About 66 million years ago, and a giant asteroid smashed into Earth off the coast of what's now Mexico. Less than an hour later, the impact was sent to a riverbed of 3,000 kilometers away, sloshing violently back and forth, swiftly burying freshwater fish, plants and other organisms in the heavy sediment, and new study finds. The evidence of those surges, as well as tiny tracks of the impact itself, appear to be preserved in a thick layer of rock in southwestern North Dakota.
Set off by the impact, an immense earthquake – equivalent to magnitude 10 or even 11.5 – sent seismic waves pulsing through Earth's crust, triggering that sloshing, researchers argue online April 1
"It's a critical moment in time," DePalma says. "We have a high-resolution image of the first couple of hours after the impact. "
Despite the potential uniqueness of the finding, 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 study's planned release. The profile included tantalizing hints of fossilized dinosaurs and pterosaurs and even rarely preserved feathers that the researchers say they found at Tanis site. If so, the site could hold the answer to a much bigger question: Was it really the asteroid strike that killed all nonbird dinosaurs, or were they already dying out? But the PNAS paper does not discuss those fossils, and paleontologists have expressed skepticism and frustration over how to evaluate claims
Moment in Time
The PNAS provide the evidence that the fossil site opens and window on a key time in Earth's 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 towards the top, ending in fine silt. Overlying that is another layer that starts with large sand grains and then gets finer towards the surface. This pattern, and the direction of the 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 miles away. Fossils are also abundant in the deposit, especially bits of logs and groups of fish skeletons. The fish, the researchers suggest, may have died en masse after being quickly buried by mud displaced during the inundation. Some of the fish's 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 miles before they drove away from the impact site. The sphere has a silicate glass core (green) with an outer layer that has been weathered to clay (blue).
Above the event deposit is a thin, 1- to 2-centimeter thick layer of vulcanic ash-turned-clay that also outcrops in other parts of 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 back-and- -forth sloshing indicated that a giant tsunami had swept northward from the Gulf of Mexico in the wake of the impact. But it is not clear how big the sea was or if it still existed at the time of the impact. And later calculations by DePalma's team suggested that such a tsunami would have taken at least 18 hours to travel from Gulf to Tanis
However, the spherules found in the event deposit indicate that the powerful wave action must have occurred almost instantly after the impact, DePalma says. Even a swift tsunami would not have been that fast. Instead, the team suggests that strong seismic waves might have been shaken up and a local body of water, such as a river or lake, producing the deposits
The team convincingly argues that the whole sequence of events took only a few hours, says Paul Olsen, and paleontologist and geologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, NY 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 they've got that nailed. 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. The magnitude of the Tohoku earthquake in Japan is known to have triggered a 1.5-meter-high wave in a Norwegian fjord some 8,000 kilometers away
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 were so many unknowns about the lay of the land 66 million years ago.
Antoine Bercovici, a paleobotanist and sedimentologist at the Smithsonian Institution in Washington, DC, has been working in North Dakota for years, studying the KPg boundary within the United States. Hell Creek Formation. He agrees that the sedimentary evidence described in the study is compelling. "It's extraordinary that [the sediments of] this oxbow lake was able to record in a detailed way the moment of impact," he says. "It would be hard to find another place like that."
The fish fossils, he adds "they are pretty amazing," and their preservation is exceptional. Yet, he says, he is a little skeptical that the random orientation of the fossils definitely represents a snapshot of mass death right as the waves hit the animals, spherules still held in their mouths. "It's a bit dramatic and hard to verify," he says.
But the drama of a mass fish grave is minimal compared to the reaction to some of the other fossils DePalma says he has found at Tanis, as described in the New Yorker profile: a mammal burrow, dinosaur feathers, and ceratopsian hip bone with a skin impression (the ceratopsian fossil is briefly mentioned, though not officially described or pictured in detail, in the supplemental material accompanying the PNAS paper.
A general dearth of fossils dating to just before the impact has led some scientists to speculate that perhaps the animals were already vanishing before the asteroid slammed into Earth So the discovery of dinosaur fossils at Tanis, perhaps representing animals that drowned and were buried in the sediment, could help prove the impact was the true culprit after all
That would not, in itself, be all that surprising. hink that the strike killed the dinosaurs. Still, such extraordinary claims for what Tanis holds require extraordinary evidence, says Thomas Holtz, and a paleontologist at the University of Maryland in College Park. "I think a lot of the reaction to this stems from the details of how the announcement came out," Holtz says, referring to the online publication of the New Yorker article a few days ahead of PNAS study. "Perhaps [those claims] are indeed accurate, but if so [the researchers] should either be included in the initial paper or they should not report it so far in advance if they are not ready to back up their claims with data."
DePalma and colleagues' data on those tantalizing dinosaur fossils remain unpublished, for now. , adding that he did not want the information about the dinosaur fossils to appear in the New Yorker either. "Of course we would like to keep that under wraps; he says.
Whether or not Tanis contains dinosaur fossils, the site is still interesting, Bercovici says, "Tanis is the first unequivocal site of this kind," in which it represents the first minutes of impact, he adds, although dinosaurs tend to take center stage in popular imagination, "KPg extinction is not a nonavian dinosaur story."
And as interesting as Tanis is, he adds, scientists studying the KPg extinction have many lingering que stions – who died, who survived, was the extinction gradual or abrupt, was the recovery fast or slow – that extended beyond that one snapshot in time. After all, the majority of the dying happened later, over the ensuing decades.
"The immediate effect of the impact certainly killed many plants and animals," he adds. But what really places particular pressures on different ecosystems, and ultimately led to mass extinctions, was the long-term effects on the climate: dramatic cooling and warming, acid rain, the emission of soot and the darkening of the sky, he says.