After dominating the planet’s surface for hundreds of millions of years, dinosaur diversity came to a dramatic end some 66 million years ago at the hot end of an asteroid impact on Mexico’s present-day Yucatán Peninsula.
It is a theory so packed with data that it is hard to imagine that there is any doubt that it actually happened. If it was a cold case, it would now have been rubber-stamped and filed under ‘Resolved’.
But scientists are a nitpick, and a tiny hole in the chain of evidence linking signs of a global apocalypse to the scene of the crime is begging to be plugged.
An international team of researchers collaborating on a study of material from the famous Chicxulub impact crater on the Yucatán Peninsula has finally compared the chemical signature of meteorite dust in its rock to that of the geological frontier that represents dinosaur extinction.
It seems to be a clear sign that the thin layer of dust deposited on Earth’s crust 66 million years ago was the result of an impact on this site.
“We are now at the level of coincidence that geologically cannot happen without a cause,” said geoscientist Sean Gulick of the University of Texas in the US.
Together with fellow geoscientist Joanna Morgan of Imperial College London, Gulick led an expedition in 2016 to retrieve a sample of shattered rock more than half a kilometer from the crater’s top ring.
Four different laboratories carried out measurements on the sample. Not only do the results help unify a major transition in the fossil record with the site, they also point to a timeline that supports a rapid decline in dinosaur populations in just a decade or two.
“If you actually made a clock go extinct 66 million years ago, you could easily argue that it all happened within a few decades, and that’s basically how long it takes for everything to starve to death,” says Gulick.
Half a century ago, the question of why the diversity of fossils representing the Mesozoic Era came to an abrupt end in the geologic record was an open question. Whatever was responsible for the sudden loss of 75 percent of life on Earth, it had to be relatively rapid and global.
Hypotheses of such cataclysmic violence tended to focus on two possibilities: one emerged as a wave of volcanic activity, the other from above in the form of a comet or asteroid strike that radically disrupts the global climate.
In 1980, American physicist Luis Alvarez and his son, a geologist named Walter, published a study of a thin layer of sediment that separates the dinosaur-populated Cretaceous from the post-dinosaur world of the Paleogene.
A characteristic feature of this millimeter to centimeter thick strip of sedimentary rock was an unusually high amount of the element iridium, a metal not found in abundance in the Earth’s crust.
One place where you will find a lot of iridium is in meteorites. So the discovery of Alvarez and Son was the first solid evidence that something from space splashed his remains all over the planet the moment dinosaur biodiversity took a plunge.
Coincidentally, around the same time, the site of that colossal collision was the focus of ongoing research, although a clear link was made between the 180 to 200 kilometers (112 to 125 miles) long scar at the southern edge of the Gulf. of Mexico with the deadly asteroid wouldn’t happen until the 1990s.
Since then, the evidence to support an asteroid impact has only grown stronger, with models going so far as to suggest the angle, as well as the location of the Chicxulub impact, playing a critical role in the extent of the extinction.
Signs that a zone of intense geologic activity in western India called the Deccan Traps was contributing massive amounts of greenhouse gases at the time meant that the volcano hypothesis has never been completely ruled out, at least as a possible contributing factor.
Whether this tectonic hotspot played a role in the famous extinction, or even helped restore biodiversity is still up for debate.
What is no longer a matter of serious debate is whether the 12-kilometer-wide stretch of rock that struck off the coast of present-day Mexico about 66 million years ago is the same that dusted off the remains of countless dinosaurs.
“The circle is now finally complete”, says study leader Steven Goderis, a geochemist at the Vrije Universiteit Brussel in Belgium.
Case closed.
This research is published in Science Advances