- Welcome, everyone, to Wednesday Nite @ the Lab. I'm Tom Zinnen. I work here at the UW-Madison Biotechnology Center. I also work for UW-Extension Cooperative Extension, and on behalf of those folks and our other co-organizers, Wisconsin Public Television, the Wisconsin Alumni Association, and the UW-Madison Science Alliance, thanks again for coming to Wednesday Nite @ the Lab. We do this every Wednesday night, 50 times a year. Tonight it's my pleasure to introduce to you Shanan Peters. He's a professor here in the Department of Geosciences. He was born in Lancaster, Ohio, and went to--  -   (inaudible)    - Went to high-- Would you like to introduce? And went to school at Fairfield Union High School, did his undergraduate work at Denison University in geology and biology, got his PhD at the University of Chicago in the Department of Geophysical Sciences, and spent three years at the University of Michigan, and then, 10 years ago, came here to the University of Wisconsin-Madison. Tonight, he gets to talk to us something that's really important for those of us who like pizza, and that's the evolution of the crust. Except for this one is the upper crust of the Earth. Biogeochemistry, it's not every day we get to have all three words together. It's impressive that you have an undergraduate degree in both geology and biology. I remember when biogeochemistry came out as a word. It was like, wow, this is amazing. Rocks, life, and reactions. If only I were younger. So I'm looking forward to hearing about this. Please join me in welcoming Professor Peters to Wednesday Nite @ the Lab.   (applause)    - Thank you very much, and thank you for braving the elements out there today and finding the tornado shelter. Hopefully we don't have to use it, but it sounds like we're safe here this evening. So I'm going to be talking to you about the long-term view of Earth's biogeochemical evolution. And, you know, the Earth's a pretty remarkable place. It's home. We're quite familiar with it, but it's really weird. And you can see a lot of what makes it weird and distinct deep within our solar system in this image. You know, we have water. That's not weird. Water is everywhere, but we have it in all three states. And you can see that here. That's pretty special. Earth is very bimodal in elevation. We have very high-standing continental crust and very low-standing oceanic crust. That's weird. We are profoundly bimodal in the elevation. That's due to tectonics. We have active plate tectonics, and that's a weird thing. And, of course, we have life, which is maybe not weird but certainly important part of the story we're going to hear about today. And so all these things come together in the story I'm going to tell you, and I think the best way to get a handle on why you have to understand all three of these things operating together as one evolving system, why that's so important. By asking one simple question. Well, not really simple, but a pretty straightforward question. Why is there so much molecular oxygen in Earth's atmosphere? That's one of the things that makes Earth just weird. And this a rough composition. The Earth's atmosphere, dry atmosphere. It's about 78% nitrogen gas, about 21% oxygen, molecular oxygen, O2. Argon and then CO2, which we spend a lot of time talking about for good reasons, is actually a trace gas. It's not very abundant in Earth's atmosphere, and we've clicked it up a little bit incrementally here. We're above .04 permanently now, probably thanks to our activity. Now, just to give you a sense of how weird this is, you need to compare the present composition of the atmosphere to where the atmospheres of planets come from. And that is devolatilization of the magmas coming out of volcanoes. In this case, on Earth they're coming out of the vents we can see active. And if you look at the composition of gases coming out of the volcanoes looks nothing like our atmosphere. Okay? It's mostly water and CO2. It turns out this is really variable from place to place from details of geology underneath volcanoes and so forth,   And so one really important question is not to say, is really why is there this long delay? Oxygen was being produced. Geology was happening. The world was doing its thing. Why was this long delay between the onset of oxygenic photosynthesis and the buildup of oxygen in the atmosphere? That's really the question. Now, I hoped I've convinced you that these things must be part of the answer, right? Rocks. There's something about the rocks. And so maybe the answer is encoded in the burial of organic carbon that's in rocks. Seems reasonable. But here's the rub. I'll cut to the chase and tell you that is correct, but it took us, as geologists in a discipline science, a while to figure this out, I think. And some people will still disagree with me just to be clear about the record. But I'm going to make the case that we kind of missed the picture here a little bit. And this is why. This is geology in your face. This is the Andes Mountains. I got back from a wonderful alumni-supported field trip there with a group that's working in the Andes, and it was fantastic, in-your-face geology. Erosion is like the thing you're struck by in the Andes. Volcanism and erosion. Just in your face, everywhere. There's rocks, they're forming new rocks by erosion, and they're being destroyed. And geologists have looked at these things, the rock record, in a way that is traceable all the way back to this guy, Charles Darwin, who is of course a geologist, right? He was well known at first for being a geologist and then secondarily become well known for his contributions to biology, which were many. And in "The Origin of Species," definitely one of the most influential scientific texts ever written, he has a chapter titled, not very subtly, "On the Imperfection of the Geological Record." And in this chapter he says, "I look at the natural geological record "as a history of the world imperfectly kept. "It's written in a changing dialect, "and of this history we have just the last volume. "And of the last volume, the last few chapters. And of those chapters, a few lines." And so on. And he portrays the geological record as this tattered "manuscript upon which the history of life, in particular, but Earth, in general, was written, but then it has been ravaged by time and geology to render it not useful, ultimately, in his argument, for really telling us about the history of life, mainly because he had peculiar ideas about what he thought he would expect to see in the fossil record, I guess. Now, what is good about this hypothesis is it makes very clear predictions. And this is basically the prediction. Here's a little cartoon of it. Erosion happens and erosion makes sediment from rocks, and those rocks, that sediment gets put back into the rock record, and this giant thing just turns, and its slowly going away, and if we watch it over time, so nothing ever changes because it's a cycle, and if we watched it over time, there's less and less and less and less of a given age as we go backwards. And, mathematically, this is really the prediction. That is the amount of rock that we have at any time, "t" say 500 million years ago, is a function of how much we started with times an exponential decay function, the decay curve. Okay? This is the formal prediction from Darwin's description of the geological record. It also, by the way, is wonderfully Lyellian and Huttonian. This uniformitarian view of the world where nothing changes. It's a giant cycle. Things come in, things come out, nothing goes anywhere. In fact, Hutton took it to the extreme to think that the Earth had no beginning. Everything was cyclical and so on. Now, this is a prediction but getting the data together to test this was very difficult and it was like, what, I can do the math, I promise, 110 or 120 years before we had any attempt to actually test this hypothesis. Okay? And this is actually it. 1970 Nature paper. It's hard to see the highlights I put here, but in reading this, I love this stuff. This is a very clever thing. This was done to address the denudation of the continents. How   Yes, we've made a mobile app. I'm also very proud of this. It's free. Android and iOS. You can download the thing. Wisconsin Public Radio has a nice little interview, if you want to dig it up and hear my view of what it is. I'll let you do that later. But if you turn it on, it just gives you, like, our best guess of what you're standing on, geologically. Okay? So hopefully it'll be fun, and it gives you really good maps and you can make observations and stuff. So check it out. If you make your stuff public, you can explore it around in an archive like this. It's for nerds like me, really, but if you're interested in just being out in the field and learning about rocks around you, this will help you, trust me. Check it out. Okay, so let's get to it. Ask the question, with these data, does sediment quantity decline with age or not? What I'm showing, over here, is just one time slice. Okay? One time slice through the stack of geology. And where there's color, or solid color in this panel or color here, there's rocks of that type, that type doesn't matter for us, but there's rocks there of that age. Okay? Where there's white there are no rocks of that age. Okay, so those are the hiatuses. And so what I'm going to show you are a bunch of measurements of quantity measured in this way. The simplest way is to just simply measure the proportion of columns that have sediment, the fraction, the proportion coverage of sediment on the continent. How many columns have sediment divided by how many columns are there. Easy. Or we could measure it in square kilometers or something like that. Either one. I'll show you both. Now, before I show you the boring time series, which will put you to sleep but I'll try not to do that, here's a little animation that shows qualitatively what we're going to do. Only from one billion years up to the present. Okay, and it's going by really quickly here. But I'll narrate it. So for a billion years, up to the, this is the start of the Phanerozoic right there. Now, remember, this isn't fossils. This isn't diversity of life. This is just rock. Okay? Just rock. And so you can see that something changes right about there pretty dramatically. And we're going to see evidence for that quantitatively in a second, but I'm going to be showing you time series that summarize this madness in numbers. And here it is. Okay? This is now going, so we saw that animation from here, and so that big jump was that. That you saw visually. And we're going to be going all the way back to two-and-a-half billion years, typically. Now, before we talk about this long period of nothing in that all the sudden a huge increase, before we talk about that and why looks completely different than most of Earth's history from, say, 600 million years ago all the way back, we want to as the question, is the Precambrian just bad somehow? Like, do we just not know it very well? And the answer is sort of. We can see some evidence that this is true. Now, it's a little subtle here, but bear with me. These packages of sediment at each column, okay, that we saw, those little chunks of color, and what we're doing here is measuring in the Phanerozoic, in blue, and in the Precambrian, in red, and we're measuring the duration in millions of years of those packages. And we can see that they're different. The Precambrian is longer in duration than the Phanerozoic. Now, the reason for this is really simple. In the Phanerozoic we have evolution of animals happening, and we use that to tell time. We're able to see evidence for missing time in rocks that is hard to see except through the signature provided to us by evolution. And so in the Precambrian we miss those things and we assume that the record is more continuous than it is, and so my columns don't break out packages like they do in the Phanerozoic. And we can see that here quantitatively. However, the Precambrian is also thicker in meters. Okay? So there's more rock in a little longer chunk of time, and if we take the combination of the two, it's what's really important for today, is the rate of sed  That oxygen can be consumed, so there's a negative sign here, that can be consumed by organic carbon. We are doing it when you go home and turn on your car, you're going to be doing this reaction part right here. And that reaction proceeds at a rate that's related to how much we have around, the amount of organic carbon that's actually in the crust. You can think of it as, like, the charge capacitor. How much that is, weakly dependent on the concentration atmosphere oxygen and a constant. And then this term is really important. It turns out that oxygen reacts with everything. Well, not everything but a lot of things in the Earth's crust itself. That's what this term is. Okay? Now, the first order of business is to calibrate this F org. How much carbon is going into the crust and when. And to make a long story short, really quickly, this is what you need to do it. This is now the same data but turned into a volume flux. Okay? This is in cubic kilometers per million years. This is now the weight percent of organic carbon. How black the rocks are, on average, basically. And this is empirically calibrated with data. I don't have time to tell you about it, but trust me, it's a good estimate for how much organic carbon is in rocks. And then this is the combination of these two to get you the burial flux. The organic carbon burial flux in moles per million years. Now, here's a question for you: how much O2 has the surviving rock record, the one that we can point to and say, "Yep, there it is." How much is that sourced to the Earth's surface environment? So here's part of it right here. How much total oxygen is in that thing? It's about 35 times the present atmospheric oxygen concentration. 35. If you put that oxygen in the atmosphere right now in a bottle, 35 times that would be what the geological record has stuffed into the Earth's surface environment. The reason we don't have 35 atmospheres worth of oxygen is because rust happens. Other exit channels for oxygen. Your car is doing its job right now rusting away a little bit in the rain. Okay, so the way we do this model is we measure that. That's this thing. The "k" constant or this constant we take directly from an estimate of oxidation of iron in the crust. This one, this "k" constant or this constant we take directly from estimates of organic carbon weathering in crust, and we combine them together to make a model, forward model, calibrated by the actual record. For oxygen, this is what we get. Now, for convenience sake, in gray, you can barely see it here, is the canonical model. There's different ways to assign carbon values and so forth. The details of how we do that are subsumed as error envelopes around this estimate here. And so there's two different models of how to distribute TOC, and the whole model space, including some Monte Carlo simulations, is shown by this red curve here. And so this is, on an absolute scale now, the fraction relative to the modern atmosphere of oxygen. And we see a whole bunch of things that look just like the empirical proxy records, including a delayed Phanerozoic rise. For the nerds in the audience, it actually turns out the Cambrian explosion happened when this first rise started to go up, but the rise of oxygen to modern levels was delayed even further. It didn't really get to modern levels until probably into the Devonian, probably. And then a big drop at the Permo-Triassic, the largest mass extinction in the Phanerozoic, right there. The thing that we see is the sustained low. This is probably the most impressive result, honestly, this transition and this long state. And we see these early possibilities of transient whiffs. Now, of course, down here it depends on if that's photosynthetic carbon or not. Okay, so the oxygenation, the timing and magnitude of oxygenation of the Earth's atmosphere. Yes, it depends on biological evolution but biology has been doing its thing forever, basically. The timing of the oxidation of Earth's atmosphere to get to a place where we can have life like us is dependent entirely on geology. It's deter  And we lost it. Now, if we take this as our calibration for the organic carbon burial history and we use that to predict what atmospheric oxygen would have done, it looks like this, using the same rules. And what we find, as some people have suggested based on proxy records, shockingly, that atmospheric oxygen may have gotten to near modern levels but not stuck around.  Which means it may not stick around in the future. Now, if you think about this for a second, it should be really obvious because burying-- Let's say we reach this hypothetical steady state that Darwin so wants and geologists still somehow cling to. If that's the case, that means organic carbon is being buried and oxygen is being sourced, but that oxygen is being consumed by organic carbon that's being removed and weathered from the system. And so you are at steady state and there is no flux of oxygen into the atmosphere. What's being produced is consumed. And that would be fine except for the problem of this term right here. Your car, basically. There are other exit channels for oxygen from the system. And it's not sufficient to have organic carbon buried that's produced by photosynthesis. You have to bury more of it and more of it and more of it so that the mass in the crust is continuously growing. That's the only way to maintain an atmosphere that has oxygen in it, and that is definitely dependent on geodynamics. Now, within the Phanerozoic, a transient hiccup in the continent's ability to do this happened. Right here at the end of the Permian. That was the largest mass extinction in the Phanerozoic. A transient interruption. In fact, geologists have called this the coal gap. And there's all kinds of reasons for the coal gap, but it had a big impact on oxygen in the atmosphere. We got out of it because, well, you might remember the power plant over here that used to haul in coal. When I first got here they were still doing this and they converted it. That coal came from rocks of this age. There's a huge amount of Cretaceous and Paleogene in coal, young coal, that rescued the Earth from this state. It grew the mass again. Now, this suggests that it's not sufficient to have oxygenic photosynthesis through this cool enzyme called RuBisCO or something like it on another planet. It's not even sufficient to have plate tectonics. It has to be combined in a way that allows you to continually go this mass, and an oxygen-rich atmosphere may not be permanent anywhere. So I'll end with just the main point I want you to take home from this, and that is the Darwinian view of the rock record, this incomplete, tattered manuscript is really missing the point. It has an archive of climate and it has an archive of life and it has an archive of tectonics in it. We can read the history of Earth through that manuscript pretty well. It turns out, however, that the formation of this manuscript is the thing that's driving the entire system. It is an active participant in the evolving Earth system. Without the sedimentary record, there is not animal life on a planet anywhere. And I'll leave you with that. Thanks.   (applause)