This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The history of complex life on Earth has been punctuated by mass extinctions. Five of these extinctions, the so-called Big Five, stand out as being particularly lethal: one at the end of the Ordovician, one in the Late Devonian, and those at the end of the Permian, Triassic and Cretaceous. But the sedimentary record also tells us that there have been a total of 31 significant disruptions to the Earth's environment since the beginning of the Phanerozoic, 542 million years ago. So why did only five of them lead to catastrophic mass extinction? And can we compare the disruptions to the environment that we see in the geological record with the one being caused now by human activities? Dan Rothman has tackled these questions by studying disruptions to the Earth's carbon cycle, as revealed by high-resolution measurements of carbon isotope ratios within carbonate rocks. His results indicate that the Earth system has a critical rate of change. Over long timescales, disruptions occur when the rate of change in the amount of carbon injected into the Earth system exceeds this rate. And he also hypothesizes that at short timescales, disruptions occur if the amount of carbon injected exceeds a critical size. Dan Rothman is Professor of Geophysics at the Massachusetts Institute of Technology. Dan Rothman, welcome to Geology Bites.

Dan Rothman: Thank you. Delighted to be here.

Oliver Strimpel: As I said in my introduction, you focused on the carbon cycle as an indicator of disruption to the Earth system. What exactly do you mean by the Earth system here? And can you remind us how the carbon cycle works?

Dan Rothman: I think of the Earth system as a combination of the physical and chemical environment with life, which you might think of as the biological environment. So Earth system dynamics, which is what this work addresses, has to do with the interaction of life and the physical/chemical environment. Your second question was about the carbon cycle. The carbon cycle, to a significant extent, is where Earth system dynamics happen. The carbon cycle is kind of the locus of the interaction of life and the environment. What it is, precisely: plants take up CO2 out of the atmosphere, and also out of the oceans, and consuming organisms like us eat either the products of photosynthesis or the animals that lived off of photosynthesis. In doing so, we return the original organic plant matter, the matter that was fixed, back to the atmosphere and oceans. So CO2 goes to organic carbon via photosynthesis, and consuming organisms return it back. The process of consumption is typically referred to as respiration.

Oliver Strimpel: So if we look at the carbon cycle, does that enable us to infer what the physical environment was at the time, things like the temperature and the composition of the atmosphere?

Dan Rothman: It allows us to identify when disruptions to the environment occurred. One can think of the carbon cycle as a loop between photosynthesis and respiration. Typically, we're given the impression that the carbon cycle is in balance; that is, nature is thought to be in balance. However, as is well known, at present, because of fossil fuel burning, we're stressing it out of balance. But there's a very important question, which asks whether the carbon cycle has, in some sense, been unbalanced in the past. And if one looks at the geologic record, the answer is pretty much unequivocally yes, it has. And then the question is, why, and how do we know this? We know this because of chemical traces that are stored in sedimentary rocks, in the form of the isotopic composition of carbonate minerals and also of organic matter. These isotopic compositions change, and sometimes they change abruptly. And, most curiously, they change abruptly every time there is a mass extinction.

Oliver Strimpel: Are the rocks that contain these records of inorganic origin or of organic origin? And where does one find them?

Dan Rothman: They're formed from carbonate, which in some cases is simply the precipitation of carbonate minerals at the seafloor, and in other cases is the remains of calcifying organisms. And that's a more recent case; when I say more recent here, I mean since the Triassic, when calcifying photosynthesizers existed. The data I use are from bulk carbonate. They're not focused on any particular species. I did that to maintain a uniformity in the record, and I've looked at data that come from a wide range of sources: outcrops for the oldest data, and deep-sea drill cores for the much more recent data.

The focus is not so much on the long-term evolution of the carbon cycle, but rather on its disruptions. In a sense, what I've done is a little bit akin to a seismologist who studies earthquakes only, but not the long-term tectonic changes associated with them. Just the burp, you know, the blast, the disruption, as I call it, to the cycle itself. And there's a distribution of sizes of disruptions and of the rates at which disruptions occur. So I made a catalog of those.

Oliver Strimpel: OK, so you mentioned isotopes. What exactly is measured in these carbonate rocks that enables you to track the disruptions in the carbon cycle?

Dan Rothman: What geochemists do is measure the ratio of the heavy stable isotope of carbon, the abundance of it, carbon-13, to the lighter stable isotope, which is carbon-12. Carbon-12 is 99% of the world's carbon; carbon-13 is nearly the remaining 1%. Carbon-14, which everybody likes to think of as the isotope of carbon, is actually very minor, and it decays with a half-life of several thousand years, so it's not in the picture. What we're looking at is ratios of stable isotopes. The point of these measurements is that these ratios change with time. And the reason they change is that photosynthesis, being a fast kinetic reaction, favors the lighter isotope of carbon. So the organic matter is isotopically lighter than the CO2 from which it comes, by about 28 parts per thousand. And when carbon is buried in sediments, if or when there is an imbalance in the carbon cycle, which is reflected in the mix of burial, then these isotopic ratios change. Most of the time, they're just changing modestly and slowly, but sometimes they change more abruptly and significantly.

Oliver Strimpel: So, as you said a moment ago, you focus on those periods when the change was abrupt. So I guess it's a spike in the ratio of carbon-13 to carbon-12. Is that right?

Dan Rothman: It's a spike when you look at it from across the room, so to speak, from a thousand miles high. It may play out over tens of thousands or hundreds of thousands of years, but it's a spike in geologic time.

Oliver Strimpel: Yes, that's a nice way of putting it. But do the geological records that you're analyzing permit you to resolve detail within each spike, or is it just like one event?

Dan Rothman: The two parameters that I focused on tend to be the best resolvable. Those two parameters are essentially the size of the disruption, which is basically how much the ratio of carbon-13 to carbon-12 changes, and the timescale over which it changes. That is the timescale over which the carbon cycle is stressed out of equilibrium: how long does it take to ramp up to its apogee, so to speak, when the disruption is at its peak?

Oliver Strimpel: OK. So the size of that peak that you were able to measure, that is, the size of the change in the isotope ratio, you're connecting that to the mass, say, of the amount of carbon that would have been injected into the system at that time?

Dan Rothman: Exactly. The underlying hypothesis, which is very, very reasonable, that motivates this is that when there's such a change in the carbon cycle, it represents an imbalance between photosynthesis and respiration. And this imbalance has its own characteristic isotopic signature. From the known isotopic change due to the imbalance, we calculate how much mass was, so to speak, reconfigured or remobilized within the carbon cycle during the course of the event. That's a straightforward geochemical calculation.

Oliver Strimpel: OK. So before we talk about how these events look when you view them over the whole of Phanerozoic time, what kind of physical mechanism would underlie the injection of enough carbon to create a spike in your isotope ratios?

Dan Rothman: Well, any of a great many. A popular one is anomalous, especially intensive volcanic activity, massive volcanism. Volcanic CO2 is isotopically light compared to our atmospheric CO2, and so if you see the carbon becoming significantly lighter, it's often hypothesized that volcanism may be the culprit. Another very, very popular hypothesis is the injection of methane into the atmosphere, methane coming from methane clathrates in the ocean.

Oliver Strimpel: Did you say methane clathrates?

Dan Rothman: It's a solid form of methane, buried beneath the seafloor. When temperatures rise, it can be remobilized within the carbon cycle. This methane is typically of biogenic origin, and it's buried, but it has a way of coming back. That's an attractive hypothesis, because methane is very isotopically light compared to volcanic CO2, so you don't need as much of it to account for the data. Another very popular way of interpreting these things is by changing the relative fluxes in the burial of organic carbon, which is isotopically light, and the burial of carbonate carbon, which is isotopically heavy compared to the average. If you change those fluxes to sediment, the so-called burial fluxes, then the isotopic ratios change. Some of these mechanisms I'm referring to can be thought of as operating within the carbon cycle, such as methane clathrates and burial. Volcanism, on the other hand, is outside the cycle. So those are a set of possible external and intrinsic stressors. And insofar as the intrinsic ones go, I tend to think of them simply as imbalances between respiration and production.

Oliver Strimpel: You have the planet-wide ratio of carbon-12 to carbon-13, if you like. Then you said the volcanic ratio is a bit lighter than that, with more carbon-12, and life even more so. Can you tease apart the various contributions?

Dan Rothman: Yeah. You can identify various sources of light and heavy inputs, but if there are n sources of light inputs and m sources of heavy inputs, and n and m are large numbers, then how can you know which ones did it? More particularly, the isotopic composition of volcanic CO2 is lighter than atmospheric CO2, but it represents the planetary average, because the carbon is deeply buried in the Earth and comes out as CO2, and it represents the isotopic composition of carbon in the mantle. But what happens is that because the carbon cycle, photosynthesis in particular, favors the light isotope, it creates fractionation, the difference in isotopic composition between light organic carbon and heavy inorganic carbon.

Oliver Strimpel: So what did you see when you plotted these events on a diagram spanning the Phanerozoic?

Dan Rothman: The most interesting plot takes all of these events and asks how they appear in a plot of the size of the event versus the timescale at which they occur, timescale being distinct from time, that is, from the date at which they occur. Did it occur over a million years, over 100,000, over 10,000 years? When you make such a plot, something interesting is already evident. Basically, you see the data all fall in one half of the plot, and the other half of the plot is empty, and the division of the plot is a diagonal line. And the diagonal line, you can think of, is the rate of change of the isotope ratios as a function of the timescale at which they occur. So it's a kind of flux, so to speak, an isotopic flux. And what that says, and this is without any kind of interpretation, it's just, let's take the data and put it on this plot, is that there's kind of a speed limit: stuff can't happen that fast. And the reason the speed limit is there is because you don't see enormous events in small amounts of time. What you see instead is small events in small amounts of time; the big events take longer to occur. So I thought that was interesting.

And so what I did was try to do something further, which is to transform this plot into physical axes, so that people like us can discuss it and everybody will understand what's there. Now the two axes in this plot: one is still timescale, but the other is mass instead of isotope change.

Oliver Strimpel: So what is the mass here?

Dan Rothman: The mass is the amount of carbon that was taking part in this isotopic change. So you integrate the flux, the influx of carbon, which I think of as a respiration flux. Integrate that respiration flux over the timescale and you get a mass. And when you make that plot, there's something which in physics we would call data collapse. It's actually quite remarkable: roughly half the data end up falling on a line. What is that line? That's really interesting. I call that line a characteristic rate, because it's the rate of a constant ratio of mass to time, or a characteristic flux. So there's this characteristic flux in the carbon cycle. That's one interesting thing.

The second interesting thing is that four of the five mass extinction events, and basically not much else, have fluxes greater than the characteristic flux. So they're associated with changes in the carbon cycle that occur faster than the characteristic rate, and the extinction event at the end of the Permian has by far the fastest rate; it stands out as an extreme outlier. And on the other side of the line are all the slow events that only specialists have ever heard of. They represent small changes occurring at long timescales; to be more precise, they represent low-flux events. I spent quite a bit of time trying to figure out what this division of the space means.

Oliver Strimpel: OK, so let's talk about that. All these events that you analyzed during the Phanerozoic, and I believe there were 31 of them, most of them actually fell near or on this straight line, which represents a critical rate of change, or a critical carbon flux.

Dan Rothman: The criticality of that line is an interpretation, and I'll tell you how I came to that interpretation. As a matter of pure phenomenology, there's a characteristic flux, or characteristic rate of change. What could that mean? Well, we talked a little bit earlier about all the ways in which the isotopic composition of the carbon cycle could change, and that translates to all the ways in which the carbon cycle could end up being disrupted. We listed a bunch: burial changes, methane burps, volcanoes becoming anomalously active, and there's a host of others. If all these things were going on, some at one time and another at another time, why would there be a characteristic rate? Well, I mean, there could be, but it doesn't really make sense that so many diverse forcings would yield the same characteristic rate.

So the alternative hypothesis is that it's the carbon cycle that is changing, and it's not really something outside the carbon cycle that's making the change. There may be something outside the carbon cycle that instigates the change, but then the carbon cycle carries on on its own. And that's a classic way, in nonlinear physics or nonlinear dynamics, for systems to behave. You stress them a little bit, and then they become, in one manner or another, unstable, and the instability has a characteristic signature. Maybe the simplest of the results of such instabilities are nonlinear oscillations, in which a system goes from being inactive to oscillating, as you often see when things start to vibrate because of the various stresses that have been placed on them. Those vibrations have a typical amplitude and a typical frequency. The hypothesis I eventually came to is that what we're seeing in this characteristic rate is a kind of shadow of that characteristic oscillation, but it's only half a cycle of the oscillation: it goes up, and it comes down, and it comes back to where it was before.

And so the question is, how could that happen? In the theory of dynamical systems, this is a well-known scenario, and such systems are called excitable. From a dynamical point of view, they're actually always stable, but if you stress them sufficiently, but not that much, they act as if they're going unstable, and they do that only for the equivalent of one cycle. Then they return to the stable equilibrium. Excitable systems were first studied in neuroscience, because this is how neurons work; sometimes it's called an action potential. You inject a small current into a neuron, and if it goes above a threshold, it fires a pulse. This pulse is called a spike. This is how the brain works, for example.

Oliver Strimpel: That's fascinating. So there could be this dynamical system that maybe underlies the carbon cycle: you stress it, and it gets excited and does one excursion, which is then one of the things you pick up with your spike in the isotope ratio, and then it goes back more or less to where it was before?

Dan Rothman: Right. It looks just like an excitation. And so this is an example of what people these days refer to as a tipping phenomenon, or a tipping point. This is a particularly interesting kind of tipping point, in which we're not crossing a change-of-state boundary, as in phase transitions. Instead, what we're doing is changing a parameter in such a way that the system acts as if it's crossing the boundary, but then it comes back to where it was.

Oliver Strimpel: And of all these events that you looked at, you said four of the Big Five extinctions are well above this line, which you interpret as a critical rate. So is there something different there that pushes them even beyond what the carbon cycle would go to?

Dan Rothman: The idea is that most of them represent some type of modest stress, which is expressed by the internal dynamics of the carbon cycle. That's a hypothesis that explains the characteristic rate. Now, why are the extinctions above it? I would qualify your statement a little bit and say that one is unequivocally above the characteristic rate; that's the end-Permian extinction. Three of them are probably above it. And what makes them special? This is a great question. I'd say that they represent excitations that have been stressed well beyond the threshold. The excitation parameter is the rate of change of CO2. If we put CO2 into this system sufficiently fast, then the carbon cycle fires a kind of acidification pulse, which is a change in the CO2 content of the oceans. If we do that in a way that is well beyond threshold, or if the carbon cycle has some special dynamics that take it into another region, then the result can be something that's effectively greater than, or worse than, the typical spiking event. It's a kind of gross instability, as opposed to the generic one.

I don't really have a new theory of mass extinction. Instead, what I am suggesting is that the carbon cycle acts in association with mass extinctions. I can't show that it causes the mass extinctions. I can't show that mass extinction caused the changes in the carbon cycle. Instead, what I can clearly point to is that there's an association, and that this association is particularly pronounced in the cases of the worst mass extinctions, four of the Big Five. The curiosity is that there's a whole bunch of other events, the ones I've studied, which you might call paleoclimate events, but not times of great biotic turnover. And they are significant disruptions by themselves. A very interesting question is what makes them different. Why aren't they mass-extinction grade? Well, in this study, I basically undertook to try to address that question, and I can separate them in this space. I can answer your question by saying that for extinctions, or worse, the excitation is pushed well beyond the threshold. But there are a lot of questions that remain unanswered.

Oliver Strimpel: Just to come back to the underlying mechanisms again. When you have one of these events that puts you on this excursion, if you like, to the spike and then back again, do we have hypotheses as to what the underlying chemistry, or biochemistry, is that first allows the excursion at the characteristic rate and then brings it back down to where it was?

Dan Rothman: That's a good question. Any system that undergoes some type of instability can't stay unstable forever. Essentially, you might say, it peters out. Why does it do that? It's because there is always some mechanism of dissipation, something that slows it down. And insofar as carbon cycle disruptions go, there's a well-studied mechanism that slows down and dampens out any kind of acidification spike, or sudden injection of CO2 into the oceans. That mechanism is sometimes referred to as carbonate compensation, or sometimes as the homeostatic response of the ocean's carbonate system. At least since the Triassic, there have been lots of shells littering the seafloor, which contain carbonate minerals. These carbonate minerals, when they are dissolved by the acidic waters, act to raise the pH. It's sometimes said that the carbonate minerals at the bottom of the sea are sort of like the ocean's antacid pills, the ones you might take when you have an acidic stomach. They act to neutralize the pH because they're being dissolved, and that is thought to play out today on a roughly 10,000-year timescale. That timescale was probably longer in the past.

Oliver Strimpel: And would that be because of the evolution of these calcareous microorganisms?

Dan Rothman: Precisely. The invention of calcifying photosynthesizers created what is sometimes referred to as a deep-sea carbonate sink, which acts as a stabilizing mechanism in the carbon cycle. The more carbonate shells there are on the seafloor, the easier it is to dissolve these antacid pills on the bottom of the sea, the easier it is to neutralize the event, and the shorter the timescale. It's a very interesting way in which nature maintains, or tries to maintain, equilibrium in the carbon cycle. The final element of the story is that there's always dissolved calcium carbonate coming into the oceans via rivers, and it's going out by burial in rocks, and simply by flushing out the system at long timescales, you also tend to dampen it out.

Oliver Strimpel: So you talked about the homeostatic timescale being about 10,000 years for the modern ocean. So that's the timescale, if you like, that it takes for these nonequilibrium processes to peak and damp out?

Dan Rothman: Yes, because, to a first approximation, the damping timescale is also the timescale it takes to reach peak disturbance.

Oliver Strimpel: So now I'd like to see if your way of approaching this problem enables us to connect the events that you see in the geological record, for which you know the size and the duration, to events that disrupt the carbon cycle on a much shorter timescale.

Dan Rothman: We have these events in geologic history that occur at long timescales, tens of thousands, hundreds of thousands, sometimes millions of years, and we have an event today which is basically occurring at a century timescale: fossil fuel burning, the injection of CO2 into the atmosphere. And so it's reasonable to think that one has nothing to do with the other, but that can't really be true. There are things you can learn from how the Earth system, the carbon cycle, behaved in the past. So what I tried to do here is show what can be done in a quantitative way. What we see in the data is this characteristic rate, and this characteristic rate provides a very interesting quantity. It says that at long timescales, if carbon is injected into the system faster than the characteristic rate, then we see serious disruption events. At the same time, as you already indicated in your question, there's also a damping mechanism. So there's a damping mechanism and a characteristic rate, and the characteristic rate times the damping timescale gives you a mass. Or the critical rate, I should say, because we take the characteristic rate as an upper bound on the critical rate. It gives you a critical mass.

Now, why is that? For that, I mostly appeal to an observation. This podcast has been going on for, I'm not quite sure what it will be after you edit it, but on the order of 30 minutes. Most people are probably listening to it inside, but outside, let's say, the CO2 levels went up 1 or 2 ppm during the podcast. There's not a single organism on the planet that will notice, all right? But its rate of change will have been effectively infinite, or at least very, very large: an impulse, or delta function. So you see right away that this idea that rates matter is kind of limited in its applicability, because here we have an infinite rate and nothing's going on. So what that means is that at short timescales, it's the amount by which the system changes, but at long timescales, it's the rate at which it changes. And the way you can get the critical mass from the critical rate is to multiply the critical rate by the damping timescale. The product of those two, because of the type of relaxation mechanisms we talked about, will give you the critical mass.

And that then gives rise to a prediction. We can calculate this critical mass. It's basically the mass that you would inject into the carbon cycle over the typical damping timescale, which in this case today is 10,000 years. We know that the critical rate is a few hundredths of a gigaton of carbon per year. You multiply it by 10,000 years, and you get a number that's on the order of three or four hundred gigatons of carbon into the oceans. And I believe we've put in about 200 gigatons. If you look at projections from things like the IPCC report, what you can see is that we will have put three or four hundred gigatons of carbon into the system probably sometime before the end of this century, three or four hundred gigatons into the surface ocean, if we stay on the present path. And so that suggests that if it gets over a certain amount, then an instability might begin. That instability is not instantaneous. Insofar as my own work goes, it would be predicted to play out over a timescale which is equivalent to the damping timescale, which is 10,000 years. So it's not something that we necessarily need to worry about at the moment. What this is saying is that there's a very logical, rational way of looking at past carbon cycle events, and from them being able to infer how much CO2 is too much. And insofar as the future of life on Earth is concerned, a result of this analysis is that anything that we can do to slow the injection of CO2 into the atmosphere may pay off at a very long timescale.

Oliver Strimpel: It's really intriguing how you managed to connect a short impulse, if you like, that's driven by anthropogenic fossil fuel burning with the geological events that we see in the record. So you mentioned a critical mass of something on the order of 300 gigatons. Are you able to figure out how much carbon was injected into the system, say, at the end of the Cretaceous, when the dinosaurs died out?

Dan Rothman: The best work in that regard has been done by some geochronologists at Princeton, who have carefully dated the magmatic deposits coming from massive volcanism in the Deccan Traps in India, just before the end-Cretaceous event. Now, most people listening to this will know that the end-Cretaceous event is the event that was the demise of the dinosaurs, and everyone is taught that it's a result of an impact with an extraterrestrial object, a bolide or meteor impact. However, if you dig into it more deeply, it's also now well known that there was massive volcanism occurring at the same time, and the best geochronology shows unequivocally that the massive volcanism was going on during this period. Moreover, it's not uniform in time: there's a significant pulse 10,000 years before the end-Cretaceous event. Now, no one's denying that the impact itself had some effect. But now you see that there's a pulse of volcanic CO2 going into the system 10,000 years before. I thought this 10,000-year number was kind of interesting, because that's the time it takes for the types of excitations I'm talking about to reach their worst, or most severe, limit. And so what you can do is put this data point on the type of plot I'm talking about, and on this plot we have the timescale of the event and the size of the event. What such a plot shows is that both the magmatic flux pulse, which produced CO2 just before the end-Cretaceous event, and the current injection of CO2 into the atmosphere basically exist near the critical rate.

Now, I should qualify that, because the critical rate is different in the case of the Cretaceous event and the modern disruption. I spoke earlier of a critical mass, but if we divide that critical mass by the timescale over which it's injected into the oceans, we get another rate. It turns out that because there is a critical mass, the shorter the timescale of the injection, the larger the critical rate at short timescales. So there's basically what we call a scaling law that one can derive from this. It says that at short timescales, there's not only a critical mass, but the critical mass can be reinterpreted as a critical rate that depends on the timescale. The critical rate today is 100 times greater than the critical rate in the Cretaceous, and that's because this Cretaceous event I'm talking about plays out over 10,000 years, whereas today's event is occurring over a hundred years. There's two orders of magnitude difference in the rate. And it's interesting to note that, yes, of course, the current event has a rate which is 100 times faster, and that's alarming by itself, but it turns out that if you analyze it carefully, they both fall on our rescaled critical rate line. In other words, insofar as the mass extinction risk is concerned, both the modern event and the end-Cretaceous event appear to be near the critical rate, even though the modern event has a rate which is 100 times faster.

Oliver Strimpel: So that's a pretty alarming conclusion. That basically says that whatever created the mass extinction at the end of the Cretaceous was caused, in some way that you've now shown, by an equivalent amount of disruption to the carbon cycle, in terms of its destructive potential. How solid is the reasoning that led you to this conclusion?

Dan Rothman: I think the reasoning is solid within the data that we have. But the most important thing that's missing is an understanding of the negative feedbacks that might occur, which are largely due to changes in ecology, or ocean ecology, or ocean biology for that matter, over the course of the 10,000 years. In the modern case, if we disrupt it beyond the threshold, what's going to happen? It may want to go off onto a trajectory that would be unstable, if the only thing governing the dynamics were the long timescales, that is, the thousand-year timescale controls on the dynamics. However, the Earth system varies at a great many timescales, and it may be that there are negative feedbacks that occur at century timescales, decadal timescales and so on, that act to damp this out. We don't know. But the conclusion I'm drawing is absolutely solid on physical grounds, given the available data. It's basically an attempt to say: we have these observations; what can we conclude from them, using elementary physical intuition and an understanding of what isotopic changes mean?

Oliver Strimpel: So what further work or observations would you like to see, to test these ideas further and constrain the predictions a little bit more?

Dan Rothman: Any kind of data that allows us to see stimuli distinct from responses would help. And that's a difficult problem, because in geology, we usually have one window through which to look at things, and it contains both the record of the stimulus and the response. So that would be something that would maybe take us beyond the carbon cycle, to look at other isotopes, other isotopic changes, and biomarkers, organic changes, the whole nine yards, so to speak, the whole periodic table, and fossil changes. It's basically what geochemists and paleontologists already do, but focused on the events themselves, these periods of disruption.

Oliver Strimpel: Dan Rothman, thank you very much.

Dan Rothman: Thank you, Oliver. It's been a pleasure.

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