Scientists put a salt marsh in a giant wave-machine: here's what they learned

Simulating salt marsh storms

0:00Welcome back to the Nature Podcast. This week, simulating how salt marshes respond to storms and learning more about what Denisovans looked like. I'm Benjamin Thompson.

0:45Salt marshes are a particular type of ecosystem. They're found in coastal areas all over the world, and they're tidal, meaning they regularly get flooded or inundated by the sea. Hence the salt. These landscapes, often dominated by grasses, are vital habitats for animals like seabirds. A lot of salt marshes have disappeared due to things like land reclamation, but they're increasingly being considered as important parts of coastal defences against storm events. In some

1:21cases, artificial defences like dikes, levees or seawalls weren't designed for a changing world, or haven't been adapted in the face of it, and big waves during a storm surge can overrun them. Salt marshes, however, can help. They're often higher ground than the surrounding seabed, and the plants that live there act as natural breaks, slowing oncoming water and attenuating, or taking the energy out of, waves. But while a lot of research has gone into understanding how salt

1:53marshes can act as coastal defences during regular storm events, there's not as much known about what effects these ecosystems might have, and indeed whether they might stand up to, truly monster storms. The kind of once in a hundred years or once in a thousand years extreme events that are expected to become more common as time goes on. One of the issues with figuring this out has been that, well, these events don't happen very often. So, to find out how salt marshes might fare,

2:24a team of researchers has come up with a different approach. So, yeah, the storm wasn't brought to the marsh, but we brought the marsh actually to a facility where we were able to create this kind of super storm, indeed. This is Pim Willemsen, from Wageningen University and Research, and from Deltares, both in the Netherlands. He's part of a group who have used an enormous wave machine called the Delta Flume to pummel a sample of salt marsh in simulated storms. I called him up to find out

2:56more about the research, and he explained the efforts the team went to to set up their experiment. We tried to get this marsh, intact pieces, transported from the Wallen Sea area, so all the way to north of the Netherlands back to Delft, which is actually close to the southwestern delta of the Netherlands. We had to really put this in a restricted time window because we had to wait until breeding season was over to not disturb the birds, and we had to get out before the storm

3:29season started because, yeah, then the chances are rather high that a marsh gets inundated again. And we had to be on the marsh with trucks with 50 tonne cranes to lift out almost 70 blocks of two by two meters with a depth ranging from 40 to 70 centimeters. So we transported 20 truckloads from the marsh towards the Delta Flume. Wow, and talk me through this facility then. In this Deltares, Delta Flume is a wave basin, so let's say a big swimming pool with a length of 300

4:03meters and a height of 9.5 meters. And there's a big paddle at one side that is called the wave generator, and that is able to generate these high waves and move all this water. So I think it can hold 9 million liters of water, which is the equivalent of almost four Olympic swimming pools. And you've used it to do a series of runs where the salt marsh you built, essentially recreated in the flume, was subjected to waves of different heights. What did you see then in these runs?

4:36So when the flume was drained, we really saw that after each run, a lot of vegetation was still straight up. And we also saw that after the really high waves of two meters, a lot of the vegetation was still straight up, but it did progressively damage over time. Still at the end of the full study, we saw at every block that we collected that vegetation stamps were standing and were able to contribute to wave attenuation. And in addition to that, we saw that the substrate also remained

5:11intact. Of course, mud particles and sand particles got up, but on average, we didn't saw a lot of erosion. The erosion was only seven millimeters on average. So the sediment survived these storms, and the plants did too, but obviously got damaged. What did these plants actually do then to attenuate, to lower these waves? The wave went traveling over the marsh, fuels friction and results in a decreasing wave height.

5:41We saw that the vegetation with waves of 75 centimeters was, at the start of the experiment, able to attenuate 20% of this wave height. But also the higher waves at the end of the experiment with a lot of vegetation damage, there was still a couple of percentage of the wave height that was attenuated by the vegetation. And can you imagine, it's just sticks standing straight up, let's say, or stems with a bit of leaves. And that is apparently able to attenuate a little bit of the wave of two meters that is, yeah, going over this vegetation.

6:15And I guess that could actually make a big difference in terms of, does the water go over the top of the human built structure, that sort of thing? Yeah, exactly. That might help a lot. Then it is for these two meter waves, it was at the potential peak of a storm, only a couple of percentages. But at the same time, for the lower waves, a lot is attenuated. So if 20% of a wave of 75 centimeters is attenuated, well, that is quite a lot. And to give a practical example, the results are also being used in a dike strengthening project

6:46in the Netherlands, where the dikes need to be improved, let's say, up in the north, where we took the marsh. And there, it really results at some locations that the clay layer on top of the dike might be 10 centimeters smaller. And then you can say, well, 10 centimeters is not quite a lot. But if you do that over a dike stretch of 15 kilometers, for a dike that has a height of, let's say, eight meters, well, then the savings go towards millions. So in that sense, if the salt marsh is already there, and we now prove that it remains in place,

7:21and that the vegetation can contribute also during these extreme waves, well, then it really starts to count. And we've talked a lot about what you've seen there in the Netherlands, I suppose, like these salt marshes are very good at attenuating the strength of the wave height of moderate storms, not quite as effective at very big storms, but still have some effect. You've actually taken the data that you've gained here, and tried to extrapolate it out to other places. Now, obviously, there are many environments like this around the globe. What was the sort of take-home message from that bit? What we then globally observed was if we extrapolate our results using a statistical model,

7:58we really saw that with these lower waves heights up to one meter, the marshes are able to contribute as much as in some locations 50% of wave attenuation. But as soon as the wave gets higher, it attenuates up to 10%. But still, we saw from this practical example in the Netherlands that it can still contribute a lot to how you build a dike at the back of this marsh then. And so you've got your data then, which is based on a salt marsh on the coast of the Netherlands in winter. From what I understand, this marsh is dominated by one sort of plant, by one sort of

8:33grass. And it obviously has the sediment that is present. I'm imagining that salt marshes around the world do look very different. So how applicable is what you found here, do you think, to other places around the world? Yeah, so that is very fair to say. What we did was wait till the end of the storm season to start the test. So the natural degradation of the vegetation through a winter season already happened. So in that sense, we got a conservative estimate of the state of the vegetation. We didn't test it at the end of the

9:04growing season, for example, and it is still strong and green and so on. And we did also additional tests in the flume that is described in another paper, where we added lots of different vegetation species. And also there we saw that the biomass only decreased by a 30% every wave run. So we really saw the same results there. And so from a vegetation species, I think we are rather on the conservative sides. The substrate we have here has been built up year after year, layer after layer. The growth is mainly

9:37driven by minerals. There are other marshes globally that are driven by organic growth, by the vegetation themselves. So that also results in a different erodibility. So the global assessment that we did, it gives first insights of where potentially vegetation can contribute to wave attenuation. But it definitely needs a local assessment as well. And that then maybe informs what built defenses need to look like in addition to the salt marshes. Is that fair? Yeah. Or you can see the other way around what we can prevent in terms of structures and how we can

10:11better use the salt marshes that are already there. And if we now prove that the salt marshes actually have this additional function, we might better think about how to keep them the way they are. This is a very positive message in how this ecosystem is not only good for biodiversity, which is already a very important to start with. But you can gain more from it by leaving it there as it is already. Pim Willemsen there. To read his paper, look out for a link in the show notes. Coming up, what the

10:46study of ancient proteins is revealing about one of our enigmatic ancient relatives. First though, it's time for the research highlights with Dan Fox.

Orangutan playdates and lead inequality

10:56Orangutans are the most solitary of the great apes. But despite this, a new study has found that orangutan mothers will go out of their way to arrange playdates for their kids. Researchers analysed 15 years of behavioural and movement data from 31 mother-offspring pairs of Bornean orangutans in Indonesia. They found that mothers with similarly aged young tended to spend more time in the same areas of forest than did those whose young were further

11:31apart in age. Young orangutans who were close in age also tended to play together more often than did those with a larger age gap. And on days when their offspring played with peers, mothers travelled farther and spent less time feeding than on other days, they also tended to move towards potential playmates the day before an encounter, returning to their usual territory afterwards. The authors say these findings suggest a previously overlooked way in which orangutan mothers support their young's social development. Arrange a date to read that research in the

12:07Proceedings of the National Academy of Sciences of the United States of America.

12:16Traces of lead preserved in human teeth have provided a snapshot of inequality in medieval Europe. The Peak District in the UK was one of the largest lead-producing regions in Europe during the Middle Ages. To better understand the labour dynamics during this period, researchers analysed teeth from 15 people who lived and were buried in the Peak District in the early 7th to early 16th centuries. The team looked at chemical signatures in the tooth enamel. Lead accumulates in enamel early in life

12:50and doesn't dissipate over time, acting as a time capsule of childhood exposure. People buried with high-status goods, such as crystal amulets and swords, had lower levels of tooth lead than did does with less decorated burials. This indicates that the latter grew up and worked in lead-tainted environments, suggesting that lower-status citizens were responsible for mining and smelting lead, whereas wealthy elites had less direct exposure. You can find that paper in Antiquity.

Uncovering Denisovan facial features

13:22There's a paper out in Nature this week that's looking to shed some more light on a group of our ancient relatives, namely the Denisovans. Nature's Rachel Fieldhouse has been covering the story, and she joins me now from Sydney, Australia. Rachel, thanks as always for being here. Thank you for having me. So yes, this story is ultimately about Denisovans, but also it's about helping to solve a mystery of sorts related to some ancient hominin remains discovered in Shujaoyao in China in the 1970s.

13:58Tell me about this trove of remains that was discovered. So we had this collection, essentially, of 20 cranial fossils and teeth, jaws that were found in northern China, and they were also found with some cultural artefacts and some animal remains. As far as we know, they're at least 130,000 years old, so they're one of the earlier examples of ancient humans that we have. But we, until recently, have not been able to identify who they were or what kind of human they were.

14:29And so what species these bones belonged to was something of a mystery then, and folk attributed them to various different ancient hominins, right? And so to try and maybe figure out what they were, in this paper, the researchers looked at a couple of bones that had been uncovered then back in the 70s and studied the proteins that they could find. Yes, so they started off by doing a technique called paleoproteomics, which is the analysis of ancient proteins. One of the benefits of this technique is that proteins tend to last a little bit longer than ancient DNA does, so it can use a bit of insight for older samples where the

15:03ancient DNA might have already degraded and be too poor of a condition to actually be analysed. What they did was they looked at tiny pieces of proteins called peptides that were from some of these fossils. And when they compared the Shujiyao fossils to other Denisovans samples, they found that, in particular, a single amino acid change on some of the peptides was also found in some of the samples that are known to be Denisovans. So from that, they were able to conclude that the the Shujiyao samples were Denisovans. And of course, contamination is always a potential

15:38pitfall in studies like these. But in this case, the researchers were looking for the hallmarks of how a couple of amino acids can become chemically altered over time. So yes, they were looking at a particular process called deamidation. So this tends to happen as proteins age. By looking for peptides that had these residues, they could be sure that the peptides were actually from the fossils themselves and had been ageing over time. They weren't contaminants. They weren't any other thing that was potentially modern human, or just anything that wasn't actually belonging to that fossil.

16:13Now, researchers only uncovered the existence of Denisovans about, what, 20 years ago. And that was via, I think, a single finger bone found in a cave in Siberia. And other bones have been found since, and researchers have been trying to piece together more about this branch of our family tree. What does this work do in that regard? What light does it shed on the Denisovans? Well, part of it is that it shows that the territory and where Denisovans were is a lot bigger than we'd previously thought. So there was more of these Denisovans in East Asia. And additionally,

16:44this work gives us a sense of who they were at like a molecular level, and also at a physical level, when they started to look at the morphology of the actual bones and how they compared to modern humans and Neanderthals and sort of what made Denisovans special. Right, because a lot of these bones that were uncovered in the 70s were cranial bones, were pieces of skull. And the researchers of what tried to piece these together to give an idea of what some Denisovans at least might have looked like? That's right. So this study gave us a general idea of what the Denisovan looked like. Maybe

17:16you frame it as sort of the average Denisovan, because previously, this team had a single individual, the Harbin individual. And they've since taken that one and the Shujeyao fossils, and they started to get a picture of the common traits. So for instance, Denisovans had a low sloping forehead, a pronounced brow ridge, and then for the bridges of their nose, it was a flat, but moderately elevated bridge. And they then compared what they were seeing there with what we can see

17:46in Neanderthal skulls and modern human skulls. And they found actually that Denisovans had a mix of different features, some found in Neanderthal, some found in modern humans, and also some other ancient species like Homo erectus. That's another one that has since gone extinct. But for example, with the Homo erectus, they shared a thick cranial vault. That's the thick part of the skull that encases the brain and protects it. And this work continues to shine a light on how our ancient relatives may have lived. And of course, now, maybe what they would have looked like in terms of the Denisovans.

18:17What have researchers you've spoken to about this work told you? They've been generally very excited. It's very exciting to them that we can finally, as one researcher put it to me, start to put a face to the Denisovans' name, really. And they were also excited because, I mean, this team has shown that using paleoproteomics is a viable and useful tool. And it can complement existing tools that we already have. The researchers behind the study, when I spoke to them, they were quite excited and really looking forward to using this technique.

18:48And we're hopeful that this technique could be really useful for identifying more fossils that were Denisovan, so fossils we've already discovered, but have an unknown identity. And also the potential use for new fossils that have emerged that are very old, but have only recently been discovered and being able to quickly identify who they are rather than leaving them in this mystery category. For the Shujiao samples, they've been in this mystery category for decades. They were also telling me too, that one of the outcomes from this study for them has been settling the debate about who these humans

19:21were, who they belong to, who their relatives were, and how they were related to us. Nature's Rachel Fieldhouse there. Head over to the show notes for links to her story and the research paper we talked about. And that's all for this time. If you'd like to stay in touch with us, you can. We're on email, podcast at nature.com, or you can find us on social media at naturepodcast. I'm Benjamin Thompson. Thanks for listening.