This is Geology Bytes with Oliver Strimple Compared to our
neighbours 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 field and in the lab. Recently, she has scaled up our
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 Bytes.
Thanks, Oliver. Nice to talk to you this
morning. 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? 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 a 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 as carbonate rock.
So really it's a sequestration process that produces rock out
of the CO2 in the atmosphere. But this process operates over
geological time, so how do you go about studying this in the
lab? 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.
Did you run these dissolution experiments by grinding up
different rock types, For example granite and basalt?
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, it can have olivine, it can have
pyroxene. But if we separate the minerals
and 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 your backyard.
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? 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, but any temperature you want to
extrapolate to down in temperature, up in 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 data set now for
mineral dissolution rates. And can you give me a ballpark
of how that rate varies with temperature?
The rate double S with 10s 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 Thermo regulates how the Earth's global
temperature has been regulated over time.
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? 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 a 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.
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?
Under some conditions be the overall rate of weathering is
limited by one process or another.
So you just said what about in floodplains sort of flatlands
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
Feldspar's 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
flatland, 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 sort of end
member situations where you're either limited by very, very
slow erosion or actually limited by kinetic weathering.
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 also have to
do something about gazettes or frozen areas.
How do you scale that up? 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 erosive transport limited, how much is kinetic
limited. But as you also mentioned,
something like 1/2, the globe also has such a small amount of
runoff going through the system. So it's 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.
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 due to the
temperature sensitivities you measure in the lab?
Does it make it more temperature sensitive, less temperature
sensitive, or some of one and some of the other?
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 tried 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 and dry lands that the temperature
sensitivity is pretty small. 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 run
off and precipitation, and by weatherability factors such as
lithology, porosity, permeability, type of
vegetation, and position within a 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? 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.
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.
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 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 and 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?
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 out, 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 Hamayas 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 effects 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.
So would it be fair to say that during the prevalence of
supercontinent, 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? I think what you said is our
baseline set of hypothesis. 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 sensitivities, again, it depends
what's the rock type. It depends where the mountain
building's 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 a 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 it causes this higher temperature
sensitivity? Is it perhaps modulated by other
processes? And 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 mountains go up.
And so there's other complexities that may actually
cause not only CO2 drawdown, but actually caused some CO2 release
during mountain building events. What's about the effect on
weathering of life forms, both microscopic and macroscopic?
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'll 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, with 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.
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? Sure.
Well, what we're doing today is totally changing this cycle that
we're talking about. South 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 going
to 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. There are silicate rocks on the
surface of Venus. Why have they not been able to
provide thermostatic temperature control there?
Venus is closer to the Sun. It's 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. And I guess on Mars the
situation is there's just not much of an atmosphere at all.
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. 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?
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 disseminated on
fields. We do that all the time with
lime. So many farmers fields are lime,
so they take limestone and grind it up and put it 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 rocks.
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 than you'd put in, in terms of
the digging, grinding, transporting and spreading.
I discussed enhanced weathering in an earlier episode with Phil
Renforth and learn 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.
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. 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? 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 as 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 and 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. What are you working on at the
moment? 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 A1 dimensional worldview
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 front 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 a geomorphologist 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 Ridge top,
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 could 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.
Susan Bradley, thank you very much.
Thank you. It was a pleasure to talk to
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