“Driving through the lonely, windswept ranchlands of southwestern North Dakota, we got out of our field vehicle, passed through through a rickety gate in a flimsy barbed-wire fence, and entered a postage-stamp-sized plot of eroded landscape. Soon we were digging out fossil fish that died 66 million years ago choking on melt-glass spherules thrown into the atmosphere by the meteor impact 1800 miles to the south in Yucatan, Mexico, that had killed the dinosaurs, opening the way for mammals, and eventually humans, to take over the terrestrial world,” planetary scientist Mark Richards wrote to The Daily Galaxy in an email about what was one of the defining events in the history of planet Earth.
“In what to non-geologists might appear to be a drab 4-foot thick layer of dirt, we would soon come to understand this to be nothing less than a stroboscopic, time-lapse image of the last fatal hour of the Cretaceous – an intact account of the birth of the modern geological era,” summarized 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 is currently leading an international team exploring the enormous Deccan Traps volcanic eruptions in India, which coincided with the great Chicxulub meteor impact.
This begins the story that started with violent shaking that raised giant waves in the waters of an inland sea in what is now North Dakota. Then, tiny glass beads began to fall like rain from the heavens. The rain of glass was so heavy it may have set fire to much of the vegetation on land. In the water, fish struggled to breathe as the beads clogged their gills, says paleontologist Robert DePalma about the killing field laid down soon after the asteroid impact that eventually led to the extinction of all ground-dwelling dinosaurs at the end of the Cretaceous Period, the so-called K-T boundary, that exterminated 75 percent of life on Earth.
It appeared as a star –paving the way for Homo sapiens
“If, on that certain evening about sixty-six million years ago, you had stood somewhere in North America and looked up at the sky, you would have soon made out what appeared to be a star,” wrote Douglas Preston in The New Yorker. “If you watched for an hour or two, the star would have seemed to grow in brightness, although it barely moved. That’s because it was not a star but an asteroid, and it was headed directly for Earth at about forty-five thousand miles an hour.”
The asteroid was so large that, even at the moment of impact, the top of it might have still towered more than a mile above the cruising altitude of a 747. “In its nearly instantaneous descent, it compressed the air below it so violently that it briefly became several times hotter than the surface of the sun,” stated mass-extinction authority, Peter Brannen. The Chicxulub asteroid impact happened 66 million years ago off the coast of Mexico at the end of the Cretaceous, creating an epic impact winter and a momentous, transitional day for the history of life on Earth.
“So Massive the Atmosphere Couldn’t Make a Scratch on It”
“The pressure of the atmosphere in front of the asteroid started excavating the crater before it even got there,” geophysicist Mario Rebolledo at the Centro de Investigación Científica de Yucatán, told Brannen. “Then, when the meteorite touched ground zero, it was totally intact. It was so massive that the atmosphere didn’t even make a scratch on it. It would have been a pleasant day one second and the world was already over by the next. As the asteroid collided with the earth, in the sky above it where there should have been air, the rock had punched a hole of outer space vacuum in the atmosphere. As the heavens rushed in to close this hole, enormous volumes of earth were expelled into orbit and beyond—all within a second or two of impact.”
K-T Boundary & the “Three-Meter Problem”
“This is the first mass death assemblage of large organisms anyone has found associated with the K-T boundary,” said DePalma, curator of paleontology at the Palm Beach Museum of Natural History, about the boundary that held the answers to our questions about one of the most significant events in the history of life on the planet.
One of the central mysteries of paleontology is the so-called “three-meter problem.” In a century and a half of assiduous searching, almost no dinosaur remains have been found in the layers three meters, or about nine feet, below the KT boundary, a depth representing many thousands of years .writes Preston for The New Yorker. “Consequently, numerous paleontologists have argued that the dinosaurs were on the way to extinction long before the asteroid struck, owing perhaps to volcanic eruptions and climate change. Other scientists have countered that the three-meter problem merely reflects how hard it is to find fossils. Sooner or later, they’ve contended, a scientist will discover dinosaurs much closer to the moment of destruction.
“If one looks at the Earth as a kind of living organism, as many biologists do, you could say that it was shot by a bullet and almost died. Deciphering what happened on the day of destruction is crucial not only to solving the three-meter problem but also to explaining our own genesis as a species.
“At no other K-T boundary section on Earth”, observes Preston, “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.”
Graveyard of the Cretaceous
The turned into a 30-foot wall of water when it reached the mouth of a river, tossing hundreds, if not thousands, of fresh-water fish—sturgeon and paddlefish—onto a sand bar and temporarily reversing the flow of the river, Stranded by the receding water, the fish were pelted by glass beads up to 5 millimeters in diameter, some burying themselves inches deep in the mud. The torrent of rocks, like fine sand, and small glass beads continued for 10 to 20 minutes before a second large wave inundated the shore and covered the fish with gravel, sand and fine sediment, sealing them from the world for 66 million years.
This unique, fossilized graveyard—fish stacked one atop another and mixed in with 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 ammonites—was unearthed by DePalma over the past six years in the Hell Creek Formation, not far from Bowman, North Dakota. The evidence confirms a suspicion that nagged at DePalma in his first digging season during the summer of 2013.
The Tanis Site Link
In a paper in the journal “Proceedings of the National Academy of Sciences”, he and his American and European colleagues, including two University of California, Berkeley, geologists, describe the site (dubbed Tanis after the ancient Egyptian city referenced in “Raiders of the Lost Ark”) and the evidence connecting it with the asteroid or comet strike off Mexico’s Yucatan Peninsula 66 million years ago. That impact created a huge crater, called Chicxulub, in the ocean floor and sent vaporized rock and cubic miles of asteroid dust into the atmosphere. The cloud eventually enveloped Earth, setting the stage for Earth’s last mass extinction.
Fossilized fish piled one atop another, suggesting that they were flung ashore and died stranded together on a sand bar after the seiche withdrew.
“It’s like a museum of the end of the Cretaceous in a layer a meter-and-a-half thick,” said Richards. Richards and Walter Alvarez, a UC Berkeley Professor of the Graduate School who 40 years ago first hypothesized that a comet or asteroid impact caused the mass extinction, were called in by DePalma and Dutch scientist Jan Smit to consult on the rain of glass beads and the tsunami-like waves that buried and preserved the fish. The beads, called tektites, formed in the atmosphere from rock melted by the impact.
Tsunami vs. Seiche
Richards and Alvarez determined that the fish could not have been stranded and then buried by a typical tsunami, a single wave that would have reached this previously unknown arm of the Western Interior Seaway no less than 10 to 12 hours after the impact 3,000 kilometers away, if it didn’t peter out before then. Their reasoning: The tektites would have rained down within 45 minutes to an hour of the impact, unable to create mudholes if the seabed had not already been exposed.
Tektites, 1 millimeter spheres of glass, recovered from the Tanis fossil bed are shown above. They were produced by the Chicxulub impact and fell within an hour of the impact. (Robert DePalma)
Instead, they argue, seismic waves likely arrived within 10 minutes of the impact from what would have been the equivalent of a magnitude 10 or 11 earthquake, creating a seiche (pronounced saysh), a standing wave, in the inland sea that is similar to water sloshing in a bathtub during an earthquake. Though large earthquakes often generate seiches in enclosed bodies of water, they’re seldom noticed, Richards said. The 2011 Tohoku quake in Japan, a magnitude 9.0, created six-foot-high seiches 30 minutes later in a Norwegian fjord 8,000 kilometers away.
“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 said. “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.”
Seismic waves likely arrived within 10 minutes of the impact from what would have been the equivalent of an impossible magnitude 10 or 11 earthquake.”
The tektites would have come in on a ballistic trajectory from space, reaching terminal velocities of between 100 and 200 miles per hour, according to Alvarez, who estimated their travel time decades ago.
“You can imagine standing there being pelted by these glass spherules. They could have killed you,” Richards said. 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 said. “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.”
At least two huge seiches inundated the land, perhaps 20 minutes apart, leaving six feet of deposits covering the fossils. Overlaying this is a layer of clay rich in iridium, a metal rare on Earth, but common in asteroids and comets. This layer is known as the K-T, or K-Pg boundary, marking the end of the Cretaceous Period and the beginning of the Tertiary Period, or Paleogene.
In 1979, Alvarez and his father, Nobelist Luis Alvarez of UC Berkeley, were the first to recognize the significance of iridium that is found in 66 million-year-old rock layers around the world. They proposed that a comet or asteroid impact was responsible for both the iridium at the K-T boundary and the mass extinction.
The impact would have melted the bedrock under the seafloor and pulverized the asteroid, sending dust and melted rock into the stratosphere, where winds would have carried them around the planet and blotted out the sun for months, if not years. Debris would have rained down from the sky: not only tektites, but also rock debris from the continental crust, including shocked quartz, whose crystal structure was deformed by the impact.
The iridium-rich dust from the pulverized meteor would have been the last to fall out of the atmosphere after the impact, capping off the Cretaceous.
“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,” said Alvarez. “Since then, the evidence has gradually built up. But it never crossed my mind that we would find a deathbed like this.”
Key confirmation of the meteor hypothesis was the discovery of a buried impact crater, Chicxulub, in the Caribbean and off the coast of the Yucatan in Mexico, that was dated to exactly the age of the extinction. Shocked quartz and glass spherules were also found in K-Pg layers worldwide. The new discovery at Tanis is the first time the debris produced in the impact was found along with animals killed in the immediate aftermath of the impact.
“And now we have this magnificent and completely unexpected site that Robert DePalma is excavating in North Dakota, which is so rich in detailed information about what happened as a result of the impact,” Alvarez said. “For me, it is very exciting and gratifying!”
Jan Smit, a retired professor of sedimentary geology from Vrije Universiteit in Amsterdam in The Netherlands who is considered the world expert on tektites from the impact, joined DePalma to analyze and date the tektites from the Tanis site. Many were found in near perfect condition embedded in amber, which at the time was pliable pine pitch.
“I went to the site in 2015 and, in front of my eyes, he (DePalma) uncovered a charred log or tree trunk about four meters long which was covered in amber, which acted as sort of an aerogel and caught the tektites when they were coming down,” Smit said. “It was a major discovery, because the resin, the amber, covered the tektites completely, and they are the most unaltered tektites I have seen so far, not 1 percent of alteration. We dated them, and they came out to be exactly from the K-T boundary.”
Tektites in the fishes’ gills –a first
“Paddlefish swim through the water with their mouths open, gaping, and in this net, they catch tiny particles, food particles, in their gill rakers, and then they swallow, like a whale shark or a baleen whale,” Smit said. “They also caught tektites. That by itself is an amazing fact. That means that the first direct victims of the impact are these accumulations of fishes.”
Smit also noted that the buried body of a Triceratops and a duck-billed hadrosaur proves beyond a doubt that dinosaurs were still alive at the time of the impact.
“We have an amazing array of discoveries which will prove in the future to be even more valuable,” Smit said. “We have fantastic deposits that need to be studied from all different viewpoints. And I think we can unravel the sequence of incoming ejecta from the Chicxulub impact in great detail, which we would never have been able to do with all the other deposits around the Gulf of Mexico.”
“So far, we have gone 40 years before something like this turned up that may very well be unique,” Smit said. “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.”
Source: “Prelude to Extinction: a seismically induced onshore surge deposit at the KPg boundary, North Dakota,” by Robert DePalma et al. Proceedings of the National Academy of Sciences (2019). www.pnas.org/cgi/doi/10.1073/pnas.1817407116
Image credit top of page: With thanks to Louie Psihoyos, a Greek American photographer and film director, who has created a powerful documentary with enough incriminating evidence to convince anyone with a shred of humanity left that we need to do something to change our own habits and tastes in order to save our planet. Interview with Louie at Google Talks. Click to view the trailer for his important documentary on the 6th mass extinction, Racing Extinction.
Editor, Jackie Faherty, astrophysicist, Senior Scientist with AMNH. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterising planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighbourhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognises scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.