- Welcome, everyone, to Wednesday Nite @ the Lab.
I'm Tom Zinnen.
I work here at the UW-Madison Biotechnology Center.
I also work for UW-Extension Cooperative Extension,
and on behalf of those folks and our other co-organizers,
Wisconsin Public Television,
the Wisconsin Alumni Association,
and the UW-Madison Science Alliance,
thanks again for coming to Wednesday Nite @ the Lab.
We do this every Wednesday night, 50 times a year.
Tonight it's my pleasure to introduce to you Shanan Peters.
He's a professor here in the Department of Geosciences.
He was born in Lancaster, Ohio, and went to-- - [inaudible]
- Went to high-- Would you like to introduce?
And went to school at Fairfield Union High School,
did his undergraduate work at Denison University in
geology and biology, got his PhD at the University of Chicago
in the Department of Geophysical Sciences,
and spent three years at the University of Michigan,
and then, 10 years ago,
came here to the University of Wisconsin-Madison.
Tonight, he gets to talk to us something that's really
important for those of us who like pizza,
and that's the evolution of the crust.
Except for this one is the upper crust of the Earth.
Biogeochemistry, it's not every day
we get to have all three words together.
It's impressive that you have an undergraduate degree
in both geology and biology.
I remember when biogeochemistry came out as a word.
It was like, wow, this is amazing.
Rocks, life, and reactions. If only I were younger.
So I'm looking forward to hearing about this.
Please join me in welcoming Professor Peters to
Wednesday Nite @ the Lab.
[applause]
- Thank you very much,
and thank you for braving the elements out there today
and finding the tornado shelter.
Hopefully we don't have to use it,
but it sounds like we're safe here this evening.
So I'm going to be talking to you about the long-term view
of Earth's biogeochemical evolution.
And, you know, the Earth's a pretty remarkable place.
It's home.
We're quite familiar with it, but it's really weird.
And you can see a lot of what makes it weird and distinct deep
within our solar system in this image.
You know, we have water. That's not weird.
Water is everywhere, but we have it in all three states.
And you can see that here. That's pretty special.
Earth is very bimodal in elevation.
We have very high-standing continental crust
and very low-standing oceanic crust.
That's weird.
We are profoundly bimodal in the elevation.
That's due to tectonics.
We have active plate tectonics, and that's a weird thing.
And, of course, we have life, which is maybe not weird
but certainly important part of the story
we're going to hear about today.
And so all these things come together in the story I'm going
to tell you, and I think the best way to get a handle on why
you have to understand all three of these things operating
together as one evolving system, why that's so important.
By asking one simple question.
Well, not really simple, but a pretty straightforward question.
Why is there so much molecular oxygen in Earth's atmosphere?
That's one of the things that makes Earth just weird.
And this a rough composition.
The Earth's atmosphere, dry atmosphere.
It's about 78% nitrogen gas, about 21% oxygen,
molecular oxygen, O2.
Argon and then CO2, which we spend a lot of time talking
about for good reasons, is actually a trace gas.
It's not very abundant in Earth's atmosphere,
and we've clicked it up a little bit incrementally here.
We're above .04 permanently now, probably thanks to our activity.
Now, just to give you a sense of how weird this is,
you need to compare the present composition of the atmosphere
to where the atmospheres of planets come from.
And that is devolatilization of the magmas coming out of volcanoes.
In this case, on Earth they're coming out of the vents we can see active.
And if you look at the composition of gases coming out
of the volcanoes looks nothing like our atmosphere.
Okay? It's mostly water and CO2.
It turns out this is really variable from place to place
from details of geology underneath volcanoes and so
forth, but that's a pretty reasonable composition
for average volcanic gas.
Mostly water and CO2
and a few other trace gases mixed in there.
Okay, so our present atmosphere looks nothing like the source
of gases to the Earth's surface environment.
At all.
And why is there so much oxygen atmosphere?
I think a lot of people would say, well, because life.
And we have this amazing biochemistry tied up in enzyme
catalyzed reactions, ultimately sourced from the central dogma
playing out to make proteins that do amazing things.
And one of these proteins is RuBisCO here.
And this one's from green alga,
but that's in the chloroplasts of plants as well.
And what is photosynthesis
in this RuBisCO catalyzed reaction do?
Well, it takes the two most abundant gases in volcanic
emissions, CO2 and water, mixes it with sunlight,
and catalyzes this reaction to drive it towards making sugars
and releasing as a byproduct molecular oxygen.
Okay, so it's certainly the case that photosynthesis,
the evolution of this key metabolic pathway that takes
the most common gases coming out of planets,
carbon dioxide and water, harnesses the most abundant
energy source in the universe, probably, light from stars,
and makes energy, makes sugars,
and releases as a waste product molecular oxygen.
Okay, so that's a necessary condition for explaining
why we have so much oxygen in Earth's atmosphere.
But here's the problem: I can take this reaction
and I can write it both directions.
And here we are, we probably had dinner today,
most of us, or maybe a beer,
and we're in the process of doing this in reverse.
We're taking and we're breathing
in the product of photosynthesis oxygen,
we're using that, there's enzymes that catalyze this reaction.
You don't need to have them present.
It can proceed inorganically too, if you get it warm enough, that reaction is spontaneous.
And it's generating energy and releasing, as a gas, CO2,
and we're exhaling it into the room now.
Okay, so this reaction we can write in both directions.
You can produce oxygen,
but biosphere can also consume oxygen.
And that's what we do back and forth with plants.
Now, to illustrate how incredibly important
photosynthesis is but not really enough,
we can do this little thought experiment here. Okay?
We're in a nice, little tornado shelter as it turns out.
We can seal the doors and we can force this reaction
in this direction up on the screen here.
We can force this reaction, which is what we're doing right
now by breathing and digesting and processing our food,
and drive it this way.
But let's do it for everything except us.
Every cell on the planet, every living thing on Earth,
all the plants, all the animals, all the biomass on Earth,
let's combust it now, seal the room,
and ask the question: What happens?
What happens to Earth's atmosphere?
Would you be scared to unseal the room
and go out into this new world?
Well, let's do it.
It turns out we have a reasonable estimate
of the mass of life, living mass on Earth.
And this is what happens. Virtually nothing.
You wouldn't even notice.
It would be less green and you might have trouble finding food
unless you were able to synthesize it cleverly somehow,
but you would live the rest of your life
and not be worried about oxygen.
Okay, so photosynthesis is not why you have oxygen to breathe.
It's the first step.
Okay, photosynthesis is necessary but it is not
sufficient to have an atmosphere like you're used to enjoying and
breathing and consuming as a high energy animal. Okay?
So, what do we need to do to build up oxygen in significant
quantities in Earth's atmosphere or any atmosphere?
All we have to do is we have to take this reaction,
we're going to assume that some sort of photosynthesis to
generate as a waste product oxygen's required.
Maybe you don't need RuBisCO, but you need some biologically
mediated process to do that.
It doesn't happen spontaneously by any mechanism in significant
quantities that we're aware of. Okay?
So life is present.
What we need to do to generate significant quantities of oxygen
is make this reaction go this way, turn the Earth green,
but not let it go back the other direction.
To cut this process off so that it doesn't get oxidized and
consumed by some animal like you and me and we leave this behind.
And where does this end up going?
It goes into sedimentary rocks.
And I have a few examples here.
Afterwards, if you want to come up here and look at them, you can.
This is why you can breathe the atmosphere.
It's life plus rocks.
And it's not just any rocks,
rocks that contain organic carbon.
And this one, it sounds like a rock, it is a rock, but it's black.
And that black comes from organic carbon compounds
that are tied up in here.
And this one is important.
As was mentioned, I'm from Ohio, and unlike Wisconsin,
Ohio is at the edge of a sedimentary basin
that has a lot of this rock in it.
And this particular one is called the Chattanooga Shale.
It's a little south of Ohio,
but there's similar rocks in Ohio of the same age,
like the Marcellus Shale you've no doubt heard about.
And this is a fracking target. Okay?
This rock has organic carbon
that was produced by photosynthesis
and locked in this rock and we go down and get it.
So fossil fuels are hyperconcentrated versions of
this organic carbon produced by photosynthesis that's been
locked in the rock record, and we're getting better and better
at finding the nooks and crannies that have lower
concentrations of this stuff for better or worse.
That's what we're doing. Okay?
So in order to produce an atmosphere like we're used to,
yes, you have to have photosynthesis
but that really doesn't do much by itself at all.
It needs to be coupled to an ability to sequester
that organic carbon into the rock record,
and we need to do this in sediments
through a geological mechanism.
Now, if you stop for just one second and think about this,
you should have been suspicious of the notion
that we owe our oxygen to the rainforest or something.
It's not true at all.
And if you think about it, it better not be true because a lot
of us drove here today, and we're going to go back outside
and turn on our cars and we're not going to have to look in the
backseat and say, like, I'm sorry, Johnny,
you're going to have to hold your breath.
You know, we're not worried about consuming oxygen
on industrial scale out of atmosphere,
but that's exactly what we're doing.
There's so much organic carbon tied up in rocks that it has
left behind a huge reservoir of oxygen in the surface
environment, and we can see us consuming oxygen
and replacing it with CO2.
This is one of the most compelling lines of evidence,
I think, for why humans are the reason for the CO2 increase
that you can possibly imagine.
You don't hear it much, but this is it.
This is from Mauna Loa. These are actual measurements.
This isn't like a model or something.
These are measurements of the gas and little samples
taken monthly at Mauna Loa going back actually
way longer than this record.
It's a rare-- You should check it out.
It's an amazing record.
And this is oxygen up here, and this is CO2.
And CO2 is going up.
Now, this is what we talk about a lot.
This has a lot of consequences for the climate.
We're not going to talk about CO2 much today,
but it's going up and O2 is going down exactly balancing
that because the going up part is us consuming oxygen
and burning fossil fuels. Okay?
Now, a lot of, when I've shown this before,
a lot of people naturally wonder what this saw tooth is,
and I'll just tell you that this is a signature of the geological
arrangement of continents on Earth.
We are asymmetric as a planet.
And even something like the distribution of land mass has a
profound effect on the chemistry of the atmosphere on a seasonal
basis as the biosphere blooms and breathes in CO2 and pumps
out oxygen in the summer, like it's doing right now,
and then does the opposite in the winter.
All right? Okay, so the answer to the question,
why is there so much molecule oxygen
in the Earth's atmosphere
is, yes, photosynthesis has to happen,
but it's because there's so much organic carbon that has been
buried and sequestered away from the surface environment
and put into sedimentary rocks.
Okay, that's great.
The next question, then, is when?
When did we get this crazy atmosphere that has this
incredible abundance of molecular oxygen that was pumped
into it because we buried organic carbon
and basically made fossil fuels?
Okay, so when did we get it?
I know this is kind of hard to see just based on the colors
here, but I'm going to be showing this timescale a lot.
And just to get you oriented,
these are billions of years ago here,
and these names are time intervals
that are sort of the major subdivisions.
And Phanerozoic just means visible life. Okay?
So this is the time when animals are around.
Protozoic means sort of first life.
We know it's actually not the first life,
but that's the historical name that this time interval has.
This is the first life.
And there's very few animals until the very end of this here.
And then this says Archean,
which is the earliest part of Earth's history here.
And, actually, it goes back to four-and-a-half billion years,
but we're going to unceremoniously truncate
that part of the hot, early Earth.
Okay, so the first part of
when did Earth get its molecular oxygen?
Well, life has to be part of the story, right?
No oxygen in significant quantities,
molecular oxygen in significant quantities
until we have this metabolic pathway in place.
And there's been a lot of work on this.
This is a very recent paper in Science that looked at the
genome that these cyanobacteria carry around with them.
And the molecular distance between the key enzymes that
drive the basics of the cell record distances that tell us
something about relationship,
and that relationship is shown here.
And what these authors have tried to do is identified when
oxygenic photosynthesis within this clade evolved.
And that requires calibrating these distances with constraints
from the fossil record for time.
And when you do that and lay that on this,
this is what it looks like approximately.
Cyanobacteria and oxygenic photosynthesis with that lineage
specifically is very old.
Okay, this molecular estimate suggests by 2.3 billion years,
2.5 billion years, or maybe even earlier there were cyanobacteria
on Earth making molecular oxygen by photosynthesis
with the same enzyme that they're doing today. Okay?
Now, this is the molecular record that these cells
are carrying with them today.
We can turn to the fossil record
to look for other evidence of this as well.
And here are one of the most well known Precambrian,
that is before the Protozoic and Archean,
the pre-Phanerazoic part of the geologic record.
That's what I mean by Precambrian.
These are stromatolites.
They're actually really abundant around town here.
I'll show you some.
And what they are are just patterns and sediment
caused by microbial mats that grow.
And these are modern ones.
They're really rare in today's ocean for reasons
that are cool but I'll talk about briefly in a minute.
You're not likely to see one today.
But they stick up out of the sea floor.
You know, this thing I can break it off and hold it like this.
And they stick up positive like this
because they're basically cemented together with minerals.
And on the top surface of this
you can see these little bubbles.
These are oxygen bubbles being given off during the day
by cyanobacteria colonies
that are encrusting the surface.
And they grow up because the molecular,
because the cyanobacteria are trapping sediment and it's
solidifying and they're kind of pushing their way up to keep in
the light and so forth and they kind of grow like this.
Now, these fossils that we find are just patterns in rock
that are very distinctive.
They don't actually have cyanobacteria in them,
but they are formed in a way that we think cyanobacteria
were involved very clearly.
And it turns out that in the old part of the rock record,
even though you're never going to see them today,
geologists have known for a long time that back here
in the Protozoic, they're everywhere and they're huge.
This is a guy standing here.
This is actually in the UP of Michigan.
And you can see these lines here.
These are giant stromatolites.
These are like making reefs, basically,
in the Precambrian. Okay?
Now, so what's the physical?
We have the molecular evidence for cyanobacteria,
what's the physical evidence for having abundant stromatolites
or abundant cyanobacteria communities?
It's not enough to have one cell some place evolved
in the nook and cranny and one little colony is growing.
In order to make a lot of oxygen, it has to be everywhere.
The Earth has to be green.
And so we can look at this record to get a sense
of how many of these colonies are there.
Are they everywhere?
Are they occupying all the marine environments, for example?
To do that, we need to go everywhere
and find a lot of stromatolites
and document where and when they occur.
That's hard to do when there's not a database for this, yet.
But geologists love writing about these things,
it turns out.
And so, actually, this is our own Bob Dodd and Charlie Byers,
retired professors here in the Department of Geoscience.
And they're actually writing about rocks here in Wisconsin.
Actually, this is the Shorewood Hills quarry.
If you go around there, there's a little quarry.
A lot of you nodding. You probably know the one.
Go up and look for stromatolites.
You'll find them there. Okay?
And they love writing about them.
And here's the text that says the basal disk has stromatolites.
And so, in principle, one could just read all the published
literature and not bother to go in the field and find mentions
of stromatolites from all over the world and compile them.
That would take a lot of time.
Even for grad students that would take a long time.
A little side story, I'm going to tell you is,
I'm involved with the project here on campus.
I'm very excited about what's called Geo Deep Dive.
It's a horrible name.
It's actually a digital library that anybody--
It's a campus resource.
And it's actually connected to a high throughput computing system
so that we can read millions of scientific papers
and actually get information out of them.
Not just find them but get information out of them.
And that's what Julia here did.
She's actually a native of Madison
but didn't go to school here
but worked in the group for a summer,
and she did the first analysis of all these papers.
In this case more than three-and-a-half million papers,
wrote a computer program to find stromatolites
and figure out their ages based on the text.
That was done here on campus, and we are able to do this
in clever ways because the computers read the sentences
and parse them into linguistic relationships
between words and all this stuff.
If you're interested, I can tell you about it later.
But here's the result after doing this for the first time,
quantitatively, for all of the published
records of stromatolites.
And this is a busy graph, but it's the frequency from--
So if every marine sediment that we have was a stromatolite,
it would be one.
If none have stromatolites, it's zero.
So it's normalized for how much is there.
And what you're seeing here is today they are super rare.
There's virtually no marine rocks that have stromatolites.
But in the past, going all the way back
to 2.5 billion years ago, they are everywhere.
And this is a very, very conservative way to measure it.
If you look at a more sensible culling,
most of the Precambrian is everywhere,
stromatolites are everywhere.
So all the way back to where we think cyanobacteria
first appeared, these signatures of cyanobacteria communities
are just everywhere on the Earth.
The communities are in place.
Okay, so when did molecular oxygen--
When did Earth get this molecular-rich oxygen atmosphere?
Well, this happened, this key step happened.
The evolution of cyanobacteria, and widespread communities
of them, is at least 2.5 billion years old.
At least.
But that, as we've already seen, doesn't mean there's
a lot of oxygen in the atmosphere.
So we need to look at other proxies for molecular
concentration of, the concentration of molecular
oxygen in the atmosphere.
And oxygen is convenient for this purpose
because when you introduce it into the environment,
it just reacts with everything.
I mean, that's why I'm able to be all energetic up here giving
this talk because I'm powering my body with oxygen burning.
It's very energetic reaction,
and it's happy to react with many things.
And so introducing oxygen to our surface environment changes not
just life, it changes the physical environment, radically.
And there's very clear signatures of the perturbation
to the geochemical system that is caused by all the sudden
having a bunch of molecular oxygen
being pumped in the system.
And that plays out in all kinds of records
we can see in the geologic record.
There's a bunch of them.
I'm going to go by really quickly here.
But there's pyrite, fool's gold, FeS2.
It's not stable today.
If you got some pyrite and put it in a glass of water and,
shook it around, eventually it would dissolve
or it would rust away. Okay?
You're not going to find pyrite beaches in the world's oceans
because there's too much oxygen around and it gets destroyed.
It turns into rust and other things.
But if you don't have oxygen, pyrite's very stable,
and it will actually make beaches.
And these are actually layers of fool's gold sand that showed up
uniquely in the Earth during the little window of time when there
was no significant molecular oxygen in the atmosphere at all.
Now, when that world exists, there's also other things.
Things like iron become soluble, much more soluble in the oceans
than they are today because oxygen grabs hold of iron
and makes it rust, immediately.
But if oxygen is not around to do that,
Fe plus 2 will just be available like chlorine in sea water.
And there's these interesting sediments called banded iron
formations that your car is quite literally derived from.
This is where most of the world's economic iron deposits
come from because there was this time when molecular oxygen's
bubbling into the Earth's surface environment for the
first time that the Earth's oceans literally rusted.
And these unique sediments show up only during this time,
presumably in response to this load of iron that was mopped up
by this molecular oxygen and put there forever afterward.
Okay, and then there's other more fancy proxies.
This sulfur isotopes in the Earth,
remember sulfur's a common volcanic gas,
in the presence of oxygen and without oxygen there's isotopes
that fraction in very different ways that show a clear signature
of divide between the old record that has no molecular oxygen,
in which case this fractionation is very visible,
and after the time when we have molecular oxygen in the
atmosphere, in which case we just lose that signature
in the sulfur isotope record.
So there are a variety of different proxy records
that say this interval right here,
this band between 2 and 2.5
is changing radically in the molecular oxygen concentration of the atmosphere.
After that we see things that tell us that there's
plenty of oxygen around.
That includes red beds. So, soils that are red.
You've probably seen them if you've paid attention.
They're red because of rust in them.
The oxygen latches on to iron minerals that are liberated
during chemical weathering, and it rusts the soils.
Those show up for the first time.
Other things, like gypsum, you may not think about it
but gypsum, drywall, that's a signature of an oxidative Earth.
The sulfate in there is sulfur that has been oxidized,
consumed molecular oxygen in the atmosphere,
put in the ocean as sulfate,
and we get it back as gypsum. Okay?
That shows up in the rock record very late and tells us, okay,
there's a lot of oxygen in the atmosphere, enough to make gypsum.
The other major record we have for the history of atmospheric
oxygen of course is biology.
Organisms care deeply about the concentration of oxygen in the
atmosphere, and there's sort of three main modes of life
reflected in these three time periods for which named after
life, not surprisingly, where we have a world that's dominated by
prokaryotes, like cyanobacteria, and then eukaryotes.
And, actually, this specimen here is a little eukaryotic
alga, and it's actually, it's about 2.1 billion years old,
and it's found just up the road in the UP of Michigan
in the Negaunee iron formation.
But it's, you know, big eukaryotic cells show up at
about 2.1 billion years.
Then there's a long period of time where not much happens,
and then we get the Cambrian explosion and the wonderful,
fast diversification of animal life at the end of the game.
Okay?
Combining all of these different proxy records together,
we've arrived at sort of this canonical model
for the history of molecular oxygen concentration in the Earth's atmosphere.
Again, this is not my work.
This is synthesis work from all these different sources.
Now, we know there's this time when there's virtually no oxygen
in the atmosphere for all those proxy records like the mass
independent sulfur fractionation I talked about before.
We know cyanobacteria weren't probably around until somewhere
in this pretty long but still, you know, this time frame.
And then we have evidence that oxygen is being produced
and released in the surface environment.
Now, there's a lot of interesting debate about
whether or not oxygen rose really quickly here,
up to near modern levels, this line up at the top,
and then fell, but what we do agree on is that sometime after
that initial what's call the great oxidation event where we
cross this divide between no oxygen and at least some
molecular oxygen happened around here.
And then there was a long period of time,
most of Earth's history, a good billion years or so,
often called the Boring Billion,
before oxygen rose rather
precipitously into the Phanerozoic.
And so one really important question is not to say,
is really why is there this long delay?
Oxygen was being produced. Geology was happening.
The world was doing its thing.
Why was this long delay between the onset of oxygenic
photosynthesis and the buildup of oxygen in the atmosphere?
That's really the question.
Now, I hoped I've convinced you that
these things must be part of the answer, right?
Rocks. There's something about the rocks.
And so maybe the answer is encoded in the burial
of organic carbon that's in rocks.
Seems reasonable.
But here's the rub.
I'll cut to the chase and tell you that is correct,
but it took us, as geologists in a discipline science,
a while to figure this out, I think.
And some people will still disagree with me
just to be clear about the record.
But I'm going to make the case that we kind of missed
the picture here a little bit.
And this is why. This is geology in your face.
This is the Andes Mountains.
I got back from a wonderful
alumni-supported field trip there
with a group that's working in the Andes,
and it was fantastic, in-your-face geology.
Erosion is like the thing you're struck by in the Andes.
Volcanism and erosion. Just in your face, everywhere.
There's rocks, they're forming new rocks by erosion,
and they're being destroyed.
And geologists have looked at these things, the rock record,
in a way that is traceable all the way back to this guy,
Charles Darwin, who is of course a geologist, right?
He was well known at first for being a geologist and then
secondarily become well known for his contributions
to biology, which were many.
And in "The Origin of Species," definitely one of the most
influential scientific texts ever written,
he has a chapter titled, not very subtly,
"On the Imperfection of the Geological Record."
And in this chapter he says,
"I look at the natural geological record
"as a history of the world imperfectly kept.
"It's written in a changing dialect,
"and of this history we have just the last volume.
"And of the last volume, the last few chapters.
And of those chapters, a few lines." And so on.
And he portrays the geological record as this tattered
"manuscript upon which the history of life, in particular,
but Earth, in general, was written,
but then it has been ravaged by time and geology
to render it not useful, ultimately, in his argument,
for really telling us about the history of life,
mainly because he had peculiar ideas about what he thought he
would expect to see in the fossil record, I guess.
Now, what is good about this hypothesis is
it makes very clear predictions.
And this is basically the prediction.
Here's a little cartoon of it.
Erosion happens and erosion makes sediment from rocks,
and those rocks, that sediment gets put back into the rock record,
and this giant thing just turns,
and its slowly going away, and if we watch it over time,
so nothing ever changes because it's a cycle,
and if we watched it over time, there's less and less and less
and less of a given age as we go backwards.
And, mathematically, this is really the prediction.
That is the amount of rock that we have at any time, "t"
say 500 million years ago, is a function of how much we started
with times an exponential decay function, the decay curve. Okay?
This is the formal prediction from Darwin's description
of the geological record.
It also, by the way, is wonderfully Lyellian and Huttonian.
This uniformitarian view of the world where nothing changes.
It's a giant cycle.
Things come in, things come out, nothing goes anywhere.
In fact, Hutton took it to the extreme
to think that the Earth had no beginning.
Everything was cyclical and so on.
Now, this is a prediction but getting the data together to
test this was very difficult and it was like, what,
I can do the math, I promise, 110 or 120 years
before we had any attempt to actually test this hypothesis.
Okay? And this is actually it. 1970 Nature paper.
It's hard to see the highlights I put here, but in reading this,
I love this stuff.
This is a very clever thing.
This was done to address the denudation of the continents.
How fast the continents are going.
And just for fun and completeness,
this person concludes that "man's intervention has
"increased the overall denudation rate of the land
"from 9 to 24 billion tons a year."
In this communication, "I attempt to verify this pre-human
"estimate and to defend its general applicability."
This was motivated in 1970 to try to figure out how important
humans are moving around sediment on the Earth
relative to geology. I thought it was pretty cool.
But this is the result over here.
These are how much rock we have of given ages, going back here.
The details aren't important.
What is important is that there were virtually no data.
Getting these data together is really hard.
You have to compile syntheses of geological knowledge
that are difficult to assemble.
You have to get a big picture view
of how the upper crust is put together.
It's not easy.
And the only actual data for this analysis were
from a Russian stratigrapher right through here,
and all this was a mishmash, okay,
of extrapolation, basically.
However, this fit the notion very clearly that geologists
had in mind that the Earth's crust must cycle continuously
and uniformly like this,
and this and other analyses cemented the notion.
It was a done deal.
And so here is a 1976 paper, controls of--
Now, everybody knew this thing,
this wheel that's turning slowly, constantly.
It's huge. Right? It's really big.
But it's turning slowly and constantly.
And here's this abstract.
Geochemical models of the Earth's surface environment,
focusing on O2 and CO2,
suggest a dynamic steady state system exists,
and that steady state is nothing changes and, if it does,
something else has to happen to the system.
It has to be perturb biologically
or by some other forcing mechanism.
The geology process by itself can't do it.
I'm going to ask a really simple question:
Does sediment quantity decline exponentially
with increasing age or not?
We need to know the answer to this question, like for real.
So the data set I put together to do this
is called Macrostrat.
I'm proud of it, but I'm not going to say much, I promise.
Basically what it is is a big compilation of basically
what you would get if you took a cookie cutter punch
through the crust and looked at the rocks
and identified their type and age, that's what it is.
And so these little points in the deep sea,
we're going to show results from the deep sea
and this is the last time you're going to see this map.
There's a bunch of them spread around the deep sea.
And we're going to really focus on this North America-centric
view of the world, and I don't have time to go into the reasons
why I can show that this is a good proxy
for continental crust globally.
It's a large enough sample that's diverse enough
tectonically to be representative of crust
everywhere, and I'm not the only one to have found this.
But I'm going to gloss over that part going forward.
So we're going to zoom into North America here
and look at this in a little bit more detail
just to have a sense of where the data are coming from.
Here it is. This is a little column here.
This is the Exshaw column.
They all have little names and stuff.
And, obviously, you can see that
this is an extrapolation around control points.
That part's also not important.
But what is important here, I've cut off the Phanerozoic,
so this is the Cambrian.
The Phanerozoic portion lasts 541 million years.
And the white chunks here are gaps, are gaps in time. Okay?
So there's no rock of this age at this column.
The colors are chunks of time here that have rock.
And the color of the rock is telling us
what type of rock it is.
At its fundamental level, this is what this database consists of.
Now, the first thing I want to make abundantly clear,
if you pay attention to Darwin or if you've read about this,
you'll recognize that Darwin also described the geological
record, not just in terms of getting worse with age,
but also being gappy.
And the gappiness of the rock record was of concern to him.
He was absolutely right there.
There's lots of gaps, and the vast majority of the time
at any given place tends to be occupied by gap. Okay?
But music is the space between the notes, as Debussy said,
so we're going to see why that's a signature of the process
we're trying to get at.
Okay, so that's what these columns look like.
They're regionally composited.
They include, importantly, the subsurface too.
So down as deep as we go, typically.
That's really important, and there's a whole bunch of data
acquired that are sort of linked to these rock types and stuff
that we're going to depend on later.
Yes, we've made a mobile app. I'm also very proud of this.
It's free. Android and iOS. You can download the thing.
Wisconsin Public Radio has a nice little interview,
if you want to dig it up and hear my view of what it is.
I'll let you do that later.
But if you turn it on, it just gives you, like,
our best guess of what you're standing on, geologically. Okay?
So hopefully it'll be fun, and it gives you really good maps
and you can make observations and stuff.
So check it out.
If you make your stuff public,
you can explore it around in an archive like this.
It's for nerds like me, really, but if you're interested in just
being out in the field and learning about rocks around you,
this will help you, trust me.
Check it out. Okay, so let's get to it.
Ask the question, with these data,
does sediment quantity decline with age or not?
What I'm showing, over here, is just one time slice. Okay?
One time slice through the stack of geology.
And where there's color, or solid color in this panel
or color here, there's rocks of that type,
that type doesn't matter for us,
but there's rocks there of that age. Okay?
Where there's white there are no rocks of that age.
Okay, so those are the hiatuses.
And so what I'm going to show you are a bunch of measurements
of quantity measured in this way.
The simplest way is to just simply measure the proportion
of columns that have sediment, the fraction,
the proportion coverage of sediment on the continent.
How many columns have sediment divided by
how many columns are there. Easy.
Or we could measure it in square kilometers or something like that.
Either one. I'll show you both.
Now, before I show you the boring time series,
which will put you to sleep but I'll try not to do that,
here's a little animation that shows qualitatively
what we're going to do.
Only from one billion years up to the present.
Okay, and it's going by really quickly here.
But I'll narrate it.
So for a billion years, up to the,
this is the start of the Phanerozoic right there.
Now, remember, this isn't fossils.
This isn't diversity of life.
This is just rock. Okay? Just rock.
And so you can see that something changes
right about there pretty dramatically.
And we're going to see evidence
for that quantitatively in a second,
but I'm going to be showing you time series
that summarize this madness in numbers.
And here it is. Okay?
This is now going, so we saw that animation from here,
and so that big jump was that.
That you saw visually.
And we're going to be going all the way back
to two-and-a-half billion years, typically.
Now, before we talk about this long period of nothing in that
all the sudden a huge increase, before we talk about that and
why looks completely different than most of Earth's history
from, say, 600 million years ago all the way back,
we want to as the question,
is the Precambrian just bad somehow?
Like, do we just not know it very well?
And the answer is sort of.
We can see some evidence that this is true.
Now, it's a little subtle here, but bear with me.
These packages of sediment at each column, okay, that we saw,
those little chunks of color, and what we're doing here is
measuring in the Phanerozoic, in blue,
and in the Precambrian, in red,
and we're measuring the duration
in millions of years of those packages.
And we can see that they're different.
The Precambrian is longer in duration than the Phanerozoic.
Now, the reason for this is really simple.
In the Phanerozoic we have evolution of animals happening,
and we use that to tell time.
We're able to see evidence for missing time in rocks
that is hard to see except through the signature
provided to us by evolution.
And so in the Precambrian we miss those things and we assume
that the record is more continuous than it is,
and so my columns don't break out packages
like they do in the Phanerozoic.
And we can see that here quantitatively.
However, the Precambrian is also thicker in meters. Okay?
So there's more rock in a little longer chunk of time,
and if we take the combination of the two,
it's what's really important for today,
is the rate of sedimentation, accumulation rate,
meters per million years.
The two distributions collapse.
Okay, so take my word for it that there's no evidence
that there's something fundamentally broken
about the Precambrian record here. Okay?
So this signature is largely real. Okay?
So let's ask the question, is this exponential? No. Right?
We can fit and exponential curve to this.
Force one. We can fit an exponential to anything.
It doesn't mean it's a good description of the data,
and this is a horrible description of the data.
And it gets a very long half-life,
but that's not important.
It's just a bad description, and why is it a bad description?
It's because the exponential models predicated on this notion
that once you have a package of rock,
it has a one-way street out of the system,
and as long as that's happening approximately constantly,
you'll get an exponential decline.
Decline should be the signature.
Actually, there's only two intervals of time
that show any decline at all
as a function of increasing age.
Right here, decrease, and then a whole bunch of up and down but
no decrease over long time and then another decrease here.
All of the net decrease in quantity with increasing age
happens in just those two little bars, which is remarkable.
And right away not what Darwin would have expected.
The rest of the time is these intervals where it's going up
and down, up and down but not going anywhere.
Not going up, not going down.
It's a state of oscillation back and forth around some average.
And that's why exponential is a horrible fit.
I'm going to speed up here,
but everybody has heard of plate tectonics.
There's ocean crust that goes away.
There's continental crust. You're standing on it.
And it's old because it never goes down,
the continental crust.
Okay, so there's a huge bimodal distribution in the ocean
and the continents that showed up
with the elevation that we talked about,
and it shows up tectonically in survivorship. Okay?
Now, let's go back onto the continents.
There's some parts of the continents that are flooded
with shallow seas, like off the coast of Florida.
This little ramp here you can see on Google Earth
is shallow water because it's on continental crust.
And this big drop, boom, down to the bottom,
that's oceanic crust.
Okay, so some parts of the continents are flooded
with seas, which means sediment is accumulating in them.
And some parts of the continent, like Wisconsin,
are not and sediments are going down the river.
There's these different domains of sediment.
We can do a better job of parsing out sediments into
different domains, and that's what I'm going to show you here
because it's sensible geologically.
So we're going to take that curve we saw before
and we're going to decompose it into deep sea, non-marine,
which is the sediment up above sea level on continents,
and marine on continents.
Here's the deep sea record.
This is the record of, measured in this same,
proportion of coverage, just like before,
and it drops down precipitously just like we would have expected
from the beginning and Darwin's point of view.
And the reason for that is the sea floors are young,
they're constantly being destroyed,
the oldest sea floor on Earth is around there.
That's still in place. Okay?
If we expand this up a little bit,
it fits the exponential pretty well.
That's a good mathematical description with oscillations
that are cool but a subject for another time.
Where we have a clear signature of formation and destruction,
we see a clear signature of formation and destruction of
sediments, just like Darwin would have guessed.
Now, how about the non-marine record.
Now, the non-marine record is an in-your-face record.
We see erosion happening all around us.
Well, that's because we live above the ocean.
We'd have a different perspective of the Earth
if we were fish.
But erosion is very heterogeneous on land.
We have rivers at high elevation and low elevation and so on,
and this is the total number, again,
proportion coverage of sediments on the continents,
and it drops precipitously back through time,
but it sticks around forever down here.
And so if we zoom in here just on the past sort of 400 million
years or so, it's sort of okay exponential fit.
Not very good but not a passable exponential fit to the data.
This makes sense because unlike the sea floor,
which all undergoing sea floor spreading and subduction,
non-marine sediments are everything.
They include glacial sediments dropped unceremoniously from
Canada by ice on Wisconsin that's slowly eroding away.
That's non-marine sediment.
The sediment in Lake Monona, it's great record
but not very long and it won't be here forever. And so on.
It's just a really heterogeneous mix of things,
and you're mixing these things together
and measuring one exponential fit,
and that's like measuring the half-life of the atoms.
It doesn't make any sense.
Some stick around a very long time, some don't,
some never go away.
And we're mixing them together here.
But still, we see a very clear signature of there's loss
because these sediments are deposited in the place
where that loss is happening.
How about marine sediments?
So, marine sediments have no such trend of decreasing
quantity with increasing age.
In fact, in North America,
there's way more Cambrian sediment,
we're standing on it, by the way,
than there is of recent marine sediment.
And so, obviously, and exponential fit to this does nothing.
It has no explanatory power at all.
So what is controlling the history of marine sedimentation
on continents, the rocks of Madison.
Bascom Hall is built on marine sediment,
not through non-marine sediment, not deep sea sediment,
marine sediment put here 500 million years ago by seas.
This signature records tectonics. Okay?
It records tectonics.
It records the-- This is that same,
in the Phanerozoic now measured as area,
rise and fall and rise and fall again, often called the M curve.
This is the super continent break up coalescence,
break up and coalescence cycle.
That's the quantitative thing
that's pushing the seas around on the continents.
And, basically, the world is going back and forth
between one that looks like this, more or less like today,
to one that is like this,
like the rocks that Shorewood Hills quarry were deposited in,
a shallow sea that lapped all the way up to Wisconsin.
And that state is changing, and that state is what's giving you
these wild oscillations in the Phanerozoic. Okay?
So this green part here, these changes are caused by
continental flooding changes which are caused by
mostly tectonics, a little bit of climate,
and that's what's driving it.
And I would argue that's what's also driving this.
This decline here is driven by deep sea, non-marine,
deep sea sediment and non-marine sediment.
What about this one?
This is the only other decrease we have.
Maybe this is caused by erosion. Well, let's go look at it.
This is, again, the shallow marine record.
That big decrease at the start of the Phanerozoic
is a marine sediment signature on the continents.
Let's go look at it in the field.
Wisconsin's actually a reasonably good place to do it.
This one is from Colorado.
And this is pretty typical of this boundary right here.
This is Cambrian marine sediment deposited on top of granite.
Very old granite, in this case a billion-year-old granite.
The Grand Canyon has at its base a surface
that John Wesley Powell called the great unconformity.
And you can see it from the rim of the canyon.
It's one mile of Phanerozoic sediment,
starting of that age right there,
sitting on top of metamorphic basement rocks
that are 1.7 billion years old.
Okay, so this decline here in sediment quantity
is not a decline at all.
It's actually an increase in flooding across the continents
that were once exposed and eroding.
And the oceans covered them and put sediments there,
and that sediment is still there to this day.
Okay, so that actually records tectonic state change
across the boundary.
What about this? A mystery for another time.
Okay, so we have three different components.
Deep sea sediments that have clear signature of cycling in them,
just like Darwin predicted
because that's the world that he described.
Non-marine sediments, very close to what Darwin described
because he's walking around on it,
probably impressed by it in the Andes just like I was.
And shallow marine sediments,
which have none of that signature at all.
Instead they have a process signature driven by tectonics
and changes in state of the world.
And, it turns out, that 75% of the volume of sediment
we have is that stuff. Okay?
So the answer to the question:
"Does sediment quantity decline exponentially
"with increasing age, is no."
It does not. Okay?
Now, this history, if you've been paying attention the whole
way through and are easily pasting things together in your
mind, this should look very familiar to that proxy history
for atmospheric oxygen and life that we saw before.
Prokaryotes, eukaryotes, metazoans.
And this curve for the metazoans in the Phanerozoic is,
this is actually what it looks like on a relative scale.
The history of atmospheric oxygen, in red here.
The canonical history, maybe it went up to one or maybe not,
but a long time or it didn't do anything and an abrupt rise in
the Phanerozoic and then wiggles around in the Phanerozoic.
In black now is the data we just saw on a logarithmic scale
for the amount of sediment on continental crust.
And it, at the sort of pattern level,
looks very similar to the history of atmospheric oxygen.
A long period of not much going on, an abrupt rise,
oscillations in the Phanerozoic.
Now, we can do better than just say, well, look, it's similar,
good enough, and go home because we can actually link this and
test the hypothesis that there's a direct causal connection by
calibrating a carbon burial model that is the ultimate
source of oxygen in the system.
And so the change in mass of oxygen in the atmosphere
as a function of time
is equal to the total amount of organic carbon
we're bearing.
The mass of it.
Molar ratio, one to one.
One mole of carbon goes in, one molecular oxygen comes out.
Now, here's the rub.
That oxygen can be consumed, so there's a negative sign here,
that can be consumed by organic carbon.
We are doing it when you go home and turn on your car,
you're going to be doing this reaction part right here.
And that reaction proceeds at a rate that's related to how much
we have around, the amount of organic carbon
that's actually in the crust.
You can think of it as, like, the charge capacitor.
How much that is, weakly dependent on the concentration
atmosphere oxygen and a constant.
And then this term is really important.
It turns out that oxygen reacts with everything.
Well, not everything but a lot of things
in the Earth's crust itself.
That's what this term is. Okay?
Now, the first order of business is to calibrate this F org.
How much carbon is going into the crust and when.
And to make a long story short, really quickly,
this is what you need to do it.
This is now the same data but turned into a volume flux. Okay?
This is in cubic kilometers per million years.
This is now the weight percent of organic carbon.
How black the rocks are, on average, basically.
And this is empirically calibrated with data.
I don't have time to tell you about it, but trust me,
it's a good estimate for how much organic carbon is in rocks.
And then this is the combination of these two
to get you the burial flux.
The organic carbon burial flux in moles per million years.
Now, here's a question for you: how much O2 has the surviving
rock record, the one that we can point to and say, "Yep, there it is."
How much is that sourced to the Earth's surface environment?
So here's part of it right here.
How much total oxygen is in that thing?
It's about 35 times the present
atmospheric oxygen concentration. 35.
If you put that oxygen in the atmosphere right now in a
bottle, 35 times that would be what the geological record
has stuffed into the Earth's surface environment.
The reason we don't have 35 atmospheres worth of oxygen
is because rust happens.
Other exit channels for oxygen.
Your car is doing its job right now
rusting away a little bit in the rain.
Okay, so the way we do this model is we measure that.
That's this thing.
The "k" constant or this constant we take directly
from an estimate of oxidation of iron in the crust.
This one, this "k" constant or this constant we take directly
from estimates of organic carbon weathering in crust,
and we combine them together to make a model, forward model,
calibrated by the actual record.
For oxygen, this is what we get.
Now, for convenience sake, in gray,
you can barely see it here, is the canonical model.
There's different ways to assign carbon values and so forth.
The details of how we do that are subsumed as error envelopes
around this estimate here.
And so there's two different models of how to distribute TOC,
and the whole model space, including some Monte Carlo
simulations, is shown by this red curve here.
And so this is, on an absolute scale now,
the fraction relative to the modern atmosphere of oxygen.
And we see a whole bunch of things
that look just like the empirical proxy records,
including a delayed Phanerozoic rise.
For the nerds in the audience,
it actually turns out the Cambrian explosion
happened when this first rise started to go up,
but the rise of oxygen to modern levels
was delayed even further.
It didn't really get to modern levels
until probably into the Devonian, probably.
And then a big drop at the Permo-Triassic,
the largest mass extinction in the Phanerozoic, right there.
The thing that we see is the sustained low.
This is probably the most impressive result, honestly,
this transition and this long state.
And we see these early possibilities of transient whiffs.
Now, of course, down here it depends on if that's
photosynthetic carbon or not.
Okay, so the oxygenation, the timing and magnitude
of oxygenation of the Earth's atmosphere.
Yes, it depends on biological evolution
but biology has been doing its thing forever, basically.
The timing of the oxidation of Earth's atmosphere to get to a
place where we can have life like us
is dependent entirely on geology.
It's determined by a geodynamic process that turned on
and suddenly allowed the crust to store fossil fuels, basically.
I've gone out of my way to argue that erosion is not a thing.
But there's clearly still erosion happening, no question about it.
And if you were forced to come up with a model for it, that would be it.
And, you know, it has a really long life.
Now, I'm going to just emphasize something here.
See this little positive residual here?
It's positive this whole stretch.
Most of the time it's negative, and so there's a pretty
pronounced positive residual here.
I'm going to re-express this axis set of
proportion coverage area.
It's the same data.
It's just that there's non-random distribution of areas.
That's all it means.
But you can see this bump gets bigger.
And now we're going to fit an exponential to it here.
And you can see that positive residual in the same way there.
Okay? Now, this is very slow erosion, right?
The half-life is 1.2 billion years.
I mean, it's basically not doing anything
at the Phanerozoic scale.
But, still, this rock is two billion years old.
It's a couple half-lives.
And so we can undo the effects of erosion mathematically,
and this is what we get.
And this is the moment where everything should come together,
and you're like, "Oh, wait a second."
This suggests that maybe we accumulated a Phanerozoic-like
mass of organic carbon in Earth's crust for geodynamic
reasons similar to this one but then it stopped.
And we lost it.
Now, if we take this as our calibration for the organic
carbon burial history and we use that to predict what atmospheric
oxygen would have done, it looks like this, using the same rules.
And what we find, as some people have suggested based on proxy
records, shockingly, that atmospheric oxygen may have
gotten to near modern levels but not stuck around.
Which means it may not stick around in the future.
Now, if you think about this for a second,
it should be really obvious because burying--
Let's say we reach this hypothetical steady state that
Darwin so wants and geologists still somehow cling to.
If that's the case, that means organic carbon
is being buried and oxygen is being sourced,
but that oxygen is being consumed by organic carbon
that's being removed and weathered from the system.
And so you are at steady state and there is no
flux of oxygen into the atmosphere.
What's being produced is consumed.
And that would be fine except for the problem
of this term right here.
Your car, basically.
There are other exit channels for oxygen from the system.
And it's not sufficient to have organic carbon buried
that's produced by photosynthesis.
You have to bury more of it and more of it and more of it so
that the mass in the crust is continuously growing.
That's the only way to maintain an atmosphere that has oxygen in
it, and that is definitely dependent on geodynamics.
Now, within the Phanerozoic, a transient hiccup in the
continent's ability to do this happened.
Right here at the end of the Permian.
That was the largest mass extinction in the Phanerozoic.
A transient interruption.
In fact, geologists have called this the coal gap.
And there's all kinds of reasons for the coal gap,
but it had a big impact on oxygen in the atmosphere.
We got out of it because, well, you might remember the power
plant over here that used to haul in coal.
When I first got here they were still doing this and they converted it.
That coal came from rocks of this age.
There's a huge amount of Cretaceous and Paleogene in
coal, young coal, that rescued the Earth from this state.
It grew the mass again.
Now, this suggests that it's not sufficient to have oxygenic
photosynthesis through this cool enzyme called RuBisCO
or something like it on another planet.
It's not even sufficient to have plate tectonics.
It has to be combined in a way
that allows you to continually go this mass,
and an oxygen-rich atmosphere may not be permanent anywhere.
So I'll end with just the main point I want you to take home
from this, and that is the Darwinian view of the rock
record, this incomplete, tattered manuscript
is really missing the point.
It has an archive of climate and it has an archive of life
and it has an archive of tectonics in it.
We can read the history of Earth
through that manuscript pretty well.
It turns out, however, that the formation of this manuscript
is the thing that's driving the entire system.
It is an active participant in the evolving Earth system.
Without the sedimentary record,
there is not animal life on a planet anywhere.
And I'll leave you with that. Thanks.
[applause]