Thu. Jan 23rd, 2020

This was the first day on Earth after the asteroid that killed dinosaurs | Science

About 66 million years ago, millennium up or down, an asteroid hit Earth on what is now the Gulf of Mexico. The clash was such that the dominant theory among scientists points out that it caused the disappearance of 75% of life, starting with dinosaurs. Now, the study of a cylinder of rock extracted from the crater that it caused has allowed us to reconstruct what happened so long ago minute by minute. And it was a real hell.

In 2016, Expedition 364 to the Chicxulub crater, in the northwest of the Yucatan Peninsula (Mexico), it drilled the impact zone. They did not pierce the same center, but at the outer edge of the crater. They removed a rocky cylinder up to 1,334 meters below the seabed. Cut into portions, your Study by a large group of geologists and scientists from other fields, tells the story in chapters as precise as the tree rings or the cores extracted from the ice do even if millions of years have passed.

Most of the crater was filled with sediments in the first 24 hours after the impact

"It is one of the advantages with impact craters. Their training follows very well defined physical laws," says the researcher at the Astrobiology Center / CSIC and co-author of the study, Jens Olof Ormö. "We can reconstruct a sequence of events (for example, see which sediments follow one above the other). By the type of sediment (size of the clasts (fragments), type and classification), we can know if they were deposited quickly or slowly, and about the time it took, "he explains.

In Chicxulub, the asteroid impacted by releasing an energy equivalent to that of 10,000 million bombs like Hiroshima's. Volatilized huge amounts of material and previous studies have estimated that it released 425 gigatons of CO into the atmospheretwo and another 325 of sulphides (one gigatonelada equals 1,000 million metric tons). A penultimate data: the subsequent tsunami brought water from the Caribbean to the great lakes of the northern US, about 2,500 kilometers from the impact zone.

But what has most interested geologists was how quickly most of the crater was filled with the remains of the brutal clash. It is estimated that in just 24 hours the hole was covered with a layer of about 130 meters of sediment, which is what they have studied now. There is written the story of the first day of life on Earth after the impact. There the geologists put the division between two ages, that of the Mesozoic and the current Cenozoic. And that's where almost everything indicates that the extinction of dinosaurs and the emergence of mammals began.

Platform from which the Chicxulub crater was drilled.

Platform from which the Chicxulub crater was drilled.

According to the study, published in PNAS, the lower 40-50 meters, formed of molten and fragmentary rock (gaps) were deposited minutes after impact. An hour later another layer of about ten meters composed of suevite, glass rocks and other molten materials had formed. Hours later, another 80 meters of finer sediments were filled. At the end of the day, the ebb of the water withdrawn with the impact dragged there huge amounts of material from the region and far more remote areas.

Among those last sediments, researchers have found a large amount of organic material, especially the fungus trail and a lot of charcoal. This must have come from the remains of the fires caused by both the impact and the fall of incandescent materials in the jungles that had hundreds of kilometers around.

"With a 12-kilometer asteroid hitting Yucatan, the local effects must have been catastrophic and also probably over distances of up to 1500 km from the impact, where the thermal pulse could cause the trees to burn. At greater distances, the ejected material would also have caused fires. by friction as it fell from the atmosphere. But those effects must have been of short duration and cannot explain the global extinction of 75% of life, "says in a mail, the main co-author of the study, the professor of the Geophysics Institute from the University of Texas (USA), Sean Gulick.

"We are facing empirical evidence of the connection between the impact of the asteroid and the great extinction"

Jaime Urrutia, UNAM researcher

That part of the story began that day, but it must have lasted for years. In the rock extracted from the inner edges of the Chicxulub crater there is a striking absence of sulphurous materials. There is no trace of sulfur in the area and moment of impact, although sulfide-rich rocks abound. These data reinforce the theory that the asteroid expelled huge amounts of sulfides into the atmosphere, preventing solar radiation and cooling the planet. The simulations indicate that the global average temperature dropped 20º and thus remained about 30 years.

"We are facing empirical evidence of the connection between the impact of the asteroid and the great extinction," says the UNAM researcher (Mexico) and one of the leaders of the research group, Jaime Urrutia, which has been investigating the Chicxulub crater for several decades. For him, the great contribution this work is the temporary resolution that it offers on the sequence of events that followed an impact that happened 66 million years ago and marked the destiny of the planet.

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