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Kristin Bergmann - Climate Extremes at the Dawn of Animal Life

AAPG Distinguished Lecture Series, 2019-20 Season

AAPG Distinguished Lecture Series, 2019-20 Season


A Distinguished Lecture talk given by Kristin Bergmann during 2019-20 AAPG DL Season. Click here for abstract.

Full Transcript

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 Development Oman.

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 pace.

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 history?

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 molecule.

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 diagenesis.

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 over time.

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 diagenetic artifact.

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 rocks.

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 research. Thanks.

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  • 55549 In the Bergmann Lab at MIT, we use a combination of approaches to reconstruct ancient climate conditions. Carbonate, phosphatic and siliciclastic rocks retain significant information about climatic conditions in Earth’s deep history in their sedimentological, petrographic and geochemical character. Determining absolute temperature ranges and seawater oxygen isotope (δ18O) fluctuations over geologic time is one crucial component of understanding Earth's long term climate history and the co-evolution of Earth and life. Carbonate clumped isotope thermometry (D47) of ancient carbonates and fossils has the potential to unravel competing effects on δ18O records. However, diagenetic effects must be considered using strategies of analysis of two or more co-existing minerals (i.e. calcite, dolomite and apatite), comparisons between co-existing fabrics (i.e. late-stage cements vs. primary shells), and micro-analytical screening techniques based on petrographic and chemical indicators of alteration. I will present results that utilize all of these strategies to assess the climate variability in the latest Precambrian and earliest Phanerozoic. Our results suggest critical differences between recent paleoclimate conditions and those of the deep past both in terms of the response of the climate system to external perturbations as well as the controls that govern the climate system's recovery to perturbations. Reconstructing Earth's Climate Dynamics at the Dawn of Animals
    Reconstructing Earth's Climate Dynamics at the Dawn of Animals


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