Scientists are assembling the most detailed timeline yet of the dino apocalypse. They are giving fresh scrutiny to telltale fingerprints left by the fateful event so long ago. At the impact site, an asteroid (or maybe a comet) crashed onto Earth’s surface. Mountains formed in mere minutes. In North America, a towering tsunami buried plants and animals alike under thick piles of rubble. Lofted debris darkened skies around the world. The planet chilled — and stayed that way for years.
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But the asteroid may not have acted alone.
Life may already have been in trouble. Growing evidence points to a supervolcanic accomplice. Eruptions in what is now India spewed out molten rock and caustic gases. These may have acidified the oceans. All of this could have destabilized ecosystems long before and after the asteroid hit. The jolt of that impact may even have boosted the eruptions, some researchers now argue.
As more clues have emerged, some seem to conflict. That has made the identity of the dinosaurs’ true killer — an impact, volcanism or both — less clear, says Paul Renne. He is a geoscientist at the Berkeley Geochronology Center in California.
“As we’ve improved our understanding of the timing, we haven’t resolved the details,” he says. “The last decade of work has only made it harder to distinguish between the two potential causes.”
The smoking gun
What is clear is that a massive die-off took place around 66 million years ago. It is visible in the layers of rock that mark the boundary between the Cretaceous and Paleogene periods. Fossils that were once abundant no longer appear in rocks after that time. Studies of fossils found (or not found) across the boundary between these two periods — abbreviated the K-Pg boundary — show that some three out of every four plant and animal species went extinct at about the same time. This included everything from the ferocious Tyrannosaurus rex to microscopic plankton.
Everything living on Earth today traces its ancestry to the few lucky survivors.
Over the years, scientists have blamed many suspects for this catastrophic die-out. Some have suggested global plagues struck. Or maybe a supernova fried the planet. In 1980, a team of researchers including father-son duo Luis and Walter Alvarez reported discovering lots of iridium in places worldwide. That element appeared along the K-Pg boundary.
Iridium is rare in Earth’s crust, but abundant in asteroids and other space rocks. The finding marked the first hard evidence for a killer-asteroid impact. But without a crater, the hypothesis couldn’t be confirmed.
Piles of impact debris led crater hunters to the Caribbean. Eleven years after the Alvarez paper, scientists at last identified the smoking gun — the hidden crater.
It circled the coastal Mexican town of Chicxulub Puerto. (The crater actually had been discovered in the late 1970s by oil company scientists. They had used variations in Earth’s gravity to visualize the crater’s 180-kilometer- [110-mile-] wide outline. Word of that find, however, did not reach crater hunters for years.) Based in part on the gaping size of the depression, scientists estimated the size of the impact. They figured it must have released 10 billion times as much energy as the nuclear bomb dropped on Hiroshima, Japan, in 1945.
That’s big.
Questions have remained, though, about how the impact might have caused so much death and destruction worldwide.
It now appears that the blast itself wasn’t the big killer in the impact scenario. It was the darkness that followed.
Inescapable night
The ground shook. Powerful gusts roiled the atmosphere. Debris rained from the sky. Soot and dust, spewed by the impact and resulting wildfires, filled the sky. That soot and dust then began to spread like a giant sunlight-blocking shade over the entire planet.
How long did the darkness last? Some scientists had estimated that it was anywhere from a few months to years. But a new computer model is giving researchers a better sense of what happened.
It simulated the length and severity of the global cooldown. And it must have been truly dramatic, reports Clay Tabor. He works at the National Center for Atmospheric Research in Boulder, Colo. As a paleoclimatologist, he studies ancient climates. And he and his colleagues have reconstructed a sort of digital crime scene. It was one of the most detailed computer simulations ever made of the impact’s effect on climate.
The simulation begins by estimating the climate before the smash-up. The researchers determined what that climate might be from geologic evidence of ancient plants and levels of atmospheric carbon dioxide. Then comes the soot. A high-end estimate of soot totals some 70 billion metric tons (about 77 billion U.S. short tons). That number is based on the size and global fallout of the impact. And it’s huge. It’s the equivalent weight of about 211,000 Empire State Buildings!
The ground shook. Powerful gusts roiled the atmosphere. Debris rained from the sky. Soot and dust, spewed by the impact and resulting wildfires, filled the sky. That soot and dust then began to spread like a giant sunlight-blocking shade over the entire planet.
How long did the darkness last? Some scientists had estimated that it was anywhere from a few months to years. But a new computer model is giving researchers a better sense of what happened.
It simulated the length and severity of the global cooldown. And it must have been truly dramatic, reports Clay Tabor. He works at the National Center for Atmospheric Research in Boulder, Colo. As a paleoclimatologist, he studies ancient climates. And he and his colleagues have reconstructed a sort of digital crime scene. It was one of the most detailed computer simulations ever made of the impact’s effect on climate.
The simulation begins by estimating the climate before the smash-up. The researchers determined what that climate might be from geologic evidence of ancient plants and levels of atmospheric carbon dioxide. Then comes the soot. A high-end estimate of soot totals some 70 billion metric tons (about 77 billion U.S. short tons). That number is based on the size and global fallout of the impact. And it’s huge. It’s the equivalent weight of about 211,000 Empire State Buildings!
For two years, no light reached Earth’s surface, the simulation shows. Not any part of Earth’s surface! Global temperatures plummeted 16 degrees Celsius (30 degrees Fahrenheit). Arctic ice spread southward. Tabor shared this dramatic scenario in September 2016 in Denver, Colo. at the annual meeting of the Geological Society of America.
Some areas would have been hit particularly hard, Tabor’s work suggests. The temperature nosedived in the Pacific Ocean, around the equator. Meanwhile, coastal Antarctica barely cooled. Inland areas generally fared worse than coastal ones. Those divides could help explain why some species and ecosystems weathered the impact while others died off, Tabor says.
Six years after the impact, sunshine returned to levels typical of conditions before the impact. Two years after that, land temperatures warmed to levels higher than had been typical before the impact. Then, all of the carbon flung into the air by the impact took effect. It acted like an insulating blanket over the planet. And the globe ultimately warmed several degrees more.
Evidence of the chilling darkness is in the rock record. Local sea surface temperatures modified lipid (fat) molecules in the membranes of ancient microbes. The fossilized remains of those lipids provide a temperature record, reports Johan Vellekoop. He is a geologist at the University of Leuven in Belgium. Fossilized lipids in what is now New Jersey suggest that temperatures there plummeted 3 degrees C (about 5 degrees F) following the impact. Vellekoop and colleagues shared their estimates in the June 2016 Geology.
Similar abrupt temperature drops plus darkened skies killed plants and other species that nourish the rest of the food web, Vellekoop says. “Dim the lights and the entire ecosystem collapses.”
The cold darkness was the impact’s deadliest weapon. Some unfortunate critters, though, died too soon to witness it.
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