The beginning of the end began 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 birdshot from the heavens. The rain of glass was so heavy it might have set fire to much of the vegetation on land. In the water, fish struggled to breathe as the beads clogged their gills.
The heaving sea 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 – on 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 another 1
This unique, fossilized graveyard – fish stacked one atop another and mixed with burned tree trunks, coniferous branches, dead mammals, mosasaur bones, insects, the partial carcass of a Triceratops dinoflagellates and snail-like marine cephalopods called ammonites – was discovered by paleontologist Robert DePalma over the past six years in Hell Creek Formation, not far from Bowman, North Dakota. The evidence confirms and suspicion that it was nagged at DePalma in its first digging season during the summer of 2013 – that this was a killing field laid down shortly after the asteroid impact that eventually led to the extinction of all ground-dwelling dinosaurs. The impact at the end of the Cretaceous Period, the so-called KT boundary, exterminated 75 percent of life on Earth
"This is the first mass death assemblage of large organisms that anyone has found associated with the KT boundary," said DePalma, curator of paleontology at the Palm Beach Museum of Natural History in Florida and a PhD student at the University of Kansas. "At the other KT boundary section on Earth you can 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." [19659003 He and his American and European colleagues, including two University of California, Berkeley, geologists, describe the site, dubbed Tanis, 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.
"It's like a museum of the end of the Cretaceous in a layer and a meter-and-a-half thick," said Mark Richards, and UC Berkeley Professor Emeritus of Earth and Planetary Science, Professor of Earth and Space Sciences at the University of Washington
Richards and Walter Alvarez, and UC Berkeley Professor of the Graduate School who 40 years ago first hypothesized that a comet or asteroid impact caused by 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 conserved the fish. The beads, called tektites, formed in the atmosphere from the 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 did not peter out before then. Their reasoning: The tektites would have rained down within 45 minutes to the hour of impact, unable to create mudholes if the seabed had not already been exposed
Instead, they argue, seismic waves probably arrived within 10 minutes of impact from what would have been the equivalent of a magnitude 10 or 11 earthquake, creating a seiche (pronounced sayh), and a standing wave, in the inland sea that is similar to water sloshing in a bathtub during a earthquake. Although large earthquakes often generate seiches in enclosed bodies of water, they are rarely noticed, Richards said. The 2011 Tohoku quake in Japan, and magnitude 9.0, created six-foot-high seiches 30 minutes later in a Norwegian fjord 8,000 kilometers away
"The seismic waves start within nine to ten minutes of impact, so they had a chance to get the water sloshing before all the spherules (small spheres) snake fallen out of the sky, "Richards said. "These spherules come in the cratered surface, making funnels – you can see the deformed layers in what used to be soft mud – and then rubble covered the spherules."
The tektites would have arrived on a balistic trajectory from space, reaching terminal speeds 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 across the entire American continent, if not around the world.
"Tsunamis from the Chicxulub impact are certainly well documented but no one knows how far something like that would go into an inland sea, "DePalma said. "When Mark arrived onboard, he discovered a remarkable artifact – that the incoming seismic waves from the impact site would have arrived at about the same time as the atmospheric travel time of the ejecta."
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.
In 1979, the Alvarez and the K-Pg boundary, marking the end of the Cretaceous Period and the beginning of the Tertiary Period, his father, Nobelist Luis Alvarez of UC Berkeley, were the first to recognize the importance of iridium that is found in 66 million-year-old rock layers around the world.
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 been rained down from the sky: not only tektites, but also rock debris from the continental crust, including the 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 on just finding anomalous concentration of iridium – the fingerprint of a asteroid or comet, "said Alvarez. "
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 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 the 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.
Jan Smit, a retired professor of sedimentary geology from Vrije Universiteit in Amsterdam in The Netherlands, who is considered the world's expert on tektites from the impact, joined DePalma to analyze and date the tektites from the Tanis site.
"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 a kind of 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 KT boundary. "
The tektites in the fishes' gills are also 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. "
Smit also noted that the buried body of a Triceratops was also a capture of the tektites. and a duck-billed hadrosaur proves beyond doubt that dinosaurs were still alive at the time of the impact
"We have an amazing array of discoveries that will prove to be even more valuable in the future," Smit said. "We have fantastic deposits that need to be studied from all different viewpoints. And we think we can unravel the sequence of incoming ejecta from the Chicxulub impact in great detail that we would never have been able to do with all the other deposits around Gulf of Mexico. "
" So far, we have gone 40 years before something like this turned up that may well be unique, "Smit said. "So, we have to be very careful with that place, how do we dig it up and learn from it, that's a great gift at the end of my career."