Important new evidence confirms extinction of dinosaurs caused by meteorite impact

Core samples drilled in the Chicxulub meteorite impact crater have revealed an abnormal amount of iridium in a clay layer. The location of the iridium in the clay layer allows scientists to determine what happened in the days and years following the meteorite’s impact.

02/25/2021 | 12:32 PM

The discovery of abnormally large amounts of iridium is the last piece of evidence needed to prove that the dinosaurs became extinct as a result of this meteorite impact. The research was conducted by Jan Smit, Emeritus Professor of Event Stratigraphy at VU Amsterdam and palaeoclimatologist Niels de Winter of Utrecht University.

The dinosaurs and many other land and sea species became extinct about 66 million years ago. Scientists Walter Alvarez and Jan Smit first discovered the reason behind this mass extinction in the 1970s, when they found high concentrations of iridium in Italy and Spain. Iridium is a metal that occurs infrequently in the Earth’s crust but is often found in meteorites. The iridium was located in a thin layer of clay between the mineral layers characteristic of the Cretaceous and Paleogene eras.

The meteorite dust cloud settles
This iridium provided evidence that a meteorite with a diameter of about 12 kilometres had crashed somewhere on Earth, releasing a huge cloud of dust containing iridium that eventually settled on the earth’s surface. This was confirmed when scientists found a 180-kilometre-wide impact crater in Mexico, partly hidden under the Yucatan peninsula. The Chicxulub crater was discovered accidentally when geophysicists found circular anomalies in the gravity field.

More than 800-metres-deep drill core
The final link between this crater and the iridium dust has now been found, and according to the researchers it provides irrefutable evidence that it was this meteorite impact that caused the mass extinction at the end of the Cretaceous. In 2016, researchers drilled into the crater’s innermost ring of hills from the sea, bringing up 835 metres of core samples. The samples revealed what happened just before, during and just after the meteorite impact. It is quite extraordinary that the layer of iridium in the crater has remained intact, because in the days following the impact, the crater was flooded with water and battered by tsunamis, followed by a series of large earthquakes months later.

“The deposits of dust from the meteorite impact made it possible to estimate very precisely how long it took for calmer conditions to return to the crater,” says Niels de Winter. “This is a unique opportunity for us as geologists; usually such mineral layers only tell us about processes that take thousands to millions of years. Although the immediate effects of the meteorite impact were relatively short-lived, it probably took thousands of years for the ecosystem to recover from this heavy blow. But of the dinosaurs, it was only the birds that survived.”

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The drill core with the iridium layer. The brown layer contains the last remnants of the filling of the crater. Most of the meteorite dust is found in the gray-green layer between white and brown (around 33-35 cm on the ruler).

“This brings us full circle”
The layer of iridium allowed the researchers to calculate how long it took life on earth to recover from the mass extinction, and what happened in the days and years after the disaster. “The drill core revealed that some 130 metres of debris accumulated during the first day after the impact. The majority of this was swept away by the waves in the first few hours, as the crater was flooded with water and violent tsunamis raged. That is incredibly fast,” say Smit and De Winter.

The upper part of the core is a 60-centimetre-thick clayey layer formed by all the mud that was disturbed and then slowly deposited from the turbid water, while the very top contains relatively large amounts of iridium from the meteorite dust that settled there. The latter layer has also been found in core samples and outcrops of rock in other parts of the world, albeit much thinner. “This brings us full circle,” says Jan Smit. “Fifty years after cosmic iridium was found all over the world, it has now been found in exactly the right place: at the source!”

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he drilling platform used to drill the core. Photo: Timothy Bralower