Susan Brantley on Earth’s Geological Thermostat
Transcript
Note, transcripts are not fully edited for grammar or spelling.
Oliver Strimpel
This is Geology Bites with Oliver Strimpel. Compared to our neighbors in the solar system, the Earth has experienced a remarkably constant temperature over the four and a half billion years since its creation. Venus has experienced runaway heating, with surface temperatures of about 460°C, while on Mars, the temperature fluctuates between 20 and -150°C. We owe this relative temperature stability to our atmosphere, which contains greenhouse gases such as carbon dioxide, that are transparent to visible and near infrared light, but opaque to thermal infrared. Too little of such gases and the Earth turns into a snowball — too much and the planet cooks. Since volcanoes continuously inject greenhouse gases into the atmosphere, the relative temperature stability of the Earth implies that some feedback mechanism regulates the amount of greenhouse gas in the atmosphere. It is widely accepted that over geological time this atmospheric thermostatic control is provided principally by the weathering of rocks containing silicate minerals, a process that draws carbon dioxide out of the atmosphere. But how exactly does this process function to regulate atmospheric temperature, and how does it depend on the changes that have occurred through geological time, such as the growth of continental crust, the formation and destruction of supercontinents, and the rise and fall of mountain chains? Susan Brantley has conducted extensive studies of the reactions between water and rock in the fields and in the lab. Recently, she has scaled up her findings to a regional and global scale so as to estimate the overall temperature sensitivity of the Earth to silicate rock weathering. She is a Professor in the Department of Geosciences at Pennsylvania State University. Susan Brantley, welcome to Geology Bites.
Susan Brantley
Thanks, Oliver. Nice to talk to you this morning.
Oliver Strimpel
Can you remind us as to what chemical processes are responsible for the weathering of a silicate rock, and how that removes carbon dioxide from the atmosphere?
Susan Brantley
Sure. When rock comes to the surface of the Earth, when it's exposed, it first has to fracture and break apart. That makes the surface exposed to air and water. And then, minerals dissolve in the air and water, and that dissolution for silicate rocks pulls carbon dioxide out of the atmosphere and then holds it dissolved in the water. And then that water runs out of the soil and joins the stream, and flows in a stream to the ocean, and, eventually, over geologic time, that CO2 that was in the water recombines with calcium or magnesium in the ocean and then precipitates this carbonate rock. So, really, it's a sequestration process that produces rock out of the CO2 in the atmosphere.
Oliver Strimpel
But this process operates over geological time, so how do you go about studying this in the lab?
Susan Brantley
You take a flask and put some rock powder — you know, rock that you've ground up — into the flask, and then you measure how the water chemistry changes with time. And then, sometimes, we might take a column and put the rock powder in the column and flow the water through. But either way, you're looking at how the chemistry of the water changes with time, and you try to do that in a very controlled way in the laboratory. And then if you look at that and change different minerals, or you change the conditions – the chemistry of the water or the temperature, you can really start to create a data set of rock weathering or mineral weathering based on laboratory data.
Oliver Strimpel
Did you run these dissolution experiments by grinding up different rock types? For example, granite and basalt?
Susan Brantley
We wanted to make it even more simple by isolating not just the rock type, but whatever mineral was in the rock. So granite, for example, is mostly feldspar and quartz, let's say. So we might just grind up the feldspar and look at how the feldspar dissolves. And basalt can have feldspar, can have olivine, it can have pyroxene. But if we separate the minerals, then we thought we could have a very precise measure of the dissolution rates of individual minerals. And then we should be able to put those back together in some kind of model to make predictions that predict how fast the weathering is happening out there in that soil that you're digging in in your backyard.
Oliver Strimpel
One of the things you were looking for was to see how temperature dependent these various processes are. What did you find on that score?
Susan Brantley
In almost every case, as you increase the temperature, certainly for the silicates, minerals dissolve faster. And so there's a standard process by which you measure some chemical process as a function of temperature and then calculate the temperature dependence – so, the temperature sensitivity. And with that temperature sensitivity, you can suggest at any temperature you want to extrapolate to — down in temperature… up and temperature… how fast the rate should proceed. So we've done that with a number of minerals, and my lab only produced a relatively small number of rates. But if you compile those with all the data collected by people around the world, there's a pretty hefty dataset now for mineral dissolution rates.
Oliver Strimpel
And can you give me a ballpark of how that rate varies with temperature?
Susan Brantley
The rate doubles with tens of degrees of temperature rise. It is a significant effect, what the temperature does to the process, and so that's very important when you start to think about how the Earth thermoregulates — how the Earth's global temperature has been regulated over time.
Oliver Strimpel
I said in the introduction that you've just been scaling up the findings in the lab up to a regional and even global scale. So how can you actually apply a lab result to the incredibly messy and complicated and diverse situation that prevails on a global scale?
Susan Brantley
There's lots of papers in the literature where people measured the temperature dependence of dissolution in the laboratory, and then there's some estimates for the temperature dependence of dissolution — you know — weathering in soils, and then there's temperature dependences that have been measured for watersheds. And, in general, they contradict one another. I would like to be able to make those numbers make sense, one to the other. And so I moved from just measuring in the laboratory to actually looking at how can we measure weathering in the soils. And then, most recently, how can we measure weathering in watersheds? And so I've tried to answer the question that you just asked me in both directions. I've done it from the direction of “can I predict it from the laboratory?” And then I've done it from the perspective of “what can you measure in the field and then pull out of that field measurement, what is the number that is comparable to something in the laboratory.” And, with those two approaches, you can start to make some sense out of this puzzle.
Oliver Strimpel
Let's look at different environments you have. Say, a relatively flat river plain or floodplain. And then you would have a mountainous region. I mean, how did you subdivide the surface of the Earth in order to be able to do this scaling-up?
Susan Brantley
Under some conditions, the overall rate, weathering, is limited by one process or another. So, you just said what about in floodplains, or the flat lands, versus mountainous areas… In the flatlands, sometimes, what we see is very, very thick soils that are depleted of the minerals of interest. So, they're depleted of feldspars, for example. And that's because the soil is shielding the reactive minerals that are at depth from corrosive rainwater, basically. So, the rain comes in and it may or may not get all the way down to the bottom of the soil where the reactive mineral is still there. So it may not ever actually see any feldspar, if all the feldspar has been removed from the soil. So in the flatlands, there actually may be very little reactive mineral surface area that's interacting with corrosive rain, whereas if you go to a mountaintop, where there's very little soil buildup, then the feldspar, let's say, or whatever reactive mineral you're interested in, is exposed to the rain. And so, in the former case, the flat land, that's erosive transport-limited, because if the erosion rate were just faster and the soil was removed, then a lot of that reactive mineral would be exposed. And then in the mountain, we say that it's kinetic-limited because the erosion is so fast, in essence, that the reactive mineral surface area is right at the surface interacting with the rain. So that's one way to simplify these complex natural systems — is to think of these sorts of end-member situations where you're either limited by very, very slow erosion, or actually limited by kinetic weathering.
Oliver Strimpel
And then did you estimate the proportion of the Earth’s surface in which each of the erosive-limited and kinetic-limited regimes prevail? And, I presume, you would have to do something about deserts or frozen areas. How do you scale that up?
Susan Brantley
When we started applying this paradigm of this kinetic limit and erosive transport limit, it allowed us to start to see why some of the data in the literature made sense or didn't make sense or was comparable across these different scales. And then we tried to make the watersheds bigger and bigger and started to think about the globe and at the global scale —then we had to come up with “How much is the rows of transport-limited? How much is kinetic-limited?” But as you also mentioned, something like a half the globe also has such a small amount of runoff going through the system — such a small amount of basically rainfall, that the fluxes are very, very small. So, we ended up having to estimate how much of the globe was runoff-limited — it's just not getting enough runoff, versus kinetic-limited, versus erosive- transport-limited. And then we had to think about how much of the Earth's surface is characterized by different rock types. So how much is granite? How much is basalt? How much is sediment, and how much is carbonates? Because, in general, we're interested in the silicate weathering as opposed to the carbonate weathering.
Oliver Strimpel
Wow. It sounds like extremely ambitious undertaking to try and quantify all those different things, but perhaps the data is already out there. But, broadly speaking then, what does introducing these different considerations do to the temperature sensitivity as you measure in the lab? Does it make it more temperature sensitive, less temperature sensitive, or some of one and some of the other?
Susan Brantley
When we went from laboratory scale up to watershed scale, and I'm talking relatively small watersheds now, I'm not talking sort of global watersheds, what we saw was that the temperature sensitivity actually increased from the laboratory up to watersheds. It's not a huge increase, but it actually makes some sense, because, just as I've expressed, in the lab we can constrain it so that the only thing that's happening is dissolution, but when we go up in scale, other processes come in that just aren't even present in the lab, and so as those other processes come in, they can be also temperature sensitive. It actually makes it look like the activation energy, which is what we call this temperature sensitivity, is higher in a watershed than it is in the laboratory. But then when we try to go up to global scale, because so much of the Earth is dry, when you try to get temperature sensitivity for the whole globe, temperature sensitivity decreases again, mostly because of this dry land. The fluxes are just so small on dry lands that the temperature sensitivity is pretty small.
Oliver Strimpel
In your recent paper, you write that the other processes that come into play as you scale-up from the lab to the watershed include solute transport, clay precipitation, biotic activity, disaggregation, fracturing, and erosion, and that these processes in turn can be influenced by climate factors such as temperature, runoff and precipitation, and by weatherability factors such as lithology, porosity, permeability, type of vegetation and position within the landscape. That's a lot of processes to consider. You characterize these processes using a standard formula for the temperature dependence of reaction rates, called the Arrhenius equation, in which the temperature sensitivity is represented by an activation energy for each process or set of processes. But in any particular setting, isn't just one of the processes likely to be the overall rate-determining step. Is that what determines the activation energy?
Susan Brantley
The activation energy does characterize something about the rate-limiting step, but if you have a complex process that doesn't really even have a rate limiting step, it can have so many processes that are all confounded to it. It can be very hard to interpret what that activation energy actually means.
Oliver Strimpel
Broadly speaking then, you try and parameterize all those many, many different effects in terms of a single activation energy that you then plug into the equation that is meant to indicate the blended temperature sensitivity.
Susan Brantley
That's right. And, in a way, it's like treating each of these larger spatial-scale systems the way a chemist would treat it, without being able to isolate every single process and measuring them all separately. In the early days, people measured very complex processes and then, over time, we simplified them more and more and try to be very reductionist. And now what we're trying to do is put it all back together again and try to understand these really big complex systems again.
Oliver Strimpel
As I mentioned in the introduction, there have been enormous changes in the Earth's surface over geological time, such as the growth of continental crusts in the formation and destruction of supercontinents, and the rise and fall of mountain chains. Would these changes be expected to affect the temperature sensitivity of the thermostat, and, if so, in what way?
Susan Brantley
In the points of Earth's history, when there were really large supercontinents, very large fractions of the continent may have been very dry because as the rain comes in from the oceans and gets rained-up, you end up having big deserts in the middle of big continents, and you can see that even on today's Earth. To do an analysis like the one I did, one would have to think about what would we expect the dry land fraction would be when there are big supercontinents. And then you mentioned big mountain-building events. There have been times like uplift of the Himalaya where, you know, mountains were going up very, very fast exposing a lot of mineral to weathering. And, there's been an ongoing argument for quite a while about how the mountain building affects weathering. From one point of view, one might argue that more reactive minerals should cause higher rates of weathering, and so it could affect the temperature sensitivity of the planet, if more of the planet was kinetic-limited.
Oliver Strimpel
So, would it be fair to say that during the prevalence of super continent, say, every 500 million years in the Wilson Cycle, that you get a period when the drawdown is less effective because you have all these dry interiors, and so that the carbon dioxide might build up in the atmosphere and you get a little bit warmer? And, conversely, when you have a lot of mountain chains forming, that the weathering is more efficient, and so you get a draw down of the carbon dioxide and it cools off. Is that something people have looked to see?
Susan Brantley
I think what you said is our baseline set of hypotheses, and, of course, when they dig into that, there's a lot of other variables. So where is this supercontinent? Is it high latitude, low latitude, northern hemisphere? It depends. And in terms of mountain-building events and higher rates of drawdown, higher temperature sensitivity, again, it depends — what's the rock type. It depends where the mountain building is occurring, and then there's this other wrinkle that we haven't really talked about and that is that there's not that much CO2 in the atmosphere and yet the CO2 in the atmosphere has been maintained at relative constancy over periods of time. So all these processes have to be balanced. So even if a mountain chain goes up, and, let's say, there's an increased temperature sensitivity because there's more kinetic-limited landscape. If that were to pull too much CO2 out of the atmosphere, the globe could quickly become ice covered and we don't always see that. So, well, what happens when a mountain chain goes up and causes this higher temperature sensitivity? Is it perhaps modulated by other processes? One of the big arguments out there right now is you know you can't just think of the world as being silicate rock. There's also a lot of pyrite out there. Pyrite is iron sulfide, and when pyrite weathers, it produces sulfuric acid. And weathering driven by pyrite also happens as the mountains go up, and so there's other complexities that may actually cause not only CO2 drawdown, but actually cause some CO2 release during mountain-building events.
Oliver Strimpel
What about the effect on weathering of life forms, both microscopic and macroscopic?
Susan Brantley
We've done some experiments where we've put in bacteria and into our flask experiments with minerals. We've done some column experiments with plants. And, in general, what you see at the lab scale, sort of at that small scale, is there's usually an acceleration of weathering. But when you go up in scale, if you think about your backyard, if you have any kind of a slope in your backyard, you're often advised to put plants on that slope because it will hold the soil in place and can move your system from something that could be kinetic-limited to a system that is covered with soil that has very little reactive mineral in it. So it could be more erosive transport-limited. So, really, that the lab-scale biology tends to accelerate weathering. But as you go up in scale, it can have both effects. It can accelerate weathering, but it can also slow down weathering, and the jury's out as to how that has played out at higher and higher spatial scales of analysis.
Oliver Strimpel
Why is it so important to know the overall temperature sensitivity of the Earth's geological thermostat? Broadly speaking, I suppose the higher the sensitivity, the quicker any temperature excursion can be damped over geological time and brought back to its steady state value. Is that something that we need to know both to understand geological history, but also to understand what's happening today?
Susan Brantley
Sure. Well, what we're doing today is totally changing this cycle that we're talking about. So geologists think about the CO2 in the atmosphere and the global temperature as being modulated by how fast volcanoes release CO2 and how fast weathering pulls the CO2 back out of the atmosphere. But that's over a million-year time scales. That's a very long, slow process. If the Earth is just left on its own to respond, CO2 will be drawn down out of the atmosphere by weathering. But it could take hundreds of thousands of years. And, right now, we're in this big dialogue around the world about how are we going to stop using fossil fuels. But what's important is that there's still so much CO2 out there in the atmosphere, and that's going to be still acting like a thermostat. The greenhouse effect is gonna maintain the warming that we're observing and it's going to be a very long time to pull that out. And so this basic understanding of the Earth system has spurred a conversation about, well, what can we do to pull the CO2 out of the atmosphere? Because the Earth is not going to do it fast enough for us.
Oliver Strimpel
There are silicate rocks on the surface of Venus. Why have they not been able to provide thermostatic temperature control there?
Susan Brantley
Venus is closer to the Sun, its rotation is slower, so part of it is facing the sun for longer periods of time. And the temperature at the surface of the planet has been such that it has lost most of its water, if not all of its water. And so the atmosphere is basically a CO2 atmosphere. So there may be rock, and there may be CO2, but there isn't the water there to allow the weathering to proceed and pull that CO2 out.
Oliver Strimpel
And I guess on Mars, the situation is they're just not much of an atmosphere at all?
Susan Brantley
Right. There has been weathering on Mars, our rovers that we've put up on the planet and looked at the geology — there's been beautiful evidence that there have been times on Mars earlier in Mars's history when there actually was very deep weathering that occurred, and now it's not occurring.
Oliver Strimpel
Coming back to the question of global warming that we're facing here on Earth, and your point that the time scale for this geological drawdown is extremely long, does your work help inform the attempts to sequester carbon dioxide by the use of enhanced weathering, which is one of the many techniques that are being proposed now?
Susan Brantley
One way to do this is to dig up a bunch of rock, grind it up, and then put it out on fields and let it weather. And that's such a simple thing to imagine. We know how to dig up rock, we know how to transport it, we know how to grind it, we know how to disseminate it on fields. We do that all the time with lime. So many farmers fields are limed. So they take limestone and grind it up and put it. It's just really common. And so people have said, why don't we dig up, especially basalt, which is a rock type that has a lot of calcium and magnesium. It's actually a very good set of minerals to pull CO2 out of the atmosphere. Why don't we dig up a lot of basalt, grind it up, transport it to farmers fields, put it out and let it pull the CO2 out of the atmosphere? And people are doing this. People are trying this in the Midwest. People are trying it in small plots around the world and people are trying to measure then, whether CO2 that's pulled out is what they might have predicted, and then they have to compare the CO2 that's pulled out to the total calculated CO2 of digging up the rock, grinding the rock, transporting the rock, and then spreading the rock. So there's like all those processes in there. That you have to figure out how much CO2 was released by the hydrocarbons that you used to do all those kinds of processes as well. This could be successful. You could run this system in the way that you'd pull more CO2 out then you'd put in, in terms of the digging, grinding, transporting and spreading.
Oliver Strimpel
I discussed enhanced weathering in an earlier episode with Phil Renforth and learned that to remove gigatons of carbon dioxide from the atmosphere per year, which is what we need to do in order to have a significant impact, we would have to do this on a truly gigantic scale.
Susan Brantley
Right. Really large percentages of the farmland in the top agricultural countries in the world would have to be used to make a dent in the amount of CO2 that we put in the atmosphere. We may say, well, this is too much rock to be dug up and transported and powdered and spread to solve the whole problem. But in certain areas, possibly near big basalt deposits, it might be a really good process, especially because the ground-up powder has nutrients in it.
Oliver Strimpel
As you mentioned, you clearly had to make a lot of simplifying assumptions in order to build a model for how weathering depends on temperature on a global scale. Are there any aspects in particular that you'd like to add to your model to make it more realistic?
Susan Brantley
One of the puzzles is the effect of biota, so plants and organisms, and how that affects weathering. I mentioned that the laboratory scale, it seems to accelerate, but then as you get up to field scales, you often see that biota can decelerate weathering. So I did work on that to some extent, in order to do this first estimate that we did at a global-scale temperature sensitivity. But I'd like to work on that more and think more about, again, scaling up biota. There's a lot of papers in all these different spatial scales. I'd like to see if I could do something similar and make sense out of why the different observations at different spatial scales don't always seem to make sense.
Oliver Strimpel
What are you working on at the moment?
Susan Brantley
One thing that I'm very intrigued by is this idea that soil can become depleted in minerals because of weathering. If you think of weathering as being top down, the rain comes from the atmosphere and moves down like one-dimensional world view, that upper soil becomes depleted in their reactive minerals. Eventually, as you dig down, you come to a zone where there are reactive minerals, and they are reacting sometimes at very, very slow rates, if they're very deep. But that zone where they're reacting, I call that a reaction front or the weathering front. And weathering fronts should be all around us in the landscape beneath our feet. So we can see the landscape that we walk on, and we can see the morphology of that landscape. That's what geomorphologists study, is why the shape of the landscape looks the way it does. What we don't know is what does the landscape of that reaction front look like in the subsurface? So for each mineral in a rock, there will be a landscape. There will be a depth at which that mineral is dissolving. And how does that landscape for each mineral vary as you move across the landscape that we can walk on? So, for example, at a ridgetop, you may only have to go very shallow before you start to see a reaction front, versus if you go down to a stream, you might have a different depth down to the reaction front. And, realistically, geochemists have never been able to study those landscapes, because that three dimensionality of it is very difficult. And so, recently, I've worked with geophysicists who have all these tools that can look at landscapes in the subsurface and start to measure geophysical properties in the subsurface. So if we could understand distributions of these geochemical landscapes of reaction in the subsurface and relate that to the geophysical signals, we can map what the subsurface looks like and where weathering is happening. And I think that in itself would help us understand this puzzle of global weathering as well, namely, how it's distributed across our landscapes.
Oliver Strimpel
Susan Brantley, thank you very much.
Susan Brantley
Thank you. It was a pleasure to talk to you.
Oliver Strimpel
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