Quirks & Quarks

Rocks recovered from ground zero reveal how the dinosaurs died

Researchers have drilled deep into the crater in the Gulf of Mexico where the killer asteroid hit

Researchers have drilled deep into the crater in the Gulf of Mexico where the killer asteroid hit

Sean Gulick, a research professor at The University of Texas at Austin Jackson School of Geosciences (right) and lead author of the study, with co-author Joanna Morgan, a professor at Imperial College London, on the International Ocean Discovery Program research expedition that retrieved cores from the submerged and buried impact crater. (The University of Texas at Austin Jackson School of Geosciences)

Researchers have used rocks recovered from the centre of the Chicxulub crater in the Gulf of Mexico to reconstruct the events that occurred immediately after a large asteroid hit the Earth 66 million years ago. 

Their new work lends powerful new evidence to the theory that this asteroid impact caused the mass extinction that wiped out 75 per cent of life on our planet  — including the dinosaurs that had ruled for about 130 million years.

"It's an expanded record of events that we were able to recover from within ground zero," research team leader and geoscientist Sean Gulick said in a release from the University of Texas at Austin. 

"It tells us about impact processes from an eyewitness location."

Rocks recovered from ground zero 

The samples were recovered in 2016, when an international team of researchers came together for an offshore drilling expedition into the centre of the crater. 

The drill site was 50 kilometres off the northwest coast of the Yucatán Peninsula in Mexico. Over a six week period, the expedition used an oil-drilling rig to retrieve samples of rock and sediment deposited in the crater immediately after the impact. The impact debris layer was more than 130 metres thick, but it in turn was covered by hundreds of metres of sediment that had accumulated in the 66 million years since the impact. 

Drilling rig from the 2016 expedition. The drill pipe is left in the photograph; the living quarters are in the center. (Timothy Bralower)

The area of the crater that they focused on is called the peak ring, which is a circular region in the middle of the 180 kilometre wide crater. The peak ring is where most of the deformation took place immediately after impact, and it formed a mountain chain that the researchers were able to drill into to help them understand what happened. 

"Rocks that were 20 kilometres in depth rose up within a matter of 90 seconds to the surface, and formed this submarine mountain chain which defines the peak ring," said team member Timothy Bralower, a professor in the department of geosciences at Penn State University, in an interview with Bob McDonald of Quirks & Quarks. 

"It's the most dramatic event in Earth's history in terms of how mountains are made." 

Core that arrived on deck from the 2016 expedition was split into 1.5 meter sections. (Timothy Bralower)

The team recovered core samples from the peak ring that revealed layers of different types of rock. Broken pieces of soft granite made up the bottom layer, and overlaying the granite was hundreds of metres of breccia — a mix of different types of rocks shattered by the impact that had become cemented together. Finally, on top of the breccia, there was the most exciting material — a thin layer of debris that had been deposited by a massive post-impact tsunami.

A mega-tsunami 100 metres tall 

Bralower said the tsunami record was key to figuring out the chain of events that led to the demise of the dinosaurs. The core sample  indicates that the impact  had generated a mega-tsunami that reached 100 metres high, and the waves reached as far as Florida and Texas. 

The story that the researchers were able to reconstruct was that the crater was emptied by the impact. However the massive hole was soon filled by the enormous tsunami and the debris it carried. A key component of that debris was charcoal.  

An artist's interpretation of the asteroid that struck Earth 66 million years ago. (NASA/Don Davis)

The charcoal, the researchers think, is from trees and other plant material burned by the enormous amount of heat the impact generated.  

The asteroid was travelling at hundreds of kilometres per second when it collided with earth, and generated a huge pulse of thermal radiation. They estimate the energy generated by the impact was equivalent to 10 billion Second World War atomic bombs.

A rock type known as a breccia was recovered from the drilling expedition. It fell into the open crater right after impact. Centimeter scale at top. (Sean Gulick)

"It would've been an intense fireball coming your way," said Bralower. "Dinosaurs were burned alive and anything on the coast would've been torched. The charcoal evidence showed that it happened very very quickly." 

A dark and cold new world

The impact according to Bralower, would've also created global wildfires.

The blast would have melted rock and ejected hot melt droplets into the atmosphere. This airborne lava would spread around the world, igniting fires that put vast amounts of soot into the air - blocking out the Sun. 

"It's highly likely that we had a period of darkness," said Bralower. 

But the longer-term and most catastrophic outcome of the impact would have been a long period of global cooling caused by massive amounts of sulfur being injected into the atmosphere.

The rock recovered from the crater floor is of a type that normally contains a large amount of sulfur, and rock from outside the crater does contain sulfur — but not rock within the crater.

This suggests to the researchers that the sulfur was vaporized by the heat of impact. Sulfur particles in the atmosphere can reflect solar energy into space, cooling the atmosphere. The massive amount of sulfur put into the atmosphere by the impact would, according to Bralower, have caused a drop in Earth's temperature of about 20 degrees. This in turn would have created an ecological collapse that ultimately led to the mass extinction that saw the dinosaurs, and many other species, go extinct. 


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