Fossil site is first ever to show deaths from mass extinction asteroid impact
A dramatic find in North Dakota has uncovered the first known victims of the asteroid impact that led to the extinction of the dinosaurs 66 million years ago. An international team of paleontologists and geologists has found a massive collection of fossils of fish and other animal that were killed by a seismic sea wave and a shower of burning glass beads less than an hour after the asteroid strike off the shore of what is now Yucatan.
The mass extinction event caused by the impact of an asteroid or perhaps a comet at the end of the Cretaceous age that wiped out 75 percent of life on Earth is widely accepted, but for most people, it has a strangely abstract quality.
It's easy to accept the idea of a giant object hitting the ocean, the massive shock waves and tsunamis and followed, as well as the concept of the sun being blotted out for years afterward, killing off plants and animals in a massive ecological chain of events. But much of it remains very clinical. It involves an ancient meteor crater on the Caribbean seabed, a sudden jump in iridium in the stratigraphy brought by the asteroid that created what is called the K-T boundary in the geological record, and an equally sudden disappearance of the dinosaurs, which had reined supreme on Earth for tens of millions of years.
It's all very convincing, but what happened at the time of impact? What was it like being in a place where a planet-wide catastrophe took place? It turns out, it wasn't very pleasant.
Thousands of miles away from Mexico, North Dakota may seem like an odd place to find evidence of the Yucatan, or Cretaceous–Tertiary (K–T) extinction event, but 66 million years ago it was part of a massive inland sea called the Westland Interior Seaway. One spot near Bowman, North Dakota, called Tanis by the geologists, was the mouth of a freshwater river teeming with sturgeon and paddlefish while triceratops and duck-billed hadrosaurs roamed the banks.
That all changed in an instant when the asteroid strike happened. According to a team led by Robert DePalma, curator of paleontology at the Palm Beach Museum of Natural History in Florida and a doctoral student at the University of Kansas, the fossil record uncovered at Tanis indicates a disaster like something out of the Old Testament.
The asteroid strike took place some 3,000 km (1.865 mi) away and a conventional tsunami would have taken up to 12 hours to reach Tanis – if it didn't die down first. Instead, the team argues that a seismic wave hit 10 minutes after impact with the force of a 10 or 11 magnitude earthquake, generating a standing wave or seiche that created a sloshing wall of water at the mouth of the river 30 ft (10 m) high that reversed the flow of the river and threw fish and other water life onto a sandbar, later to be buried by successive waves of mud and debris.
Meanwhile, the initial Yucatan blast vaporized the asteroid and hurled millions of tonnes of debris into the atmosphere. The superhot remains of the asteroid cooled into burning hot beads of glass called tektites that were about one to five millimeters in diameter.
In about 45 minutes, these rained down on the fish at Tanis at up to 200 mph (322 km/h), filling their mouths and gills. Others left pits in the uncovered mud and those that touched land started fires as they came down in a fiery torrent that lasted for up to 20 minutes. Some landed in resin and were preserved as it turned into amber. It was a scene that must have been common around the planet, but the first evidence is at Tanis.
"This is the first mass death assemblage of large organisms anyone has found associated with the K-T boundary," says DePalma. "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.
"Tsunamis from the Chicxulub impact are certainly well-documented, but no one knew how far something like that would go into an inland sea. When [Mark Richards, professor of earth and space sciences at the University of Washington] 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 research will be published in the Proceedings of the National Academy of Sciences.
The video below shows Robert DePalma excavating at the Tanis fossil site in North Dakota.
Source: UC Berkeley