Geologists believe they have closed the case of what killed non-avian dinosaurs at the end of the Cretaceous period.
This painting depicts an asteroid slamming into tropical, shallow seas of the sulfur-rich Yucatan Peninsula in what is today southeast Mexico. The aftermath of this immense asteroid collision, which occurred approximately 65 million years ago, is believed to have caused the extinction of the dinosaurs and many other species on Earth. Shown in this painting are pterodactyls, flying reptiles with wingspans of up to 50 feet, gliding above low tropical clouds. Image credit: Donald E. Davis / NASA.
About 66 million years ago, the end-Cretaceous mass extinction eradicated roughly 75% of the animal and plant species on Earth, including whole groups like non-avian dinosaurs and ammonites.
In the geologic record, this extinction event is marked by a thin layer of clay with elevated concentrations of iridium — a metal that occurs in higher concentrations in meteorites but only in low concentrations in Earth’s crust.
First detected in Gubbio, Italy, and Caravaca, Spain, in the 1980s, the iridium layer was used to propose a large-scale impact of an extraterrestrial body as the cause for the end-Cretaceous extinction.
In the 1990s, high-energy deposits were identified around the Gulf of Mexico and the Caribbean, culminating in the discovery of the 180- to 200-km-wide Chicxulub crater on the Yucatán Peninsula of Mexico.
In a new study, an international team of researchers found iridium in drill core recovered by IODP-ICDP Expedition 364 from the Chicxulub impact structure.
In the crater, the sediment layer deposited in the days to years after the strike is so thick that the scientists were able to precisely date the dust to a mere two decades after impact.
“The circle is now finally complete,” said Professor Steven Goderis, a researcher at the Vrije Universiteit Brussel.
“We are now at the level of coincidence that geologically doesn’t happen without causation,” said Professor Sean Gulick, a researcher at the University of Texas at Austin.
“It puts to bed any doubts that the iridium anomaly in the geologic layer is not related to the Chicxulub crater.”
The Chicxulub asteroid impact. Image credit: Victor Leshyk.
The highest concentrations of iridium were found within a 5-cm section of the rock core retrieved from the top of the Chicxulub crater’s peak ring.
In addition to iridium, the crater section showed elevated levels of other elements associated with asteroid material.
“This asteroid was vaporized and ejected from the impact site at high speed,” said Professor Joanna Morgan, a researcher at Imperial College London.
“Iridium, and other asteroidal material, then circled the Earth above the stratosphere within a fast-moving dust cloud, and may have taken up to two decades to settle through the atmosphere and ocean before being deposited at the impact site.”
“We combined the results from four independent laboratories around the world to make sure we got this right,” Professor Goderis said.
The findings appear in the journal Science Advances.
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Steven Goderis et al. 2021. Globally distributed iridium layer preserved within the Chicxulub impact structure. Science Advances 7 (9): eabe3647; doi: 10.1126/sciadv.abe3647
