This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Stratigraphic field studies, by their very nature, cover a particular geographical region. Linking these studies into a global record is difficult, because there is enormous spatial variability in the rate at which sedimentary rock thicknesses accumulate, if they accumulate at all, and if they survive erosion to the present day. But that is precisely what Shanan Peters set out to do. Since about 2012, he and his team have been compiling stratigraphic data from around the world and have assembled it into a platform called Macrostrat. This has enabled them to address some fundamental questions about the long-term secular evolution of the Earth–life system in a quantitative manner. Shanan Peters is a professor in the Department of Geoscience at the University of Wisconsin–Madison. Shanan Peters, welcome to Geology Bites.

Shanan Peters: Thank you, Oliver. It's great to be here.

Oliver Strimpel: Compiling a global database of the spatial and temporal distribution of sedimentary rocks is an extremely ambitious project. What motivated you to embark on Macrostrat?

Shanan Peters: There were really two motivating factors. The first, I would say, really stems from my background as a fossil collector and paleobiologist. Based on my experience poring over the scientific literature and out in the field looking at rocks, I had this intuitive sense that there was covariation between what we would see in the temporal pattern of the sedimentary rock record and the diversity and turnover of life that we see in the fossil record. And this covariation, it seemed to me, was very important to our basic understanding of the history of life on Earth and all the factors that have governed it. So I wanted to quantify it and to understand the nature of the relationship between the sedimentary rock record and macroevolution. The second reason I set out to compile a global database on the rocks of Earth's crust is that I knew we really needed one. A quantitative understanding of the rocks in the Earth's crust is really important not only to a bunch of questions about Earth and life history, but to a wide range of things that human society cares about and really needs, ranging from mineral resources for the energy transition to groundwater and everything in between.

Oliver Strimpel: How did you go about building Macrostrat and populating it with data?

Shanan Peters: It was really a two-step process. The first step was figuring out exactly what it was I was going to measure in the first place, and then constructing a database that reflected that structure. It actually took me a little while to arrive at the best approach, which I now call macrostratigraphy, and from there it was a matter of finding the appropriate sources of data. Originally, I wanted to use these things called measured stratigraphic sections, which geologists produce all the time when they go out into the field; they're high-resolution, detailed transects through the Earth's crust. But I quickly realized that this was way beyond what I could accomplish on my own, because there are probably millions of these things out there in the literature. So I started with what are essentially correlation charts. These are regional summaries of geology, in the form of rock columns, that are put together by field geologists to provide a quick guide to the local geology of a region. Starting from there, I was able to assemble Macrostrat, the database, which now consists of tens of thousands of individual rock units from thousands of those regional columns, from all over North America and other parts of the world.

Oliver Strimpel: That's already an impressive collection. Roughly what fraction do you think you've managed to assemble into the database?

Shanan Peters: I think it might be best to phrase it this way: of all the pertinent geological knowledge that's out there, roughly what fraction do we capture so far? And I'd say it's pretty small. Although the columns and the geologic maps that we compile in Macrostrat are basically foundational geological products, they're really difficult to produce, and they're not usually published in ways that are best for entry into the database. We're actually working right now to build web tools that allow a global community of geologists to contribute their knowledge and expertise to the system more easily and grow the data holdings. And I hope that this exercise is successful, and that Macrostrat becomes more of a global, community-data-driven resource that others can participate in more readily.

Oliver Strimpel: It's an amazing resource. Is there a way for people to access it without being a member of the research community?

Shanan Peters: Yeah, absolutely. There are several ways that you can do this. The first is by visiting our website, macrostrat.org. For the more technically and scientifically inclined, we also have a very powerful API, which stands for application programming interface, that gives you command-line access to the data. That said, by far the best and easiest-to-use resource for Macrostrat is our mobile app, called Rockd, R-O-C-K-D. This app was designed to put Macrostrat's very rich data holdings in your palm when you're out in the field and on the go. It's really fun to use. It's focused on the rocks around you, though you can explore the geologic map globally by panning around with your finger, and I hope everyone listening takes advantage of it. It's available on the Google Play and Apple iOS app stores. It's free, but of course we welcome donations.

Oliver Strimpel: A key reason you wanted to take a global look at the temporal and spatial distribution of the rock record was to separate out biases in the fossil record caused by variability in the volume and extent of sedimentary rock. Can you explain the logic as to how these data enable you to do that?

Shanan Peters: There's this old idea, going all the way back to Darwin, that the sedimentary rock record is very incomplete, gets worse with age, and provides a rather poor accounting of evolution. In fact, in the 1970s, David Raup and Jack Sepkoski were among the first to really explore the quantitative relationship between the rock record itself and what we see in the fossil record of evolution, like the diversity of life, for example. How does that vary through time, and how does that compare to the amount of rock we see preserved in the sedimentary rock record? What they found was something that seemed to be quite consistent with Darwin's notion that the preservation of the rock record might be distorting, biasing, overprinting our understanding of the true history of life. In other words, there's this history of life that has played out over time, and the rock record has captured incomplete bits and pieces of it, so when it gets stitched together, it looks completely different. Basically, less rock, fewer fossils, less diversity; more rock, more fossils, more diversity. It kind of boils down to that. The big advance of macrostratigraphy is that it allows both the spatial and the temporal dimensions of variability to be measured a lot more completely, and this allows us to discern what might be sampling bias versus changes in the Earth system that could drive both real variation in evolution and variation in the rock record.

Oliver Strimpel: Suppose you were to find that the area of sedimentary rocks on the Earth's surface showed a similar pattern over time to that of the diversity of marine animals. How would you distinguish between a rock sampling bias on the one hand and a common cause for both of these on the other?

Shanan Peters: Yeah, that's a good question, and it's exactly what we find. As you said, we find a correlation between sedimentary rock area and the diversity of marine animal life, among other parameters; the two things vary together. And there are several ways to get at what might be causing this variation and this correlation in the rock and fossil records. One of them actually involves going to the fossil record and trying to standardize our sampling effort of fossils directly. This has been one of the main tasks enabled by what's called the Paleobiology Database. This is a system that my group runs at a technical level, and it's the product of an international team of paleobiologists who have been assembling fossil occurrence data from around the world. So you can pluck fossils out of rocks, analyze the fossils in their context, and basically adjust for sampling intensity in the fossil record. That's one avenue to take. The other avenue is to actually measure different properties of the rock record, such as the gappiness of the rock record, that is, how tattered the manuscript is, measuring the tatters themselves, and to see if that gappiness predicts the turnover of lineages that we see in the fossil record. So it's a different approach to the problem.

Oliver Strimpel: And what did you find?

Shanan Peters: What we find is that the history of sediment abundance does look a lot like key features derived from the fossil record. That part is very clear and has been recognized for some time. For example, there's a correlation between the occurrence of gaps in the sedimentary rock record and turnover in the fossil record: mass extinctions tend to coincide with intervals of time that are followed by big drops in rock quantity. But it turns out that the durations of those rock gaps are not sufficient to cause an artifact in the fossil record as large as the turnover pulses that we do see. It's kind of cool, actually. Darwin was right in one sense: the rock record is actually full of gaps, and those gaps can be very long. But what he missed is that those gaps actually mean something. They formed in response to Earth processes that are very important. They tell us about changes in the Earth system, and those changes in the Earth system actually prompted environmental conditions that life itself responded to. So the gaps have meaning, just a different meaning from the rocks. As the composer Debussy said, music is the space between the notes, and that's very much the case with the rock record. Both the rocks, the notes, and the unconformities, the silences in between them, have significance, and Macrostrat allows you to measure both.

Oliver Strimpel: Wow, that's fascinating. So what did you find is the meaning of these gaps?

Shanan Peters: Funnily enough, one of the key mechanisms that links patterns in the rock and fossil records is changes in sea level and the extent of continental flooding by shallow seas. This parameter has changed a lot. For example, I'm sitting right here talking to you in Wisconsin, in the United States, which is in the middle of the continent, and the rocks below me were deposited in shallow seas. This is a very characteristic feature of the rock record. And so what we've done is link variation in the rock record to changes in the extent of those shallow seas. Ironically, this is also the mechanism that was first proposed as the cause of mass extinctions back in the 1960s, when they were first recognized. So we've kind of come full circle. The pattern is pretty obvious: the rock record and the fossil record covary, so it can be recognized qualitatively. What Macrostrat does is formalize that qualitative relationship, allow us to measure it, and give us a platform for exploring it further.

Oliver Strimpel: So were you able to delineate the relationship between changes in sea level, then, and the mechanisms by which diversity or evolution would take place? What is the driver there that connects these two things?

Shanan Peters: At the longest timescale, it's tectonics. Tectonics is operating as a sort of background process on Earth, and it drives big expansions and contractions in how much of the continents are flooded by shallow seas. That environmental parameter is like a dipstick; it's a good environmental dipstick for planet Earth. So when we see variation in that dipstick, it's telling us about surface changes on the planet that have big macroevolutionary consequences. That's where the common cause really gets united: through a mechanism that is fundamentally underpinned by tectonics.

Oliver Strimpel: So are we talking here about sea level rise? I mean, sea level rise would seem to me, naively, to be the cause of a great increase in deposition, as you get more marine surface area, and the sea brings in erosional materials that can be deposited.

Shanan Peters: Yeah. The expansion and contraction of seas has several different impacts on the surface environment. The simplest of these is the area in which shallow-marine critters can live. As you say, when sea level is high, you have a large amount of sedimentary rock deposited over a large area. And when the area for organisms to live in expands, there's a well-known relationship that diversity tends to go up when area is big. So those expansions and contractions of shallow-marine shelf area are one of the proximal causes of the variation we see in the fossil record. But it turns out, perhaps more interestingly, that those variations in shallow seas covary with other changes in the Earth system, biogeochemical cycling, climate and even the oxygenation of the atmosphere, that also impact the biosphere. So it's more than a simple area relationship with biodiversity; it's a more nuanced connection to the state of the Earth system that's connecting sedimentary rock record patterns to macroevolutionary processes.

Oliver Strimpel: Could you talk about the oxygenation of the atmosphere? Are you suggesting that it is causally linked to changes in sea level?

Shanan Peters: Well, now you're basically hitting on why I went through the trouble of constructing Macrostrat in the first place. The quantity and composition of rocks in the Earth's crust is just so important to so many different dimensions of the Earth system, and this question gets at that. As it turns out, the reason we can all breathe the Earth's atmosphere is that it has so much molecular oxygen, almost 22% molecular oxygen, which is absurd when you think about it: it's this reactive gas that causes fire and all kinds of crazy reactions to take place. It's remarkable that it's so abundant in our atmosphere, and in part the reason for that is photosynthesis. Photosynthesis is this remarkable reaction that takes CO2 and water and the energy of sunlight, splits the water, releases oxygen as a waste product, and produces organic material that then goes on into the biosphere. But what I think a lot of people don't fully appreciate is that photosynthesis is really only a necessary condition for generating an atmosphere that's rich in oxygen, like ours. It's not sufficient operating on its own, because the photosynthesis reaction is easily reversible. Photosynthesis can produce oxygen, but life, like you and me right now, is breathing in oxygen, consuming the products of photosynthesis and running the reaction fully in reverse. And that, of course, can take place inorganically. Fire is an example of that reversible reaction taking place, and the rotting of a log is an example of that reaction going in reverse. And so photosynthesis operating on its own, life operating on its own, can't do much, actually, to the composition of the atmosphere. To build up oxygen, what needs to happen is that you need to couple that photosynthetic pathway to a mechanism that buries carbon in the Earth's crust, in sedimentary rocks. And that burial of organic carbon leaves behind oxygen in the atmosphere. So it's a remarkable fact, but the growth of the sedimentary rock record, which is what you're measuring in Macrostrat, is why you can breathe the air. Macrostrat allows us to measure that growth, and I think that really helps us understand the history of atmospheric oxygen.

Oliver Strimpel: Are those sequestered products of photosynthesis mainly what we have today in the form of coal and petroleum?

Shanan Peters: Those are the hyperconcentrated versions of that carbon, but there's also disseminated carbon, in low concentrations that are not currently economically exploitable, that is distributed in things like shales, which have small percentages of organic carbon in them but are volumetrically really important.

Oliver Strimpel: In a recent episode of Geology Bites, Paul Smith talked about the Cambrian explosion, which, of course, is the very best-known time in Earth history when we see diversification. And I guess your analysis would suggest that the diversification is real, that it's not an artifact of the sedimentary rock bias. And does it also correspond, then, with sea level rise?

Shanan Peters: It does. It's quite remarkable, actually. The start of the Phanerozoic, and Phanerozoic just means visible life... The transition between that world and the Precambrian, the Proterozoic, "first life," part of Earth's history is really dramatic from a biological point of view. And what's fascinating is that it's equally dramatic from the physical point of view of the rock record. At first blush, you might think, well, maybe there is a bias there. But using the methods we talked about before, analyzing the gaps and looking at the pattern in space and time of the rocks, you can quickly rule out that it's a simple artifact relationship, and come to something more in tune with the common-cause mechanism: that there's a perturbation of the Earth system, bound up with oxygen history and everything else, that's encapsulated in that changing rock record and fossil record, which suggests there's a common causal association there.

Oliver Strimpel: Is what you're saying, then, that the time at which we see the Cambrian explosion actually also corresponds to a time when we see a much bigger volume of sedimentary rock globally?

Shanan Peters: Yeah, it's really quite wild, actually. When you compile the amount of sedimentary rock, at least in North America, what you see is quite amazing. A good first-order description of the rock record in North America is this: lots of Cambrian and younger Phanerozoic sedimentary rocks overlying Precambrian crystalline basement rocks. And that fundamental pattern is dominant in the rock record. So you can pull out the fossils, look at the rocks, and identify that interval as the dramatic transition. And the fact that the fossils are along for that ride suggests very strongly to me that there's a common-cause mechanism: that the changes to the Earth system that drove the flooding of the continents and the re-establishment of continental seaways and sedimentation in continental environments are linked to the evolution of life in an important way. And it can be direct, like the area effect we discussed. It can also be indirect, through the burial of organic carbon and the oxygenation of the atmosphere. Both go hand in hand.

Oliver Strimpel: So that big deposition that we see at the start of the Phanerozoic, over crystalline basement rock: are we talking about a conformable boundary there, or is there a gap?

Shanan Peters: There is a gap, a gap in time. The rocks below the surface tend to be very old, a billion-plus years old oftentimes, although the duration of that gap is hugely variable depending on where you go. So in that sense, there's a gap. But as I said before, that gap is recording a process. It's not very nuanced to look at that as the geologic record being broken. It's not. It's a geomorphic surface that's recording a different set of processes from the processes that record sedimentation. And so both are important in the evolving Earth system, and that boundary is very profound from that point of view. It tells us that something changed across that boundary. We actually call it the Great Unconformity in North America. It's been recognized for a very long time.

Oliver Strimpel: In one of your papers, you said that the Great Unconformity is identified by macrostratigraphy as the most important transition in the entire rock record. What did you mean by that?

Shanan Peters: It really is a step-function change in the abundance of sedimentary rock on the continent, again, at least in North America, where we have the best data. For most of Earth's history, including the interval that's affectionately known as the boring billion, sedimentary rock abundance is quite low. And then, all of a sudden, beginning in the Ediacaran and continuing into the Cambrian, there's a huge rise in sedimentary rock abundance. Most of the continent is covered by Cambrian and younger Phanerozoic sedimentary cover, again including where I'm sitting here in Wisconsin. The rocks below me are Cambrian, and then there's an unconformity, the Great Unconformity, that separates them from billion-plus-year-old igneous rocks. And these igneous rocks, formed deep within the interior of the Earth, are completely different beasts from the sedimentary rocks above them. What we know is that these deeply formed rocks were in the subsurface during the formation of the Great Unconformity; they got up to the surface, and then they were covered by younger sedimentary deposits. And so that's why it is such a profound break in the rock record in North America: when you measure the abundance of sedimentary rocks, it's the boundary that stands out above everything else, by a very large margin.

Oliver Strimpel: Is that true globally as well as in North America?

Shanan Peters: Good question. We would love to have the data to really measure that quantitatively. Anecdotally, and through various published reports, there is evidence to suggest that the Great Unconformity is global in scope. However, in the absence of hard quantitative data, it's very unsatisfying to say that we think it's there. What we do know is that there are going to be regional differences in the patterns of sedimentation on the continents, because each continental block has its own unique tectonic history. At the same time, there's also a global overprint, with coordinated expansions and contractions of seaways across continents, and rises and falls in sea level and so on, that affect the sedimentary rock record. So we expect there to be a global component of variation, with regional variation superimposed. And the question is, which is more important, the global signal or the regional signal?

Oliver Strimpel: And would you say there is a global mechanism that might be at work here?

Shanan Peters: Unfortunately, Macrostrat can't really say anything directly about that. What Macrostrat does is allow us to measure patterns in the rock record in a robust fashion. Coming up with mechanisms for those patterns is a separate exercise. Macrostrat identified the Great Unconformity, in that sense, as something we need to look at and understand, and that realization prompted a bunch of work trying to work out the history of formation of the Great Unconformity and what caused it. There are a couple of interesting hypotheses out there. One idea, and in fact I'm a co-author on the paper, is that Snowball Earth, this interval of global glaciation, with continental-scale glaciers that persisted for a very long time, scraped off the continents and reset the sedimentary clock. So this is a glacial mechanism for the Great Unconformity. Side note: I actually think that the glaciation may be more a symptom of the processes that formed the Great Unconformity than the cause. Separate discussion. There are other people looking at the Great Unconformity from the point of view of the igneous rocks below it and their exhumation and tectonic histories, and it's quite diverse. Depending on where you go, the same Great Unconformity can have quite different expressions regionally, and there's some great work on thermochronology, and other approaches to interrogating the history of exhumation across that boundary, that suggests a more complicated, tectonically induced, multiphase history. In reality, I think both are probably contributing. The Snowball Earth episode probably was erosive; it may have helped remove thin sedimentary cover from the continents. But probably the tectonic driver that got us into that mess in the first place is more important. So the jury's still out on what caused it, in part because we need a better global signal, but it's an interesting area of work right now.

Oliver Strimpel: I think you've already alluded to this, but does your analysis using Macrostrat actually support the specific conclusion, then, that the Great Unconformity was implicated in the environmental, geochemical and biological changes that led to the Cambrian explosion?

Shanan Peters: Well, the correlation is very striking. Again, the biggest transition in all of the rock record, at least in North America, is spot on in timing and in magnitude with arguably the biggest transition in the history of life on Earth. This could be a coincidence, I suppose, but when it's all taken together, including the oxygen story we mentioned earlier, it seems like there is a lot linking the physical and the biological together across the Great Unconformity. For example, exposing that much crystalline basement rock over essentially an entire continent, uniquely, probably, and a continent located at low latitudes, which is where North America was for much of the late stages of it, would have a big effect on biogeochemical cycling. Earth systems and life systems are intertwined, and that intertwined relationship, I think, shows up in a big way across the Great Unconformity. We're still working out exactly what the causal connections are there, but the correlation is striking.

Oliver Strimpel: That's fascinating. And that really supports what Paul Smith was saying in the earlier episode of Geology Bites I referred to. It seems like your work actually provides support for two of the specific mechanisms he talked about. One of these was an increase in habitable area through sea level rise and continental flooding, and the other was an increase in the nutrient flux through the erosion of the continental surface. He also mentioned several other processes, food web complexity and the origin of biomineralization, for example. These are the kinds of mechanisms that you were saying might be beyond the scope of Macrostrat, but do you have a sense as to whether your work might eventually be able to shed light on these questions?

Shanan Peters: I think it can help. It basically serves as a very good boundary condition that has a bunch of different environmental implications. Take biomineralization, for example. The evolution and origin of biomineralization is kind of perplexing from a Darwinian point of view, because evolution can't anticipate the fact that having a skeleton might be good. It can only work with the material and variability that's available to it at the moment. And so the origin of a biomineral, which for most organisms today takes energy, is a bit of a conundrum. How do you get it when you don't know it's going to be useful? One possible hypothesis that's come out of Macrostrat and analysis of the Great Unconformity is that maybe there's a flux of biogeochemically sensitive elements into the system that's promoted by that surface, phosphorus, for example, being one of them. And so it's fun to imagine that the evolution of biomineralization might have been prompted by the final stages in the formation of the Great Unconformity. I think that's a reasonable hypothesis; proving it is a different matter.

Oliver Strimpel: As your platform becomes richer over time, what other questions do you hope to address with it?

Shanan Peters: I think it might be more efficient to say what questions I can't address with it. I really do view the quantitative understanding of the composition, quantity and age of materials in Earth's crust as a first-order feature of our planet that is central to our understanding of all sorts of natural phenomena, from macroevolution to the oxygenation of the atmosphere that we've touched on, and many others. There are just so many practical, industrial and commercial applications too. I'm hopeful that my group will go in some of those directions, but I'm even more hopeful that the platforms we are building, from a cyberinfrastructure point of view, are going to enable a global community of very clever geoscientists and paleobiologists out there to take advantage of the data that are in Macrostrat and incorporate those data into their ever-evolving understanding of the Earth system. It's really then, I think, that we'll start to gain some new insights into how Earth has come together.

Oliver Strimpel: Shanan Peters, thank you very much.

Shanan Peters: All right. Well, thanks a lot for having me. It's been fun.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find transcripts and a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.