Massive volcanic eruptions 66 million years ago occurred almost exactly when the dinosaurs were dead

Everyone knows that an asteroid attack has wiped out the dinosaurs, is not it? There is ample evidence that the impact of Chicxulub has had terrible consequences for dinosaurs. But the photo is a bit more complicated than that. Extreme volcanic activity may have contributed to extinction.

At the end of the Cretaceous period, about 66 million years ago, the dinosaurs disappeared. And not only dinosaurs; about 75% of all plants and animals have disappeared. The avian dinosaurs survived.

At the same time, a huge asteroid or comet hit the Yucatan peninsula in present-day Mexico. Called the Chicxulub impact event, it obscured the atmosphere with water vapor and a particularly long dust, due to the structure and type of rock at the site of impact.

It was easy to conclude that Chicxulub caused extinction. There is a lot of evidence beyond the impact site itself.

First of all, extinction itself calls for Cretaceous-Paleogene extinction (extinction K-Pg), as it marked the late Cretaceous and the early Paleogene period. . (It is also called Cretaceous-Tertiary extinction (extinction K-T.)

The geologic record contains a sediment layer dating from 66 million years ago called K-Pg limit. The K-Pg boundary is present in marine and terrestrial rocks around the world. It contains a lot of iridium metal, which is rare on Earth, but abundant in asteroids. The conclusion is that the impact of Chicxulub has spread iridium in the global atmosphere and that it is now preserved in the geology of the Earth, a kind of smoke cannon for the event.

But it is becoming increasingly clear that volcanic eruptions contributed to mass extinction 66 million years ago, and that this evidence comes from a rock formation called the Deccan Traps. A new study reinforces the link between K-Pg extinction and volcanic activity at the origin of deccan traps.

Deccan Traps is a region in India known as the Great Igneous Province. It is one of the largest volcanic features on the planet. The traps consist of several layers of basaltic lava and together they are more than 2 km (1.2 miles) thick. Deccan traps cover an area of ​​500,000 km. m² (200 000 sq mi), although at one time they covered up to 1.5 million km2. (600 000 sq mi) The volume of the lava is 1 million km 3 (200 000 km 3).

The name "Traps" comes from the Swedish word "trappa" which means stairs. It refers to the stepped structure in the landscape of the region.

But these traps are not just rock. The amount of volcanic activity required to create the Deccan traps would have polluted the atmosphere with toxic gases. Two geoscientists from Princeton University have established a closer connection between Deccan traps and K-Pg Extinction with the first high-resolution chronological scenario of the eruptions that created the Deccan traps of India. Their research appears in the Feb. 22 issue of Science.

The two scientists are Blair Schoene and Gerta Keller, both from Princeton University. They led an international team for this study, which attempted to date the different layers of Deccan traps more precisely than ever before.

"Everyone has heard that dinosaurs died of an asteroid striking Earth," said Schoene, an associate professor of geoscience. "What many people do not realize is that there have been many other massive extinctions over the last 500 million years, and many coincide with great floods volcanic "massive volcanoes known as flood basalts or large igneous provinces.

This is not the first time that traps are involved in extinguishing K-Pg. But the accuracy of this new study is a key element.

The effort to date geological formations is called geochronology. Geochronology uses the inherent features of the rocks themselves to determine their age, usually based on isotopic ratios and radioactive decay.

The best-known geochronology technique, commonly referred to as carbon dating, uses the decay rate of radioactive carbon 14 to find fossil ages. But carbon dating only works for living tissue a few thousand years old, making it useful for archeology but not for 66 million year old basalts.

For rocks of the time of mass extinction, geoscientists have some choices of naturally radioactive materials. Uranium-lead geochronology gives very precise ages, but uranium-containing minerals are rarely found in basalt, the rock that makes up the massive lava flows at the Deccan hatches. Uranium zircon is another method of dating ancient rock, but is found more often in the explosive eruptions of Mount St. Helens volcanoes, which have a chemistry richer in silica.

Keeping in mind these restrictions on dating, the team of scientists was cautious about predictions of success. They had never planned that their first trip into the deccan traps would yield the desired results.

"I think none of us had planned that our first trip in Deccan Traps would lead to the type of dataset we were able to produce," said Mike Eddy of the promotion of 2011, now a postdoctoral researcher in geosciences. and a co-author of Science paper.

But they were lucky.

During their first days at the Deccan Traps, scientists collected samples of what are known as coarse-grained basalts. Basalt is the most common type of volcanic rock on Earth. They were looking for samples containing uranium minerals, because the radioactive decay of uranium is a reference method in geochronology. At first, they found none, as these rocks are rare in formations such as Deccan traps and more common in volcanic ash.

But after a few days, they found the type of silica-rich rocks they were looking for.

"During our first week in India, we found a bed of ash with high silica content between two basalt flows, which allowed our system to run," said Eddy. The researchers knew that silica-rich volcanic ash could easily contain the tiny zircon crystals preserving radioactive uranium. "The real breakthrough came a day or two later, when Blair realized that fossil soils could also have collected this type of ash in small quantities," Eddy said.

The team has changed centers. Instead, they looked for ash deposits between basalt flows, in search of radioactive uranium contained in zircon, contained in the ash. Uranium makes zircon, and therefore layers of ash, easy to date. By dating the layers of ash above and below a lava flow, they could more accurately date the lava itself and the date of the eruption.

The team spent three field seasons in Deccan Province and returned samples from 141 sites to the Princeton laboratory. Twenty-four of the samples contained what the team needed: zircon crystals containing uranium. The analysis of the samples showed that Deccan traps were created by four distinct eruption impulses. Each of these impulses condemned dinosaurs and most other forms of life on Earth at that time.

Whenever a volcano erupts, it alters the atmosphere. Huge quantities of sulfur and carbon dioxide are released into the atmosphere by their long-term sequestration inside the rocks. Sulfur has a short-term cooling effect on the atmosphere, while carbon dioxide has a long-term heating effect. Both handsets can lead to wild climate swings.

"These can cause climatic variations between the hot and cold periods that make life on Earth very difficult," said Schoene. But to know more clearly how these eruptions would have affected life on Earth, the moment had to be well chosen. A mass quantity of CO2 will have a very different effect if it is injected into the atmosphere in a hundred years than if it took a million years.

Of the four eruption impulses that scientists identified, two occurred before massive extinctions. The second of these two years did not begin until tens of thousands of years before the impact of Chicxulub, almost at the same time geologically. "The first two impulses … correspond to a period of time when the climate has fluctuated from cold to hot, many scientists believe this indicates an initial climate disruption that may have contributed to the extinction event of mass, "said Schoene. "Our data shows that perhaps the second impulse could have played a significant role in extinction itself because it happened just before."

"Deccan volcanism is the most likely cause of the massive extinction of dinosaurs," said Gerta Keller. "The impact Chicxulub may have contributed to their disappearance, although the timing and the environmental effects of this impact remain to be determined."

Another recent study published in the same issue of Science used a different method to date Deccan traps and proposed different dates. This study concluded that the Princeton study did not identify four distinct volcanic impulses and that more than 90% of the volume of Deccan traps had exploded in less than a million years. He also concluded that about 75% of this phenomenon occurred after the extinction of K-Pg and that climate change at the end of the Cretaceous coincided with the smaller phases of eruptions at the traps of the Deccan. If this were the case, Deccan Trap volcanism could not have caused extinction.

Other scientists, aware of the nature of the debate between incidence and the volcano of which this study is a part, are more circumspect in their conclusions.

"In general, I think this article is meaningful and interesting," said Pincelli Hull, an assistant professor of geology and geophysics at Yale, who did not participate in this research and stood up against the role of the volcano in the mass extinction. "Paper is a huge step forward in synchronizing the [Deccan Traps] eruptions, but the relationship between this and the time of degassing remains a major issue to resolve to determine exactly what were the relevant roles of volcanism and impact. "

It is rare that a study of a long series puts an end to the scientific debate, and this one is not different. Science is progressing as scientists measure things better and think about them. This will not be the end of the debate.

It may be that it took one out of two shots to cause the K-Pg to go extinct. The dinosaurs may have been overthrown by the impact of Chicxulub and on the way up, and finally by the volcanoes. Or it can be even more complex than that.

A study conducted in 2016 shows that dinosaur species were already extinct millions of years before the extinction of K-Pg, and that new species did not seem to replace them. At the same time, mammal species diversified and were perhaps better placed to adapt to the changes caused by volcanic activity and the impact of Chicxulub. Perhaps during the course of evolution, the dinosaurs followed their course, and the impact and volcanoes were only the point of exclamation.

And maybe we will never know for sure.

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