In the Age of the Upper Devonian, between approximately -380 and -360 million years before our era, the Earth suffered mass extinction resulting in the loss of more than 70% living species of the time. The cause of the event remains unknown to this day. Volcanic activity, asteroid impact, ocean anoxia or global warming ? All assumptions are plausible. Today, a group of researchers proposes another theory : the culprit could be the explosion of a supernova.

It is one of the five mass extinctions that our planet has experienced in its history. The best known (and widely used in cinema) is undoubtedly the Cretaceous-Paleogene extinction, there is 66 million years, which ended the reign of the dinosaurs. Today, experts agree it was triggered by the impact of an asteroid (of which the Earth still bears the stigmata : Chicxulub crater, in Mexico). For the extinction of the Denovian, it is more difficult to decide. But a recent American study, led by astrophysicist Brian Fields, suggests a new hypothesis that may end the debate ...

The Devonian extends from −419.2 ± 3,2 to −358.9 ± 0,4 million years. This period is subdivided into three parts : the Upper Devonian, the Middle Devonian and the Lower Devonian. The time is notably characterized by the presence of a great diversity of species of fish on the planet. The Devonian is also sometimes nicknamed "the Age of Fish". The oceans are home to many brachiopods and large coral reefs. On the ground, the first forests appear, quickly populated by insects and chelicera (a subphylum of arthropods).

To explain mass extinctions, most of the time scientists look for "local" causes. for example, a devastating volcanic eruption, whose ashes and dust projected into the air would deprive the Earth of light, gradually leading to the disappearance of all forms of plant and animal life. Another frequently mentioned scenario is a collision with an asteroid.

The different cycles of natural history, mass extinctions and the main astroblems. Credits : Wikimedia Commons

In the case of the extinction that took place in the Upper Devonian, researchers point to a more distant phenomenon. A dying star, located light years from our planet, who would have ended his life in a major explosion. "We are citizens of a larger cosmos, and the cosmos intervenes in our lives - often imperceptibly, but sometimes badly ", says Brian Fields of the Department of Astronomy at the University of Illinois.

Indeed, Fields and colleagues show in their study - published in the journal PNAS - that cosmic rays from a nearby supernova (located approximately 65 light years from Earth) could be the cause of the dramatic drop in ozone levels that caused the disappearance of nearly three quarters of species. The discovery of certain radioactive isotopes in sedimentary rocks of the Earth could confirm this scenario.

However, this is not the first time that supernovas have been found guilty of such events.. The accusation goes back years 1950 ! German paleontologist Otto Schindewolf was the first to suggest that the mass extinctions may have been caused by cosmic impacts or nearby supernovas. More recently, researchers have even determined the critical distance at which a supernova could be fatal to us : they have thus established a "risk zone" between 25 and 50 light years.

For the extinction of the Devonian, suggesting the death of a star as an origin, however, is a first. And in this specific case, Fields and his team are even pushing back the border of dangerousness estimated by their colleagues ...

Their study reveals that the explosions of stars located beyond 50 light years could significantly impact life on Earth, via a combination of more or less long-term effects. Supernovas are the effect of major sources of ionizing radiation : Extreme UV, X-rays and gamma rays. When the star dies, the shock of the explosion causes the particles to accelerate ; the supernova thus causes the formation of cosmic rays, made up of atomic nuclei and high energy particles.

Or, these cosmic rays could be powerful enough to deplete the earth's ozone layer, which would be catastrophic for the terrestrial biosphere, which would be burned by the UV of the Sun. This is presumably what happened in the Upper Devonian. The loss of species diversity and the deformations observed in the spores of ancient plants, unearthed in the deep rocks corresponding to this period, could be a proof. Fossils even indicate a 300,000-year decline in biodiversity leading to extinction, suggesting not a, but several explosions.

otherwise, the other possible causes of this fifth extinction (asteroid impacts, solar flares, etc.) are relatively brief events. It is therefore unlikely that they caused the depletion of the ozone layer. The authors of the study point out, however, that the hypothesis remains to be confirmed. They intend to find other evidence before confirming the scenario.

The Betelgeuse star, observed here with the Atacama Large Millimeter / submillimeter Array (ALMA), is about to become a supernova. He is one of the greatest stars known to date (its radius is 1400 times larger than that of the Sun). Located approximately 600 light years, its explosion should have no consequences on Earth. Credits : THAT

For all scientists interested in the theory of killer supernovae, the search for ancient radioactive isotopes, which could only have been deposited by exploding stars, is decisive. This is why a rare isotope of iron, the iron 60, found in several places on Earth, has already been the subject of much research. In the case of the Devonian, other radioactive isotopes could confirm the hypothesis, especially plutonium 244 and the samarium 146. As Zhenghai Liu explains, co-author of the study, none of these isotopes occur naturally on Earth. Only cosmic explosions could explain their presence in Earth's soil.

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On the same subject : The most important mass extinction took place ago 2.4 billion years

So, if traces of these isotopes are detected in the rock at the level of the Devonian-Carboniferous limit, the scenario proposed by Fields and his collaborators becomes very plausible. Will the stars be seen as a new threat ? The closest star to us likely to become a supernova is Betelgeuse, a red giant located more than 600 light years. Nothing to panic about then, we are well over the estimated limit !

The sixth mass extinction, the extinction of the Holocene, started 13,000 years ago. But the stars have absolutely nothing to be ashamed of : this time, only human activities are responsible. Deforestation, pollution, overexploitation of natural resources, gradually lead to the disappearance of species. Certainly, in numbers of species, we are far from the level of previous extinctions, but the extinction rate is increasing dramatically.

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World news – THAT – Fifth mass extinction could be due to the death of a star

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