When a giant asteroid or comet crashed into the ocean floor off the coast of Mexico's Yucatan Peninsula 66 million years ago, the world ended. The event quickly killed off 75 of all life on Earth. For the first time, scientists have found a graveyard that offers evidence of how quickly it all happened. In North Dakota, is filled with a wide variety of plant and animal species—fish, burned tree trunks, conifer branches, dead mammals, mosasaur bones, insects, the partial carcass of a Triceratops, marine microorganisms called dinoflagellates and snail-like marine cephalopods called ammonite—that all died at once.
"This is the first mass death assemblage of large organisms anyone has found associated with the ," says Robert DePalma, curator of paleontology at the Palm Beach Museum of Natural History in Florida and a doctoral student at the University of Kansas, in the press statement. "At no other K-T boundary section on Earth can you find such a collection consisting of a large number of species representing different ages of organisms and different stages of life, all of which died at the same time, on the same day."
The site, which DePalma and others have deemed Tanis, has been studied for six years now. Writing in the academic journal Proceedings of the National Academy of Sciences, DePalma and an international team including two geologists present evidence directly tying the graveyard in North Dakota to the crater caused by the catastrophic space rock 66 million years ago, known as Chicxulub.
"It's like a museum of the end of the Cretaceous in a layer a meter-and-a-half thick," said Mark Richards, a UC Berkeley professor emeritus of earth and planetary science who is now provost and professor of earth and space sciences at the University of Washington.
Richards, along with scientist Walter Alvarez, have authority on the subject: Some 40 years ago, they were among the very first to declare that a rock from space killed the dinosaurs. In 1980, Alvarez, along with his father, found of such a global attack in Italy, when they discovered that 66 million-year old rocks had unusually high traces of the rare element iridium. The pair was called in by DePalma and Dutch scientist Jan Smit to consult on the North Dakota project. What they found was more evidence.
In North Dakota, Richards and Alvarez focused on the preservation of the fish in the graveyard. They're not just buried, but buried with glass beads known as . These tektites formed in the Earth atmosphere in the aftermath of the asteroid or comet smashing into the Earth. Dispersed into the air, fragments of shale, quartz sandstone and dirt flew into the atmosphere where they quickly heated and cooled upon landing. The result was a rain of glass beads onto the Earth.
There were multiple phenomena occurring during the period after the asteroid or comet hit. Richards and Alvarez also pointed to the seismic waves that likely arrived within 10 minutes of rock's impact, which would have been devastating by today's standards. They estimate it would have reached a 10 or 11 on the Richter scale. To put that in perspective: the earthquake that in 2015, killing almost 9,000 people, was a 7.8.
A seismic event of that scale would have created what's known as a seiche, or standing wave. Seiches don't need strength to form, tiny version are created every time water is splashed against the back of a tub. They're common after earthquakes, although do not usually get much attention. The 2011 Tohoku quake in Japan for example, which was a magnitude 9.0, created six-foot-high seiches 30 minutes later in a faraway Norwegian fjord.
"The seismic waves start arising within nine to 10 minutes of the impact, so they had a chance to get the water sloshing before all the spherules (small spheres) had fallen out of the sky," Richards says, speaking about the K-T boundary event. "These spherules coming in cratered the surface, making funnels -- you can see the deformed layers in what used to be soft mud—and then rubble covered the spherules. No one has seen these funnels before."
The tektites would have been nearly falling from space, falling down to Earth at terminal velocities between 100 and 200 miles per hour, the scientists estimate.
"You can imagine standing there being pelted by these glass spherules. They could have killed you," Richards says. "Many believe that the rain of debris was so intense that the energy ignited wildfires over the entire American continent, if not around the world."
"Tsunamis from the Chicxulub impact are certainly well-documented, but no one knew how far something like that would go into an inland sea," DePalma says. "When Mark came aboard, he discovered a remarkable artifact—that the incoming seismic waves from the impact site would have arrived at just about the same time as the atmospheric travel time of the ejecta. That was our big breakthrough."
The scientists estimate that there were two seiches, around twenty minutes apart. Overlaying them is a layer of iridium, the same radioactive mineral that Alvarez and his father found in ancient rocks in 1980.
"When we proposed the impact hypothesis to explain the great extinction, it was based just on finding an anomalous concentration of iridium—the fingerprint of an asteroid or comet," says Alvarez. "Since then, the evidence has gradually built up. But it never crossed my mind that we would find a deathbed like this."
"So far, we have gone 40 years before something like this turned up that may very well be unique," Smit says. "So, we have to be very careful with that place, how we dig it up and learn from it. This is a great gift at the end of my career. Walter sees it as the same."