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North Dakota Site Shows Wreckage from Same Object That Killed the Dinosaurs

2019-03-29 00:00:00

cartoon of waves and animal images

A meteor impact 66 million years ago generated a tsunami-like wave in an inland sea that killed and buried fish, mammals, insects and a dinosaur, the first victims of Earth's most recent mass extinction event. The deathbed created within an hour of the impact has been excavated at an unprecedented fossil site in North Dakota.(Graphics and photos courtesy of Robert DePalma)

An excavation site in North Dakota sheds new light on what happened when a giant meteorite struck planet Earth, 66 million years ago.

On that day, violent ground shaking first raised giant waves in the waters of an ancient inland sea. Then tiny beads began to fall, created from molten rock cooling at the edge of space to make glassy spheres. The rain of glass was so heavy it may have ignited much of the vegetation on land, while in the water, fish struggled to breathe as the glass beads clogged their gills.

Once the surge of water reached a river mouth, it transformed into a 30-foot wall of water, tossing hundreds if not thousands of freshwater fish -- sturgeon and paddlefish -- onto a sandbar and temporarily reversing the flow of the river.

Stranded by the receding water, the fish were pelted by glass beads up to 5 millimeters (almost a quarter of an inch) in diameter. Some beads got buried inches deep in the mud. Rocks and beads continued to rain down for another 10 to 20 minutes before a second large wave inundated the shore and covered the fish with gravel, sand and fine sediment, sealing them until the present-day discovery.

brown rock with embedded fish print

Fossilized fish piled one atop another, suggesting that they were flung ashore and died stranded together on a sand bar after the seiche withdrew.Robert DePalma

In a paper to be published April 1 in the Proceedings of the National Academy of Sciences an international team of authors, including University of Washington Provost Mark Richards, share the discovery of a site that tells another piece of the story from the day a meteor strike is thought to have led to the end of the dinosaurs.

“It’s like a museum of the end of the Cretaceous in a layer a meter and a half thick,” said Richards, who is also a professor in the UW Department of Earth & Space Sciences.

This unique fossilized graveyard - fish stacked one atop another mixed with burned tree trunks and conifer branches, dead mammals, a pterosaur egg, a mosasaur and insects, the carcass of a Triceratops and seaweed and marine snails called ammonites - was unearthed over the past six years in the Hell Creek Formation in North Dakota by lead author Ro bert DePalma.

“This is the first mass death assemblage of large organisms 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 doctoral student at the University of Kansas. “Nowhere else 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 new study describes the site, dubbed Tanis, and the evidence connecting it with the asteroid or comet strike off Mexico’s Yucatan Peninsula. That impact created a huge crater, called Chicxulub, on the ocean floor and sent vaporized rock and cubic miles of asteroid dust into the atmosphere.

The impact would have melted the bedrock under the seafloor and pulverized the asteroid, sending dust and melted rock into the stratosphere, darkening the sun for months if not years. Debris would have rained down from the sky.

Richards, who did the research as a professor and dean at the University of California, Berkeley, and Walter Alvarez, a UC Berkeley professor of the graduate school who first hypothesized that a comet or asteroid impact caused the mass extinction, analyzed 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.

three researchers at field site

Jan Smit, Mark Richards and Walter Alvarez at the North Dakota site of dinosaur-killing meteor's first victims.

Richards and Alvarez determined that the fish could not have been stranded and then buried by a typical tsunami, a single ocean wave that would have reached this previously unknown arm of the Western Interior Seaway hours after the impact 3,000 kilometers away, if at all. 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.

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) wave in the inland sea, similar to water sloshing in a bathtub during an earthquake.

Though large earthquakes often generate seiche waves in enclosed bodies of water, they’re seldom noticed, Richards said. For example the 2011 Tohoku quake in Japan, a magnitude 9.0, created 6-foot-high seiche waves a half hour later in a Norwegian fjord 8,000 kilometers away.

In the case of the Chicxulub impact, the timing works out for the seiche waves’ arrival.

“The seismic waves start arising within 9 to 10 minutes of the impact, so they had a chance to get the water sloshing before all the spherules had fallen out 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.”

small green beads on fish

Tektites, 1 millimeter spheres of glass, recovered from the Tanis fossil bed.Robert DePalma

The tektites would have reached terminal velocity of about 200 miles per hour, according to Alvarez, who decades ago estimated the travel time of these objects through the atmosphere.

“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 ignited wildfires over the entire American continent, if not around the world.

Richards estimated that the seismic waves, creating the seiche waves, would arrive in North Dakota at roughly the same time as the projectiles from above.

See also: “The Day the Dinosaurs Died” in The New Yorker

At least two huge seiche waves inundated the land, perhaps 20 minutes apart, leaving six feet of deposits covering the fossils. Overlaying this material 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 KT or KPg boundary, marking the end of the Cretaceous Period.

“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.”

The new discovery at Tanis is the first time the debris produced in the impact was found along with animals killed in the impact’s immediate aftermath.

Jan Smit, a retired professor of paleontology from the Vrije Universiteit in Amsterdam in The Netherlands, who is considered the world expert on tektites from the impact, analyzed and dated the tektites from the Tanis site. Many tektites were found in near-perfect condition embedded in amber.

“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 needs to be studied from all different viewpoints. And I think we can unravel the sequence of incoming ejecta from the Chicxuulab impact in great detail, which we would never have been able to do with all the other deposits around the Gulf of Mexico."

Co-authors are David Burnham of the University of Kansas, Klaudia Kuiper of Vrije Universiteit, Phillip Manning of the College of Charleston in South Carolina, Anton Oleinik of Florida Atlantic University, Peter Larson of the Black Hills Institute of Geological Research in South Dakota, Florentin Maurrasse of Florida International University, Johan Vellekoop of KU Leuven in Belgium and Loren Gurche of the Palm Beach Museum of Natural History.

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Adapted from a UC Berkeley press release. Reporters can contact Richards through Hannah Hickey (206-543-2580/hickeyh@uw.edu).

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