October 20, 2020

The meteorite did not kill the dinosaurs | Science

The meteorite did not kill the dinosaurs | Science

Every so often, on Earth there is a gigantic holocaust that is usually also a regime change. 2.8 billion years ago, a new group of microorganisms, the cyanobacteria, began to produce oxygen by doing photosynthesis. They transformed the world and made life possible as we know it, but they annihilated the organisms that had dominated the planet until then because oxygen was toxic to them. As one of the leaders of the ultraconservative revolution recounted in The story of the maid, by Margaret Atwood, "better never means better for everyone, it always means worse for some". And what is valid for political revolutions, it is also valid for biological revolutions.

Of the five great extinctions that arrived later, the most lethal happened 252 million years ago, at the end of the Permian. Then, a huge eruption in Siberia flooded the atmosphere of CO2 and produced an intense greenhouse effect that aggravated the activity of some methane-producing microbes. The oceans became more acidic and lost oxygen and the partial destruction of the ozone layer allowed the ultraviolet radiation to sweep the earth's surface. It is estimated that 96% of the species that inhabited the Earth perished in less than one million years, a brief time if the geological scales are considered.

A gigantic eruption in Siberia is at the origin of the greatest extinction in the history of the Earth

Despite annihilating life almost completely, the great mortality, as this massive extinction is known, is not the best known of all. That honor corresponds to that happened at the end of the Cretaceous, about 66 million years ago, the cataclysm that took ahead to one of the most fascinating animal groups that ever set foot on Earth. When they dig the ground in search of fossils with which to reconstruct the past, scientists observe that at that moment they disappear most dinosaurs and practically 75% of the living beings of the time. In that stratum, Luis Álvarez and his son Walter discovered in the eighties a large amount of iridium, a very rare material on our planet but abundant in meteorites and asteroids. From the iridium calculated that a rock of 10 kilometers in diameter from space was probably the culprit of that hecatomb. Soon after, the theory was strengthened when a crater was found in the Mexican Yucatan Peninsula that was identified with the impact site.

But life does not wobble on a planetary scale by a single blow, no matter how strong, and it has long been argued that a series of volcanic eruptions over hundreds of thousands of years, as has happened in similar events throughout of the history of the planet, they were changing the climate and atmospheric conditions of the Earth preparing the ground for the extinction of the Cretaceous. The place of these eruptions are the stairs of the Deccan, one of the largest volcanic regions of the planet located in India. Today, two teams of scientists publish in the magazine Science High precision measurements of the area to try to reconstruct the course of events that killed the dinosaurs.

On the one hand, a team led by Blair Schoene, from Princeton University (USA), used a dating method that takes as a reference the rate at which the uranium is radioactively disintegrating to become lead. Thus they calculated that the eruptions of the Deccan began tens of thousands of years before the great asteroid. The large amounts of methane, carbon dioxide and sulfur dioxide released into the atmosphere by volcanoes would have caused planetary disturbances capable of extinguishing much of Earth's life long before the arrival of the asteroid.

It is likely that dinosaurs took tens of thousands of years to succumb to the cataclysms that annihilated them

In a second study, led by Courtney Sprain, of the University of California at Berkeley (USA), uses radioactive argon to calculate the moment in which the eruptions occurred. Although the results are not very different, there are different interpretations of the data and it is argued that the collision in Mexico, practically in the antipodes of India, accelerated the eruptions and produced a gas emission responsible in part for the extinctions.

The catastrophe, which facilitated the arrival of mammals and in the end of our lineage, maybe you should not imagine how Hollywood movies usually do it, with an imminent impact that will end life on Earth in a few days. "It's very difficult to say what the exact time scale of the extinction was," says Paul Renne, a Berkeley researcher and co-author of one of the studies. "In fact, it is likely to be variable for different animals and plants, depending on their position in the food web. It seems clear that marine plankton was the fastest to disappear, probably in less than 10,000 years. For other animals, especially terrestrials, like dinosaurs, it could take more time, but it is very controversial, "he says. And he concludes: "Something is true, the extinction did not happen in an instant like in the movies".

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