This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The Amazon basin is the most biodiverse region on Earth. It is the home of one in five of all bird species, one in five of all fish species, and over 40,000 plant species. How did this come to be? Can we answer this question just by looking at the present-day climate, soil and other aspects of the environment? Or do we have to look at the region's history?

Carina Hoorn has shown that we need to go back at least 23 million years, to the early Miocene, to consider two key geologically caused drivers of biodiversity: the rise of the Andes, and marine incursions. She is an associate professor in the Institute for Biodiversity and Ecosystem Dynamics at the University of Amsterdam. Carina Hoorn, welcome to Geology Bites.

Carina Hoorn: Thank you very much for inviting me. It's a great honor to be part of this series.

Oliver Strimpel: The Andes rose on the western shore of South America, while the Amazon basin lies far to the east, extending all the way to the Atlantic coast. How can the rise of the Andes affect biodiversity on the other side of the continent?

Carina Hoorn: Yeah, that's an interesting point. We know from tectonic models, and also from geophysical models, that the central Andes and the northern Andes had a strong uplift in the Neogene, and this affected the climate. It generated an orographic barrier, and it also fomented erosion processes. So we see this effect of uplift in the Amazon basin. There was also another effect, which was subsidence on the eastern flanks of the Andes, and so there was accommodation space for sediments to arrive. Those processes played a very important role in reshaping not only the landscape and the topography, but also in creating a lot of geological changes and paleoenvironmental changes in the Amazon basin. And for that reason, we have to see the history of the Amazon in the context of the uplift history of the Andes.

Oliver Strimpel: So let's talk a bit more about some of the aspects you mentioned. How does the increasing topography of the Andes affect the climate in the Amazon basin?

Carina Hoorn: If you think of the central and northern Andes now, they're a very important orographic barrier. They're about 7,000 meters at the highest peaks in Peru, and 6,000 in the northern Andes. So this orographic barrier intercepts the Intertropical Convergence Zone and these humid air masses that come from the Atlantic. Because they're intercepted, precipitation rates increase on the intercepting flank of the Andes, and a rain shadow is created on the other side. And with that, they also generate, of course, erosion processes.

So this orographic barrier also creates an elevation gradient and a climatic gradient. We have high precipitation and high temperatures on the lower flank, and then as we go up the mountain, of course, temperatures decrease. Especially at mid-elevation, we have very high precipitation levels, and that generates the cloud forest, which is super rich. And we have the alpine zone, which is much colder and more arid. So we already have high diversity on the mountain flanks themselves. But this effect of intercepting the humidity also leads to increased precipitation in the western Amazon. So we see a differentiation between the eastern and the western Amazon. And there's also a geological difference, of course, between having these mountains in the west and the craton, the old crystalline lowlands, in the eastern Amazon. So there's really a big division in terms of climate, but also in geology, for the Amazon, that is generated by the Andean uplift.

Oliver Strimpel: Let's talk a bit about the geology, because obviously the topography increases the rate of erosion and provides sediments that are carried by rivers into the Amazon basin. Are these sediments different from those that were already present in the Amazon at the time?

Carina Hoorn: Yeah, that's a very interesting aspect. When you walk around, or when you travel by boat along the rivers, there's a huge difference in the waters of the Amazon region. You have the white-water rivers, which are derived from the Andes, and you have the black-water rivers, for instance, that are derived from the craton, or the clear-water rivers that are derived from the other highlands that you have in the Amazon. So the rivers themselves already reveal a difference in sediment composition.

The eastern part of the Amazon, dominated by the craton, has weathered surfaces. It's a very weathered profile, so those sediments are very stable in terms of composition, and also depleted of nutrients. So they have a very distinct geochemical signature. Geochemists can differentiate them: sediments older than 1.5 billion years are cratonic, and those younger than 1.5 billion are Andean. They also have a characteristic heavy mineral composition, if you were to look at them that way: stable minerals from the craton, and unstable minerals from the metamorphic rocks in the Andes. So you can see that by measuring the chemistry of the sediments in outcrops, and you see it in the rivers. When you travel around in the field, you see, for instance, clays that are typically blue, very bright, with a very different composition from, for instance, the siliciclastic sediments, pure quartz and gray clays, that are derived from the craton. So the Andes is like a big fertilizer of the Amazon and the western Atlantic.

Oliver Strimpel: Were new ecosystems created specifically as a result of the Andes-driven changes in climate and the riverborne sediments that you just talked about?

Carina Hoorn: Yeah. There were big changes happening in the Amazon in relation to this Andean uplift. A lot of the changes are driven, on one side, by this particular geological configuration, by changes in mantle convection, and flat-slab subduction, which we learned from the geophysicists, has played an important role. That has generated, on one side, the uplift, but on the other side, the subsidence. And through that, we've had a very unique system there, which is this fluviolacustrine system, which we call the Pebas mega-wetland, that really transformed the environment. It extended over more than 1 million square kilometers, and it had a long existence. We think from the geological record that it existed for almost 10 million years.

Oliver Strimpel: Pebas, you said?

Carina Hoorn: Yes, it's called the Pebas system, based on the type outcrop at Pebas, which is in Peru. It's an amazing setting, because it seems it was like a cradle of speciation. It opened up the opportunity for taxa that were not in the region, that settled there because of dispersal from elsewhere, and they found a suitable environment. That applies to mollusks and ostracods, these tiny crustaceans. They diversified very much in that system, and they created a completely new ecosystem that wasn't there before. And of course, there were all sorts of other organisms related in this trophic chain. There was a very big variety of reptiles, for instance, that were able to feed on these mollusks, and there was a very diverse aquatic fauna. In terms of vegetation, we know there was already a diverse rainforest at the periphery of the system, but also marshland, swamps with palms that are now extinct.

So that system existed because of these special geological circumstances, but after some time, it also disappeared. At some point, because of this subduction process, there was an eastward tilt. The sedimentary basins in the western Amazon got overfilled, and this system, this Pebas system that existed for so long, then transitioned into a fluvial system, into the transcontinental Amazon. We know that from the sedimentary record in the western Amazonian basins, but also from the marine basin on the Atlantic side, where the Amazon River sediments arrive.

From the western Amazonian basins, we know that this effect of the uplift of the Andes started to be recorded in the Miocene. By about 16 million years ago, we see already that there are high-elevation Andes, because of the type of deposits we get. We get pollen and spores, which are the reproductive material of plants, deposited in the sedimentary basin, and they show us that there was high montane vegetation in the Andes at the time. And then we see in the marine record that we get sediments of Andean origin; we get them at around 10 million years. So based on the sedimentary record on land, we see how this wetland disappears, and we see the emergence of the Amazon River, and we see the arrival of this river in the sediments of the marine record. So that's how we follow the story.

Oliver Strimpel: We don't see a similar burgeoning of biodiversity next to newly formed mountain chains in all other locations, so there must have been other factors at play here as well. In my introduction, I also mentioned that marine incursions were an important driver of biodiversity. Is that part of what makes the Amazon different?

Carina Hoorn: Yeah, definitely. But we have to go back again to this complex geological history first, because it's in the first place this subsidence, triggered by the special mantle convection and the flat-slab subduction, that generates the opportunity for marine incursions to reach the area. But also, besides Andean uplift and flat-slab subduction, the sea level rise caused by global warming during the early and middle Miocene helped facilitate the marine incursions. For sure, that's a very important driver. And at some point, this process stopped, and then marine incursions were no longer possible, also because of blockages along the pathway, which we think was from the Caribbean, and maybe even from the Pacific. So it is in the first place driven by geological processes.

And then the marine incursions have a very important role in, well, also fertilizing the western Amazon. That's the geochemical aspect, of bringing in new nutrients. But on the other side, it's also a very interesting aspect of the evolutionary history, because we have all these organisms that thrived there, and some of them adapted there. Some organisms disappeared because the conditions vanished and that wetland was no longer there, but some of these species adapted. A nice example of that is the pink river dolphin. That's a very iconic organism that we all love, and it arrived from the oceans. It came with the marine incursions, as did some fish as well, and they adapted there, and they persist. So there is a legacy of these marine incursions in the fauna, but also in the flora. There are some species that we think were really brought in by the marine incursions and settled. There are certain coastal elements that are found and established in the Amazon.

Oliver Strimpel: Is it possible to say how far into the continent the salt water actually penetrated during these incursions?

Carina Hoorn: Oh yeah, thousands of kilometers. Some people have suggested this was just by transport by birds, but we think there is a lot of evidence all along the pathway to the Caribbean. We have made an inventory of localities, and we find marine organisms of different kinds. We find marine protozoans that settled there, and that we know existed for many generations. We find marine algae, like dinoflagellate cysts, and we also find mangrove pollen in large quantities. Mangroves are typical of the coastal environment, but they managed to settle there as well, and they really formed an important vegetation unit for certain periods of time. And there are also some sedimentological features, and the fauna, that tell us that the marine incursions reached thousands of kilometers, into western Brazil.

Oliver Strimpel: That's fascinating. So that's really good evidence for the marine incursions. What about the erosional and soil change processes that you talked about? Can we see the changing sediment composition, and also the changing climate that you mentioned, caused by the rise of the Andes? Can you see that in the geological record?

Carina Hoorn: Yeah. It was particularly important to use some of the available core sections for that, from the Brazilian Geological Survey. We see this transition from stable cratonic minerals at the base of the core, in the early Miocene, transitioning in the course of the early to middle Miocene into Andean sediments. So what we had seen in outcrop, these contrasting types of sediments, we could also follow in core sediments. At the same time, by using pollen and spores, we could set up a biostratigraphic scheme to attach some ages to these sediments. That led us to propose this model of having a transition in the early to middle Miocene, and when you look at the geological models of the Andes, that fits with what's happening there. Subsequently, in the Atlantic, we also see a similar change. We see a change from cratonic sediment to Andean sediment when the Amazon River arrives there. Again, the geochemical imprint is left in the sediment, and we see that at 10 million years. With the help of the subsurface sections, together with outcrops of course, we can little by little piece together the story.

Oliver Strimpel: As you mentioned, most of the Amazon region is actually on the Amazon craton, which is extremely old. And yes, OK, it would be somewhat starved of nutrients, but on the other hand, it would have had plenty of time to develop a lot of biodiversity. So were we starting from a fairly biodiverse base, if you like, when all these events we're talking about with the rise of the Andes took place?

Carina Hoorn: For sure. The Amazon has actually been very diverse since the Cretaceous-Tertiary events, the famous impact, and the transition to a tropical rainforest. OK, that's not exactly the Amazon; it's a bit further north. But the transition to the forest that we now think of as tropical rainforest really initiates in the Paleogene. Also, it's important to realize that the Amazon was previously linked with what is now the Orinoco basin, so it was like a mega-Amazon region. This division only appeared in the late Miocene, so the area was much bigger prior to the late Miocene. So we know that that region was very rich, and also that the craton has already been rich. However, this is a relative concept. Between the eastern and western Amazon, the western Amazon is much richer. And at a global scale, it is the biggest rainforest and the most biodiverse region in the world.

Oliver Strimpel: You mentioned palynology a moment ago, and a lot of your research has been in the field of palynology, which is the study of pollen and spores. Does the geological record in the Amazon basin contain a fossil record of pollen and spores, and was that really how you determined the increasing biodiversity during the Miocene?

Carina Hoorn: Yeah, pollen and spores are very good indicators of change in vegetation, and they can also give us hints about climate. The reason we can work with them is that they are everywhere. They are in your hair, in your nose, and in the Amazon. They are also deposited in the fluvial and the fluviolacustrine deposits. The main thing pollen and spores don't resist is repetitive weathering, oxidation, so they need an anoxic environment for preservation. So what we're always looking for is organic material to extract pollen and spores from. And then you need a good lab to process the samples. But when you have everything lined up, then you can extract this huge biodiversity of the past from just a cubic centimeter of material.

We find thousands and thousands of species, and we are only little by little understanding this diversity of the past now. It gives us an interesting clue. On one side, we know from studies of the Paleogene that it was a diverse forest, but we can also tell something about this huge diversity over the last 23 million years in the Amazon. We see big changes depending on the environmental conditions, on the chemistry of the system, but we see a high diversity throughout. We're now looking into a later part of the history, with a new core that has been drilled, and it will be very interesting to see if there's actually a decline in the late Miocene, when there's global cooling happening.

Oliver Strimpel: Are these spores the actual original organic material of the spores, or are they fossilized spores where the material has been replaced by minerals?

Carina Hoorn: The pollen and spores are made of a very durable component, which is sporopollenin. That is the pollen wall that encapsulates the DNA, the plant DNA. That part is gone, but the sporopollenin is a relict of the original plant.

Oliver Strimpel: And retains the structure.

Carina Hoorn: Yeah, exactly. It retains its composition and structure. And in fact, we work with that structure, because pollen have a very distinct morphology. Based on those morphological elements, we can classify them and also relate them to modern species, so we can assign a botanical affinity to them. And in that way, we can understand more about the composition of the forest, and how the forest changed, or how the swamp palms changed.

Oliver Strimpel: What else did you use to measure the history of biodiversity, especially of fauna, in addition to the flora?

Carina Hoorn: Yeah, the sediments of the Pebas system are very rich in mollusks and in ostracods, but also all kinds of vertebrates. These components tell us about how rich this environment was, and how diverse, and also how different it was: we had a very rich system there that was different from the present. It has been a very dynamic environment. The Amazon is seen now as the richest forest, and one that we should protect. But we learn from the fossil record that it has also been a different system, and that it has evolved a lot through time, responding to climate change, from global warming to global cooling, and transitioning into the present.

Oliver Strimpel: So obviously you were able to access the geological record to be able to find these spores and the fossils of the animals and so on. But the present-day Amazon is extremely densely vegetated. How were you able to find any outcrops there to make your measurements?

Carina Hoorn: First of all, we had a location in the western Amazon, in Colombia, that was our base camp. It was an old prison, almost like Papillon's in French Guiana. From there we could access the river, and we traveled along the river to find outcrops, and at the same time worked together with local assistants, indigenous people who knew the region well. So a lot of it was based on interviewing them and asking them for advice. At some point, of course, they knew what I was looking for, so that made it easier. So we would travel to find outcrops, but they are far apart, and it's not always easy.

But of course, a very important element as well is working with core sections. Obtaining that material was transformational for understanding the Amazon, because you have to tie outcrops and records together to get the bigger picture, to really understand the stratigraphy, and use biostratigraphy to put ages to it. So the combination of outcrops and core sections is crucial.

Oliver Strimpel: There are some oil wells in the region, I believe. Did the oil companies cooperate with you and give you access to some of their drill cores?

Carina Hoorn: Yeah, it's pretty rare for companies to do that, because it's all confidential information. I've tried many times, but I was very fortunate, and I'm also grateful for the possibilities that were given by the Brazilian oil company Petrobras. There were several windows of opportunity that allowed us to work with core sections in the western Amazon, but also at the mouth of the Amazon. And those opportunities have really allowed us to advance our knowledge of the Amazon.

Oliver Strimpel: What are you working on at the moment?

Carina Hoorn: At the moment, I'm trying to better understand the composition of the mangrove coastal system. We talked about these marine incursions and the mangroves that existed there. Nowadays, the mangroves of the Neotropics, of northern South America, are quite impoverished compared to, for instance, the Asian mangroves. But when we look at the past, for instance the Miocene, we already start noticing that there was more diversity in mangroves than there is at present. So we're intrigued by that, and trying to better understand it. I'm also trying to understand the relation between geodiversity and the modern distribution of organisms. So I'm trying different things to keep advancing knowledge of the region.

Oliver Strimpel: Carina Hoorn, thank you very much.

Carina Hoorn: Thank you so much. It was very nice to talk to you.

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