It is now 66 million years before the era of the dinosaurs on Earth ended in one fell swoop. However, scientists can determine exactly when of the year this occurs.
It seemed like a spring day like any other, in the place where after 66 million years a recently evolved mammal called “Tanis”. a river meanders through the lowlands; Dinosaurs, amphibians, and fish worked to make up for the lack of food in the winter and to reproduce.
Then suddenly it was over.
A distant continent, near a place now called Chicxulub, in Mexico, an asteroid struck. The ten-kilometre object dealt the Earth one of the greatest blows in its history, with the force of about a million atomic bombs. The heat released caused wildfires all over the world, after which dust particles in the atmosphere caused a “nuclear winter” as also feared after a nuclear war: years of darkness and cold.
And so life after the collision took a turn: the dinosaurs disappeared, as did three-quarters of all kinds of animals and plants. Species lucky enough to survive suddenly got a chance to become dominant, in an era that paleontologists renamed it: the Cretaceous period ended, and the Paleogene began.
But the Tanis ecosystem received none of this. That was already buried under meters of mud in the first minutes after the impact. In recent years, that spot has become famous among paleontologists as “the place where chalk died.”
And as of this week, we know that it happened in the spring.
Searching in the mud layer
Dutch researcher Melanie Dhan came to this conclusion in an article published in the professional journal nature† During, now a doctoral student at Uppsala University in Sweden, she did research in Tanis for her master’s thesis. Using various techniques, she examined sturgeon and spoon sturgeon bones that had been preserved in the clay layer. In it you can see how the fish grew larger from year to year and their bones thicker: a halt in the winter and a spurt of growth in the summer. In their last days of their lives, she saw that that spurt of growth had just begun: it must have been spring.
Obviously when you walk around the sturgeon died the day the collision occurred, during a newspaper interview. She excavated in North Dakota in 2017. “You see the riverbank sand, and over those wild structures. You’ll see all the fish going left, and a higher layer all facing right. There are tree branches around which fish are being cut or opened. Sounds like the worst car accident I’ve ever seen. Not at all, but he froze, like a block of ice had suddenly formed around him.”
What happened on that unusual day 66 million years ago is that dozens of miles away, beyond the mouth of a river, the now-vanishing Inland Sea of the Americas felt the shock of the impact. It caused a Brought, Roll that the water was rushing high on the hoof. As a result, a wave no less than ten meters high swept upstream towards Tanis. He took the material from the bottom and everything that lived on and above that bottom. Part of it ended up on the sandy inner bend of the zigzag.
Secondary tick bombing
Further evidence that the fish perished in the mud layer while the world was ablaze is that tiny glass beads, called tektites, were found in their gills. They are created when molten rock is thrown into space by a meteorite impact into droplets, where they solidify and eventually recede. After the Chicxulub collision, that secondary bombardment from the tektite was massive. It is clear that the sturgeon, feeding on the bottom, was drawn into the water containing the tektite. But they did not have the pellets in their stomachs, a sign that there was almost no time between the arrival of the pellets and their death.
This aspect of the Tanis site has already been known since it was reported in 2019 by researcher Robert DePalma of the University of Manchester in England, and he has been excavating there since 2012. At the end of last year, de Palma also came up with an estimate of the season in which the asteroid struck: spring or Northern summer. While estimating is more accurate: it must have been spring. If she had to guess a month: April.
With this knowledge, paleontologists can start to work wondering how exactly such a global catastrophe led to the extinction of so many species. Because the season makes a huge difference, says During. “To get past that nuclear winter, you first had to survive the impact itself. If you were on the surface when the impact happened, you would likely die. If you were hibernating, or staying in a den, that increases your chances. In the spring, this is less likely. It was autumn in the southern hemisphere, and perhaps more animals sought shelter. There the chances of survival were greater. ”
This is an obvious theory, but it is very difficult to confirm or refute. Because there are much fewer fossils than in the southern hemisphere. But there is evidence that at least a significant part of the current diversity of animal species in the world is due to the survivors in the south, whose descendants later repopulated the north. This can be especially true of the branch of small dinosaurs that learned to use their feathers to fly: birds.
Clearing the sky didn’t help
Could a dinosaur that happened to search for the horizon 66 million years ago see that asteroid coming?
Planetary scientist Christian Koberl of the University of Vienna calculated it, looking for a way to show how long a catastrophe from space could remain hidden. The Center for Near-Earth Object Studies (CNEOS) monitors asteroids and comets whose orbits could bring them close to Earth. The United Nations space agency regularly invites experts on such objects to the Planetary Defense Conference. For this conference, Cooperl searched for the answer to his question.
“Science is not always taken seriously,” he explained his small project in October last year at the Geological Society of America conference in Portland, Oregon. “The big question is, how do we explain the danger to the public when we detect an object on a collision course with Earth.”
How bright Chicxulub asteroid in the sky depends on its size, material, and distance. “We have a pretty good idea of the size,” Cooperl said. “And it was a carbonaceous chondrite – an object made of that material that only reflects a small portion of sunlight.”
As it hurtled toward Earth at a speed of about 15 kilometers per second, the asteroid became brighter. When can a man with good eyes see him on earth?
“It’s surprisingly late,” Coberl concludes. On a good dark night just when he was a little farther from the moon. Only during the day they were already as close as the geostationary satellites, at an altitude of 36,000 km. “If you want to rely on your eyes to meet this danger, you have about ten hours of warning at night and half an hour during the day.”
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