About 66 million years ago, a giant asteroid smashed into Earth off the coast of whats now Mexico. Less than an hour later, the impact sent water in a riverbed 3,000 kilometers away sloshing violently back and forth, swiftly burying freshwater fish, plants and other organisms in the heavy sediment, a new study finds. Evidence of those surges, as well as tiny traces of the impact itself, appear to be preserved in a meter-thick layer of rock in southwestern North Dakota.
Set off by the impact, an immense earthquake — equivalent to a magnitude 10 or even 11.5 — sent seismic waves pulsing through Earths crust, triggering that sloshing, researchers argue online April 1 in the Proceedings of the National Academy of Sciences. If true, the scenario would add a new kill mechanism to the mass extinction event that marks the boundary between the Cretaceous and Paleogene Periods, often called the KPg. Some 75 percent of land-based species, including all nonbird dinosaurs, died in the event (SN: 2/4/17, p. 16).
The site, found in the Hell Creek Formation and dubbed Tanis, represents a unique snapshot of what happened on land in the immediate aftermath of the impact, says paleontologist Robert DePalma of the University of Kansas in Lawrence.
“Its a critical moment in time,” DePalma says. “We have a high-resolution image of the first couple of hours after the impact. That level of detail is not really known elsewhere.”
Despite the potential uniqueness of the find, the publication of the PNAS paper was overshadowed by a profile of DePalma that had been published online on March 29 in the New Yorker, just a few days before the studys planned release. The profile included tantalizing hints of fossilized dinosaurs and pterosaurs and even rarely preserved feathers that the researchers say they have found at the Tanis site. If so, the site could hold the answer to a much larger question: Was it really the asteroid strike that killed all nonbird dinosaurs, or were they already dying out? But the PNAS paper doesnt discuss those fossils, and paleontologists have expressed skepticism and frustration over how to evaluate the claims.
Moment in time
The PNAS paper does provide evidence that the fossil site opens a window on a key time in Earths history. At Tanis, a river once drained eastward from a vast inland sea. Sandy deposits reveal where the meandering river carved a deep channel into the rock. Above that channel lies an unusual rock sequence that DePalma and his colleagues call the “event deposit.” That 1.3-meter-thick layer of rock has two distinct sublayers. The bottom layer has large pebbles at its base and finer-grained sediment toward the top, ending in fine silt. Overlying that is another layer that starts with large sand grains and then gets finer toward the surface. This pattern, and the direction of water flow preserved by the grains, point to some kind of massive inundation, DePalma says.
The deposit also contains tiny glass spheres, remnants of vaporized rock cast into the atmosphere from the impact that then rained back down potentially thousands of kilometers away. Fossils are also abundant in the deposit, particularly bits of logs and groups of fish skeletons. The fish, the researchers suggest, may have died en masse after becoming rapidly buried by mud displaced during the inundation. Some of the fishs bellies contain the tiny spherules, possibly snagged from the water just before death.
As an asteroid struck Earth, it vaporized rock, sending particles (one from a new fossil site in North Dakota shown in a micro-CT image) into the air. Those particles traveled for thousands of kilometers before raining down far from the impact site. The sphere has a silicate glass core (green) with an outer layer that has weathered to clay (blue).
Above the event deposit is a thin, 1- to 2-centimeter-thick layer of volcanic ash-turned-clay that also outcrops in other parts of the central United States. This layer contains impact spherules and dates to the KPg, helping connect the Tanis site to the extinction event.
Because a vast shallow sea covered much of the ancient central United States at one time, the team first suspected that the back-and-forth sloshing indicated that a giant tsunami had swept northward from the Gulf of Mexico in the wake of the impact. But its not clear how large the sea was or if it still existed at the time of the impact. And later calculations by DePalmas team suggested that such a tsunami would have taken at least 18 hours to travel from the Gulf to Tanis.
However, the spherules found embedded in the event deposit indicate that the powerful wave action must have occurred nearly instantly after the impact, DePalma says. Even a swift tsunami wouldnt have been that fast. Instead, the team suggests, strong seismic waves might have shaken up a local body of water, such as river or lake, producing the deposits.
The team convincingly argues that the whole sequence of events took only a few hours, says Paul Olsen, a paleontologist and geologist at Columbia Universitys Lamont-Doherty Earth Observatory in Palisades, N.Y. Fish consumed the spherules that rained into the water, became entombed in sediment displaced during the violent sloshing of the water, and were then covered by a second layer of sediment bearing iridium, an element enriched in asteroids. “I think theyve got that nailed. Its hard to imagine how that would occur any other way,” Olsen says.
Jessica Whiteside, a geochemist at the University of Southampton in England, agrees that the sedimentary evidence supports the idea that the impact produced violent sloshing at Tanis. And the possibility that a massive earthquake provoked those waves is plausible, she adds. As the authors say in the study, the 2011 magnitude 9.2 Tohoku earthquake in Japan is known to have triggered a 1.5-meter-high waves in a Norwegian fjord some 8,000 kilometers away.
“But its not the only plausible sequence of events that could have happened,” Whiteside says. And there may be no way to know for sure if the scenario is the right one, or what the exact timing would have been of the seismic waves arrival, because there so many unknowns about the lay of the land 66 million years ago.
“That said, I find it really exciting work,” Whiteside says.
Antoine Bercovici, a paleobotanist and sedimentologist at the Smithsonian Institution in Washington, D.C., has worked in North Dakota for years, studying the KPg boundary within the Hell Creek Formation. He agrees that the sedimentary evidence described in the study is compelling. “Its extraordinary that [the sediments of] this oxbow lake were able to record in a detailed way the moment of the impact,” he says. “It would be hard to find another place like that.”
The fish fossils, he adds “are pretty amazing,” and their preservation is exceptional. Yet, he says, hes a bit skeptical that the haphazard orientation of the fossils definitively represents a snapshot of mass death right as the waves impacted the animals, spherules still held in their mouths. “Its a bit dramatic and hard to verify,” he says.
But the drama of a mass fish grave is minimal compared with the reaction to some of the other fossils DePalma says that he has found at Tanis, as described in the New YoRead More – Source