How did the dinosaurs die out? New evidence from asteroid impact site confirms global climate change the cause
The finding supports the hypothesis that the impact started wildfires, triggered tsunamis and blasted a lot of sulfur into the atmosphere that blocked the sun, causing global cooling and leading to the extinction of the dinosaurs
Around 66 million years ago, an asteroid smashed into Earth, triggering a mass extinction that ended the reign of the dinosaurs and snuffed out 75% of all life on the planet. Evidence recovered from the site of the impact — which created a massive crater — contains information on what happened on the first day of the mass extinction event that led to the end of the dinosaur era.
"Hard evidence" found by a research team in hundreds of feet of rocks that filled the impact crater within the first 24 hours after the impact includes bits of charcoal, melted and broken rocks such as sandstone, limestone, and granite brought in by the tsunami's backflow.
The evidence contained no sulfur-bearing minerals though, despite the area's high concentration of sulfur-containing rocks.
The finding supports what has been proposed so far — the asteroid impact started wildfires, triggered tsunamis and blasted so much sulfur into the atmosphere that it blocked the Sun, which caused the global cooling that ultimately doomed the dinosaurs.
The findings are from an expedition conducted by the European Consortium for Ocean Research Drilling (ECORD) as part of the International Ocean Discovery Program (IODP) and was supported by the International Continental Scientific Drilling Program (ICDP).
According to Sean Gulick, a research professor at the University of Texas Institute for Geophysics (UTIG) at the Jackson School of Geosciences, what has been recovered are all part of a rock record that offers the most detailed look yet into the aftermath of the catastrophe that ended the Age of the Dinosaurs.
"It is an expanded record of events that we were able to recover from within ground zero. It tells us about impact processes from an eyewitness location," says Gulick, who led the study. He also co-led the 2016 International Ocean Discovery Program scientific drilling mission that retrieved the rocks from the impact site offshore of the Yucatan Peninsula.
The study, published in the Proceedings of the National Academy of Sciences (PNAS) reconstructs the events of the first day of the Cenozoic Era. The asteroid impact at the Chicxulub crater, approximately 66 million years ago, marked the boundary between the Mesozoic and Cenozoic Eras and most probably caused the associated mass extinction event.
Researchers have estimated that the asteroid hit with the equivalent power of 10 billion atomic bombs of the size used in World War II. The blast ignited trees and plants thousands of miles away and triggered a massive tsunami that reached as far inland as Illinois.
To better understand the immediate aftermath of the impact, researchers analyzed a core from the peak ring of the Chicxulub impact crater in the Gulf of Mexico, which is a circle of elevated topography surrounding the center of the crater, containing approximately 130 meters of impact melt rock and melt-bearing breccia deposited on the first day following the impact.
The peak ring – the mountain or hills around the impact – consists of rock that comes from deep within the crust. It is now covered by water and limestone from the modern gulf floor.
"The Expedition 364 core of the Chicxulub peak ring contains the most complete and expanded record of the immediate aftermath of the mass extinction to date. Based on the analysis, we infer that the bottom 40-50 meters impact melt and breccia were rapidly deposited within minutes of the impact. Another 90 meters of the material was deposited by ocean water flowing back into the crater and the subsequent settling of debris over a period of hours," the findings state.
Inside the crater, researchers found charcoal and a chemical biomarker associated with soil fungi, within or just above layers of sand, which shows signs of being deposited by resurging waters. This suggests that the charred landscape was pulled into the crater with the receding waters of the tsunami, according to the findings.
"Most of the material that filled the crater within hours of impact was produced at the impact site or was swept in by seawater pouring back into the crater from the surrounding Gulf of Mexico," the study states.
It further says, "Just one day deposited about 425 feet of material, a rate that is among the highest ever encountered in the geologic record. This breakneck rate of accumulation means that the rocks record what was happening in the environment within and around the crater in the minutes and hours after impact and give clues about the longer-lasting effects of the impact that wiped out 75% of life on the planet."
Gulick describes it as a short-lived inferno at the regional level, followed by a long period of global cooling. "We fried them, and then we froze them. Not all the dinosaurs died that day, but many dinosaurs did," he reveals.
According to researchers, an important finding of the study is what was missing from the core samples. While the area surrounding the impact crater is full of sulfur-rich rocks, there was no sulfur in the core.
This finding, according to the team, supports the theory that the asteroid impact vaporized the sulfur-bearing minerals present at the impact site and released it into the atmosphere, where it wreaked havoc on the Earth's climate, reflecting sunlight away from the planet and causing global cooling.
"The presence of charcoal in the uppermost layers suggests the existence of impact-induced wildfires, whereas the absence of sulfur-rich evaporites from the deposit suggests that the impact released a large amount of sulfate aerosols that could have caused global cooling and darkening," say researchers.
The researchers estimate that the impact would have released at least 325 billion metric tons. "To put that in perspective, that's about four orders of magnitude greater than the sulfur that was spewed during the 1883 eruption of Krakatoa, which cooled the Earth's climate by an average of 2.2 degrees Fahrenheit for five years," reveals the study.
The research team explains that while massive destruction resulted from the asteroid impact, it was the resulting climate change that led to global mass extinction, killing dinosaurs and most other life on the planet during that period.
Jay Melosh, a Purdue University professor and expert on impact cratering, said that finding proof of wildfire helps scientists know that their understanding of the asteroid impact is on the right track. "It was a momentous day in the history of life, and this is a very clear documentation of what happened at ground zero," says Melosh.