Welcome to an AAPG Distinguished Lecture. I'm Kristin Bergmann, a carbonate
sedimentologist and assistant professor at MIT. And today I'll be talking about
the climate extremes at the dawn of animal life, but before I begin, I'd like
to think AAPG and the AAPG Foundation for making this lecture series possible.
I'd also like to acknowledge the students and postdocs who work with me at MIT
who make this research possible-- I'll highlight some of their work today-- as
well as my collaborators and funding agencies, including Shell and Petroleum
Every year, scientists and journalists document extreme weather events, in
precipitation, temperature, and wind, that are more significant and extreme
than any in recorded history, events like Hurricane Harvey hitting Houston in
2017 and extreme melting in Greenland in 2019. These weather events are a
direct effect of the increases in atmospheric CO2 that we are all contributing
to in our everyday lives.
One record of atmospheric CO2 from Mauna Loa in the lower left shows the
gradual increase year by year over the last decades. And as we experience these
extreme weather events, we're all wondering how will the earth respond to more
increases in atmospheric CO2. What will the effects be?
And there are two main ways that we have to predict the future. One is
through modeling efforts. And there are a range of climate modeling
predictions. I've highlighted this one by Schneider et al. In 2019 because it
suggests that as CO2 rises, we'll lose a cooling agent, the stratocumulus
clouds, in the equatorial regions and that lots of clouds will lead to a
threshold and significant increase in Earth's temperatures, something like 7 to
9 degrees at the equator.
We can also probe deep-time records-- deep-time records, like ice cores,
that tell us about climate change across glacial-interglacial cycles, deep-sea
records that tell us about some of the best analogs for today-- extreme warming
events like the Paleocene-Eocene Thermal Maximum. If you look at the picture on
the right, it's a deep-sea sediment core. And the PETM event is a very sharp
transition from the white carbonate-rich sediments to the red clays. That
warming event can be studied across the ocean environments and on land to give
us a sense for the wide range of effects from increasing CO2 at a very rapid
Studying deep-time records allows us to have a model for how Earth's climate
operates. And this model includes three main components, and they help govern
Earth's temperature over long geologic time scales. The first component is CO2
addition to the atmosphere through volcanic activity, mid-ocean ridge
volcanism, very similar to what we're doing today with our daily activities.
That CO2 addition to the atmosphere reacts with silicate minerals, minerals
that are found in igneous rocks.
As those minerals weather, they add cations and anions to solution that then
become sequestered in carbonate rocks. So those carbonate rocks then get
reintroduced to the mantle and complete the cycle. And so this cycle, through a
temperature dependence at that rate of silicate weathering, helps keep Earth's
climate within tens of degrees of present temperature.
One of the questions that motivates my research is this question of why life
was microscopic for so much of Earth's history, over two billion years, and
what changed to drive macroscopic animal evolution. And so if you actually look
at rocks that capture the Precambrian, you'll find very few fossils, and
they're microscopic. You'll only see them in thin section. As you transition
into the Phanerozoic, macroscopic animals, soft-bodied organisms, these
Ediacaran fossils, and finally organisms that make their skeletons out of
calcium carbonate all evolved in relatively short order.
The question that I'm curious about is could changes in Earth's climate have
played a role? Temperature has a very strong control on macroscopic eukaryotes
and animals. If you look at the plot on the left, you can see that the
eukaryotic organisms and animals have a much narrower temperature distribution
than archaea and bacteria, the organisms that existed for much of Earth's
history, over 2 billion years. Temperature would also affect the amount of
dissolved oxygen within the oceans, and oxygen is another component that
eukaryotes need in large volumes for respiration.
In part one of today's talk, I will explore the long-term temperature
history over the last billion years. In part two, I'll look at the temperature
extremes in the latest Precambrian, just before the dawn of animals. And then
in part three, I'll explore a mechanism to sequester CO2 into carbonates over
long geologic time scales.
So if we go back to this rich fossil record and significant changes in
animals from predation to biomineralization, we might ask what was the context.
What was the environment that was hosting these early animals? And how might
the environment have driven changes in their evolution?
And we can do that by going to locations around the world that capture this
time period in rocks. If you do that, one of the fascinating types of rocks
that you'll see in places like Svalbard, which is north of Norway, north of the
Arctic Circle-- light throughout the summer, dark throughout the winter--
you'll find these rocks that host many different clasts of different sizes in a
fine matrix. And to interpret the environment that that rock formed in, all you
have to do is look to the environments directly adjacent to those rocks in
modern day Svalbard.
Active glaciers create this type of rock by mixing up large and small clasts
and very fine muds and depositing them all in the same location. In this
picture on the right, you can see it's a terminal marine. We can also find
evidence for icebergs. So on the right, you'll see the icebergs floating in the
seas of Svalbard. And on the left, you'll see a picture of what we call a
dropstone, a large, outsized clast that fell out of an iceberg as it melted and
landed in the sediments at the bottom of the sea. This also suggests glaciers
were a component of the latest Precambrian climate.
If we actually drill and analyze some of these rocks for their magnetic
minerals, we can estimate the paleolatitude that these rocks formed at. And
when we do that, we see something striking. We see a change in the
paleolatitude of glacial deposits over geologic time so that there are these
two glaciations within the latest Precambrian that we call snowball earth
glaciations because we think the entire Earth was covered in ice. And that is
very different than the glaciations that we see within the Phanerozoic, where
glaciers got to about the same latitude that they did in the most recent
glaciation, say, Kansas.
So to understand how and why glaciers were reaching the equator and what
were the consequences of this glacial style, we can look up close at some of
these deposits. And when we do that, here in Svalbard, you'll see the glacial
deposits on the far left in red. And they're capped by these carbonate rocks in
beige. And the carbonate rocks have some very unusual sedimentary structures,
like giant wave ripples, ripples bigger than anything you'd see at a beach
today, as you see in the picture on the right with the hammer for scale.
And to go from glacial deposits to carbonate rocks in short order, it
suggests that we're sampling very cold conditions to warm, tropical conditions
in short order. To go in, then, and model getting these two types of deposits
at the equator, extreme temperatures are required. Models predict that the
glacial temperatures were something like minus 5 to maybe even minus 35 degrees
C, and that the warm temperatures that the carbonates after the glaciation
experienced were anywhere from 15 to 50 degrees C.
These are modeled temperatures. We would love to have more constraints from
the rocks themselves. So how can we reliably reconstruct Earth's temperature
For this, I turn to marine carbonate rocks. And marine carbonate rocks are
great because they capture information about the biological community living
there, the physical environment, as well as the chemical environment of the
oceans at that time. The biological environment, you could think about reefs,
including the modern reef shown in the upper left, Acropora palmata corals that
have been the main component of today's tropical reefs, but also modern reefs that
are made out of microbes, just like the Precambrian reefs might have been,
these stromatolites that you can find in the Bahamas as well as in the ancient,
like the stromatolites I'm showing in the lower right from Oman.
We also see information about the physical environment. And, in carbonates,
that often means sediments that are made out of mobile grains, grains that
move. And in particular, in carbonates, they're often ooids, these beautiful,
polished, round grains that precipitate directly from seawater. And you can
find them today, and they often preserve ripples. And you can find them in the
past, like this thin section image from Oman in the lower left.
And lastly, they include information about the chemistry of the water. That
could be elemental concentrations substituting into the carbonate lattice, like
iron shown in the electron microprobe map on the left. Or it could be chemical
information captured in isotopic distributions of a single element. So the
ratio of heavy carbon-13 to carbon-12. And for that, we refer to it as delta
C-13 or delta O-18, if we're thinking about the heavy isotopes of oxygen
relative to the light isotope, oxygen-16.
Why isotopes? So isotopes in carbonates are great because they can get us
information about the climate and even the biological activity present. The
delta O-18 of carbonates is directly dependent on temperature and the seawater
oxygen isotope composition.
And so if you think about this equation, first written out by Harold Urey in
1947, we have an equilibrium equation. So it has a temperature dependence
through the equilibrium constant, that KT at the top of the equation. And on
the left side, we have unsubstituted carbonate reacting with water with a heavy
oxygen-18 in it. And on the right, that heavy oxygen-18 is in the carbonate
And so if we go in, and we measure that carbonate today, we get a delta O-18
measurement. And that delta O-18 measurement, that light blue contour on the
plot, could represent a wide range of temperatures or fluid compositions if we
don't know either. And so it can be difficult to understand the delta O-18 of
rocks in the past, and yet if you make the measurements, there is a very
striking pattern in them.
And that's that the delta O-18 of carbonate minerals as well as other types
of minerals has, in general, become heavier towards the recent. So here, four
billion years ago is on the far left, and the modern is on the far right. And
this pattern in the delta O-18 of different minerals has led to two very
different interpretations debated in the literature for over 40 years.
And in one interpretation, this shift in delta O-18 represents a
well-buffered climate that's kept within 25 degrees C at the equator by
silicate weathering. To do that, the delta O-18 of seawater must have changed
to something like minus 6, minus 7 per mil, when today it is 0 per mil. On the
other side, scientists have argued that this delta O-18 signature is evidence
of a changing climate system and that seawater delta O-18 compositions have
remained near 0 per mil through reactions at the mid-ocean ridge systems.
I look at this compilation, which is one of the most recent that currently
exists, and I think it's inadequate for evaluating trends in delta O-18 through
time. So if you look at the delta O-18 data on this plot, you'll see a wide
range of delta O-18 compositions, something like 30 per mil, for any given time
interval. And if you go into the data that's presented within this compilation,
it is from a wide range of locations around the globe, including some that have
seen significant burial and post-depositional over printing, or what we call
So what we've been building is a new compilation of carbonate delta O-18.
And we've thrown in delta C-13 as well. Luckily, actually, that's something
that has already been compiled because people think it's less susceptible to
diagenetic alteration. The important components of this compilation are that
it's high-resolution, that it's time resolved with an age model so that there's
one record through time. We've screened for a significant burial and
diagenesis, and we include fabric-retentive dolomite in the Precambrian.
And this is what it looks like. So once you add the Precambrian delta O-18
record to the Phanerozoic one, you can see that there is significant structure
to this record. So the delta O-18 record of the Precambrian actually spans the
full range of compositions of the Phanerozoic that have that slow, gradual rise
towards the modern.
And so if we go back to this equilibrium equation, we can actually make the
two assumptions that have been made in the literature over the last 40 years.
So on one hand, we can take the assumption that Earth's temperature has been
largely invariant and assume a 25 degrees C temperature. On the other hand, we
can assume that seawater has been largely invariant and take, instead of 0 per
mil for today, take an ice-free estimate of minus 1.4 per mil from the period
before we had glaciers because much of Earth's history has been unglaciated.
So if we take those two assumptions and our record, we can create two
end-member models. And in the upper model, we've assumed that the climate system
is well buffered and use that 25 degrees C estimate for equatorial
temperatures. And in this model, you can see what has been predicted in the
literature, a decline in the delta O-18 of seawater to values like minus 5,
minus 6 per mil as you approach the Precambrian to Phanerozoic boundary.
And yet, in the Precambrian, there's actually now a significant amount of
structure. So what do those perturbations represent? Are they diagenesis? In
the second model, we use this estimate for seawater delta O-18 and calculate
what the expected temperature would be. And there, you see the other
interpretation in the literature, that Earth's climate system has warmed as you
go back in time, and perhaps those excursions represent climate perturbations.
Obviously, both end-member models are clearly imperfect. There are effects
of local evaporation on the delta O-18 mineral composition. Glaciation could
affect the delta O-18 mineral for a given period of time. And yet the size and
the magnitude of the delta O-18 change is such that these two end-member models
are a good first step for evaluating the driving mechanism.
So then how do we actually deconvolve temperature and seawater delta O-18?
And so for this, we're turning to a relatively new measurement, clumped isotope
thermometry. And this measurement is one where we're looking at two heavy
isotopes within the carbonate molecule. And we can make this measurement at the
same time that we make our delta C-13 and delta O-18 measurements.
So if you notice, there are two equations now. And that upper equation is
also an equilibrium exchange reaction, but, this time, we're tracking when both
C-13 and O-18 are together in the same molecule, as you'll see on the right
side of the reaction, versus times when they are singly substituted. And that
equilibrium exchange reaction is temperature dependent, but it's not dependent
on the delta O-18 of the fluid the carbonate is precipitating from.
So what we can do is we can pair that reaction with the one that we've all
been thinking about since Harold Urey first described it in the 1940s and use
the assumption that the carbonate precipitated at the same temperature and of
the same delta O-18 composition. And thus, we can calculate the delta O-18 of
the fluid that is no longer with us. So in the plot, we take our clumped
isotope measurement. We take our delta O-18 mineral measurement. And then we
calculate out a delta O-18 water measurement.
How do we actually get a clumped isotope measurement in practice? In
practice, we take 400 micrograms of carbonate, and we produce CO2 gas. We
ionize that gas within the source of the mass spectrometer, and we measure the
different mass fractions. And one thing I'd like to point out is just how small
of a component of the CO2 that clumped molecule is. It's parts per million as
opposed to percentages of the CO2 gas. And so that's why it really took
innovations in mass spectrometry for us to make this isotopic measurement.
Once we have our clumped isotope measurement, how do we recognize whether
that signal is a climate signal or a diagenetic signal, something that happened
to the carbonate after it was deposited, after it was buried? Carbonates are
susceptible to diagenesis. They can dissolve. They can reprecipitate.
So what our approach has been has been to pair these delta O-18 and clumped
isotope thermometry measurements with micro-analytical tools where we can look
at the petrographic character of the rocks, the elemental character, the
isotopic character at the micro scale to assess for evidence of diagenesis.
With modern carbonate biominerals, this is very easy. They make predictable
macroscopic structures that are large enough to sample.
So you can see a modern brachiopod on the left with its open punctae. On the
right is a tool that we've been using in my lab that is synchrotron based. And
we're looking at the crystal orientation of carbonates in mollusk shells. This
technique allows you to map the orientation of the sea axis.
And so in this mollusk shell from 13 million years ago, you see a narrow
range of crystal orientations. Whereas, in the mollusk shell from 187 million
years ago, you actually can see that it's undergone a lot of diagenesis. So
those aragonite nacre tablets have dissolved where you see black. And where you
see bright purple, they've actually reprecipitated it as calcite. So this
technique is very good at identifying diagenetic alteration of that aragonite
nacre mollusk shell.
It becomes much harder as we move into the Precambrian, this interval of
time that we're very interested in. Carbonates do not make large primary
predictable structures in large part, and so we're left to carbonates that
capture a series of events. And these events could be everything from
dolomitization, micritization, pressure solution, and stylotization that could
alter the isotopic composition of our rocks.
If you look at these pictures on the left, you would be hard-pressed to
think we might get a climate signal out of these materials. And yet I think we
can be smarter about how we target our materials. And so we can try to find
places around the world that have experienced very minimal burial history,
maybe are within the oil window or suboil window, and places that allow us to
target different mineralogies so that we can help target minerals that are
differently susceptible to dissolution and reprecipitation. So aragonite versus
calcite versus dolomite have very different solubilities.
And we can appreciate that all rocks are no longer the sediments that they
started as, and yet there are rocks that give me hope. So if you look at the
two pictures on the left, you'll see what's called a fenestral mudstone,
fenestrae, from the French word "window." And this rock has open
pores that reflect gas bubbles that were formed from microbes likely very early
in the sediments. And those open pores remain. If you look at the thin section
at the top, they're not filled in with later cements, and the crystal size of
that mud remains very small.
There are actually some macroscopic carbonates from the Precambrian that do
have very predictable physical structures, and those are ooids. So these two
sets ooids have growth mechanism where they grow outwards. And so we can
evaluate that precipitation again using our crystal orientation maps.
The PIC map is shown at the upper right. And EBSD is another way that we can
do this on an SEM. And so it makes it much more accessible, although the
resolution is not quite as good.
OK, so what have we done? Well, we've gone around the world to try to find
these types of materials, and we've looked at a range of different fossils as
well as bulk rocks from the Phanerozoic and Precambrian. The strata that host
these materials, we try to find flat-line strata. They say that the best
locations are the ones where when it rains, you can go and your boots get
covered with carbonate mud. These locations become harder to find the further
back in time you go, and yet we're doing our best.
So if we look at the range of different materials that we've analyzed for
their clumped isotope composition and the delta O-18 and the fluid that they
formed from, you can see a range of fossils, calcite, aragonite, even apatite
fossils like these inarticulate brachiopods. We can also see that we've
analyzed limestones, and we've analyzed dolomites.
And if you work in carbonates in the Phanerozoic, you might think of
dolomites as this picture in the upper right, of course crystals, rhombohedral
in structure. But in the Precambrian, a lot of the dolomite that you find is
what I call fabric retentive, with very small crystal sizes that preserve
stromatolite morphologies beautifully or ooids like the bottom two pictures on
the right. And this dolomite has the potential for having formed very early or
perhaps even as a primary precipitate out of seawater.
So if we overlay all of the data that we've gotten from a range of materials
onto the two models that we're testing, you can see a striking difference. So
in the upper plot that predicted delta O-18 seawater change, constant climate,
all of the clumped isotope data falls above the dashed line for a constant
water composition. So it's not fitting that model well. At a time when the gray
data predicts seawater delta O-18 should be minus 6, minus 7 per mil, none of
the clumped isotope data suggests that that's what seawater delta O-18 was.
In contrast, that plot on the bottom, we can see that the prediction of a
warmer climate, as you go further back in time, matches the clumped isotope
temperatures very well, and thus this idea that seawater delta O-18 has not changed
significantly over the last billion years. We also see that when we look at a
population of genetically related samples, say, that are from a single location
closely associated stratigraphically, that those samples have very little
spread in the delta O-18 mineral composition, and they create a tight array in
clumped isotope temperature space. And this suggests rock buffer diagenesis.
And so we can actually use that as a tool to look towards the low end of our
temperature range and use a moving minimum approach to think about this record
And so if we think about what temperatures were getting in the Precambrian
and using this moving minimum approach, I'd like to highlight a couple of data
sets from postdocs working in my lab, Tyler Mackey and Adam Jost. And they've
been going in in the Svalbard succession to look at the preglacial stratigraphy
and the glacial stratigraphy from the snowball earth deposits. And here, you
can see that the preglacial stratigraphy in orange is much warmer than the
synglacial stratigraphy and that the coldest clumped isotope temperatures from
the synglacial stratigraphy are something like 17 degrees C.
OK. So here, on the bottom, is a moving minimum now across the delta O-18
data and the clumped isotope data in stars. And we've always found that, during
glacial intervals-- we've looked at three of them-- the clumped isotope
temperatures are colder than the surrounding stratigraphy. So let's turn to
part two, temperature extremes of the last billion years. And for this, I'd
like to look at one excursion, the Shuram excursion.
If we now go back to our record, I've reversed the axes, and we have delta
O-18 on the top, looking like the temperature record, rising and becoming more
variable. Delta C-13 is on the bottom, and it also becomes more variable as you
go further back in time. And you can find those periods where there's an
excursion in delta O-18 coincides with an excursion in delta C-13.
One such event is the Shuram excursion. This is the most negative carbon
isotope excursion in Earth's history, and it's found globally in over five
sections. And it's found in the middle Ediacaran period and is closely
associated with the appearance of soft-bodied organisms, like these shown here
on the left.
Traditionally, scientists have predicted that these large negative carbon
isotope excursions represent a diagenetic process, and so they tend to divide
the record from a primary processes in delta C-13 operating in the Phanerozoic
to diagenetic processes operating in the Precambrian. And yet I think it's
important to remember that diagenetic alteration-- that's altering the bite
bulk isotopic composition, delta C-13, delta O-18 of a carbonate rock-- leaves
physical and chemical predictions at the macroscopic and microscopic scale.
If there's a fluid flushing event, you should see that, or a phase ingrown
within the sediments-- you should be able to find it. In the Phanerozoic, in
contrast, the null hypothesis, if there's covariation between delta C-13 and
delta O-18, is that there is a coupled climate carbon cycle perturbation. And
the classic example of this is the example I mentioned earlier, the PETM, which
is a great analog for the modern.
And so there you see delta C-13 and delta O-18 changing together, becoming
more negative, and then recovering. If we then look at the Shuram excursion,
the delta C-13 and the delta O-18 of the carbonates in the stratigraphy, as
shown in the plot on the right, also capture this behavior of covariation. They
become more negative together, and then they recover.
And yet, side by side, if you compare the PETM and the Shuram excursion, the
magnitude of change is much larger for the Shuram excursion. So the carbon
isotopic values change by 17 per mil versus 3 and 1/2 per mil. The delta O-18
values change by 6 per mil versus 2 per mil. If we take that delta O-18 change,
the temperature change expected is 4 to 6 degrees C for the PETM versus 15 to
20 degrees C for the Shuram excursion.
And so this really makes us question is the climate system capable of this
size perturbation. How would it work? We can answer this question by going to a
place that hosts the Shuram excursion, in this case the Sultanate of Oman, and
we can try to evaluate whether it represents an extreme climate event or a
In Oman, the stratigraphy includes the Khufai Formation, the Shuram
Formation, and the Buah Formation. The rocks that host this excursion are some
of the shallowest buried Ediacaran rocks globally, so much so that they are
within the oil window and, in some cases in Oman, even below the oil window.
And Petroleum Development Oman produces oil from adjacent stratigraphy. It's
one of the oldest producing oil reservoirs in the world.
And all of those examples that I highlighted earlier of exceptional
preserved microscopic fabrics in carbonates come from the rocks that host the
Shuram excursion. So we see the fenestral mudstones before the onset of the
excursion, and then these two cases of ooids are actually two different
mineralogies that preserve crystal orientations, even though they are now
And what we can see in the clumped isotope data from these rocks is evidence
of warming. The temperatures increase within the dolomite section of the
stratigraphy, and then they maintain those temperatures into the limestone
section. The high-resolution data that I am showing on the left also shows very
similar delta O-18 fluid compositions across the entire excursion,
seawater-like compositions. On the right, I've also included the last bit of
recovery temperatures. And so you can see the warming and then actually return
to cooler temperatures as you recover the delta C-13 composition.
So to summarize the these two parts of the talk, we're finding evidence that
the seawater composition of the global oceans has not changed significantly
over the last billion years of Earth's history. We're also finding evidence for
warmer temperatures in the Precambrian and much more extreme temperatures, both
cold in the snowball earth events and in hyperthermal events, like the Shuram
excursion. And yet these hyperthermal events are warmer than the PETM, our best
known example of a hyperthermal event from the last 65 million years.
So how do we understand this? We need a new model for Earth's long-term
climate. And so this last part, I'll go through my current thinking on what
might be causing Earth's climate to change.
So if we go back to this idea that we have of Earth's thermostat being
governed by CO2 outgassing, silicate weathering, and carbonate sedimentation
before returning to the mantle, something must have changed over geologic time.
In fact, there are lots of things that have changed in uniform, one-direction
ways over Earth's history, and yet that last component, the CO2 out gassing
history, weathering dynamics, and carbonate sedimentation-- I think carbonate
sedimentation might be critical and something that we've underappreciated. So
if we go back and we focus in on carbonate sedimentation over this billion year
period of Earth's history, and we zoom out-- we expand our view of carbonates--
we can see that actually, today, there are three main components of carbonates
that capture and sequester CO2, the deep sea carbonate record, shallow marine
carbonates, and then the carbonate rock reservoir that's tied up on continents.
Today, we see CO2 being easily and responsibly sequestered into marine deep
sea carbonates. So planktonic organisms, like foraminifera and coccolithophores,
sequester CO2, deposit it on the deep sea, and then reintroduce it to the
mantle at subduction zones, and yet these organisms have evolved since the
Mesozoic. And so if we take those away, suddenly we might ask are we
reintroducing the same amount of carbonate to the mantle at subduction zones.
So have we changed the fluxes in that Earth's thermostat? And then we could
also ask have we changed the reservoir size over time.
Today, a huge volume of CO2 is stored in the carbonate rock reservoir that
sits on our continental crust. And so we can ask the question when and where
was this sedimentary carbonate reservoir built. What would the climate state of
this earth look like, where we have very little carbon tied up in carbonate
rocks? And so, instead, you might imagine that the amount of carbon in the
oceans and atmospheres was much larger, thus the climate was warmer.
In the literature, a common interpretation is one of geochemical
uniformitarianism, that is, the amount of carbonate that we see today was similar
to the amount of carbonate coming out of the oceans in the past. And so if you
look at the proportions of carbonates to other sediments within any given time
period, it should look uniform. This is not an easy thing to do, but I've taken
a couple of attempts that I'd like to show you.
So the first set of data is the percentage of carbonate to total
siliciclastic rocks preserved in a database gathered by Soviet scientist
Alexander Ronov. And so he tried to account for all of the different rocks preserved
on continents over a 30-plus year time period. And what you can see if you
ratio these two types of rocks from his data is a stepwise change at the
Precambrian to Phanerozoic transition about 540 million years ago. And he moved
from something like 10% of the remaining rocks being carbonate to something
over 20% in the Phanerozoic.
A much less perfect attempt is one that uses this global database of
geochemical records, earth chem. And here, what we've done is divide the
percentage of carbonate analyses to total sedimentary rock analyses. And again,
you can see the same jump in more carbonate rock analyses in the Phanerozoic.
And the last attempt is a Ronov 2.0 for North America, pioneered by Shanan
Peters at the University of Wisconsin in Madison. And here, you again can see
that, in North America, the proportion of carbonates in the Precambrian to
total sedimentary rocks was much less than the proportion of carbonates in the
Phanerozoic. How can we explain this? How did the carbonate sedimentary record
change from the Precambrian to the Cambrian?
We see a stepwise shift in biological innovation. So as we move from a world
of stromatolites and microbes into a world of animals that make their skeletons
out of calcium carbonate, we see an evolution of strategies to biomineralize.
Those strategies include making their skeletons within membranes, proton
pumping to change the pH of the fluid, and using enzymes like carbonic
anhydrase to make carbonate precipitation happen more rapidly. And the
consequence of that is reefs today, like Acropora palmata reefs that can keep
pace with sea-level rise from the last glacial maximum. But even in the
Cambrian and the Ordovician, we can also imagine that those organisms were
doing it better than their Precambrian counterparts.
And so this idea that as you transition into the Phanerozoic-- the platforms
themselves became more massive-- is one where both Precambrian and Phanerozoic
carbonate platforms a grade to sea level. But then because Phanerozoic
platforms produce more carbonate, they're more productive. You have more
progradation. So they span larger areas. And so this combination of more
voluminous carbonate platforms associated with continental flooding allowed the
earth to build this carbonate reservoir that we have today.
I think you can see this change in the platform volume if you look at the
rock record in the field. So this is a picture from the Nopah range in
California outside of Death Valley, and in the foreground is the Ediacaran
stratigraphy. And there, the carbonate rocks are a component, but they're
within a thick siliciclastic record.
In the background, those gray and white striped rocks you can see in the
distance are the Paleozoic or early Phanerozoic carbonate platforms. And they
represent kilometers of carbonate deposition and CO2 sequestration. And so if
we think about this finding or this idea that the Phanerozoic represents a
long-term cooling through carbon sequestration into carbonate rocks through
biological innovation, we can think about the problem we face today, this
annual rise in atmospheric CO2 from our everyday activities.
And efforts so far with carbon sequestration have focused on things like the
Oman Drilling Project and sequestering carbon into the Semail Ophiolite and
other igneous rocks. And yet perhaps another strategy that we should be
exploring is carbon sequestration into lake systems through
biologically-mediated carbonate precipitation. There are organisms that do the
same thing as forams and coccoliths in the oceans in lakes, like this picture
that I've included at the left. To me, this gives us some hope for the future
in solving today's climate crisis.
And so I'll just end with this thought that Earth's temperature history is a
dynamic one and that carbon sequestration into carbonates is not always
operated as it does today. And this idea that biological organisms that
biomineralize have figured out ways to do it better, to do it faster, to
sequester more CO2 into carbonate, can actually help us for the problems that
we face today.
And so with that, I'll just thank you for listening and watching the
lecture. And don't hesitate to reach out if you have questions about my