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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. We're familiar with the fact that there have been mass extinction events during the Earth's history. The best known of these is the one that took place about 66 million years ago, when the dinosaurs were wiped out, making way for the mammals to prosper. As a result of much painstaking research, we're now fairly confident that we know what caused that extinction, as well as other earlier ones. But there is one major extinction, about 360 million years ago, at the end of the Devonian period, that we have not been able to explain.

John Marshall is a professor at the School of Ocean and Earth Science at the University of Southampton. He is a fossil expert specializing in mass extinction events. His recent research has uncovered new evidence that may finally explain what caused the end-Devonian mass extinction. John Marshall, welcome to Geology Bites.

John Marshall: Well, thank you for the invitation, and pleased to be here.

Oliver Strimpel: In the introduction, I said that we can fairly confidently explain most mass extinction events. So what caused such extinctions?

John Marshall: Well, the collective work of geologists and others over the last 30 years has revealed that we're quite clear about the Cretaceous–Tertiary boundary and the end of the dinosaurs being an asteroid, and obvious about the other five or six mass extinctions. Most of them are very large igneous eruptions on a continental scale.

Oliver Strimpel: So we have two major causes. One is the meteor impacts, and the other is the giant lava eruptions. Let's take them one at a time. When a meteor strikes, what exactly becomes lethal to life?

John Marshall: There's many different ways it's lethal. The first thing is you get a giant cloud of debris up into the atmosphere, high up, and they behave like meteorites in that they rain back to Earth again. And when they rain back to Earth, they melt and they warm up the atmosphere. So you get something like the heat, twice the temperature of a grill you might use in cooking. So if you're at the surface, you're dead. If you're large, you're dead.

Oliver Strimpel: What about in the water?

John Marshall: That's where the survivors are. So it's no surprise that very archaic animals like crocodiles come through, because you can hide in the water, breathe through your nostrils and so on. And probably the others are burrowing animals, or ones which could take temporary shelter.

Oliver Strimpel: Can we see any direct evidence of the specific impacts that caused these extinctions?

John Marshall: Oh, yes. Well, it took a while to find a crater at Chicxulub.

Oliver Strimpel: Which is a largely submerged crater about 93 miles in diameter, centered just off the coast of the Yucatán Peninsula in Mexico.

John Marshall: Yes. But before then, there was indirect evidence, which was in fact the first recognized evidence, which was the iridium spike globally. So you're finding a rather strange element, which you only find in high concentrations in things like extraterrestrial bodies, meteorites and so on.

Oliver Strimpel: And we have evidence for these meteor impacts, say iridium layers, for previous extinctions as well?

John Marshall: People have searched and searched, and no, it appears to be quite a singular thing, the mass extinction with the asteroid. That's unusual. There's been a gigantic chase across the geological column, with people looking at all the extinction levels, searching for iridium.

Oliver Strimpel: So the other cause, the lava eruptions. How does that wind up killing life?

John Marshall: It's very interesting, because you probably know of eruptions like Mount Saint Helens and Krakatoa, which make a huge bang, and they cause global cooling for one, two, three years or something like that. Whereas these are very passive eruptions. They're like Hawaii, in that you can go and stand next to them. But what they're doing is erupting over hundreds of years, persistently, every year, all the year, and they just produce a vast quantity of volatile matter up into the atmosphere: dust, and clouds of volatile sulfur dioxide and so on. And they're on the scale of a continent. So it would be like covering the entire United States land area with lava. It's that sort of size. So they upset the Earth system, they upset the atmosphere, they dim the Sun and so on. So initially, you get lots of particles in the atmosphere, you get global cooling, a sort of nuclear winter, and then once the eruptions have stopped and the ash clouds clear, you'll then get a high-CO2 atmosphere, and you'll get warming. So a lot of change. And it's probably something subtle in the rate of eruption. So some of these eruptions will be a bit slower than others, and the Earth system can cope with it, whereas others are probably too many eruptions too fast, and that's what causes the damage.

Oliver Strimpel: Can we tie specific lava eruptions that we can see around the planet to particular extinctions? In other words, can we see that they happened at the same time? And perhaps you could give us some examples.

John Marshall: We find them at the end of the Triassic, where we find, in fact, the lavas are right down the eastern side of the United States and then into North Africa. And what we find then is we can date the lava. We can date it with isotopes. But more significantly, I mean, you never find fossils in lavas. You find fossils in bedded sedimentary sequences. That's where you identify the extinction. And there's a new sort of way of signifying it, in that you can measure mercury. An eruption gives you mercury; it's about the only source of mercury of any quantity in the Earth. And so if you have a major eruption, then if you go into bedded sedimentary sequences through the mass extinction, you'll find a high, an anomaly in the amount of mercury in the rocks. So we can tie them very directly.

Oliver Strimpel: So is there a different large igneous province, so-called, that's associated with and thought to be the culprit for each of the major mass extinctions, except the one we're about to talk about?

John Marshall: Yes. You can find the mercury trace, and you can actually identify exactly the eruptive province of the area. But they're huge things; you can't hide them away. You can't hide a lava flow the size of, say, the United States. You can't bury it. You can't conceal it. It's there; it's evidence. I mean, you can date them, and then you can track it with the mercury. And we can certainly find a very good one for the end-Permian mass extinction, and a very good one for the Triassic–Jurassic mass extinction.

Oliver Strimpel: OK, so that sounds like a pretty watertight case. Now let's talk about the extinction that happened about 360 million years ago, at the end of the Devonian. Why is that different?

John Marshall: It's somewhat unusual in that it was two extinctions, separated by something like 300,000 years. And the lower extinction mostly happens in the sea, and it's marine creatures which become extinct, many groups, things like trilobites, corals and so on. And then there's this later extinction, 300,000 years later, and that's mostly terrestrial. There are some marine extinctions, but what you lose are effectively the forests. Forests collapse. You lose the big forest trees, you lose the understory, the lower layer of plants in the forest. And we also lose one entire group of fish. The fish at that time, the biggest group and the dominant group, were the armored fish, and some of these get up to huge size, and they're covered in plates, so their name is quite apt, and they all become extinct quite quickly. And after that, the main fish groups which we're left with are the sharks and the bony fish. The bony fish are things like herrings, mackerel, plaice and so on, tuna, which you're familiar with.

Oliver Strimpel: So as I understand it, the earlier of the two extinctions is associated with extreme cooling at the start of an ice age, just before the end of the Devonian period, and you were studying the period corresponding to the second extinction 300,000 years later by looking at sedimentary rocks that were laid down at that time. Tell us about that.

John Marshall: We identified the Devonian–Carboniferous boundary layer in East Greenland, and it was the first known terrestrial section, which was totally terrestrial, which wasn't on the margin of the sea, where we had the mass extinction layer, and we had mostly a good record through it. So it was obviously something which was very worth studying, which would help us very much tie down what the cause of the mass extinction might be. So the first thing to do, of course, is to record the data of the organisms which become extinct through the extinction level. And I study fossil pollen grains, fossil spores. So that's a way of looking at what's happening to land plants. It's quite hard to find a big tree. You don't find them very often, and they're not complete. But you can find millions and millions of little reproductive bits, and these are all quite exotic little objects, and you can identify them and give them names and so on. So that was the way of looking at the mass extinction, and we could count the groups as they disappeared.

Oliver Strimpel: So you were looking at spores at the end of the Devonian, around the time that the extinction happened. What did you see there?

John Marshall: Well, it was quite startling, really, because I expected it to just be a sort of routine counting, the percentages, proportions, and putting the names on them and so on. And I looked down a microscope, and I can remember it now, and I thought, "What's that? I've never seen anything looking like that before." And when you move the microscope stage around, you see another one and another one. I mean, you realize what they are: they're spores which you normally know, but they're all malformed. And they're malformed by not growing properly. In other words, their ornament and sculpture on the surface (because often they're covered in little spines and things) is all sorts of the wrong sorts of shapes, and hasn't formed properly, is all twisted and so on.

Oliver Strimpel: So you see higher-than-normal mutation?

John Marshall: Absolutely. Normally, you wouldn't see malformation. People have looked at it in modern pollen, and it's about 3%. So you find an odd one every now and again. It's like finding two grapes stuck together, or two strawberries stuck together. Maybe you've seen it in an orange: you can get an orange on the back of an orange. And so it is rare. And I was counting, and it was up to 60, 70% were malformed in different ways, and these would have been sterile. They wouldn't have been able to reproduce or function as little reproductive objects. And the other interesting thing was the color changes. So pollen is yellow, and it's yellow because there's a protective material in the wall to give it protection from ultraviolet light. And a lot of these were brown and orange, and so the amount of protection had increased, with a thickening in the material.

Oliver Strimpel: Hmm. So that's like the spore getting a kind of suntan. Why do we think there was a higher mutation rate and this color change?

John Marshall: Well, it's direct evidence. Fossil pollen and fossil spores are the original organic matter, so they're not turned to stone or fossilized; it's just that little bit of organic carbon which is preserved. It's a very tough little polymer, so they come down to us sort of intact, with the same shape and form. And people have done experiments with living plants, because they're interested in UV, and so they'll plant crop plants under a sunlamp, irradiate them with measured dosages of UV light, and they start to malform. What you can see is it happens within that particular few months of that plant growing. So if you turn on the light, the things will not form properly. And it's because the ultraviolet light is getting at the DNA of the cells of the reproductive material, and it's forming defects in that DNA. So when it tries to make its little wall, it can't make it properly.

Oliver Strimpel: So what kind of mechanism could increase the amount of UV that reaches the ground and mutates these spores?

John Marshall: This was interesting, and we had to look through the literature. And the one thing we really knew was that this maximum malformation was in the middle of a lake bed, and this lake bed represented a time when it was warmer, and there was more heat in the climate system, and the monsoon was much stronger. And so this coincidence of maximum malformation in the middle of a lake suggested it was maximum malformation with the highest temperatures. And there's experiments and modeling done today, where people have looked at the mid-continent of the United States, and what you can show is that as you get very hot summers, there's an increased transport of water vapor, in storms and so on, into the upper parts of the atmosphere. And that water vapor carries with it naturally occurring compounds which will destroy the ozone layer. There's various chemicals produced by plants, fungi and so on, algae, which are things like methyl chloride and methyl bromide. And those will get into that upper atmosphere, and then they start to get the energy from the Sun, and they will dissociate, and it will give you these little free radicals, chlorine and bromine, and they will zip around then, knocking the third oxygen off the ozone molecules and turning it back to normal oxygen.

Oliver Strimpel: And since the ozone layer is the shield that protects the Earth's surface from the Sun's ultraviolet, damage to that layer would lead to an increase in the amount of UV reaching the ground. Is there quite a bit of evidence, then, that we had a swing to very high temperatures at that time?

John Marshall: Yes, we've got, obviously, a direct climatic record, and so that's the value of working on terrestrial sediments. If you work in marine sections, it's harder to know, and it's harder to measure past temperatures, using particular carbon isotopes. But we can read it in the way the sediments react. And so we see a lake at this time, effectively like no other in our sedimentary sequences. So in our basin, the lake's probably about the size of Belgium, and it's probably well over 100 meters deep. So it's an enormous volume of water, and it's as if Death Valley suddenly started to flood with water because it was raining every year. Suddenly it got seasonal rains, and that rainfall was enough to give it a flow to the sea and maintain a standing body of water which never dried out. And so it really is quite unusual.

But it's a pair of climatic events, because this other extinction 300,000 years earlier is more or less coincident with the last of the Devonian glaciations. And so that's where you get a glaciation reaching quite far south, effectively. So the analogy now would be that the ice got down to southern Tennessee, that sort of place. So the globe was generally starting to frost over. And so you had a maximum cooling, and then you have a maximum warming. These are often related, because they're driven by natural climate cycles in the Earth. And when you get the climate cycles coinciding, you can get one of quite strong amplitude one way, which is then succeeded by a strong amplitude the other way.

Oliver Strimpel: OK, so just to recap: we have malformed spores of land plants that washed into a huge lake, and the presence of this large body of water contrasted with what preceded it in the geological record and indicates that these malformations took place at a time of high global temperatures.

John Marshall: So the two true facts are that we see malformation, and that can only be UV-B. And the other true fact is that it's a time of quite significant warming. And so we can then put those two together. We've looked for mercury through the sections, and there was no mercury, and other people have systematically searched for mercury through different sections globally and not found any significant anomaly. So we don't find any evidence for any large igneous eruptions coincident. We also know it's not a meteorite impact, because there's no iridium. So we can rule that one out. So then we're looking for a mechanism which basically ties together warmth and the true fact of UV-B: the ozone shield is thinned sufficiently to allow damage to come in.

Oliver Strimpel: So the usual suspects can't be invoked here. So what do we think might be the cause of such an unusually large swing in temperature of the planet?

John Marshall: It may be that the Earth behaves a little bit differently, in that during the Devonian, for probably about 50 million years before, you've been developing forests. And so you start off with plants on land, coming into the Devonian, that are basically the size of pins, and as you then go through, you start to get proper forest trees, which are 40 meters in height. So it's this locking up of a lot of carbon. It probably very much changes the way the world works. And you're also changing it below ground, because if you put a big tree above ground, you've got to root it. And so this rooting then changes that below-ground environment and opens it up to all sorts of fungi, microbes, little animals and so on, and they all conspire to break up the rock to make it what you might call a soil, which is an organic sort of factory. And what they're then doing is releasing for the first time large amounts of phosphorus, which is obviously this crucial nutrient element, and you're also releasing lots of calcium and magnesium, and all of this will get flushed to the sea. So you're probably starting to slowly destabilize the Earth system, such that its ability to be perturbed is much greater. In other words, it's easier to flip into a cooler state and a warmer state. So there's something in those feedbacks we don't understand.

Oliver Strimpel: Hmm, it's complicated. So the Earth was already susceptible to the impact of climate change as a result of other processes that were already happening.

John Marshall: Yes. There was a background of other processes which probably made it easier to perturb. And we've got the orbital cycles, which are inherent in the Earth today, or inherent back then, of precession, obliquity and eccentricity. So eccentricity is: the Earth doesn't have a circular orbit; it has an elliptical orbit, but how much it's squashed off the circle basically changes, with a 100,000- and a 400,000-year rhythm to it. And you just basically need to get these, about 20,000, about 40,000 and about 100,000-year cycles, to line up in a certain way, and then you can start to tip it to very warm and very cool.

Oliver Strimpel: You explained how the eccentricity of the Earth's orbit will have a forcing effect. What about the obliquity and the precession? What's the mechanism for that?

John Marshall: Yes. It's like a little spinning top. When we spin a top, it starts to wobble when it slows down, and it's wobbling like that as it goes round, and the obliquity, the angle of that, also changes. So it wobbles and moves. And so these have different sorts of instabilities, but they're what you might call predictable instabilities, and they've got periodicities, 20,000 years, 40,000 years. And so if you can get all these things to line up, you can get them all acting one way and then acting back the other way.

Oliver Strimpel: So the actual climate change that took place, which may have been more extreme than usual, from that severe ice age that took the ice caps all the way down to the latitude of Tennessee, followed by a warm period, that would have been triggered perhaps by several different astronomical processes coming into confluence by chance at that time?

John Marshall: Yes. If you align them, and you align them to go cold, then when it comes back, because it's a symmetrical curve, when it goes back the other way, you'll align them to be warm. So they just come into alignment for that short period of time, and then they'll be counteracting each other for another geological period of time, and then they'll come in again. So it's probably a regularity at the wrong time, when you have systems more likely to tip.

Oliver Strimpel: Are there any alternative theories as to what caused the end-Devonian extinction?

John Marshall: Yes, there have been any number. It's like the dinosaurs: the list is as long as your arm. But the astronomers have always had this idea, for as long as I've been a professional geologist, and that is that it was to do with a supernova, basically an exploding star, and that would give you a cosmic ray burst, and that would then purge life on Earth. And that would have exactly the same effect: it would strip off the ozone layer, and the UV for a short time would be lethal to life on Earth. I'm really quite happy that we're having this debate, because we've got the ability to test it, because they've predicted which chemical elements, which radioactive elements, would be produced by the star as it explodes, and that dust cloud would eventually reach the Earth, and you would find things like plutonium-244, which would be actually out on the surface and causing all the trouble, and that's measurable. And so it's an element which you only find manufactured in an exploding star, so therefore it would be its own unique signature. So that would be a way of testing it. So we've now got a section where we've got an extinction which isn't the usual suspects. I've suggested a hypothesis, they've suggested a hypothesis back, and we can now actually try and measure these elements and prove it one way or the other. So that's how science works.

Oliver Strimpel: So the big picture here is that you're proposing a third cause of mass extinctions, or perhaps the fourth cause if we include the supernova explosion argument, in which high global temperatures may have been sufficient to cause the destruction of the ozone layer, which in turn led to a mass extinction. Perhaps this has implications for what we can expect in the near future as a result of present-day anthropogenic global warming.

John Marshall: Well, exactly. What we're not having today is the same world. Our world is different to the Devonian world. But what we're going to face is a very rapid warming event. So we have to be very watchful that we're not going to get the same mechanism in play, with high temperatures, and the mechanism by which the ozone layer is thinned and destroyed. You can call it, if you wish, a warning from deep time for our present world.

Oliver Strimpel: John Marshall, thank you very much.

John Marshall: Thank you.

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