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So You Think You Know Plate Tectonics? Part 7 - Rocks and Climate · PlanetGeo: The Geology Podcast

• 0:02
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:13
  SPEAKER_00: Christoffer.
• 0:15
  SPEAKER_00: What's up, man?
• 0:16
  SPEAKER_00: Hey, how are you doing?
• 0:18
  SPEAKER_00: I am doing pretty well.
• 0:20
  SPEAKER_00: Recording with my buddy Chris.
• 0:22
  SPEAKER_00: How can we be back?
• 0:23
  SPEAKER_00: Your little buddy.
• 0:24
  SPEAKER_00: My little buddy Chris.
• 0:25
  SPEAKER_00: What's up, man?
• 0:27
  SPEAKER_00: Oh man.
• 0:29
  SPEAKER_01: Not a whole lot.
• 0:30
  SPEAKER_01: It's good.
• 0:30
  SPEAKER_01: All's good.
• 0:31
  SPEAKER_00: Yeah, well, this is uh this is a good one because this is a wrap, basically, on our plate tectonics question list, which you got uh very worked up about in what was it, September or something like that, I guess.
• 0:46
  SPEAKER_00: This was a long time ago that we started this whole process, maybe maybe later than September.
• 0:50
  SPEAKER_01: September, October.
• 0:51
  SPEAKER_00: Yeah, it was it was right away, but the beginning of this school year.
• 0:54
  SPEAKER_00: So this is, I mean, we'll probably come back to this with part eight in you know next year or something like that.
• 0:59
  SPEAKER_00: But but uh this is part seven.
• 1:00
  SPEAKER_00: Eight through ten.
• 1:05
  SPEAKER_00: But this is a big topic.
• 1:06
  SPEAKER_00: And and this is a big episode, too.
• 1:07
  SPEAKER_00: I think this is a really important episode.
• 1:09
  SPEAKER_00: So we we kind of have to cover it and we want to end on it.
• 1:13
  SPEAKER_00: Let's say what this is.
• 1:14
  SPEAKER_01: This is about rocks and climate.
• 1:17
  SPEAKER_01: Can we determine what Earth's ancient climate was when we look at ancient rocks?
• 1:22
  SPEAKER_01: That's really what this is all about today.
• 1:25
  SPEAKER_00: And so let me add one thing to that, Chris, because I think we're we're gonna kind of come from this.
• 1:29
  SPEAKER_00: I think we we sort of have to come from this from two sides.
• 1:31
  SPEAKER_00: Like, how do rocks interact with climate?
• 1:34
  SPEAKER_00: Like, we should we should at least touch on that briefly, how rocks interact with climate, which sets the stage for how can we use old rocks to understand climate as well.
• 1:42
  SPEAKER_00: Like those are kind of two sides of the same coin, this rock climate interaction.
• 1:47
  SPEAKER_01: This was kind of my idea, this episode, but it it came from one of my students actually in my astronomy class.
• 1:54
  SPEAKER_01: That's great.
• 1:54
  SPEAKER_01: Yeah, she was doing a presentation, like a 15-minute presentation that I have all the kids do.
• 1:59
  SPEAKER_01: They get to pick the topic that they want that is astronomy related, right?
• 2:04
  SPEAKER_01: And she chose this one, and I was amazed by it.
• 2:07
  SPEAKER_01: I had I'd actually never thought about this in this way, about how rocks are affected by the atmospheric composition.
• 2:16
  SPEAKER_01: And so this episode is tied to plate tectonics, you know, how how the atmospheric composition affects the rocks, but then there's parts of it that are not directly tied to plate tectonics.
• 2:28
  SPEAKER_01: This is a perfect way to end this series.
• 2:30
  SPEAKER_00: Yeah, I I agree completely.
• 2:32
  SPEAKER_00: And so I think Chris, maybe uh the best way to start out here is to just kind of level set from how this happens today, how do rocks interact with atmosphere today, and and sort of a little brief thing about how that affects climate or really how it is the thermostat of Earth, which is a term that's been used before.
• 2:51
  SPEAKER_00: But we talked about this going back to one of our really early episodes on uh hard water and the carbon cycle, the long-term carbon cycle, basically.
• 3:01
  SPEAKER_00: And so I think should we start there?
• 3:03
  SPEAKER_00: Should we just kind of give five minutes of background here?
• 3:07
  SPEAKER_01: Sure.
• 3:08
  SPEAKER_01: I'm not sure exactly what you what direction you want to go.
• 3:12
  SPEAKER_01: So go ahead, just throw something my way.
• 3:14
  SPEAKER_00: So I I guess what I'm thinking of is let's talk about CO2, because we're gonna really talk about CO2 and oxygen and how they interact with rocks.
• 3:22
  SPEAKER_00: So can we just give the the intro level Campageo level style of how do rocks interact with CO2?
• 3:30
  SPEAKER_00: Or how do rocks impact the atmospheric CO2 content?
• 3:34
  SPEAKER_01: Well, that's a huge topic, Jesse.
• 3:38
  SPEAKER_01: Oh my gosh.
• 3:39
  SPEAKER_01: You really uh I mean we have I we have to do this.
• 3:43
  SPEAKER_01: We have to spend a couple of episodes and talk about this carbon cycle, the fast carbon cycle, the slow carbon cycle.
• 3:49
  SPEAKER_01: But the bottom line is that you know, carbonates and carbon dioxide is drawn out of rocks through weathering, and it's brought then to the oceans.
• 3:59
  SPEAKER_01: And then I don't know, Jesse, if we want to get do you want to get into the geochemistry?
• 4:04
  SPEAKER_01: Or that was actually a really dumb question here that I asked a geochemist if you want to talk about geoch geochemistry.
• 4:10
  SPEAKER_00: But obviously, I mean it's the best, but uh no, I I think we don't need to go into too much detail here.
• 4:15
  SPEAKER_00: I I think you've covered it uh basically like you know that CO2 is pulled out of the atmosphere by weathering.
• 4:21
  SPEAKER_00: I think weathering is the key, the key thing here.
• 4:24
  SPEAKER_00: That is exactly what we're we're looking for.
• 4:27
  SPEAKER_01: Okay, hold on though, Jesse, because you said something that I want to ask you about.
• 4:30
  SPEAKER_01: I do want you to explain it because I think you skipped a couple steps.
• 4:34
  SPEAKER_01: You just went from from one to the other, right?
• 4:36
  SPEAKER_01: You said that carbon dioxide is drawn out of the atmosphere by weathering.
• 4:41
  SPEAKER_01: Okay, but I think you need to walk us through that step or the steps that are in between there.
• 4:48
  SPEAKER_00: Sure.
• 4:49
  SPEAKER_00: And this is a really big and important topic, which you're exactly right.
• 4:52
  SPEAKER_00: We should dedicate many episodes to, and we should interview some people who maybe are part of these startup companies who are trying to do this as a uh carbon sequestration mechanism.
• 5:01
  SPEAKER_00: But basically what happens if you take ultramafic rocks, which we've talked about before, this is the interior of the earth, this is pretty tight, high in magnesium and iron and calcium, those things, those types of rocks break down easily at the surface, meaning they're weathered chemically and physically very easily.
• 5:16
  SPEAKER_00: They're not happy at the surface.
• 5:18
  SPEAKER_00: So when they are at low pressure, low temperature, they're unhappy minerals, they break down.
• 5:22
  SPEAKER_00: What happens then is calcium, very importantly, also magnesium and iron to some degree, are the term is solubilized, but basically they're picked up in water and they're carried into the ocean.
• 5:32
  SPEAKER_00: And when they're carried into the ocean, what they do is they bond with an acid, a CO2-bearing acid.
• 5:40
  SPEAKER_00: Usually it's uh HCO3 or one of these acid types, but CO2 goes into the ocean and is a little acid in the ocean, and they bond.
• 5:49
  SPEAKER_00: Calcium plus this CO3 bond together to form calcite, the mineral calcite in that instance.
• 5:54
  SPEAKER_00: Or dolomide or any of these organisms can do this.
• 5:57
  SPEAKER_00: It's biologically mediated uh today.
• 5:59
  SPEAKER_00: But basically, so the minerals break down these ultramafic minerals, we put them on the surface, they basically, through weathering, the ions are transported, they interact with CO2 gas and or ga they interact with CO2 that's been dissolved in water and they form minerals, and then they're locked up for millions and millions of years in those very stable minerals like calcite or aragonite or dolomite.
• 6:22
  SPEAKER_01: And not to nitpick at all with you, but how is this carbon dioxide drawn out of the atmosphere through this process?
• 6:28
  SPEAKER_00: Yeah, so that's a sort of a partitioning.
• 6:31
  SPEAKER_00: That's something that is is basically they're in equilibrium.
• 6:34
  SPEAKER_00: The atmospheric CO2 and the oceanic CO2 are going to be in equilibrium.
• 6:38
  SPEAKER_00: So if we take more bicarbonate ions out of the ocean, the ocean will start to suck up CO2 out of the atmosphere.
• 6:44
  SPEAKER_00: So they kind of maintain this chemical equilibrium at the surface of the ocean.
• 6:48
  SPEAKER_00: So that's how this happens.
• 6:50
  SPEAKER_00: But I I want to tie this off at plate tectonics.
• 6:53
  SPEAKER_00: Let's bring it back to plate tectonics a little bit because this is a big cycle.
• 6:57
  SPEAKER_00: There's a actually a paper published by some colleagues of mine at Penn State University recently, just this year, that got a lot of uh press actually.
• 7:04
  SPEAKER_00: And the title here is How Temperature Dependent Silicate Weathering Acts as Earth's Geological Thermostat.
• 7:10
  SPEAKER_00: I mean, it's a cool title, geological thermostat.
• 7:14
  SPEAKER_00: It's a good one.
• 7:14
  SPEAKER_00: And but basically that CO2, it does not go down all the way into the mantle and stay there forever.
• 7:19
  SPEAKER_00: Those carbonate minerals, those come back up in volcanoes, but that cycle takes millions and millions of years.
• 7:25
  SPEAKER_00: So this is you said it earlier, Earth's long-term carbon cycle.
• 7:29
  SPEAKER_01: Well, okay, Jesse, bringing this back to plate tectonics, okay.
• 7:33
  SPEAKER_01: Let's just just simplify this because I think you you went into the weeds a little bit, and I'm going to bring it back a little bit, is basically plate tectonics is the force that lifts things up, right?
• 7:42
  SPEAKER_01: And and so as that happens, that accelerates the weathering and erosion process, which accelerates the removal of carbon dioxide.
• 7:52
  SPEAKER_01: And so that's kind of how these gases, atmospheric gases, are tied to plate tectonics.
• 7:58
  SPEAKER_01: The rate of removal, the rate at which things happen is tied to elevation.
• 8:04
  SPEAKER_01: That's exactly right, Chris.
• 8:05
  SPEAKER_00: And and I just want to double-click on that because you're exactly right.
• 8:08
  SPEAKER_00: The more the faster things go up, the more CO2 is going to be drawn back down.
• 8:11
  SPEAKER_00: Also, what happens is the hotter things get, the more CO2 gets drawn back down because hotter temperatures, faster reactions, faster chemical weathering, this reactions that are drawing down CO2 happens faster.
• 8:24
  SPEAKER_00: So over really long timescales, this thing kind of goes up and down and is modulated like a thermostat where you know it goes up and then turns off and then goes down and then turns on.
• 8:34
  SPEAKER_00: And this is how this works over really long timescales.
• 8:37
  SPEAKER_00: That I think brings us nicely into the discussion of the early Earth and comparisons with a planet like Venus, which Marty Gilmore, who is amazing and we were lucky enough to interview a while back, she called it Earth's toxic twin, which kind of brings us back home to this paper that I think you wanted to center this discussion around a little bit about ancient rocks and climate.
• 9:01
  SPEAKER_01: That's right.
• 9:02
  SPEAKER_01: Talking about our neighbors, Mars and Venus, Earth is peculiar.
• 9:06
  SPEAKER_01: We're definitely Mars and Venus, their atmospheres, although very different, right?
• 9:12
  SPEAKER_01: Venus has an extremely thick atmosphere.
• 9:15
  SPEAKER_01: Everything about Venus has to do with the atmosphere.
• 9:17
  SPEAKER_01: I mean, it's it's such a harsh, harsh place because of this.
• 9:21
  SPEAKER_01: But Mars has a very thin atmosphere comparatively.
• 9:25
  SPEAKER_01: And and so, but they have very similar atmospheric compositions.
• 9:29
  SPEAKER_01: They are dominated both of them by carbon dioxide, and then the second most abundant gas being nitrogen, on the order of two to three percent nitrogen.
• 9:38
  SPEAKER_01: So 96 to 97 CO2, and then nitrogen.
• 9:43
  SPEAKER_01: Okay.
• 9:44
  SPEAKER_01: Earth is very peculiar in this because we're dominated by nitrogen, and it's like 78% of our atmospheric composition.
• 9:53
  SPEAKER_01: Then oxygen comes in at 21%, and then we have, although very important, our carbon dioxide concentration.
• 10:02
  SPEAKER_01: We're talking about things on the order of like 415 to 420 parts per million.
• 10:07
  SPEAKER_01: You know, it's a it's a hugely important gas, but a very small constituent of our atmosphere.
• 10:14
  SPEAKER_01: So Earth is really, really peculiar in this situation, but we don't think it was always that way.
• 10:20
  SPEAKER_01: And that's what comes back to plate tectonics and in this whole paper.
• 10:24
  SPEAKER_00: Yeah, and we kind of we've talked about this a little bit before.
• 10:27
  SPEAKER_00: We talked about this in I think part four of this plate tectonics series, which is when did plate tectonics start on Earth?
• 10:33
  SPEAKER_00: We talked about this in part three of this with planetary tectonics.
• 10:36
  SPEAKER_00: What is the tectonic regime of other planets?
• 10:40
  SPEAKER_00: And this is part of the big question about the early Earth is what type of tectonics was going on on the early Earth?
• 10:46
  SPEAKER_00: Was it plate tectonics?
• 10:48
  SPEAKER_00: Was it different?
• 10:49
  SPEAKER_00: Earth and Venus and Mars are extremely different right now, both in plate tectonics and in atmospheric composition.
• 10:56
  SPEAKER_00: The question is, were they always different?
• 10:58
  SPEAKER_00: There's pretty good evidence that Mars had water.
• 11:00
  SPEAKER_00: We don't really know what Venus had going on in the atmosphere over the last four and a half billion years.
• 11:05
  SPEAKER_00: So it's a really interesting problem and deeply tied to plate tectonics, because as we just started out talking about, plate tectonics influences climate and influences the atmospheric composition of a planet.
• 11:18
  SPEAKER_01: Okay, so let's start talking then about what we think our early atmosphere was like.
• 11:23
  SPEAKER_01: Can we do that now?
• 11:24
  SPEAKER_01: Yeah, all right.
• 11:24
  SPEAKER_01: So first of all, we think that we probably had two atmospheres in ancient Earth.
• 11:31
  SPEAKER_01: One was when Earth began, you know, when it formed 4.54 billion years ago.
• 11:37
  SPEAKER_01: Our atmosphere, whatever it was at that time, it went away because we got smacked by a Mars-sized object, Thea, that process formed our moon.
• 11:49
  SPEAKER_01: And that that violent collision would have also degassed Earth.
• 11:54
  SPEAKER_01: I mean, we law our atmosphere would have been blown into space at that point.
• 11:57
  SPEAKER_01: So whatever our earliest composition was, that went away.
• 12:01
  SPEAKER_01: But then it started over again because of this process, right?
• 12:05
  SPEAKER_01: I mean, you know, when that event happened, Earth was entirely resurfaced, it was molten again.
• 12:12
  SPEAKER_01: And then that ocean of magma was degassing and creating a new atmosphere.
• 12:20
  SPEAKER_01: And that's really what this is about today is what do we think that atmosphere was like?
• 12:25
  SPEAKER_01: And how can the rocks tell us about that?
• 12:28
  SPEAKER_00: I just want to paint a little bit of a visual here, Chris, because I want to get a cross the point of how violent these things are.
• 12:35
  SPEAKER_00: I mean, the planet is melted, and in order for the planet to be molten for any period of time, for the the top of the planet to be molten at any period of time, we have to have like a thick atmosphere.
• 12:46
  SPEAKER_00: You can't just be exposed to outer space because the thing will solidify.
• 12:49
  SPEAKER_00: If the temperature at the surface of this magma pond is space temperature, it's going to cool down and crystallize really, really quickly.
• 12:58
  SPEAKER_00: But if you have an atmosphere on top of that, you can kind of insulate this, much like Earth is insulated, and it can stay warm for longer.
• 13:05
  SPEAKER_00: But this is like an atmospheric composition that right after this impact, you have silica in the atmosphere.
• 13:11
  SPEAKER_00: So you have like basically this is a rock dust atmosphere.
• 13:14
  SPEAKER_00: It's like a volcanic eruption, you know, it's just full of particles and gas.
• 13:19
  SPEAKER_00: It's so hot that stuff that should not be gas is gas.
• 13:23
  SPEAKER_00: And it's an extreme atmospheric composition in that first stage that you're talking about.
• 13:28
  SPEAKER_01: Okay.
• 13:29
  SPEAKER_01: Let's talk about this then.
• 13:31
  SPEAKER_01: Like what kind of research is being done?
• 13:34
  SPEAKER_01: You know, the papers are saying that we think our early atmosphere was similar to what Venus and Mars have today in terms of like overall percentages.
• 13:42
  SPEAKER_01: In other words, we were dominated by carbon dioxide, then probably nitrogen.
• 13:46
  SPEAKER_01: Okay.
• 13:47
  SPEAKER_01: What's being done?
• 13:48
  SPEAKER_01: How how in the world can ancient rocks tell us about the chemistry of our atmosphere?
• 13:57
  SPEAKER_01: Yeah.
• 13:57
  SPEAKER_01: Let's let's go in that direction.
• 13:59
  SPEAKER_01: I think this is like, I mean, you you live in this world a whole lot more than than you do.
• 14:04
  SPEAKER_01: So what's going on?
• 14:05
  SPEAKER_01: Because I think this is just fast.
• 14:06
  SPEAKER_01: This is so cool.
• 14:08
  SPEAKER_00: So, okay, there's a whole bunch of different ways to do this.
• 14:11
  SPEAKER_00: And I think this is actually a really nice leading one because we're gonna have an episode coming up in the next month or so here on banded iron formations, and banded iron formations are a sedimentary rock type that don't exist on the modern earth or don't form on the modern earth, but they formed a lot back in early Earth, and that tells us that Earth's atmosphere was different.
• 14:30
  SPEAKER_00: So we know for sure Earth's atmosphere is different.
• 14:32
  SPEAKER_00: For instance, there was no oxygen or barely any oxygen in Earth's atmosphere prior to two and a half billion years ago.
• 14:39
  SPEAKER_00: We can use sedimentary rocks that kind of record this interaction, they record the climate conditions a little bit, or they record certain aspects of the climate conditions.
• 14:49
  SPEAKER_00: We can also look in minerals that are formed with fluid around.
• 14:54
  SPEAKER_00: So things like quartz inclusions.
• 14:56
  SPEAKER_00: Sometimes quartz has little gas bubbles in it, and quartz can form near the atmosphere and it can crystallize, and there might be some atmospheric gas around.
• 15:03
  SPEAKER_00: And so there's really old quartz crystals that people argue have a sort of a record of the early atmosphere, or at least something close to the early atmosphere.
• 15:13
  SPEAKER_00: That's really interesting.
• 15:14
  SPEAKER_01: Can I interrupt you a second?
• 15:15
  SPEAKER_01: Yeah.
• 15:16
  SPEAKER_01: Don't lose your train of thought, but that does bring up a point, too, that we're talking about going back billions of years and looking at atmospheric composition.
• 15:24
  SPEAKER_01: We're able to go back maybe hundreds of thousands of years by looking at little bubbles that are trapped in ice cores.
• 15:31
  SPEAKER_01: Okay.
• 15:32
  SPEAKER_01: You know, we have the Greenland ice sheet, we have the Antarctic ice sheets, and these are basically time machines for us.
• 15:38
  SPEAKER_01: You know, the deeper that we drill down into the ice and we are able to analyze these little bubbles of tens of thousands of year-old atmosphere.
• 15:49
  SPEAKER_01: So we can do that.
• 15:50
  SPEAKER_01: That's not what we're talking about today.
• 15:52
  SPEAKER_01: We're talking we have to go back much further in time.
• 15:56
  SPEAKER_00: And we have to look at rocks then.
• 15:58
  SPEAKER_00: And we have to also look at not really the atmosphere.
• 16:01
  SPEAKER_00: Like when a quartz crystal grows, it's usually growing, it's not growing right at the surface like ice does.
• 16:06
  SPEAKER_00: You know, it's not actually grabbing a piece of the atmosphere.
• 16:09
  SPEAKER_00: It's maybe some gas that's in chemical communication with the atmosphere, but is modified uh, you know, maybe a couple hundred meters down in the earth.
• 16:17
  SPEAKER_00: Like the gas is different than the atmosphere, so it's not a perfect sample of atmospheric gas, uh, but it is, you know, we can make a relationship.
• 16:26
  SPEAKER_00: There's a relationship between the composition of this little bubble in quartz and the actual atmospheric composition.
• 16:31
  SPEAKER_00: But uh those samples are few and far between.
• 16:34
  SPEAKER_00: The other thing that we can do is look at rocks, as we've talked about here, and as the sort of we're centering this conversation on.
• 16:40
  SPEAKER_00: And people have looked at rocks, and basically what this comes down to is looking at the chemical composition, usually of iron, sometimes of other elements.
• 16:50
  SPEAKER_00: Uh, for instance, zircon has some chemistry in it that can get at the broadly the composition of the atmosphere, but it's not perfect, um, but usually of iron.
• 17:01
  SPEAKER_00: And the key here is that rock composition has a little bit to do with the atmosphere.
• 17:07
  SPEAKER_00: Certain rocks have a little bit to do with the atmosphere.
• 17:10
  SPEAKER_00: And so let me just ask you a question here, Chris.
• 17:12
  SPEAKER_00: If I was to go through pressure's on.
• 17:16
  SPEAKER_00: Yeah, perk up, exactly.
• 17:17
  SPEAKER_00: Uh yeah, straighten up, here we go.
• 17:19
  SPEAKER_00: Uh so if I was to say, I want you, Chris, to go find me a rock type that has a record of an interaction with the atmosphere, what broad general category of rock would you be most likely to select?
• 17:33
  SPEAKER_00: And I'm thinking, you know, the intro-level physical geology categories here.
• 17:38
  SPEAKER_01: All right.
• 17:39
  SPEAKER_01: So I'm I'm gonna look for something that's mafic or ultra mafic because I'm looking to be looking for something that is loaded with iron.
• 17:46
  SPEAKER_01: You just got done talking about the importance of iron.
• 17:49
  SPEAKER_01: Basically, we're looking.
• 17:51
  SPEAKER_01: Did I get that right by the way?
• 17:52
  SPEAKER_00: Yeah, I actually want to go one ramble on.
• 17:55
  SPEAKER_00: I want to go, I want to go one level deeper though.
• 17:57
  SPEAKER_00: Or do you want the name of the rock?
• 17:59
  SPEAKER_00: Well, I'm thinking how like where is this rock found?
• 18:02
  SPEAKER_00: What would this rock have experienced?
• 18:04
  SPEAKER_00: I mean, are you are you looking for a gabbro?
• 18:07
  SPEAKER_01: Are you looking for uh you know mafic intermediate composition, but can you I'm looking for a rock that was extrusive at the time that it formed.
• 18:17
  SPEAKER_00: Perfect.
• 18:18
  SPEAKER_00: That's right.
• 18:18
  SPEAKER_01: I mean, yeah, because we want to have something.
• 18:21
  SPEAKER_01: Oh, I feel so good, like the pressure's off me now.
• 18:23
  SPEAKER_01: Um, we want to have something that was directly interacting with our atmosphere, right?
• 18:28
  SPEAKER_01: And so what we're basically talking about is what happens to iron.
• 18:33
  SPEAKER_01: Everybody knows this.
• 18:34
  SPEAKER_01: Iron rusts, okay.
• 18:36
  SPEAKER_01: And this is if we're talking about this in a chemistry context, that's the oxidation of iron.
• 18:41
  SPEAKER_01: But there are various different oxidation states that can happen.
• 18:46
  SPEAKER_01: And uh by looking at the the oxidation state of iron, then we can determine what the composition of the atmosphere was that was interacting with that rock at the time that it formed.
• 18:59
  SPEAKER_00: Right.
• 19:00
  SPEAKER_00: I think a good way to sort of set this stage, and so what we're talking about here is how much oxygen is in the atmosphere.
• 19:08
  SPEAKER_00: So think of it this way: you have iron, and iron is hanging on to electrons on the outer shell, and oxygen really loves to grab electrons.
• 19:17
  SPEAKER_00: Oxygen super loves electrons, and so oxygen is gonna strip away electrons and make iron lose electrons, and iron can lose two electrons, that's iron two plus, so it becomes two plus because it lost two electrons, or iron can become three plus and it lost three electrons.
• 19:33
  SPEAKER_00: And when it becomes three plus, that's the oxidation, quote unquote oxidation state of iron.
• 19:39
  SPEAKER_00: It means that there's more oxygen around because more oxygen is grabbing more electrons, basically.
• 19:44
  SPEAKER_00: I mean a really rudimentary sense, that's what it means.
• 19:47
  SPEAKER_00: But I think it's important, Kristen.
• 19:49
  SPEAKER_01: Can I just interrupt you a second?
• 19:51
  SPEAKER_01: So, what that would form then with an abundance of oxygen is it's gonna form Fe2O3, as opposed to when you have less oxygen, it'll form a one-to-one ratio because it only lost two electrons, it'll form FEO.
• 20:07
  SPEAKER_00: So you're exactly right, but I think people might get lost in the numbers there.
• 20:10
  SPEAKER_00: Can you balance those equations?
• 20:12
  SPEAKER_00: Because I just said Fe3 plus gets formed with more oxygen, and then you said Fe2O3.
• 20:16
  SPEAKER_00: So why why the numbers here?
• 20:18
  SPEAKER_00: Can you just explain that?
• 20:20
  SPEAKER_01: I think so.
• 20:20
  SPEAKER_01: Um, so when ions combine, when they come together, the electrical nature of matter opposites attract.
• 20:26
  SPEAKER_01: So the the iron, which is Fe3 plus in this situation, is gonna bond with oxygen in the negative state, but they have to come together to form a neutral charge.
• 20:38
  SPEAKER_01: So if I have two ions, FE2, that gives me a total plus six charge, right?
• 20:45
  SPEAKER_01: I need three oxygens, each of those are negative two, and then I have a negative six, and I can put them together then in that ratio.
• 20:53
  SPEAKER_01: So it's Fe2O3 that forms a neutral compound.
• 20:58
  SPEAKER_01: But in order to do that, in order to do that, you have to have an abundance of oxygen.
• 21:04
  SPEAKER_00: So I think if we just back up really quickly here, because if we take iron three plus, so we've got a lot of iron three plus, which as you said, two of those plus three oxygen two minuses makes an even thing.
• 21:16
  SPEAKER_00: If you have some iron two plus around, you can't just stuff that in the same mineral because you're gonna have too many oxygens.
• 21:21
  SPEAKER_00: So you have to make a different combination, a different recipe to get a neutral bond here.
• 21:25
  SPEAKER_00: So the type of iron in a rock will tell us something about the quote unquote oxidation state, the amount of oxygen around when that rock formed.
• 21:36
  SPEAKER_00: That's really what we're talking about here, right, Chris?
• 21:40
  SPEAKER_01: Absolutely.
• 21:40
  SPEAKER_01: We're looking at whether or not we have FEO, which means there was less oxygen in the atmosphere that was interacting with the rocks, or whether we have FE2, O3, which means there was more oxygen.
• 21:53
  SPEAKER_01: So let's just cut to the chase.
• 21:55
  SPEAKER_01: What do we find with these ancient rocks?
• 21:57
  SPEAKER_01: What kind of oxidation state are we finding?
• 22:00
  SPEAKER_00: Well, we're finding pretty low oxidation state in most of the uh ancient rocks because again, we've talked about the fact that there's less free oxygen, less O2 in the early atmosphere than there is today.
• 22:14
  SPEAKER_00: So by today's standards, fairly low.
• 22:17
  SPEAKER_00: But uh that's not necessarily totally low, I guess.
• 22:22
  SPEAKER_00: Right?
• 22:23
  SPEAKER_01: All right, so Jesse, um, before we get into some of the other details, I do want to ask something that hopefully you can clear up.
• 22:31
  SPEAKER_01: We're talking about the oxidation state of iron.
• 22:34
  SPEAKER_01: So, what kind of oxygen are we referring to?
• 22:37
  SPEAKER_01: Is this just free oxygen in the atmosphere, or can this be oxygen that's tied to carbon in the form of, let's say, carbon dioxide or a carbonate, something like that?
• 22:46
  SPEAKER_00: It's a really good point, Chris, because free oxygen is what everybody thinks about when we're having this discussion about oxygen grabbing electrons from iron.
• 22:53
  SPEAKER_00: But it's not just free oxygen, O2, that we're talking about, because a lot of the molecules we have in our atmosphere have oxygen in them, H2O, CO, CO2, they all have oxygen in there.
• 23:03
  SPEAKER_00: And so those things have higher what are called oxidation states, meaning there is more oxygen, there's more potential for oxygen to grab some electrons from iron.
• 23:12
  SPEAKER_00: So oxygen fugacity, or FO2 is a term we use, oxygen fugacity, quote unquote, or oxidation state, that's a really complicated thing that depends on the certain combination of gases you have in your atmosphere, like what the oxidation state, quote unquote, of the atmosphere is, can depend on a lot of different stuff.
• 23:31
  SPEAKER_00: CO2, CO H2O, H2, CH4, N2, all that stuff goes in and you make a lot of things.
• 23:36
  SPEAKER_01: It can come from all the oxygen that's available, right?
• 23:39
  SPEAKER_01: That's what you're saying.
• 23:40
  SPEAKER_00: Exactly.
• 23:40
  SPEAKER_01: Okay, so let's talk about then this experiment, because in order for them to make conclusions about what our early atmosphere was like, we have to be able to do this in lab, right?
• 23:51
  SPEAKER_01: I mean, and so you said earlier there are a lot of different ways this can be done, but let's talk about what was done with the paper that we kind of read as a primer to get us ready thinking about this episode.
• 24:03
  SPEAKER_01: So, bottom line is I'm gonna try to paint a picture of what this was like, and I'm I'm really simplifying this a lot.
• 24:08
  SPEAKER_01: But basically, in the lab, they used the laser and they melted the rock peridotite.
• 24:14
  SPEAKER_01: Okay, and peridotite is this is ultra mafic, which just means this is a rock that is loaded with iron, a lot of magnesium too, but we're talking about the oxidation state of iron.
• 24:27
  SPEAKER_01: So, and then they used a variety of gases, different compositions, different ratios of the gases that are meant to kind of simulate what our early atmosphere was like.
• 24:37
  SPEAKER_01: Okay.
• 24:38
  SPEAKER_01: And this is the part that I found interesting and weird, and maybe I don't know what to think of it, but they were actually able to levitate this melt in the gas.
• 24:51
  SPEAKER_01: In other words, they melted the rock and it was levitated inside this chamber.
• 24:56
  SPEAKER_01: So the gas was available all around the melt, not just the surface of it.
• 25:00
  SPEAKER_01: It was actually all around.
• 25:02
  SPEAKER_01: Is that a problem with this experiment?
• 25:04
  SPEAKER_01: Like, why is that necessary?
• 25:07
  SPEAKER_01: Is it just so they can do it faster, you think?
• 25:09
  SPEAKER_00: I think it's partially to do it faster.
• 25:12
  SPEAKER_00: And also, I think um, I mean, I'm not an experimental person, so I don't exactly know how these experiments are done or what the limitations of other types of experiments are.
• 25:21
  SPEAKER_00: But with this thing, you can quench this thing super quick.
• 25:25
  SPEAKER_00: So it's basically think of a little tiny bead of predatite that's being melted and levitated up in space, and then there's atmosphere kind of swirling around it.
• 25:33
  SPEAKER_00: You can cool that down instantaneously.
• 25:35
  SPEAKER_00: And that's really valuable because while you cool bigger experiments, think of like a little glass vial full of liquid magma, that takes longer to cool down.
• 25:45
  SPEAKER_00: And when stuff cools down, it changes the composition.
• 25:48
  SPEAKER_00: And what we're after is the composition of magma and atmosphere in equilibrium when it is magma at super high temperatures, not what happens after it cools down with all those back reactions that might happen and perturb that in the experiments.
• 26:02
  SPEAKER_01: And so what they did is the you know, the variable in the experiment was the composition of the gas.
• 26:08
  SPEAKER_01: They kept changing the composition until the rocks oxidized in the way that we see ancient rocks today.
• 26:17
  SPEAKER_01: Yeah.
• 26:17
  SPEAKER_00: Does that make sense?
• 26:18
  SPEAKER_00: Yeah, for sure.
• 26:19
  SPEAKER_00: I mean it's a it's a really uh uh sort of you know, we're coming at this from the experimental way because as we've talked about, there's not many rocks preserved from early in Earth history, and most of the rocks that are preserved are completely altered.
• 26:32
  SPEAKER_00: So if we went back and found a 3.8 billion-year-old basalt, which do exist, there are basalts that old, very few of them, but those things have been altered so much that any gas, little bubbles that were entrapped, think vesicles or think anything that could be gas in that rock when it formed 3.8 billion years ago, that's all been overprinted by later metamorphism.
• 26:53
  SPEAKER_00: So we rely on this sort of experimental approach to say what is possible on the early earth, and then compare that, compare the oxidation state, which is hard to perturb, compare the oxidation state of iron uh to what we see on Earth.
• 27:07
  SPEAKER_01: So And so bottom line is what they found to get to replicate the oxidation state in lab with what we find in ancient rocks.
• 27:18
  SPEAKER_01: Our atmosphere was similar then to what it's what it is on Venus and Mars today.
• 27:23
  SPEAKER_01: In other words, we're talking about upwards of 97% carbon dioxide, about three percent or a little bit less N2 or nitrogen, gave us the proper oxidation state in the lab.
• 27:36
  SPEAKER_01: And so that's the same basically as what we have on Mars and Venus today.
• 27:43
  SPEAKER_01: So this brings us full circle then back to plate tectonics.
• 27:48
  SPEAKER_01: So if this was Earth's early atmosphere, where did all the CO2 go?
• 27:55
  SPEAKER_01: You know, we're obviously very, very different then.
• 27:58
  SPEAKER_01: And that comes back to a massive part of carbon dioxide removal is tied to plate tectonics, right?
• 28:05
  SPEAKER_01: Yeah, you know, so over time we've removed carbon dioxide because of our oceans and because of plate tectonics.
• 28:15
  SPEAKER_00: Yeah, and it's a really uh important question of where is this stuff all tied up in?
• 28:20
  SPEAKER_00: Where does it exist?
• 28:21
  SPEAKER_00: We know there's diamonds down in the mantle.
• 28:23
  SPEAKER_00: Some of those diamonds have a record of surface carbon down there, probably was CO2 at some point.
• 28:28
  SPEAKER_00: So, you know, there could be loads of carbon stored in diamonds.
• 28:31
  SPEAKER_00: There is loads of carbon stored in carbonate rocks, so think any limestone or marble, if you have marble countertops, if you have a limestone out your back door, that's a lot of CO2 stored in that baby right there.
• 28:42
  SPEAKER_00: And there's not a lot of those rock types on Mars or Venus necessarily that we know of.
• 28:47
  SPEAKER_00: We might find some later which gives us a record of ancient Mars or Venus, what that might look like.
• 28:52
  SPEAKER_00: But I don't know, Chris, it kind of highlights for me the most interesting data point we could ever get from our solar system would be a rock from Venus, because we could do these same sort of calibrations, these same experimental tests on a rock from Venus.
• 29:04
  SPEAKER_00: If we had a sample from Venus in a lab here at home on Earth, we'd be able to tell a lot about the history of Venus, which I think is a really important comparison.
• 29:13
  SPEAKER_00: We'd be able to kind of test this model, maybe even.
• 29:16
  SPEAKER_01: And that's not gonna happen, at least not in the foreseeable future.
• 29:19
  SPEAKER_01: You know, we're not gonna get a rock back from Venus.
• 29:21
  SPEAKER_01: We're gonna go to Venus, you know, go back to our Marty Gilmore interview, which was amazing.
• 29:27
  SPEAKER_01: And we're gonna put a probe on the surface again for the first time in decades.
• 29:33
  SPEAKER_01: But we're not coming back with it.
• 29:35
  SPEAKER_01: It's just gonna give us as much data, right?
• 29:40
  SPEAKER_01: It's gonna give us as much data as we can possibly glean from that little bit of time that it's gonna be able to survive on that harsh uh surface.
• 29:47
  SPEAKER_01: But yeah, we're not gonna get a rock back anytime soon anyway.
• 29:50
  SPEAKER_01: One other thing, right?
• 29:52
  SPEAKER_01: Because, you know, we talked about how plate tectonics is tied to carbon dioxide and the carbon cycle, right?
• 29:59
  SPEAKER_01: But there's another part of that carbon cycle too, and that that has to deal with our oceans, okay?
• 30:05
  SPEAKER_01: And so why is Earth unique that we still have water?
• 30:09
  SPEAKER_01: I mean, we know Mars had water at one time, it it lost its oceans.
• 30:14
  SPEAKER_01: We really think Venus had oceans as well.
• 30:17
  SPEAKER_01: It lost them.
• 30:18
  SPEAKER_01: Why is Earth able to retain for such a long time its oceans?
• 30:25
  SPEAKER_00: Great and very important question about which I don't think there's a huge amount of consensus.
• 30:31
  SPEAKER_00: Lots of ideas about it, but it might have something to do with the distance from the sun.
• 30:35
  SPEAKER_00: You know, it's not our oceans, our atmosphere is staying intact, or is it staying in this really uh Goldilocks habitable zone, quote unquote habitable zone where it's not getting blown away, it's not getting too thick, it's not burning off because the planet gets too hot.
• 30:49
  SPEAKER_00: So there's a lot of uh potential reasons, a magnetic field, you know, lots of potential reasons for that um that that could exist.
• 30:58
  SPEAKER_00: But it's a very important one.
• 31:00
  SPEAKER_00: Or it could be plate tectonics, you know, water is cycling a lot, and plate tectonics, there's this is sort of a chicken or the egg argument.
• 31:06
  SPEAKER_00: Some people would like to think that water makes plate tectonics possible.
• 31:09
  SPEAKER_00: Some people would like to think that plate tectonics makes our oceans uh long-lived.
• 31:14
  SPEAKER_01: And hey, funny thing, it all comes back to plate tectonics.
• 31:18
  SPEAKER_00: Like it just does, makes the world go round, you know.
• 31:22
  SPEAKER_00: So fun.
• 31:23
  SPEAKER_00: Well, I think, Chris, that's a good place to wrap up our plate tectonics series here.
• 31:28
  SPEAKER_00: This was a fun exercise.
• 31:30
  SPEAKER_00: Um, as hard as it was to convince me that it was a good thing to do, you were totally right.
• 31:35
  SPEAKER_00: And um, this was really fun and a good exercise.
• 31:38
  SPEAKER_01: I I will say this, that we might not be done yet, because we might have office hours coming your way about questions.
• 31:45
  SPEAKER_01: We've gotten a lot of questions, emails about tectonic-related issues.
• 31:51
  SPEAKER_01: So we might not be totally done with it yet.
• 31:54
  SPEAKER_01: But in terms of like our series, yeah, that's a that's a wrap for that.
• 31:57
  SPEAKER_01: But we'll we'll definitely be coming back to tectonics-related issues for sure.
• 32:02
  SPEAKER_00: Absolutely.
• 32:02
  SPEAKER_00: And that's a great point, Chris.
• 32:03
  SPEAKER_00: So if you have questions about anything we've covered in play tectonics or otherwise, send us an email.
• 32:08
  SPEAKER_00: We are planetgeocast at gmail.com.
• 32:10
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• 32:13
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• 32:16
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• 32:22
  SPEAKER_00: And Chris, what else?
• 32:25
  SPEAKER_01: And the only thing I would add, Jess, rate us.
• 32:28
  SPEAKER_01: Give us a rating.
• 32:29
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• 32:34
  SPEAKER_00: We love it.
• 32:35
  SPEAKER_00: All right.
• 32:35
  SPEAKER_00: Yeah.
• 32:36
  SPEAKER_00: Peace.
• 32:36
  SPEAKER_00: Cheers.