 So thank you. It's an honor to be invited to give this presentation and Tell you what we don't know and a little bit about what we know about the great oxidation event one of the great Transformations maybe after the origin of life the greatest transformation in the history of life on earth Just to orient you a little bit to what the what it is. We're trying to understand This is a quick cartoon showing you the relative abundances of the gases in the atmosphere Most of the earth's atmosphere is nitrogen, but the next most abundant gas is oxygen at about 21% today So the question is how did it get to be that way? Wasn't always 21% and it's rather important that it's 21% because we wouldn't be here otherwise to enjoy this Enjoy this conference and enjoy everything So this is this is a first-order challenge in earth history trying to understand how the atmosphere got to be the way it is The answer to that We don't fully understand, but we know that the way we get to it is through the rocks I think was Carlo yesterday showed a picture of a banded iron formation. So this is a Spectacular setting in Western Australia where you can see these banded iron formations These are rocks that appear uniquely towards the middle part of earth history then they go away and We essentially never see rocks like this in the geologic record afterwards and we think this has to do with the rise of oxygen I show you this particular As a website this particular picture as a website because this is a teaching resource here that I think might be useful to some of you That a group that I lead at Arizona State University the Center for Education through exploration Produces we produce a virtual field trips among other things we produce immersive interactive virtual field trips that take you To places where your students unfortunately well and most of us will never get to go But we've gotten to go to some of these places and capture some pretty exciting renderings and imagery and and data for these places and and Have these as resources that you and your students can go visit and in some cases We've built interactive and adaptive lessons that take students through these locations and teach particular scientific concepts So that's the there's a link to this in the brochure in the pack in the packet if you go to my page there There's some links and one of them is to VFT dot as you dot edu Which is a website that has dozens of these virtual field trips and growing a little bit every year So what we want to understand in this particular case is The origin the cause of the great oxidation events. This is a plot showing you oxygen through time So here's four and a half billion years ago Here's today and this is a summary from a few years ago of oxygen in the atmosphere over time So here we are today at around 20% oxygen in the atmosphere Here's the first half-worth history with very little or no oxygen And then there's this sharp change that happens around 2.3 billion years ago We argue about the exact date roughly halfway through Earth history We have this sharp increase that we refer to as the great oxidation event So this is what we want to we'd like to understand why this happened big transition in the history of the planet Not the only transition involving oxygen and worth noting that this is if you're teaching a math class This is worth pointing out. This is a log plot here the access here is log logarithmic and on a log plot This looks like the big increase But if you were to plot this is a linear plot, this would be the big increase Right, so sometimes, you know when you switch from linear to log things can look quite different So so there's a second big increase here in the neoproterozoic the neoproterozoic oxygenate oxidation event And this Really made the rise of animals possible But this rise wouldn't have happened if this rise hadn't happened first So we refer to this one as the the great oxidation event even though an actual amount of oxygen is smaller Because here we went from almost nothing to something that we argue about how much but something appreciable So before we launch into what happened it's and why maybe we should talk a little bit about why we care Certainly an education. That's something your students will ask you about I get asked all the time Not just by students, but why do we care about this and I'm gonna give you two reasons that both Come from this orientation about thinking about the earth as a planet. This is a pic This is the pale blue dot. This is earth You're all in this picture all of us are in this picture a few years ago No is taken this is from the Cassini orbiter when it was orbiting Saturn and they point it back to earth to take a family portrait About all of us. So here's earth as a planet and when you think about earth as a planet There are two big areas that pop into our minds many of us who are geoscientists. One is to think about Life on other planets and how we would look for it. How do we look for life on this planet from a distance like this? So the the astrobiology questions that motivate many people and the other questions that motivate many people when you're a geoscientist or Just a citizen of the planet and occupant of the planet thinking about the future are Anthropocene questions about the future of this world and in both cases the oxygen question has some some very big picture relevance So from the astrobiology standpoint as most of you probably are aware We're discovering planets outside our solar system at a crazy crazy pace. There are now over 3,000 over 3,500 confirmed planets orbiting other stars So 25 30 years ago now as a graduate student it was science fictions Even you can consider kind of crazy to try to go into that field of science and now it's almost boring It's like easy to find planets. That's kind of amazing when you think about it Almost a thousand of the planets that have been found are terrestrial planets meaning they're rocky planets They aren't quite necessarily earth-sized most of them are super earths But these are planets where we can start thinking about life like ours on and and this is growing all the time Every time I give a version this talk I go to the JPL website and update these numbers because they change weekly So we're finding exoplanets all over the place This is the Kepler space telescope Which is one of the main ways we found a lot of these planets and it finds planets by Looking at dips in the brightness of light from the star as a planet passes in front of the star and That sets up the ability to do once we get better telescopes spectroscopy You can imagine if you have a planet passing between us and a star that the light that's going through the atmosphere of that planet will be Dimmed differently will absorb certain wavelengths of light do the gases in the atmosphere and you can do spectroscopy and tell What the atmosphere is made of we can't really do that now for terrestrial planets orbiting other stars But we will be able to it's a big goal that NASA and and ESA have the so-called spectroscopic search for biosignatures and Oxygen looms very large among the ways we might do this So here looking at our solar system as an example. Here's Venus earth and Mars And if we look in the infrared wavelengths We see that the atmosphere of Venus has this big feature due to the absorption of carbon dioxide So there's the Venus atmosphere is about 90 times earth's atmosphere and pressure and it's almost all carbon dioxide and carbon dioxide is a very strong infrared absorbers is a very strong infrared feature and not much else Mars Mars has almost no atmosphere It's an atmosphere of six millibars of pressure It's it's it's six thousandths of the earth's atmosphere, but it's also almost entirely co2 So this is big absorption feature due to co2 and there's really nothing else If you make the same kind of observation of earth in the infrared You again see a co2 feature the earth has 400 ppm co2 and rising But you also see this interesting feature here from ozone in the Earth's atmosphere You don't see this in the Venus or the Venus or Mars atmospheres and Ozone is a byproduct of having a large amount of oxygen in the atmosphere and so this is arguably diagnostic of life The spectroscopy of earth's atmosphere looks different in the spectroscopy of the atmospheres of Venus and Mars because earth has life Which pumps out oxygen through photosynthesis and and gives us this this feature So this is one of the sorts of data that astronomers want to obtain from planets orbiting other stars or to look for Right, and if we found something like this, we'd get very excited And then we'd argue about whether or not they're non biologicals to make ways to make that and we'd have you know 20 more years of controversy, but at least we'd have we'd have moved the ball down the court in terms of what we're arguing about So so oxygen looms large in this astrobiology field And so wanting to understand it's kind of important and understand what controls oxygen in the in the atmosphere From the Anthropocene perspective, you know, we're entering this epoch of earth history where the future the planet is very much in the hands of us of beings walking around with big brains and hands who are big evolutionary innovation on this world perhaps as big an evolutionary innovation as the As the photosynthesizers that evolved we'll talk about when they evolved and change the surface of the planet so the earth is very much in our hands and that's leading to all sorts of Amazing and scary ideas like deliberate intervention in the earth's climate system is a way of coping with what we've been doing Accidentally to the earth's climate system. So ideas of deliberately removing co2 from the atmosphere sort of cleaning up our waste or Deliberately modifying the earth's atmosphere to reflect sunlight These are ideas that are being taken very very seriously in the most by the most august scientific bodies and natural National Research Council of the US You should report on this a few years ago working on another one now So there are ideas like this that are out there as we enter this sort of Hopefully adulthood at least teenager hood of humanity on the planet And their ideas like this they're floating around Elon Musk wants to terraform Mars Now we're a long way from doing this, but this is the kind of this kind of talk is out there And it's going beyond science fiction into people who actually have industrial capacity to try to do things This is where Elon Musk is aiming. He wants to make Mars green Okay, if we're going to start modifying the earth let alone modifying other planets If this is something we're really going to think about we kind of need to understand the system that we're modifying and If we don't understand why the earth has 20% oxygen the atmosphere and how it got to be that way Which is one of the first sort of things you could ask about in our atmosphere Then I think you could rightly say that our knowledge is pretty primitive compared to our ambitions and our abilities And this is something we might want to try to understand if you want to understand the earth system One of the first sort of things you might want to be able to explain is when did the earth's atmosphere become 20% And why did that happen in oxygen? It's like a first-order feature of the atmosphere. I can't explain that one You know, we're in bad shape trying to do anything deliberate to earth let alone try to replicate earth somewhere else So so for both these reasons, but the astrobiological and the anthropocentric reasons This oxygen pursuit is a very big picture important thing to try to figure out So so what caused the great oxidation meant and I'm just noticing that the timer didn't start So I have no idea if I'm on pace or not. Just so Carlo, please or max Mako. Yeah, I know I'm okay right now But later on let me know so what caused the great oxidation event. How do we get oxygen in the air in the atmosphere? So there's one process that nobody talks about very much, but is worth Worth noting so that you're aware of There is a continual source of oxygen in the earth's atmosphere from the loss of hydrogen out the top of the atmosphere This is a process that happens all the time. It's happened throughout earth history. It's happening right now. We can actually see Emission of photons from this process Hydrogen is being lost at the top of the atmosphere We heard a little about it yesterday from John Tarduno in the context of the Martian atmosphere Atmospheric loss happens on earth to for hydrogen and if you lose hydrogen You leave behind oxygen most that hydrogen is coming one way or another It was kind of paired with with oxygen in water at one point and as you lose hydrogen you gradually oxidize the planet So there's a continual sort of geophysical source of oxygen over time and people argue about its magnitude And it certainly plays a role But there's no reason to think there was some big inflection or anything like that some big change halfway through with history and the Rate of hydrogen loss, so we don't think this is the story about the great oxidation We don't think this is this is it, but it's a factor that needs to be thought about and considered The textbooks will say something like this, especially the biology textbooks We'll say something like this the rise of oxygen is due to the evolution of photosynthesis Life figured out how to take CO2 and water and React them to make oxygen and organic carbon. This is the geochemist way of writing organic carbon We don't actually mean the molecules ch2o. We mean the stoichiometry one carbon to two hydrogens to one oxygen in a whole array of different molecules My chemistry friends especially my biochemistry friends are horrified when I ever I write this equation down But stoichiometrically it makes sense one co2 to one water to makes one oxygen and an organic carbon molecule So the the biology textbooks will tell you well this evolved and oxygen rose and a story We can drop the mic and go out right that's the end of the game But the geologists if you look at the geology, it's more complicated than that and since we're here at a gift workshop not a Bift workshop since the gift workshop. We're gonna delve a little into the geology So this is a virtual field trip to shark Bay in Western, Australia To Carbola Beach specifically and this looks kind of nice and pretty but kind of nondescript Until you go under the water when you see these mound-like things all around you and If you and these are really interesting to swim around with it's kind of cold also too But once you get over the shock of how cold it is they're really interesting to swim around And if you look at these things very closely what you find is that while they look like rocks. They're really not Or they're not purely rocks. They're layers of Microbial microbes and and sediment interbed it and the microbes that are living here are dominantly cyanobacteria and There's cyanobacteria that are making oxygen and we call these things when they're fossilized. We call them stromatolites These are modern versions of what we find in the ancient fossil record that are called stromatolites And they are these very complex microbial communities that leave behind a very distinct Geological marker in the form of these things which when they get buried and lithified they're pretty distinctive in ancient rocks So and when we go back in the geologic record what we find is that before the rise of animals you go back before half a billion years ago Or so you find the fossil remains of stromatolites are quite common You find them even if you go back before the Great Oxidation event 2.7 billion years ago This is in Western Australia go up this ridge here. I don't know if the lighting works well You can see this all that well, but but we'll zoom in To this cross-section and you can find these fossilized stromatolites That are we're gonna zoom in over here That are absolutely spectacular and you can again the lighting a little better. You could see all the glorious detail in here I'm not a paleontologist Let alone a geomicrobiologist, but those who are look at these and say oh, yeah We can we can pretty convincing argue that these are fossilized forms of those stromatolites that we were swimming amongst in Shark Bay that you can find living today and When you go back into the the Precambrian before the the rise of animals This is the dominant form of life that you see in terms of anything macroscopic that you can see that's biological Today these stromatolites that we see today make oxygen and so it's quite conceivable that these are making oxygen as well And notice the date here. This is 2.7 billion years ago. The Great Oxidation event is 2.3 billion years ago so we're already Quite a bit of time before the Great Oxidation event here, and we see these stromatolites We can go to some of the oldest sediments that we that we know of oldest nicely preserved sedimentary rocks These are the these are rocks in the dresser formation 3.5 billion years ago in Western Australia, and they aren't nearly as well preserved But they are not Terrible and you can make an argument that in some of these rocks we see structures features that are That have been interpreted again as being stromatolites at 3.5 billion years ago And if you look at the detailed morphology of these of these stromatolites the argument has been made that that indeed They were oxygen-producing stromatolites now. That's not a slam dunk That's very contentious But you can at least make the case and it's not a weak case that there could have an auction production as early as 3.5 billion years ago just based on the fossil record So the Great Oxidation that happens at about 2.3 billion years ago But well before that there's there was certainly biology There was certainly phototactic biology that was making use of sunlight and there's a good case to make any way that that biology was making oxygen So it's not as simple as saying oh photosynthesis Turns on and oxygen takes over the world Now we being geochemists. We don't believe all that morphological Fossil stuff some of us well so that we don't believe it It's that we we trust magical things that emerge from our mass spectrometers more So so we work with our paleontological partners to do things That are geochemical to try to confirm verify and extend what what the paleontologists find the paleobiologists find So what I'm gonna walk you through here now is a little bit of stuff that my group and collaborators have done Looking at of all things the element molybdenum Why molybdenum if any you read hitchhiker's guide to the galaxy in this room hitchhiker's guide to the galaxy ever read that? Yeah What's the answer to life the universe and everything? 42 it's element 42. So of course you'd use a molybdenum. What else would you do? But that's not the real reason we use molybdenum the reason we look at molybdenum in ancient rocks is because it turns out the Geochemistry of molybdenum in the environment is very sensitive to the amount of oxygen that's around It's a proxy for oxygen the amount of molybdenum in certain kinds of sediments is a proxy for the amount of oxygen in the environment We can't record ancient oxygen directly in rocks billions of years ago, but we can Reconstruct the amount of molybdenum that was in rocks laid down billions of years ago and from that infer What the chemistry of the atmosphere and oceans was or draw some inferences? And molybdenum is not the only element, but it's probably the one that's been looked at the most carefully in this regard So there's a number of of molybdenum related arguments that I could make to you But this is probably the simplest one to get across in a short talk So molybdenum in in in crustal rocks, especially in igneous rocks Which are the primary rocks of the make up the crust molybdenum is found inside sulfide minerals dominantly Sometimes is an impurity in pyrite and Sometimes as molybdenum sulfide molybdenite is a sulfide mineral that is a molybdenum bearing mineral but either way those sulfide minerals are major reservoirs of molybdenum in the crust and They react quite vigorously with O2 right sulfide minerals oxidize quite readily if you expose them to oxygen They don't necessarily fall apart right away sitting on your desk But give it geological time and you know your fool's gold your iron pyrite sitting on your desk is not going to last all that long Because it's reacting with the oxygen in the atmosphere So the logic of using molybdenum is a paleo redox proxy goes something like this Imagine I've got a continental crust that has molybdenum in Sulfide minerals in it If there's no oxygen in the environment if oxygen is absent then that molybdenum basically stays locked in the sulfides And if it stays locked in the sulfides, there's really not much opportunity for molybdenum to get into the oceans and So the molybdenum concentration of the oceans is low and If you go to ocean sediments that could scavenge molybdenum if it was there you'll find that there isn't much molybdenum there But if I turn oxygen on if there's oxygen around in the environment Then these sulfides oxidize they fall apart they deliver their sulfur and their molybdenum into the oceans And the molybdenum content of the oceans rises and I can find evidence of that in the geologic record That's the that's the the simple logic and we can make it more complicated with equations in math and talk about Solubilities and stuff like that, but the logic is basically this Right, so no oxygen molybdenum stays locked up in the crust Oxygen around molybdenum can accumulate in seawater and all the rest is important commentary So we can go into ocean sediments that are ancient We look at the kind of sediments in which molybdenum Can accumulate if it's in the water in the first place not all sediments will scavenge molybdenum for example Calcium carbonate rocks are very molybdenum poor. Molybdenum doesn't really go into calcium carbonate very well But molybdenum goes very nicely into black shales Into carbon-rich organic carbon-rich rocks that make that sediments that eventually make up these kind of sedimentary rocks called black shales and So there's a small cottage industry of us going around drilling ancient sedimentary sequences to get these nice black shales out That are well preserved And then measure the heck out of them for molybdenum and other trace elements and isotopes to try to reconstruct what the Chemistry of the waters was waters were where these sediments were accumulating billions of years ago And when we've done that we've been able to do things like this. So here. We're looking through time Here's today. Here's three billion years ago. So we're not going all the way back here yet So zero to three billion years. Here's a molybdenum concentration in black shales. So what's been done here is to get black shales through time measure the molybdenum contents And then plot them up and what you can see is in the last half billion years or so the living contents are quite high Which makes sense because in the last half billion years or so there's been a lot of oxygen in the environment 10 20% oxygen Molybdenum should be a very mobile element in that world. It should be abundant in seawater like it is today today Molybdenum is the most abundant transition metal in seawater. It's still very scarce But it's but it's but compared to other metals It's quite abundant in seawater and that was probably true for most of the last half billion years or so And that's why you see good enrichments all the way up here Then you go earlier remember I've mentioned that that neoproterozoic oxidation events So we go before that time and molybdenum concentrations are lower And then we go further back and there's not a lot of data and the concentrations are lower still So you have this sort of first-order confirmation of this kind of threefold or two-step change low Something goes up and then it goes up again kind of like what I showed you before for oxygen And this is this is this is one line of evidence that's taken to say to taken as support for the hypothesis I gave you that we can use molybdenum as a way of tracing oxygen through time But let's dig into this a little bit more so that great oxidation event happens about 2.3 billion years ago That's right around here Here's 2.5 billion years ago, and you can see that molybdenum is actually somewhat it's actually not there's not a big difference here There's not a big step here Hmm what's going on here So let's look at this more closely So this is a this is data that that my group and collaborators generated about 10 years ago at this point We we wanted to look at sorry at whoops Back at that point of time. We didn't have these data that was just very low Do we said let's start looking before the great oxidation and see is there a molybdenum around in the environment? And what we found is in these rocks from about 2.5 billion years ago where there's these beautifully preserved black shales When we looked with depth in these rocks, we found this enrichment of molybdenum. It's kind of transient But as an enrichment of a little it goes up to around 40 parts per million at around 2.5 billion years ago This is a healthy amount of molybdenum and you can look at other tracers We won't go into all this because it gets into all sorts of geochemical nerdy nerdy-ness that we don't need to go into Don't have time for but all sorts of things kick here at the same time and You can argue about Alteration stuff like that, but but we and most people are pretty convinced at this point. This is all primary It's all telling you something about the original seawater and what we think we see here is a transient a Pulse of molybdenum into the environment and we've argued that that actually reflects a transient whiff of oxygen Small amounts of oxygen wafting into the environment in sort of variable ways a bit like today Right today methane is a rare gas in the atmosphere But but it's around and it varies in concentration certainly over over tens and hundreds and thousands of years It can vary up and down and so we would argue that in this pre great oxidation event environment We had oxygen as a minor gas But one that varied around and sometimes you might get a pulse of it for a few million years depending on vagaries of Biological production and other things that are going on So we call this the whiff of oxygen and The picture that emerges from this and other studies is that it during that first half of its history Whereas today the it's quite simple more or less oxygen penetrates through the water column in most places In most parts of the world the environment as the oceans are thoroughly oxygenated We think that before the great oxidation event. It was a more complicated picture We had large volumes of ocean water that are were so-called ferruginous They had very little oxygen, but lots of iron. That's why you could get those banded iron formations And then as you went to the near shore environment you had we argued and this is not just us This is a classic argument actually you had cyanobacteria living close to the seashore producing oxygen creating an oxidized Near surface region and sort of oxygen oases So so regions large but contained regions that were oxygenated in the shallow waters Overlying and paradoxical reasons we could talk about later on you get these very reducing sulfide rich regions below that but the the basic story here is a kind of heterogeneous story with some oxygen around in these near shore environments probably being produced by cyanobacteria probably Many of them in communities like those stromatolites So we see fossil record of stromatolites and we see geochemical evidence that we argue is evidence of the being oxygen around in the environment in small amounts before the great oxidation event so again, the geochemistry is telling us something similar to the Fossil evidence, which is that it's not as simple as life figures out how to make oxygen and boom away It goes we had a long period of time It looks like when life was around possibly making oxygen, but the environment was not taking off Into a 20% or anything close to that oxygen rich atmosphere Right, so here is this taking this diagram again for my friend Tim Lyons and adding into it oxygenic photosynthesis We think oxygenic photosynthesis is old. I think today. We might extend this back to 3.5 based on some other data that this out there now But the rise of oxygen was delayed and didn't happen until around 2.3 billion years ago So in text summary so far before we get to the conclusion here. How am I doing on time? Ten more minutes perfect So somebody so far the great oxidation event occurred around 2.3 billion years ago We kind of took that as a given. I didn't really show you the evidence for that but just take that as a given evidence of microbial mats Which when fossilized are stromatolites that might have produced oxygen are found as far back as 3.5 billion years ago Malibu them and other elements in Asian ocean sediment suggest a slightly oxidizing surface environment and hence Arguably and this is all arguable, but arguably auction production at least by two and a half billion years ago And I didn't show you this evidence, but but there's other things that get a little more complicated to show Around molybdenum and a little bit of isotopes and chromium isotopes and stuff like that all of which Let you extend this kind of argument back to almost 3.5 billion years ago this geochemical evidence and So yes photosynthesis was necessary for the great oxygen oxidation event We don't think you can get a 20% or even close to an oxygen atmosphere on earth without having biology pumping oxygen in the environment but It originated much earlier Another way of putting this is photosynthesis is a necessary condition But it's not sufficient on its own to explain why we wind up with a heavily oxygen atmosphere So this question what caused the great oxidation event we can be a little more sophisticated about it now The question we really need to be asking is not what caused the event, but what kept oxygen low before the great oxidation event It was being produced we think But something was keeping it down So There's broadly two arguments that people make and one that I'll argue is is more likely or to be more important That to try to account for this so we need to get into one more step of geochemistry to to Understand the first these arguments So we talked about oxygen the atmosphere But it turns out that oxygen in the atmosphere the accumulation of oxygen the atmosphere is a consequence of burial of organic carbon How does that work so the very simple way to sort of conceptualize it So so this is such a tight relationship that in the literature you'll find people talking about organic carbon burial as if it's Synonymous with oxygen production unless you know what's going on. It's like why they talking about organic carbon burial We're talking about again. What is this so I want to get this across to you here So CO2 and water are the reactants in photosynthesis in oxygen-producing photosynthesis So they react to make oxygen and organic carbon that's the photosynthetic reaction But as you all know unconsciously if not consciously because you're doing this right now with your breakfast the back reaction of aerobic respiration Takes oxygen and organic carbon and turns it back into CO2 and water. We're all doing this right now as we speak and On a global basis, these are pretty finely balanced actually Pretty much all the organic carbon and oxygen that are produced every year by biology are re consumed by respiration I mean, why wouldn't it be that way the organic carbon is there is oxygen the atmosphere So if if this is being made there are bacteria and other organisms that are going to do this back reaction and respire it So how do you actually then accumulate any oxygen in the environment at all? How do you end up building up oxygen in the atmosphere? What happens that there's a trickle of organic carbon that gets buried in sediments on Geologic time scales and gets protected for for hundreds of millions or billions of years from re-oxidation and for every mole of organic carbon that you bury You leave behind a mole of oxygen in the atmosphere So there's this so burial in sediments. This is geology now, right? So everything here is biology, but this is geology and So if you do things that change the efficiency with which you can bury organic carbon in sediments over time You can change the source the effective source of oxygen in the atmosphere So one classic argument that has been made is that the rate of burial or organic carbon in sediments might have changed Either because of geological changes or maybe because of changes in evolution that changed the efficiency with which organic carbon gets packaged and sent into sediments And so perhaps early on earth history We were simply burying less organic carbon than we are today and maybe that maybe there was a big change that caused the great oxidation event So this gets too complicated to go into I just want to summarize the the result people have looked at this hard They've used carbon isotopes and this is a whole hour-long lecture to explain how you use carbon isotopes to get at this But they've tried to reconstruct the fraction of carbon in the surface environment that gets buried as organic carbon through time Here's 3.5 billion years ago here's today and there is indeed a bit of a trend We are burying a bit more organic carbon today than we used to But and this is an attempt this this study by Christensen Totten is an attempt to really put rigorous statistics on this And the statistics tell you that yes, there is a change here, but it's not enough to account for the great oxidation event So there has been an increase in the burial organic carbon Which means there is an increase in the effective source of oxygen the environment But it's not enough to explain this big change that we see 2.3 billion years ago So we need to look for other things instead or in addition and so here again. We come to geology What about the geological sinks for oxygen? So over geologic time we produce oxygen through burial of organic carbon which leaves the oxygen behind and we also have a Escape of hydrogen space, which is a source of oxygen that talked about at the beginning But what happens to that oxygen that gets accumulated in the atmosphere? Well, it reacts with things it reacts with reduced minerals on the continents during weather a it reacts with Volcanic gases and with rocks on the seafloor it reacts with volcanic gases on land and with metamorphic gases these are all sinks ways of consuming oxygen and So the question that has loomed large in the last 10-15 years is well, can we better understand all these sinks and? Did how do they change? And if you think about it the earth's interior is a very chemically reducing place It's effectively an infinite sink for oxygen and Most of the planet is the interior Right the oxygen that we're talking about the atmosphere is this very thin scum here It's a tiny tiny tiny fraction of the mass of the planet and so Changes even small changes in the interaction between the interior and the surface can make a big difference in the oxygen sink strength on the surface So you could have things the most controversially What if the actual redox state of the mantle changed slightly and in Steve's talk when he summarized when he summarized Steve Moig's work He talked about possible changes in the the oxygen fugacity of the mantle That's a way of talking about how reducing the mantle is has this changed over time in some ways Has the composition the continental crust changes change in ways that would change the sink strength of oxygen And what about the composition and flux of volcanic gases for the interior? How is have those things changed? So all these things have become areas of serious investigation over the last decade or so and it turns out there's support for each of these ideas Against our it's argued about but but there's a blizzard of papers And this is only some of them There's more where people have attacked various ideas like this and shown Hey, this could be true this could be true This could be true and we're only now getting into the stage where people are string to attack some of these papers And see which ones can be shot down This is an area of very of a lot of intellectual ferment right now of looking at these geological sinks and trying to understand which ones are important and which ones aren't and What I'd like to leave you with is the thought that perhaps they're all important Perhaps these are all just different parts of the elephant That we're all feeling in our blindness And imagine this kind of thought experiment imagine we begin with a hot earth an Earth where there's where there's a very rapid exchange between the surface of the interior because it's hot and it's it's Motion the mantle is it's convecting faster and things like that And so you have a lot of interaction between whatever oxygen is produced here and they've reduced interior And so oxygen can never really rise can't really take off because you have too rapid interaction with the surface in the interior And then as the planet cools down you kind of slowly freeze that out right you slow down the interactions between the surface of the interior and Oxygen can increase and perhaps all these things we're talking about change in crustal composition change the amount of the crust change Volcanic gases all those really are symptoms changes in those things are going to be symptoms of Cooling of the planet, so maybe what we really need to do if we really want to nail this question This topic is to sort of develop a theory of the earth There have been some previous attempts to write books called theory of the earth Perhaps what we need to do is as a community try to put together a theory of the earth system that really integrates That will the story that's emerged from the surface and what we're starting to figure out about the deep interior and its Interaction with the surface and we really need to sit down as communities together and try to figure this out It turns out that surface earth scientists and deep earth scientists don't tend to talk to each other all that much Don't tend to do research all that they don't even understand each other's questions and problems and vocabulary at the time all that much But maybe it's time we need to pull that together in order to resolve this problem, and I'll leave you with that. Thank you