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May 3rd: Tracing Planetary Atmospheric Evolution Using Isotopes from Life, Part 2


Date:May 3, 2011

Title: Tracing Planetary Atmospheric Evolution Using Isotopes from Life, Part 2

Podcaster: Adam Fuller

Link: Astrobiology Forum – http://astrobiology.jhu.edu

Description: To detect life in space, we must first understand how life changes its environment and how it changes with its environment. And the best place to study the interaction of life with its environment is right here on Earth. For today’s 365 Days Of Astronomy podcast, the JHU Astrobiology Forum’s Adam Fuller spoke with Dr. Naomi Levin, an assistant professor in the Earth & Planetary Science department at Johns Hopkins University, about paleo-climate research, the study of how Earth’s atmosphere and climate have changed since its formation and the clues plant and animal fossils leave us that reveal these ancient climate patterns. This is part two of the interview started on April 26th.

Bio: Adam Fuller is a graduate student in the Earth & Planetary Sciences department at Johns Hopkins University in Baltimore, Maryland. For over a year now he has worked with Dan Richman and Veselin Kostov, both graduate students in the Physics & Astronomy department at JHU, to establish the JHU Astrobiology Forum, a cross-disciplinary group that promotes astrobiology research among undergraduates, graduates, and faculty at Hopkins and the surrounding research institutions. More information about the JHU Astrobiology Forum can be found at their website, http://astrobiology.jhu.edu.

Dr. Naomi Levin is an assistant professor in the Earth & Planetary Sciences department at Johns Hopkins University. Her research focuses on understanding how landscapes and terrestrial organisms respond to past climate change. She primarily uses stable isotopic records to study interactions between mammals, vegetation, and climate in past ecosystems.

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Hi, my name is Adam Fuller, and I’m a graduate student in the Department of Earth and Planetary Science at Johns Hopkins University in Baltimore, Maryland. Along with Dan Richman and Veselin Kostov, I helped create the JHU Astrobiology Forum, a cross-disciplinary group that promotes astrobiology research among undergraduates, graduates, and faculty at Hopkins and the surrounding research institutions. As part of our group’s mission, this year we’re highlighting the cutting edge research being done by our faculty. Last week you listened to the first half of our discussion with Dr. Naomi Levin from the Department of Earth and Planetary Science about her research on paleo-climates. This week we finish that discussion.

AF: Okay, what can, how do we take the carbon-12 that we measure and the carbon-13 that we measure in plants and extrapolate backwards and figure out what the climate is back, say, in a thousand years or two thousand years ago or in a million years?

NL: The amount that those plants discriminate against carbon-13 versus carbon-12 can be a big function of plant type. There are most tropical grasses today use a C4 metabolic pathway which is kind of a way to actually—if you take the perspective of the last 5 million years, we’re actually now, and probably conservatively, let’s say, in the last 2 million years the parts per million, the atmospheric concentration of CO2 varied between 180 parts per million in 280 ppm like clockwork based on the glacial and interglacial cycles. So we were in very, very low CO2 conditions and we seems to be constricted to these 180 ppm and 280 ppm. After around—and it’s just starting to be documented since the 1950s and Mauna Loa—after the 1850s we started to march past that current boundary condition of 280 ppm. So now we’re actually at 390 ppm which is 100 ppm more than this boundary condition of during the glacial/interglacial was in the last million years or so, which is actually shocking if you think of it. The earth has been maintaining this steady state and it’s never gone beyond that 280 ppm—

AF: —in the past few million years.

NL: Correct as—basically since we’ve had glacial and interglacial cycles in 100 years or 150 years we have increased atmospheric CO2 by the same amount that it would oscillate between a glacial and an interglacial cycle. That’s 100 ppm. So while I say it’s low in geologic time period, for the life on earth as we know it, as humans have evolved on earth, we have—and as really actually mammals have really taken off on earth we only know this low CO2 condition. We are actually starting to leave the boundary conditions in which humans evolved, in which mammals started to really evolve and become more dominant. The earth has seen high CO2 levels and low CO2 levels, but in terms of the conditions that humans know how to negotiate and the distribution of the present sort of climate regime, it’s actually kind of shocking. It’s a matter of scale.

AF: Do we see any evidence of this shifting of a ppm of CO2, like—do isotopes in plants tell us anything about this? Like, when you go and do the paleo…

NL: They do. They do for a number of reasons. Some plants are basically adapted to low CO2 conditions. This means if you think of CO2, carbon dioxide, as plant food and if there’s a lot of plant food around, a lot of CO2, so very high CO2 conditions like thousands of parts per million—10 times more than we have today—then plants, basically that plant can be as piggy as it wants to be. It can pick and chose the 12C over the 13C. However, if you have low CO2 conditions, basically you would have lots of fractionation, lots of offset between the atmosphere and the plant carbon isotope value. If that plant needs to get all the CO2 that it can possibly get, then it’s going to be less picky about the carbon isotope fractionation. Other things that could—so that’s the concentration of the atmosphere. Other ways that the isotope value of plants would change based on environment is—and it’s the same sort of—you can kind of think of it in similar parameters, but water availability, the fractionation of plants will depend on how much water is available. This also in some way depends on the type of plant and its specific physiology. Water stress, temperature, plant type—some plants have different fractionations than other plants. The C4 grasses, most tropical grasses, are actually a lot less picky than trees in terms of the carbon that they uptake. That’s another way to kind of parse out the different isotopes. Plant ecophysiology and isotopes mapped onto them is a whole field in and of itself. What we do in terms of using those are patterns that are observed in today’s climates and under different environments today and how it affects carbon isotopes in plants. We then in turn try to extrapolate back to the past to try to understand—we use the different isotope distributions to try to back up, “Okay, were there grasses here or were there trees here? Is this indicative of low CO2 conditions or high CO2 conditions? Could this be—could the isotope variation that we see could be a functional water stress or maybe high temperatures?” We’ll often use multiple—we don’t only use carbon isotopes—we’ll use multiple proxies to put together a story. But ultimately it has to be consistent.

AF: So then, when you are looking at, when you’re looking back, say a million years, are you looking at fossils, like, is there evidence of what the ratios were in fossils of plants or, I’m not sure, like, how–

NL: How we transfer from plants to actually something that we can measure—in some cases plant matter is preserved and you can actually measure the actual matter. That is, you have to run all sorts of tests about to argue why the original plant matter is preserved. Other ways in which plant matter preserves, if you don’t have a fossilized leaf but still leaf remains that are intact organic matter, a general sort of organic matter from the soil, that can be analyzed. What’s been done in terms of origin of life studies is actually looking at specific biomarkers, specific compounds themselves. You can look at hopanes and steranes and actually look at the carbon isotopic composition of those specific compounds. Or now, specific amino acids are being analyzed for their isotopic values. You can actually look at organic matter. But the neat trick that geochemists have come up with essentially is that those organic signatures get incorporated into non—they get incorporated into minerals that then are preserved over much longer spans of time. So, for instance, teeth are preserved over very long periods of times. We are walking around with a bunch of minerals in our bodies. Our bones and our teeth are going to last a lot longer than our tissues, and teeth are going to last longer than bones. We can analyze the carbon isotope composition of a giraffe tooth that’s 5 million years old and figure out, well, were giraffes really eating grasses 5 million years ago or..they also had shorter necks so maybe, you know, is that possible? We think of giraffes as browsing, but, well, there’s actually these short-neck giraffes that grazed and ate grass. We can test that with carbon isotopes. Other minerals that are preserved are things like stalactites and stalagmites. These shells that are preserved—you can preserve carbonate, the same calcium carbonate mineral that gets preserved in soils. I work a lot with that calcium carbonate. The carbon that gets incorporated in that calcium carbonate in the soil is a function of the vegetation that was growing in that soil when that carbonate formed. So it kind of seals in and locks in that carbon isotope signature. A lot of what we do as geochemists is try to figure out, try to continue to make as many links between these mineral forms that get preserved. We can measure millions of years later versus the actual active processes that are going to contribute to their isotopic composition.

AF: Okay, so then if I were to bring you two samples of soil and I didn’t tell you one of those samples is from Mars, you would be able to tell me yea or nay, like, just doing, like, isotopic, like, just looking at the isotopes in the soil samples? Could you say, this sample here, conclusively, has never had life in it, or life has never existed…

NL: If there’s carbon—if you gave me a soil sample from Mars, there’s multiple things that you can use from it, not only with isotopes but just in terms of the content of the actual sediments themselves and what they’re made out of. The land formations of Mars are made up of different minerals from that of earth, so probably there would be something diagnostic about just the minerals themselves in the soil. In terms of isotopes, is there life in it? Well, the first thing you would want to be careful about, and this is something all return mission have to be careful about, that is, if you’re looking for signatures of life you better be darn sure that you didn’t introduce any signatures of life when you went to collect it. That’s a huge problem and something you have to be aware of. Soils on Earth have so much organic matter, if some Martian came and got some of our soil and even if they just got their little martian paws on it, the organic matter in our soil would probably overwhelm any influence of their little Martian paws unless it was very different from what our signatures on earth. So you would first just look for how much organic matter is there. Is there very much? And then you could say, and then if you’re absolutely positively sure that you didn’t introduce any, you did not introduce any organic matter into it, then you can start to analyze that organic matter for carbon isotopes, let’s say, which would be a first pass. What people are doing today and which is more diagnostic is they’re actually looking at specific amino acids or specific biomarkers and analyzing the isotopic composition of those. They tell you a lot more information if you actually know exactly what you’re analyzing. If you analyze—if you pick up a bunch of, let’s say, soil with sticks and leaves and worms and grass in it, you just analyzed the whole pile of soil, you don’t know really what you’re getting. But if you analyze the individual parts of that soil, then you can kind of get a better sense for you’re seeing. But the problem of carbon isotopes is that there’s more than one way to get these low delta 13C values. You have to come up with, if you do get low delta 13C values, which in most cases is indicative of life, then you’re going to have to go through also a series of—start to throw out all the other possibilities of how you could get those low delta 13C values. There are some processes, metamorphic processes on earth and some exchange processes on Earth, that can result in low delta 13C values which have been made it difficult in many ways to use carbon isotopes on earth to actually identify the earliest evidence for life, because you have to prove beyond doubt that only life form these low carbon isotope values, not anything else. You would have to go through a very long rigorous procedure to kind of knock out all the other possibilities of what could form those values.

AF: So then this dovetails nicely to the part of the paper I sent you the other day where we had a probe—the Huygens probe—made a measurement of the, looking at the methane, it made a measurement of the carbon isotopes in the methane and they were able to say, “the ratios do not match what we would see for life here on Earth.” If we were to see a ratio that matched a ratio somewhat like what we see for methane here on earth, how could we know that this other planet wasn’t, that its methane ratio was due to life versus geological processes. I mean, are there geological processes that occur here on earth that could occur on other planets that could fake an organic signal in the isotopes?

NL: Yeah, so methane is kind of classic as a very low carbon isotope sort of signature. So I think to, I would think that the way to actually have more confidence in what those carbon isotopes values of methane actually mean would be to also try to find, get carbon isotope measurements from other parts of the carbon cycle of the body that you’re looking at and to look at those sorts of offsets. Okay, so how do we know what the carbon balance on Titan should be, right? Maybe what we’re seeing for the methane is just average bulk composition of how much 13C there actually is on Titan. We happen to have 1.1% on earth, but maybe there’s a lot less on Titan or maybe there’s a lot more on Titan, right? I think the first step to actually trying to use carbon isotope as some sort of the signature for life elsewhere is to understand the isotope, carbon isotope landscape on that body that you’re looking at. And I realized that that’s a hard question, but even if you have a couple of other points, is there CO2 being emitted in places and what is the carbon isotopic composition of the CO2 relative to the methane? And, if you are ever to get some sort of return samples, what would you expect if you had any carbon preserved? What, so I think, you know, it’s not going to be—

AF: There isn’t going to be one magic bullet—

NL: Yeah, there’s no magic bullet to basically answer the question of it, and it’s a complicated system. There’s multiple ways to basically—even on earth, to produce low carbon isotope signatures, basically you have to start trying to check off all the other possibilities that it could be and be very prudent and very sober about how you go about systematically honing in on what really is contributing to the isotopic value that you measure of that methane. Part of it is understanding the systems. So just like that low carbon isotope signature in that closed canopy forest, there’s nothing different going on really in that closed canopy forest. Plants are still metabolizing that CO2. They just see different CO2, they just they just—

AF: Different CO2 is available to them to use—

NL: Right, so that’s what they use. If there’s a different abundance of 13C on a different body, then you’re going to have to kind of recalibrate your system. We know Earth. We know Earth really well. We have a good sense for—we have observations of the patterns, the isotopic patterns that exist on earth, but we also have a good sense for what are the physiochemical reasons for why we have isotope fractionation. So we can applied a lot of what we know about Earth to different bodies, but we have to also be prudent in terms of making sure that we see that whole system.

AF: Okay, I think that’ll do it there. Thanks a lot for your time.

And that concludes the interview. If you’d like to hear the interview in its entirety, it’s available on our website, http://astrobiology.jhu.edu. A big thanks goes to Dr. Levin for taking time out of her busy schedule to speak with us. Everyone else, have a great day and keep listening.

End of podcast:

365 Days of Astronomy
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