The Chicxulub impact is famed for having killed the dinosaurs and most other species alive on Earth at the time, and it left behind a thin layer of dust rich in rare elements. Modeling of the impact has suggested almost too many ways it could have killed: massive tsunamis, a magnitude 11 earthquake, global wildfires and searing heat, months of frigid darkness, acid rain, a massive surge of carbon dioxide, and more. While we've had confirmation that some of these events occurred, we don't have a strong sense of their impact because we haven't found fossils that tell us much about what happened to the ecosystems of the time.
That may have changed, according to a report in PNAS. The paper describes a large deposit residing just under the iridium-rich dust that marks the impact, apparently formed while heavier debris was still falling from the sky. The site, in western North Dakota, contains a mix of fresh and saltwater species, and it seems to have formed when water rushed ashore from what was then a nearby ocean.
On the edge of your sea
At the time of the Chicxulub impact, the Earth looked very different. A long-running hothouse period had likely left the planet without any significant polar ice, meaning extremely high ocean levels. A large area of the US Midwest was underwater as an arm of the oceans formed the Western Interior Seaway, extending from the antecedent of the Gulf of Mexico up to roughly the Canadian border. Its western shores were dotted with valleys cut by rivers that flowed to the nearby ocean.
The remains of one of these valleys contains what is being called the Tanis site. It's marked by layers of light-colored rock that fill part of the valley side. What drew attention to this rock is the fact that its uppermost layer terminated right at the iridium-rich dust that marks the time of the Chicxulub impact. That by itself is not unusual; we wouldn't have known of the global reach of Chicxulub if it weren't for many examples of this iridium layer. However, the layer contained a rich collection of fossils, largely of aquatic species, but with a lot of trees mixed in.
That is unusual, as the fossil record from immediately before the impact is very sparse. The researchers excavating the site thus tried to determine how long before the impact the Tanis deposit formed; the answer they came up with is that it was during the immediate aftermath of the impact.
While we tend to think of the iridium layer as marking the instant of the impact, it's really a bit of a latecomer. The layer was formed by tiny, dust-like particles blasted into the atmosphere during the impact, and these stayed aloft long enough to spread evenly around the globe. By the time they settled, many of the impact's worst effects—the tsunamis and earthquakes—had come and gone, and the global wildfires were in full swing.
If the iridium layer is a bit late to fully understand Chicxulub's impact, other sites have been too early, consisting of rocks that had already been set in place long before Chicxulub.
Just right
The Tanis deposit consists of about a meter of material that looks like it was deposited during two floods, with larger, heavier particles in the lower layers and finer, lighter materials above them. The bodies of animals and trees, along with flow marks in the deposit itself, are also consistent with a flood, being aligned with the prevailing current. There was just one thing odd about all of this: the lower layer seems to have been deposited as water was rushing inland from the ocean. The layer on top was built as it shifted back to the downstream direction.
Based on the elevation where the deposit was left, the flood is estimated to have been at least 10 meters deep.
Strikingly, the deposit seems to have been deposited after the Chicxulub impact but before the iridium layer was deposited. There are small fragments of material scattered throughout the deposit that are typical of impact debris: shocked minerals and spheres of materials that formed glass as it cooled rapidly in the atmosphere. Chemical analysis indicates that these were indistinguishable from similar material found elsewhere that originated at Chicxulub. Argon dating of the material produced a date (65.76 million years old) that matched that of the impact.
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Impact debris trapped in the gills of a fish.DePalma et. al.
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The disruptions in the sediment layer caused by Chicxulub debris that dropped onto the flood debris.DePalma et. al.
The next key question is how this material got to the Tanis deposit. If the flood had eroded a Chicxulub-era deposit elsewhere, you could see a similar pattern of material. But the researchers make a convincing case that this isn't what happened here. Some of the Chicxulub material is embedded in amber, suggesting it had hit tree sap that was being carried by the flood. Other material was found in the gills of fish that died at the site. And a number of the particles were found at the bottom of several disruptions in the sediment, with patterns that "suggest that a descending spherule fell at terminal velocity on an exposed, soft surface between the two main surge pulses."
Timing is everything
All of this puts constraints on when the flood took place. It had to happen quickly enough that the iridium layer hadn't been laid down yet. It also had to take place before the heavier material—the shocked minerals and glasses—had fallen out of the sky. This creates a likely window of between 15 minutes and two hours after the impact.
That time frame rules out the flood being caused by a tsunami, as it would have taken 18 hours for that to reach North Dakota from the impact in the Yucatan. (It's also not clear whether a tsunami could find a contiguous path of open water through the Western Interior Seaway.) Instead, the authors suggest that the flood was the result of a seismic phenomenon called a "seiche," in which a resonance from seismic waves causes waves to rise in a constrained body of water. They point to the example of the 2011 Tohoku earthquake in Japan, which caused 1.5 meter waves to form in Norwegian fjords.
That earthquake was a magnitude 9.2; Chicxulub was estimated to have caused a seismic disturbance that may have reached magnitude 11.5. The waves would have started reaching North Dakota six minutes after impact, with the last passing at about 15 minutes post-impact. So the timing is right for this theory to work. However, the researchers acknowledge that they can't rule out the flood being caused by extreme winds driven by the sudden temperature surges or the seismic event causing landslides that drove the floods inland.
End of life
One of the big questions regarding the Chicxulub event was its role in the mass extinction. Had the dinosaurs been teetering on the brink of extinction, pushed there by the massive Deccan Traps eruptions? Or was life thriving right up until the moment that the impact occurred? The Tanis site won't answer these questions, at least not immediately. There's no mention of any dinosaur bones in the paper itself. Instead, the vast majority of species described are either plants or aquatic life.
On the freshwater side, the fish include paddlefish and sturgeons. Species from both of these groups have survived to the present day. The marine species include many ammonites, a shelled animal that looked a bit like a nautilus and is nearly ubiquitous in marine fossil deposits from before the impact. The ammonite lineage, however, ended in the mass extinction.
But the biological material is given a few scant paragraphs in what's a very substantial paper. They're likely to end up being described separately in multiple papers, so there's a chance that we'll be able to say more about pre-extinction ecology as those details are revealed. But the real value of Tanis may end up being how much it can tell us about how to find similar sites elsewhere.
The researchers who discovered Tanis, however, are already facing some controversy about how they've parceled out the details. Robert DePalma, the grad student who made the discovery, has been sharing its details with a journalist since 2013; only recently did he present any of the results at a scientific meeting. The researchers, PNAS, and a publisher then worked together to time the paper's release to an article in The New Yorker. For several days last week, the paper itself was only available to journalists who had signed up for emails from PNAS; scientists and the rest of the press were left to scramble to come up with a copy.
(An image provided to the press by DePalma also shows a triceratops swept up in the flood waters, something that the paper presents no evidence of.)
None of this takes away from the excitement of the discovery or the prospect that further work will affirm that this deposit was made within hours of the Chicxulub. But that affirmation will require the larger scientific community to get greater access to the materials.
PNAS, 2019. DOI: 10.1073/pnas.1817407116 (About DOIs).
https://arstechnica.com/science/2019/04/scientists-say-they-have-deposits-formed-hours-after-dino-killing-impact/
2019-04-02 17:31:00Z
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