identified more than 300 meters below the seabed, about 400 kilometers off the coast of Guinea, in West Africa, a structure they dubbed the Nadir Crateraccording to the volcano neighbour Nadir Seamount. Its diameter is about 8.5 kilometers and its age is certainly very close to that of the Chicxulub crater in Yucatán, that is to say about 66 million years.
However, the characteristics of the crater Nadir are also those of an impact crater, characteristics from which it appears that it is probable that the asteroid which created it measured a little less than half a kilometer in diameter. Although much less significant, its impact on the biosphere would have been comparable to that of the event in Yucatán.
Just 40 years ago, scientists discovered a widespread layer of iridium on our planet, proof of a cosmic impact on the surface of the Earth. A look back at an incredible investigation that revealed evidence of a massive extinction 66 million years ago with a teacher-researcher at the Paris Sud Geosciences laboratory at Paris-Saclay University, Sylvain Bouley. He deciphers the planetary surfaces in order to reconstruct the history of our Solar System. A specialist in the planet Mars and impact craters, he is also co-responsible for the FRIPON and Vigie Ciel programs. © Fleurance Astronomy Festival
An example of serendipity
As they say, the discovery of the Nadir crater was made by serendipity end of 2020. The researchers initially simply wanted to clarify the geological structure of the Atlantic seabed as part of a project to reconstruct the separation tectonics of South America from Africa during the Cretaceous period, due to continental drift.
For this, they used the well-known technique of “seismic reflection” which has been used for decades to also do marine prospecting in search of deposits of oil and of gas. To do this, guns air compressed, water cannons or acoustic vibrators generate shock waves, as in the case of an explosion, shock waves which are reflected and refracted by their passage through the layers of sediment and rocks before being subsequently captured by hydrophones or other sensors seismic waves distributed along cables pulled by a ship. We can then do the equivalent ofultrasound in medicine.
Many of the features of the crater-like structure discovered are consistent with an impact crater, such as the aspect ratio and height of the crater rim, as well as the presence of chaotic deposits outside the crater floor that resemble “ejecta” – material expelled from the crater immediately after a collision.
As in the case of the impact of Chicxulub, it was possible to simulate numerically on computer what may have happened, in search of the best parameters for a model capable of reproducing the characteristics of the Nadir crater.
The scenario that best allows for this is an asteroid 400 meters in diameter hitting an ocean 800 meters deep.. L’energy released would have been about a thousand times greater than that of the recent Tonga eruption. The shock would then have produced a earthquake of magnitude 6.5 or 7, which would probably have triggered submarine landslides in the area and certainly a tsunami accompanied by the vaporization snapshot of the asteroid and a substantial volume of sediment – with a large fireball visible hundreds of miles away, as explained in the article by The Conversation.
A Shoemaker-Levy 9 scenario?
The question now arises of the relationship between the astrobleme of Nadir and that of Chicxulub. They are of comparable ages but like the size of theimpactor of the Nadir crater is smaller than that of the Yucatán crater, the estimate of the frequency of impacts of a small celestial body about 400 meters in diameter is about one every 700,000 years on Earth. A remarkable coincidence cannot therefore be excluded, since the age of the Nadir crater is estimated with an uncertainty of plus or minus one million years compared to that of the KT limit.
However, it could indeed be two simultaneous impacts produced by a small celestial body about ten kilometers in diameter which would have fragmented such Shoemaker-Levy’s Comet 9 under the influence of tidal forces in the vicinity of the Earth. Moreover, we find on the Moon chains of impact craters testifying to an event similar to that seen on Jupiter in 1994 with Shoemaker-Levy 9.
This is the “little brother” or “little sister” hypothesis if it is assumed that the Chicxulub impactor was a comet.
Last hypothesis, that of the “little cousin”. The Nadir crater would then have been caused by one of the fragments of a major collision in the asteroid beltfragments of which another would be at the origin of that of Chicxulub.
Again, as explained in the article by The Conversation“ we can only test the little sister and little cousin hypotheses by accurately dating the crater using these samples, as well as looking for other candidate craters of a similar age “.
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