Season of the asteroid impact determined

Season of the asteroid impact determined

Artist’s depiction of the catastrophe that befell life on a river in present-day North Dakota one spring around 66 million years ago. © Joshua Knüppe, based on research and commissioned by Melanie During

Fate struck the Cretaceous world about 66 million years ago – but at what time of the year? Apparently, the famous dinosaur-killing asteroid struck when it was spring in the northern hemisphere—which means it was autumn in the south. This emerges from an examination of the fossils of fish that perished in the inferno. The result of the study thus provides indications of the development phases in which the living beings in the different areas of the earth were currently. This, in turn, may shed light on specific patterns of extinction, the scientists say.

The earth bears a huge scar: in the area of ​​the Mexican peninsula of Yucatan, the remains of a crater about 150 kilometers wide bear witness to a massive asteroid impact. According to dating, this cosmic bolide hit the Cretaceous world about 66 million years ago. The consequences were catastrophic: the so-called Chicxulub asteroid caused firestorms, tsunamis raged around the earth and the atmosphere was heavily polluted. Living beings that had been spared the direct consequences then had to survive the drastic climatic effects. Because it is assumed that a cold phase began due to the darkening of the atmosphere. The result was one of the worst extinction waves in the history of the earth: About 76 percent of the Cretaceous animal species were swept off the stage of evolution – the most famous victims being the dinosaurs.

Other lines of development also suffered great losses – but there were survivors: apparently some species of mammals, birds and reptiles survived the catastrophe somewhere on earth and subsequently formed the basis for the development of today’s biodiversity. It is still unclear what factors made this possible for them. It could also have played a role in which seasonal development phase they have just found themselves. So far, however, there have only been rough chronological classifications as to when the Chicxulub asteroid struck. The study by scientists led by Melanie During from the Free University of Amsterdam has now changed this.

Seasons reflected in fish bones

Their findings are based on examining fish remains discovered at a site in North Dakota. The representatives of the sturgeon family lived there at the end of the Cretaceous period in a river ecosystem of the northern hemisphere. As the researchers explain, findings showed that the fossils were victims of the immediate aftermath of the asteroid impact: they found small glass globules in the sediment and even in the gill areas of the fish. These are structures formed from the molten material that the impact carried high into the atmosphere. They then rained from the sky over a wide area – including the river in what is now North Dakota.

At about the same time, powerful waves caused by the shaking of the earth’s crust devoured the home of the fish and buried them under sediment. The researchers assume that they died only about an hour after the impact. “This showed that their bones could contain valuable information about the time of the impact,” says During. In order to examine their fine structures, the scientists prepared thin sections, which were then examined using high-resolution synchrotron X-ray tomography.

inferno in spring

“Through our analyzes we were able to show that the bones show signs of annual growth – similar to trees: a new layer has formed on the outside of the bone every year,” explains co-author Sophie Sanchez from Uppsala University. The detailed images also showed traces of the bone cells, whose characteristics are known to vary with the seasons. “In all the fish examined, changes in bone cell density and volume can be tracked over the course of several years. They use it to indicate whether it was spring, summer, autumn or winter,” says co-author Dennis Voeten from Uppsala University. A look at the last stage of development when the fish died revealed: “There was an increase in cell density and volume, but this had not yet reached its peak. This means that growth stopped abruptly in the spring,” says Voeten.

A second detection method confirmed this result: The team carried out a carbon isotope analysis on the bone layers in order to trace the seasonal feeding pattern of the fish. As they explain, the amount of food usually peaked in the summer. This temporary increase in food supply enriched the fish’s bones with more of the 13C-carbon isotope compared to the lighter 12C-carbon isotope. “The carbon isotope signal in the course of growth of the examined fish confirmed that the amount of food had not yet reached its peak. So death came in the spring,” summarizes During.

As she and her colleagues explain, this information can now provide important clues for paleontologists studying the mass extinction. Because the result shows that the reproductive cycles of animals and plants in the northern hemisphere were currently in the relatively sensitive development phase of spring. In the southern hemisphere, on the other hand, it was autumn and many organisms were probably preparing for winter. Thus, they could have been better prepared: where the living beings adapted to harsher conditions anyway or even crept into burrows, there were better chances of survival in the time of the catastrophe. “Our results could now help to find out why the dinosaurs became extinct while birds and early mammals escaped extinction,” concludes During.

Source: Uppsala University, professional article: Nature, doi: 10.1038/s41586-022-04446-1

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