From plankton shells to past oceans: How biominerals record seawater physics | Stergios Zarkogiannis

But, thanks for coming. I want to talk uh to you about like my work, which has been um for quite some years now. Uh so, the presentation will be about uh plankton shells and cells and how they were mineralized maybe can give us some hints about the physics of the ocean. We have been um using microfossils for quite some time for the chemistry of the ocean. Uh but, I think there may be information about the ocean physics on those fossils as well. I'm currently a Humboldt um fellow uh at the Leibniz Center of Tropical Marine Research uh Bremen. And Should this go by clicking? Okay. Good. So, just a bit of uh background. I did my bachelor's degree in marine sciences in the University of Aegean, and then I did uh a master's in environmental coastal engineering. I wanted to be a little bit more um applicable and to apply more the the marine sciences. Uh but, after that, I decided to take a second degree in architecture. And uh 2015, I started uh my PhD in oceanography at the University of Athens. So, after my PhD, I had a Royal Society fellowship for 2 years at Oxford working things on things that they were related to my PhD. These are like uh on planktonic uh foraminifera, which are uh microfossils. Then, I went to on while I was at I I get them I got a Marie Curie fellowship and I stayed there and worked a little bit more on some nanofossils this time. So, these are like also calcifying organisms that they are at nanoscale. And then uh after this fellowship finished, I got my uh Humboldt fellowship to uh work a bit on coral. So, all these um three organisms, they share the same um um the same mechanism that is calcification. So, they use uh calcium and uh and they bind carbon and oxygen to produce their uh skeletons. So, I spent most of my time, as I said, uh since my PhD working with uh planktonic foraminifera. So, what are planktonic foraminifera? They are uh single-celled uh marine protists. And that they build their shells out of calcium carbonate. And in there, the history of the ocean uh is recorded. What is nice about these uh organisms is that they are tiny and they are we can have a lot of them in the sedimentary in the paleoceanographic record. So, they become uh statistically important when we want to extract information um uh past climatic information. Uh so, they are they have been used for uh quite some time and there there are they can be um uh indicators based on their morphology, the way that they live in the water column, and the way they have spines or not, if they are uh globular or not plan spiral and the characteristic of these organisms is that they are passive floaters. So, they don't they cannot they cannot actively swim in the water column, but they only like float themselves and they float themselves at specific depths, which is a characteristic of its species along with other um characteristics. So, they have been around since um the Jurassic. And although they are not that many in the water column, they are very well preserved in the sediment. So, after they die, uh their their material is um is well preserved in the sediment. And they are in this micro scale that they don't they don't easily dissolve. So, we they have been used for quite some time now for their chemical composition because we can gain information about the salinity of the ocean uh also using like trace elements in this calcite, we can tell something about the temperature of the ocean or if we use boron on these shells, we can tell something about the pH of the past ocean. And they their distribution and their ecology can also tell us about roughly now to now today we know it's roughly, but for a long time they have been used to get information about the ecology who tell us information about the general idea of the temperature at different places of the planet of the ocean. And they also because they lock carbon within their shell, they are important they play an important role in the carbon cycle. And this particular uh process, the calcification, is what drives the carbon to be stored in the sediment. So, compared to the uh two neighboring planets, which they already they have like about 98% uh calcium car uh uh carbon dioxide in the atmosphere, the Earth currently has only 0.04%. And the reason for that is because uh the carbon dioxide, it does not just um dissolve in the water, it reacts with the water to form a weak acid. And from this weak acid, the carbon is dis dissociates, and then organisms can use this dissolved inorganic carbon to build their skeletons. By doing so, they lock this carbon into a solid state, which is then preserved into the sediments, and then we have a lot of carbon that leaves the atmosphere through the water to gets into the uh geological record. Uh so, as I said, we have been uh using those as chemical indicators, but I was uh intrigued mostly about they are the the mass of the shell. And this is like a very prominent feature, as prominent today at least as the size of these organisms is, which is like a very easy metric to start working on. And today we have microbalances, but they have like they are like very sensitive, so we are able to actually calculate um the the masses, the average masses of these individuals we are which are on at the size of uh a which are at uh sand grain So, um what is generally uh what we generally do is like we sieve the sediments, and then we pick from certain sieve fractions a few individuals, we weigh them all together, we average we take an average, and this is how we uh create the record of their um shell weight in the past. So, for in the beginning when these like sort of analysis were possible, there were uh there were there were works that found that the weight of these organisms changes through time, and they become heavier during glacial times, and they become lighter during interglacial times. And most of the work has been uh though focused on the last glacial interglacial cycle. But, and then the idea to explain this change in the shell weight is primarily on the chemical basis, and that during glacial times when the atmospheric carbon dioxide is less in the atmosphere, um the waters are less acidic, so these organisms they build heavier shells, and the opposite happens in the during interglacials we have more CO2 in the atmosphere, we have ocean acidification, so these organisms are not able to uh build the skeletons. Uh but, these um relationship starts to break if we look like deeper in the in the in the record. So, this is like the the longest record that we have from these organisms, and it's 1 million years old, and we see that there are times when the CO2 and this is um the uh isotopes, which is an indication of um let's see salinity and temperature of the ocean. This is the the shell mass the shell weight of this particular um uh species. This is Glo- Glo- bigerina bulloides. It's a It's a cosmopolitan species. It's the one that we mostly um use because it's it's found in many places around the world. And I'm going to be talking about this species a lot and then we see yeah that this um relationship starts to break and we have like uh change when the CO2 in the atmosphere is low. So, the waters are not acidic, but the weights are lower as well. So, there was already some uh problems with the with this theory behind the behind the chemical behind the chemical uh forcing on the on the masses. And then what happened to me, and this is like where it all started, I was given a core from this uh equatorial um tropics of uh tropical side uh offshore Mauritania. And the idea was to do a weight analysis of at least two glacial cycles. So, going back to 200 uh million years. [clears throat] Uh I averaged the weights. I I picked the 50 of them for all of like for every time uh step here and then I found no signal. So, there was like no change in the weight. Um for 200,000 years. And then this was striking because I couldn't finish my work because the work The idea was that okay, you weigh, you reconstruct the CO2 in the atmosphere, and then we have something about the acidity of the ocean in this region, but this couldn't be done. So, I had to explain what what I was finding and why there there there is no signal and the signal is um so flat. So, what I did is then I combined all the weights of the of the foram nifri planktonic weights that we had in the literature and found out that the difference between the glacier their glacial weight and the interglacial weight is a function of latitude. So here I plotted latitude and here is the um Yeah, it's the delta mass. So this is like the mass of the glacier their glacial mass minus the Holocene mass. So the difference between uh the termination, how much this uh the weight changes uh through termination. And you can also uh you can almost fit a straight line here and we see that at northern latitudes we have a change of about 50 more than 50% in the weight between glacial and interglacials, whereas if you go closer to the equator where my site was, the change can only be like maximum 8%. And I was like still struggling to understand like how can we explain that? And just note this number here and average the change from glacial to interglacial about 23%. These are like different species, different sizes and different locations. So everything that I could find in the literature by then. And I was talking to another friend about this and then he also told me, "Okay, you know he was studying livestock and cattle and then he said that we know that like in the in the northern latitudes the the cattle has like heavier skeletons. This is because they have to carry a little bit more fat for the temperature." So I said like if this is like a gravity thing, how does this work with the with the with the ocean? Can this be like a buoyancy thing? So I started thinking um how can I check this? Uh I was again very lucky because by the time I was thinking about that, we already had a quite good uh tomography um instruments. And why do we need tomography in this case? So, in order to for me to calculate the buoyancy of these that each of these organisms feel from the ocean, I need to use the Archimedes principle which actually tells us that the buoyancy equals the weight of the displaced liquid. So, the weight of the displaced liquid So, this is the So, this is the amount of ocean that this organism displaces when it is alive. It is um the density of the displaced uh liquid times the gravitational acceleration. And then we can break the the mass of the displaced water even into the density of the of the water times the volume of the displaced water. And the volume of the displaced water is like the volume of the of the foraminifera. If we fill in all these voids with uh protoplasm, this is what um this is the organic part of the cell. And good. So, I have a way to calculate um buoyancy. And what do I need? I need the volume. Cool. I can have the volume from tomography at a very high precision. And then I need some chemical information from the shell, which I can get. So, from the same specimen, I can have the total volume, the mass of the shell, and also if I look at the elemental analysis of magnesium over calcium, I can have an estimation of the temperature of the uh of of the waters when this organism was produced. And then if I look at the isotopes, I can extract So, the isotopes are a composite signal of temperature and salinity. So, if I have an independent proxy of salinity, I can correct the isotope to give me some indication of the salinity. So, if I have if I have temperature and salinity, I have density. And so, this is how I then calculated the density of the water, the volume of the displaced liquid. And this is how I I was able to calculate the the the buoyancy of these that these organisms feel from the ocean. Um this is the relationship between the buoyancy and the and the mass of this particular species, the Globorotaloides. And then I said, "Okay, if this is if this is the thing, maybe I don't need actually to go and see this kind of the specimens. I can go directly to calculate the density using geochemistry, the magnesium calcium and the isotopes. And try to see if there is a relationship with the mass. So, what I did is like I combined again bibliographic data. And these are like measurements from a higher size fraction. So, this is 300 to 350 microns, and this is uh um measurements like reconstructions, density reconstructions of smaller specimens, 250 to 300 microns. And then I saw that there is like a very good uh relationship between the mass of the specimen and the seawater and the seawater density, the geochemically reconstructed seawater density. And also has a very good um physical explanation because these are smaller uh specimens, smaller organisms of the same species, but we know I live uh they live higher in the water column. So, I'm just like reconstructing here lower densities because these were living at shallower depths. So, I said, "Okay, if this is the case, then we can then I formed the hypothesis that the change in the cell mass was due to the change in the density of the seawater. So, during glacial times when we have water pure water leaving the ocean to form continental ice, particularly continental ice, the change is also the volume of the of the ocean. Um we have all the particles, all the all the ions in the water exerting greater forces to these organisms now because the volume has declined. So, all these extra all these concentrated particles will will try to squeeze the organism towards the the surface. So, my hypothesis was that in order for the organism to maintain the same depth, let's say that its optimum carbon depth is 50 m, in order to maintain this 50-m depth against the the force exerted from a glacial denser ocean, they need to build a bit heavier shell. And then I just yeah, found this um relationship. I just I want to mention this earlier that can get us from the weighing these specimens, single weight measurements, to the density of the ocean. And the density of the ocean is also very uh is a fundamental uh characteristic of of um of material and of of the ocean, of course. So, I said, "What happens if I use this proxy that I created to do a little bit large-scale oceanography?" So, I just uh weighed specimens from the North Atlantic, the equatorial ocean, and the South Atlantic for 200,000 years, and put them together. These are the weights here as it is, and this is like when I turn them into seawater densities, and then I found out this nice graph. And there are like three uh nice uh and striking things in this graph graph. First of all is this is the very first graph that I showed you in the beginning that started the whole thing from this area where there was no um signal, and the weights here are the lowest. And this makes sense because the equatorial uh the equatorial ocean is low is like the density of the equatorial ocean is the lowest because it gets most of the energy of the sun, and it also gets a lot of precipitation. So, the waters are light, and this is why the weights here are like the lightest. And also, as we go away towards like higher and lower latitudes, we see this sort of mouse activity uh happening. And I explained this cyclicity to be a cyclicity in the in in the seawater density. And the other cool thing is that there are times where the densities of the Eastern Atlantic, the weights at least, they converge momentarily. And not only they converge, but they converge to the same number. And these are times where I found in the literature are that the the the whole Atlantic circulation momentarily uh ceased, or it was like very sluggish. And the other the other nice thing is that we see that after the last uh deglaciation, the the the gradient between like the different basins is minimum. So, which also this tells me the circulation in the Atlantic has been slowing down. It It is slower than in its past. And so, this was like a first check that I did to see whether we can use as a proxy as cheap and easy and fast as the weights to do uh large-scale paleoceanography. But, I wanted to test this uh this hypothesis further and because you can say, "Okay, maybe this change this non-change in the signal here is because like there is uh no change in the temperature and the way that the CO2 dissolves in the water is a function of temperature. So, acidification would not affect the whole the all parts of the ocean the same way and maybe high latitudes get more or less acidified during uh glacial um cycles. So, the only way to disentangle between this is if we look in depth in in the sense that we look what happens in the water column. So, what we What I did here is like I weighed um eight different species. Now, seven different species in this case and that I knew that they live at different depths. And then I found out that the the weight of the cell is a function of their position in the water column. There is There are some discrepancies here and this because this is not normalized to the volume. And this is where tomography comes in in the next week. We can have like volume-normalized cell weights. But, in principle, the the organisms that they live deeper in the water column, they have a heavier shell. And this goes against the acidity of the water column because we know that the acidity of the water column increases with depth. So, if these organisms were so much um affected or if their calcification was primarily driven by the seawater chemistry, then the shells should have been lighter in the deeper more acidic waters, but they they don't care as as long as they have food. They are uh they are happy to to to maintain some chemical gradients and produce their skeletons. And then I said, if this is true for one taxa, it should be true for other taxa as well. What happens to coccolithophores? These guys that I told you uh in the beginning uh that they also calcify, but they are nano scale. So, and then I got this graph from uh um a colleague, and this is the amount of inorganic to organic carbon in the shell of these organisms. And then, what this graph shows here, this is the depth, and this is latitude. Um what this graph shows here is like again, these organisms, they are heavier, they have more inorganic carbon in their in their when they are deeper in the water column. So, again, they don't so much care about the acidity of the of the of the surrounding environment. So, this is where I had uh I the best evidence that I could for my uh hypothesis. Um So, earlier I talked about the fact that we need to normalize the weights to the total volume of the of the of the specimen. And when when I did that, I compared these three different species together, I found a relationship that I could fit all species together. Uh and so I can then convert the volume normalized weight or as I call it bulk cell density because it's like mass over volume, which is an indication of density, uh to the density of the seawater. I had a similar graph in the beginning, but it was only for one species. So now this is for many species. So what I wanted to to check here is like what this graph actually shows us is that if we get any size normalized weight of any species in the water column, we can have a density indication at this specific depth. And this is where I started uh on my next fellowship step, which fellowship step, which is like to look at organisms from different basins. So and then I went to the black to the Red Sea because the Red Sea is a a nice um density end member uh because this is a very saline and and the waters here are are pretty dense. And also there is this very nice gradient from the north to the south of this um of this basin. And with the hope that I will find a change in the in the weight of these of these organisms, of these two different species as I go from north to south. So this is um work in progress. So yeah, tie everything back to architecture, what I wanted to show you with this here is like for example uh the the fact that these organisms have or don't have availability of material doesn't mean that they need to produce their skeletons according to material availability. They have to produce their skeletons according to their living needs. And this is what I want to show you that today this is the limit up to which we can build. And this is not because we don't have the resources, it's because we have some physical limitations that do not allow us to go further in the map. Um yeah, this is I think some um future steps that I would like and I think these are like the results and some consequences of the relationship between um the weight of the of plankton shells to density and how this can be used for example to do physical oceanography because if we go this is um cross-section across the Gulf Stream between the Florida, Miami, and the Bahamas, yeah. And this is the velocity field that um we get and this is the density gradient. So, always in the geostrophic flows >> [snorts] >> the the the the strength of the velocity, the the strength of the current, so its velocity it's uh it's a relationship, it's a function of the density gradient between um these two points. So, the greater the the the gradient, the faster is the velocity. So, my hypothesis this is this is what what what uh I did something that I did this year is like if I go and take some samples from here and there, maybe the the change in the calcification in their weight is enough to let to allow me to reconstruct in the end the the the velocity of the Gulf Stream. And this is if this is true for today, then maybe I can do this uh in the sediments in the past. Uh this is one application and the other application that I think uh this relationship between calcification and seawater density extends to is the fact that maybe we will maybe able to explain the amount of carbon that goes up and down during during climatic cycles. So as I say as I said earlier, we know that the the carbon dio- carbon dioxide in the atmosphere has been going up and down during the climatic cycles. And at least for the last two, maybe more four, but then goes down to about 100 ppm. But still we don't know how this happens and where does this carbon go. And so my hypothesis here is that it might be that if you decrease if you increase the density of the waters in the during the glacial ocean and then you have more carbon built around the cells and you have like in the end more carbon carbon being stored in the sediment, this also means that maybe these organisms this is they get their carbon from the atmosphere. So they they they drive the the general concentration of the atmosphere down. And then we lose this carbon to the sediment. So we we we get it out of the of the atmosphere. And to know that maybe we also need to decrease a little bit the alkalinity. And this can well happen because during glacial times we don't have a lot of precipitation because you have a lot of water being made into solid. So the alkalinity that we get from the from the runoff what what we need to produce the to have calcification declines. So there might be a point at which these organisms do consume all this alkalinity and then they start to degas because during calcification you have one molecule of of carbon going down, but there is also one molecule of carbon that gets released back to us carbon dioxide. So, if you squeeze the um system to the extreme, maybe we will start having outgassing here. This then would turn the cycle to the next interglacial uh to the next interglacial phase. And I just I I I'm not a modeler, but I used like AI for this and then I for a 20% like according to the literature change in the alkalinity, we just need to have 15 to 20% change in the pelagic calcification to have outgassing of CO2 from the water to the atmosphere. And if you remember the number when I told you in the beginning where it was about 23% change between glacials and interglacials, then we are very good into these numbers that um can perhaps explain these changes in the atmospheric concentrations. Um I know that here I'm talking to an audience which probably knows about that. I don't know I hope that I covered some things for the audience that uh are not so familiar with oceanography and then I want to tell you like one next step and this is why I think my visit here at Links was very important is like how can we use X-rays and remote analysis to calculate to um study material that are in museum collections and this is what I tried to do using some material that are collected from the ocean in the 1870s. So, these are like material from the almost pre-industrial ocean. And these material are locked within these glass slides. And then the only way to access This is the zoom in. They actually It's amazing how well preserved they are. And the only way to access this material is by using synchrotron because we cannot put this into the lab CT because of the size of the uh of the specimen of the of the glass slide, it cannot rotate. So, we need like the synchrotron. And we also need the synchrotron because of course we we need better resolution and greater um scanning times. And this is how we can use synchrotron for the analysis of um uh of the um of the specimens. And we can perhaps also use neutron analysis to analyze specimens that they are loose, some of them, but perhaps also the ones that they are in here as well for boron uh concentrations. And this is uh because neutrons are pretty sensitive um to boron. And by calculating uh the boron in these cells, we can have some estimates uh of the of the pH of the of the in pre-industrial ocean by analyzing uh these specimens. And yeah, I think the the way to go forward with that is using prompt gamma activation analysis. And I was hoping to uh intrigue some people working in the neutron area to uh initiate some discussions while I'm here. Um this is where I'm more or less at. And thank you very much for your attention.