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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The hypothesis that the mass extinction at the end of the Cretaceous was caused by the impact of a large asteroid was first proposed by Luis and Walter Alvarez, and independently by Jan Smit, in 1980. The impact hypothesis is now widely accepted by the scientific community, but many details as to how this one-of-a-kind event affected the Earth and living things remain unclear. Then in 2019, the announcement of a new find in North Dakota created its own big splash: an astonishingly detailed snapshot of the immediate aftermath of the impact event had been found.

Jan Smit is Emeritus Professor of Event Stratigraphy at the Free University of Amsterdam. Initially a paleontologist studying planktonic foraminifera, he became intrigued by what appeared to be very sudden events in the fossil record, and increasingly focused his research on the boundary between the Cretaceous and the Paleogene, the so-called KT boundary. He is one of the lead authors on the paper announcing the North Dakota discovery. Jan Smit, welcome to Geology Bites.

Jan Smit: Thank you.

Oliver Strimpel: You were one of the first to suggest the impact hypothesis for the end-Cretaceous mass extinction that destroyed the non-avian dinosaurs, as well as a myriad of other terrestrial and marine organisms. What were the competing hypotheses at the time?

Jan Smit: I think somebody tabulated a number of 50 explanations, either sensible or nonsensical, for the disappearance. But most of them started from something which is Earth-bound, like climate change. It could have been colder. It could have been wetter. It could have been warmer. The seas might have changed. A lot of theories were concerned with the opening and closing of gateways in the sea, like the separation between South and North America. A lot of people thought, OK, there was a gateway open, and somehow the ocean currents ran through between North and South America, and they started to create havoc among organisms. Of course, volcanism was suggested: if you put enough toxic gases in the atmosphere, CO2 or SO2 or something like that... We know big volcanic eruptions can have consequences.

And then, at the time I met Walter Alvarez, in 1979 at a conference, there was an explanation which was even more ingenious. Right now, the North Atlantic is open to the Arctic seas, but they thought that, due to plate tectonics, at that time the gap between Norway and Greenland was closed, and so probably was the Bering Strait. So the idea was that there was a huge ocean right at the spot where the North Pole is. According to them, it was probably a freshwater body, comparable to a giant-sized Great Lakes, like in North America. The opening of the North Atlantic was probably happening around the Cretaceous-Tertiary boundary. So the idea was that fresh water, which, even if it's colder, is still lighter than normal ocean water, was spilling out over the northern Atlantic and eventually over the entire ocean, and was probably shutting down the circulation between the deep ocean and the shallow waters.

Well, that's an elegant idea, of course, because that way you can kill off the plankton, which for the most part are tropical or subtropical species. So it happened in a fairly short time. And of course, the dinosaurs were killed by the Arctic flooding, as they called it, because it would cool the climate, and since dinosaurs are reptiles, the thinking was that they couldn't survive a cold spell following the spillage of the Arctic Ocean over the entire ocean. So these were the sorts of theories floating around at that time.

Oliver Strimpel: So then you and the Alvarezes came up with the asteroid impact hypothesis. Before the recent discovery in North Dakota, what did we infer about the nature of this impact, and what was the evidence based on?

Jan Smit: Well, of course, the first evidence was the finding of the platinum group elements. Iridium is the foremost among those platinum group elements. They are all what we call iron-loving, siderophile, elements, so iron and nickel and chromium and cobalt are enriched as well. And the ratio between the platinum group elements was very close to what we call a chondritic asteroid body. So that was the first indication from the iridium. Although there were theories that it could be spilled out of a volcano, a deep-seated one, maybe a hot-spot volcano, even then the iridium would deviate from what we call the chondritic ratio.

And before I found the iridium, I was looking at the tiny layer which separated the Cretaceous and the Tertiary. In the meantime, I started looking at many places, like Denmark, Italy, Germany, France, the north of Spain. Wherever I looked, there was a clay layer which consisted of two parts. The lower part is a very thin layer, about 2 to 3 millimeters, and it doesn't matter where you look, in the Pacific Ocean, in New Zealand, in Europe or in the Atlantic, it's more or less 2 to 3 millimeters. And it's also the most enriched in platinum group elements and iridium. The second layer is much thicker, and that layer is created by the supply of clay from land, windblown and from rivers, et cetera. Because if you kill off the planktonic organisms, which bear a calcareous skeleton, they don't sink to the sea floor and make a limestone layer. Now what you get is a layer of clay, from the windblown dust and the clay derived from riverine sources.

So that was the second indication: that we have a twofold layer. And within that lower 2-millimeter layer, I found earlier a whole suite of spherules, tiny spherules, about half a millimeter for the largest. They turned out to be crystalline. I already suspected that they were the remains of what we call microtektites, and microtektites are the product of a big meteorite impact, where the melted stuff is squeezed out underneath the penetrating meteorite and thrown out over a large area of the globe. So not only did we find the iridium, we knew it had to produce these microtektites, and that excluded that we simply have an accretion of extraterrestrial matter, where you can imagine that instead of a big meteorite, we have an enormous amount of smaller meteorites landing on the surface of the Earth. These spherules proved that it must have hit somewhere in the world. We didn't know where.

And it kept on going. People found strange isotopes, and Bruce Bohor, a few years later, found in the same 2-millimeter-thick layer what we call shocked quartz: tiny grains of quartz which, under a microscope, show signs of a severe shock, like a plane going through the sound barrier. They're highly diagnostic for shock pressures, which can only be created by nuclear explosions or by meteorite impacts. So one by one, the evidence in the early '80s was accumulating that we had to deal with a very big impact.

Oliver Strimpel: So how did the discovery of the so-called Tanis site in North Dakota come about?

Jan Smit: The Tanis site first came to my attention because Robert DePalma just sent me an email, and he said that he had found evidence for a tsunami deposit in North Dakota. Well, at first I forgot about it, because I had so many mentions of so-called tsunami deposits. But then he showed me a couple of pictures, and one of them was very important. He showed me a piece of rock, a very big piece he collected, with three fishes in it, a sturgeon and a paddlefish, together with an ammonite. Now, an ammonite is a purely marine animal, a squid-like organism, but the sturgeon and especially the paddlefish are strictly freshwater, and both of them are beautifully preserved, with all the details of the fin tips, et cetera. So they had to be washed together by some event, and also be perfectly preserved, so that convinced me that there was something very unusual there.

Oliver Strimpel: Why were you so skeptical, though, initially, that there was another tsunami deposit there?

Jan Smit: Well, it's far away from the Gulf, and it's very hard to imagine. Jody Bourgeois calculated the height of the tsunami. She came up with a couple of hundred meters, but even at a couple of hundred meters, it would just diminish and disappear within a few hundred kilometers, and you still have to travel 2,000 kilometers all the way up to Tanis. So I was quite skeptical. But because he showed me the ammonite and the fish, and he also showed me the structure of the deposit, which is very similar to what we discovered around the Gulf of Mexico, I became more and more convinced. And then he sent me pictures of the tektites he found inside the deposit, so I offered: OK, we can date those tektites in our lab here in Amsterdam. And that's what we did, and they turned out to be exactly the same age as the tektites in Haiti. So I was convinced, and I had to get there.

Oliver Strimpel: So at this point, you were convinced that it was somehow part of the same event that had been well documented around the Gulf of Mexico and so on. So what happened when you visited the site and took a closer look?

Jan Smit: We had a conversation, Robert and me, and when we were there, he showed me all the carcasses, and the whole deposit was sloping on the river bank. It had all the typical hallmarks, for me as a sedimentologist, of a high-energy deposit, with two pulses, and in the top one are perfectly preserved fishes. And then he started scratching the earth away from a big fish, a paddlefish lying upside down, and he said to me, look here, here's the upper and the lower jaw. And he started to dig between both sides of the lower jaw, and before my eyes he pried out the gills of the fish. And in the gills, and more importantly the gill rakers, there was a sort of catch, a lot of those tektites stuck there. So this fish must have been swallowing tektites while it was still alive. So whether it died from the event or not, it had to die very close to the impact itself, within hours or something.

Oliver Strimpel: Let me see if I get the logic. Because you found the tektites in the gills of these filter-feeding paddlefish, how does that tell you they died within an hour or two of the impact?

Jan Smit: A couple of things. Because they're very effective in swallowing their food. Whatever it catches while swimming, it does as a pelagic fish: open mouth, it swims in the river. So it must have caught them when they were still raining out of the sky. It's not a bottom feeder, like a sturgeon. And so it caught them in the gills, and they didn't get any farther than that. We have X-ray scans in high detail, in 3D, and you can see that the gut, the beginning of the gut of those fishes, is also preserved, but none of it contains any tektites. I'm not a specialist in fishes, but I could imagine that it swallows whatever it catches within minutes and ingests it into its intestine. So we didn't see that.

Oliver Strimpel: So in other words, there was time for it to get caught in the gills, but no time, before it died, for it to get from the gills into the digestive system.

Jan Smit: And I think it has mechanisms to remove those objects from its gills, by just swimming harder, or swimming in reverse, or whatever. But they're still in there. In 60% of the fishes, we see that they have the tektites still inside their gills. Combine that with the perfect preservation, the bodies not being swollen, and also, above the tomb of the fishes, there are additional tektites. So while they are dying, additional tektites come from the sky and land on succeeding layers of tsunami deposits, while the fishes are already buried. So that's the time window you have: it's still within the rain of tektites coming from Chicxulub. And that's a very narrow window. The first ones arrive in, say, 15 minutes, and the latest come within an hour or so, maybe some hours, but that's really stretching it. So that's a very narrow window in which they have to die. So I would submit that they are the direct victims of Chicxulub itself. There's probably no other way to explain it.

Oliver Strimpel: OK. So both the distribution of the tektites inside the fish, but also the distribution of tektites around the fish in that deposit, show that this whole event had to have happened within about two hours or something, at most.

Jan Smit: Exactly. Yeah.

Oliver Strimpel: And yet we're 3,000 kilometers away from the impact, so that just doesn't compute for the travel time of a tsunami. So how did you resolve this conundrum?

Jan Smit: I happened to travel around the same time in India, looking for the effects of the Deccan Traps.

Oliver Strimpel: Just by way of background: the Deccan Traps are a very extensive, 2-kilometer-thick sequence of lava flows in central India that erupted in the space of a few tens of thousands of years, more or less coincident with the end of the Cretaceous. The volcanic gases released during the formation of the traps have been thought to be implicated in the end-Cretaceous extinction event, both via the sudden cooling of the climate they may have induced, and through the toxicity of the gases. I think you told me you were traveling there with a geophysicist friend called Mark Richards.

Jan Smit: Yeah. And he had just conceived a theory that the Deccan Traps might have erupted more severely as a consequence of the impact. The impact didn't start the Deccan Traps, but it accelerated them. And so I told him the whole story, because I'd been there just in the fall, and in the winter we were in India, so a couple of months apart. We were still thinking, and he said, Jan, there's no way you can get your tsunamis there in time. And secondly, he said, I may have a solution for you. And then he showed me a picture of a fjord in Norway, where people filmed a tsunami in the Sognefjord and another fjord, which originated about half an hour after the giant earthquake in Japan in 2011. And what you see is really amazing: one-and-a-half-meter tsunamis running up the shore in a fjord in Norway, thousands of kilometers away from that severe magnitude 9 earthquake in Japan. So there is a way, he said, to trigger a body of water, probably by putting it in resonance with the earthquake waves which are arriving there.

And he said, the earthquake caused by Chicxulub is about 50 times, or two orders of magnitude, more powerful, and that's the minimum estimate, than the largest earthquake which can be caused by plate tectonics, which is the 9.6 in Chile. He believes 11.6 or 7 is the maximum for the impact. The waves, and the amplitude of the waves, at that distance, 3,000 to 4,000 kilometers away from the impact, could have triggered, could have pulled, the body of water into a resonant mode and caused a surge in situ, here in North Dakota itself, if there was a shallow sea there, and run up the Tanis river within the required amount of time, between half an hour and a couple of hours.

Oliver Strimpel: How thick is the surge deposit, and can you talk a little bit about the different layers in it and the tektites, and how much detail that provides us about what actually would have happened in the first hour or two after this impact?

Jan Smit: What we clearly see in the field is that the first surge is the most energetic, because at the bottom of the river channel we find boulders about half a meter in size, but they quickly become finer. And then we get a sand body with all the indications of an enormous sediment load, which grades upwards within half a meter to a meter into the deposits where the fishes were buried, where the logs are, where the other trophies are of the riparian fauna and flora that existed at the time, in the topmost Cretaceous. So apparently the surge running up the river was powerful enough to rip off branches, to kill animals, to kill the fishes, for instance, and not only the big ones; there are smaller fishes. We see shark teeth. There are insects; there are wasps surprised by the water there, and nests, and mammal burrows. They provide a sort of snapshot of living things in the area, which besides are perfectly buried in the tombstone of Tanis. So besides being an intriguing deposit related to the impact, it offers us a glimpse of what life was like at the close of the Cretaceous.

Oliver Strimpel: Does it also explain how the ammonites, a marine species, got into this freshwater environment?

Jan Smit: Yeah, that's still a problem for the model. They're perfectly preserved; they're nice shells. We have evidence that the fish were clearly alive when they were buried, but we have no live ammonites. So it could have been like today on the beaches of Indonesia, where you find those nice mother-of-pearl shells of nautiloids. So the shells could have been lying on the beach and then been swept up. But you still need a shoreline, a marine coast, where the saltwater moves inward. Our thinking is that the fishes were not killed by the tektites; they had to deal with a sudden surge of saltwater, which they didn't like at all.

Oliver Strimpel: So you have this picture of something like a giant tidal bore sweeping up the shallow river from the sea, bringing with it ammonites, and then creating a huge run-up on the bank and basically just washing all the life out of the water. Now, what exactly would have been the cause of death in this situation?

Jan Smit: Yeah, you can think of a variety of things. But what we also find is evidence on the tree trunks. There are tree trunks about 5 meters in length, the largest ones, and the surfaces of the tree trunks are scorched; there is a thin layer of charcoal around them. And on the tree trunks are pieces of amber, with tektites inside the amber. The only way you can make amber sticky enough and liquid enough to be able to catch the tektites while they were flying in is by making it very soft, by a fire or something like that. So that's one of the initial kill mechanisms. Those zillions and billions of returning spherules must have created as many shooting stars at the same time, which caused a sort of broiler effect. You see the incoming tektites at a height of 20 to 40 kilometers, and they created a blanket of strongly radiating stuff, which must have zapped at least the surface of the Earth just before the tsunami came up. So it's perfectly possible that a lot of the victims were created just before the tsunami. They were sort of suffocated at that time, and then they were swept up by the tsunami.

Oliver Strimpel: So if they were cooked in the first hour or two, perhaps then you had climate change, and then they could freeze, I guess. What happened then?

Jan Smit: Yeah. Due to the thrown-up dust, there's a global layer of dust in the stratosphere. And added to that is the very rare circumstance that the meteorite in Yucatán hit a layer of gypsum, which of course is highly rich in sulfur. The sulfur is also vaporized. So the dust and the sulfur, which is a sort of whitish powder, are very capable of bouncing off and reflecting sunlight back into space. So the second effect, after the first half hour of zapping the plants and charring them, is a freezing spell. There is still a scientific debate going on as to whether the freezing spell lasted for a couple of months. We stretched it to a maximum of 10 years; that may be overdoing it. And there is other evidence for it: migration patterns of dinoflagellates, which are planktonic species that did not go extinct. And there is evidence in the chemical composition of the cell walls of certain bacteria, which are found in situ in the layers in Texas. The quality of the membranes, the number of cyclic components, is a measure of the ambient temperature they lived in, so that's supporting evidence for the cold spell.

And then, when that rains out, we are left with all the greenhouse gases, like methane, CO2, et cetera, staying behind in the atmosphere. So the third phase is a phase of warm climate. It's not that warm, I mean, not as extreme as the first spell, but it certainly makes the atmosphere 10 degrees centigrade warmer for a couple of thousand years.

Oliver Strimpel: And how do we explain the survival of the species that did survive, I guess the birds, and maybe some detritus feeders in the water?

Jan Smit: Exactly. Yeah. The detritus feeders have a feast, because they're not dependent on the primary food source. And one of the main kill mechanisms proposed, and I still think it's very valid, is the cessation of photosynthesis for a couple of months. Besides the cold spell, sunlight is being effectively blocked. So the whole food chain, starting from tiny algae dependent on photosynthesis, goes up the food chain ladder. And that's exactly what we see: the big species are dependent on smaller species, and they feed on algae, and that's exactly what the interruption of the food chain is.

Oliver Strimpel: I just can't help being struck by how amazing it is that from about 66 million years ago, you have a very thick layer, about two and a half meters, that was deposited all within the space of about an hour. So you've got enormous temporal resolution, literally of minutes, 66 million years ago.

Jan Smit: Yeah, that's amazing. But it's not unique. We have, in the deep sea now and in the past, avalanches from the fronts of deltas, which come down as an avalanche onto the deep sea floor, and those can be sand bodies meters in thickness, deposited in the same time frame, a couple of hours at most. So yes, we're lucky, but these sorts of event deposits are created in a very short amount of time, while the remaining sediments above and below accumulate very, very slowly.

Oliver Strimpel: So if this seismic-wave-induced river surge happened at Tanis, do we think it must have happened elsewhere along this river, and perhaps along other rivers all over the continent? In other words, can we expect to find many more such sites?

Jan Smit: I think so too, but it may require the right geometry of the water body. Just like with the Japan earthquake: in some fjords it did not, and in other fjords it did create a tsunami. So the morphology and the orientation of the seawater body is very important. So it can be that it just happened in one place, because there is a funneling effect of the estuary. It's just the fortuitous, lucky circumstances that Tanis was lucky. But along the Tanis river, we might expect to have more deposits like that, definitely. But so far we just have this one sole spot.

Oliver Strimpel: There have been at least five major extinctions during the history of the Earth, with perhaps the one at the end of the Permian, about 252 million years ago, being the largest one. Is there any evidence linking a possible asteroid impact to any of these other extinctions?

Jan Smit: Well, of course people have looked in that respect. The end-Devonian extinction, which is one of the big five extinctions: was it due to a major impact, making the sea non-transparent? All the late Devonian reefs, which we know from Australia to the Canadian Rockies to Europe, were killed at exactly the same time. One way you can do that is by blocking sunlight, by making the waters turbid, so the sunlight can't reach the corals and they go extinct. OK, that was a nice idea, but of course many people have looked in a good place in New York, upstate New York, and found nothing there.

We have looked at the Permian-Triassic boundary in the Italian Dolomites. There's a very nice clay layer there, which separates shallow-water limestones from the Permian from even shallower or slightly deeper limestones from the Triassic. There is a nice clay layer there, a couple of decimeters thick, but there's no iridium, there are no spherules, no shocked quartz or anything like that. So in that way it's disappointing. We have not been able to find definitive evidence for any contribution of a meteorite impact except at the Cretaceous-Tertiary boundary. You can say, yeah, if you want to find a common denominator for all the extinctions, and people have been looking for that, then it comes back again to volcanism as the culprit.

Oliver Strimpel: So what are you working on at the moment?

Jan Smit: I hope to write a book about the KT boundary, my personal view on it, and see how everything interrelates, whatever the resistance was. Of course, there was lots of criticism, which makes it a very lively debate. People think that it was gradual, and that they made a mistake, or I made a mistake. So that's a lively debate, and it is still going on. The volcanists are clearly convinced that the Deccan Traps ended it all, and they see their evidence in the light of the Deccan, and I say, no, you have the wrong view. We see it in the light of a major impact.

Oliver Strimpel: Jan Smit, thank you very much.

Jan Smit: It was a pleasure.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, you can go to geologybites.com.