Date: April 26, 2011
Title: Tracing Planetary Atmospheric Evolution Using Isotopes from Life
Podcaster: Adam Fuller
Links: 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 1; part 2 will air on May 3, 2011.
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.
Sponsor: This episode of “365 Days of Astronomy” has been brought to you by Wayne Robertson.
It is often said that the atmosphere of Saturn’s largest moon, Titan, is very similar to Earth’s primitive atmosphere before life appeared over three billion years ago. This is based on evidence found in Earth’s oldest rocks. By breaking down and analyzing the abundances of certain chemical constituents in these ancient rocks, we’ve pieced together models of what our atmosphere and climate used to be like. By analyzing animal and plant fossils, we’ve also been able to reconstruct how our atmosphere has changed over the life of the planet. The chemical composition of giraffe teeth and pine tree fossils may seem like a far cry from the search for life in space, but it’s actually very relevant to astrobiology. Understanding how planetary atmospheres and life evolve together and recognizing the chemical signatures that come from this complex interaction are absolutely critical in the quest to find extraterrestrial life. Not only that, the physics used to analyze fossils is very similar to, among many things, the physics used to detect planet formation in huge clouds of gas and dust in the interstellar medium.
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. For today’s podcast, I spoke with Dr. Naomi Levin from the Department of Earth and Planetary Science about her research on paleo-climates. Our discussion runs a little long, so today is the first half of our interview. The second half will be available next Tuesday, May 3rd. And so we begin!
NL: I’m Naomi Levin and I’m an assistant professor in the Department of Earth and Planetary Sciences at Johns Hopkins University. I am an isotope geochemist and I am also a sedimentary geologist. My main goal is to reconstruct what the environment and life in the climate was like in past episodes of Earth’s history.
AF: Do you mind telling us then a little bit about what your lab does?
NL: So the lab, basically, it’s main goal is to generate isotope records of paleo-climate and paleo-environment—paleo meaning past. Basically everybody in the lab goes out and collects samples from anywhere—from rocks to waters and sometimes to fossil teeth and fossil bone—and takes them back to the lab, prepares them, and gets them ready for isotopic analysis. The kind of isotope analysis that we do in our lab is called gas source mass spectrometry, which means that we have to get all of those solid or liquid samples into some sort of gas form in order to analyze them.
AF: Like a nautilus shell, you’re grinding it up…
NL: Grinding it up, but that still doesn’t create gas, right. The next thing you have to do is come up with some sort of reaction that’s going to get the nautilus shell into gas form. For nautilus shells we would look at the carbon isotopes and the oxygen isotopes. In this lab we use a reaction with phosphoric acid. The nautilus shell is calcium carbonate, predominantly, so we dunk the powdered or ground up nautilus shell, calcium carbonate, which is the same mineral that a stalagmite is made out of, let’s say, and most shells are made out of that. We react it in phosphoric acid, and that produces two things, primarily: CO2, which is carbon dioxide, and other is water. We want the carbon dioxide. We’re basically looking at the carbon isotopes and the oxygen isotopes in the CO2, the carbon dioxide, that’s retained the same oxygen isotope and carbon isotopic composition that the nautilus shell did. Once we have the gas that we want, that it’s clean to the degree that we want to do it, then we introduce it into the mass spectrometer, which has an ion source. It basically bombards that gas with ions. That gas travels down a flight tube past an electromagnet. That electromagnet basically splits the molecules—those CO2 molecules that have different isotopes in them. So you might have a CO2 molecule with a 13C and two 16O, or you might have a CO2 molecule with one 12C and two 16O. That’s the most common. So the common isotopes that we’re dealing with are 12-carbon and 16-oxygen, but there is some 13-carbon, around which is the ratio of 13- to 12-carbon that we’re interested in. So how do you tell the difference between a molecule with a 13-carbon versus a 12-carbon? Well, the mass is slightly different. The mass of that carbon dioxide molecule with a 12-carbon is 44, which is 12-carbon plus one 16-oxygen plus another 16-oxygen. That’s mass 44. Then, if you want to see that molecule that has the 13-carbon in it, that’s going to be mass 13 plus mass 16 plus mass 16, oxygen-16, oxygen-16. That’s mass 45. If we’re looking at carbon isotopes, which is a really important isotopic ratio that’s actually used in origin of life questions, then we’re actually looking at the ratio of that of mass 44 to mass 45. Once we zoom these molecules past the electromagnet and they curve, the lighter molecules, that mass 44, actually take a tighter curve around that mass spectrometer, around that magnet. It’s just like a sports car taking a turn versus a big truck taking a turn: the sports car takes a tighter turn. If you can imagine a bunch of people holding bins just at the right parts—so we basically have two in the mass spectrometer to hold a bin where we know all the 44 mass is going to come. We have another bin that’s basically getting all of the charges of where the 45 mass is going to come. We compare them, so we have that. It can be expressed by voltage, so the more 44 you get the higher voltage you’re going to get in that Faraday cup versus 45. So what we do is we compare how much we get in the 44 cup versus how much did we get in the 45 cup.
AF: Alright, let’s take a step back. Explain what isotopes are real quick.
NL: Yes. So isotopes are—all elements are defined by how many protons they have. Taking this example of carbon: carbon has six protons. What makes it the weight, though—so that’s only half of the piece to the puzzle. It gets its atomic weight from the other part: its neutrons. Most carbon on Earth actually has six protons and six neutrons. It is defined by its six protons. You can add and subtract neutrons and it is still carbon—as an element it still behaves as carbon. So an isotope of carbon is just a carbon that weighs differently. The most common isotope of carbon is carbon-12: the carbon with six protons and six neutrons. The second runner up is going to be carbon-13, which has six neutrons and—sorry six protons, because it’s carbon, six protons and seven neutrons. And then carbon-14, which people hear about a lot. It’s not stable so we actually don’t mess with. It’s very infrequent, but that weighs 14, and we know it has to have six protons, so it has eight neutrons. So the mass of an electron is minuscule compared to the mass of a neutron or the mass of a proton. A lot of the isotopic effects that we actually see in nature, because of these small differences in weight between—and it’s actually called an ass-dependent isotope fractionation, okay. So mass-dependent isotope fractionation is dependent on the mass. You have different processes that are occurring. When plants actually eat CO2, when they take up CO2, carbon dioxide, they prefer the lighter carbon; they prefer carbon molecules with the 12C, not the 13C. That’s just some sort of—there are different processes in nature that like to pick out isotopes, different isotopes. There’re also processes that are not necessarily a function of mass that have to do bond strength and size of the nucleus. But for most of the cases, here everything is a function of how much that molecule weighs. And so, if the different isotopes affect the molecules, then you can parse out—then you can—what we call fractionate. You can select some molecules based on their weight and end up with—when you have two sides of a process but, say, atmospheric CO2 versus the carbon in a plant, that two sides of the process—if the plant is choosy about which carbon it likes then, there’s going to be a fractionation between the atmospheric CO2 and the plant itself.
AF: Okay, so let’s come back to the mass spectrometer. So you’re separating out the 44 from the 45. How are you measuring the 45? Are you measuring it relative to the 44? Are you trying to get an absolute—
NL: So we’re measuring these ratio,s so that’s what…the kind of mass spectrometry is gas source, isotope ratio mass spectrometry. So what we do is we measure a ratio—we end up measuring the 45 over 44 because the 44 is the common one in our sample. And then what we also do is, every time we measure something on the mass spectrometer, the 45/44 ratio of our sample, we also measure the 45/44 ratio of a reference standard. And so we’re constantly calibrating the machine. What we do is report everything with respect to that reference standard.
AF: So there’s all these isotopes floating around…
NL: So if you think of all the carbon in the world, 98.9% of all carbon is carbon-12, which means that it has six protons and six neutrons. Most of what we’re dealing with is carbon-12. 1.1% of that carbon is carbon-13. If you take an average of the whole, all the carbon on Earth, 1.1% of it is going to be carton-13. And carbon-14 relative to that is completely minuscule; it’s very hard to—
AF: It sounds like there’s almost no carbon-13, but that’s just saying out of every 100 atoms of carbon, there’s going to be one C13. So it doesn’t look like it’s very common, but it’s actually quite common, and that’s how we can do stuff like radio—
NL: Yeah, 1% of all the carbon atoms that you are going to come across are going to be carbon-13. In different, and specifically in biology, but even across different physical processes, in terms of diffusion, you’ll have some processes that are going prefer that carbon-13 and some processes are going to prefer that carbon-12. If you look at the world as a whole or the earth as a whole, its carbon-13 is 1%. But if you look at plants as a whole, you’re going to actually have a lot less carbon-13 in plants as a whole because plants like carbon-12. So they’re going to be very picky when they metabolize CO2 and avoid that carbon-13.
AF: Okay, so how does that happen? Do isotopes serve any function in life? Why does life, like plants, why do they choose carbon-12 over carbon-13?
NL: They choose it because, mostly, the reason why—you can think of it and—the easiest way to think of it is in terms of some sort of diffusion process or some sort of selective process. So that, let’s say, you have gas diffusing through some sort of pinhole, the gas that’s actually going to make it through easier is going to be that carbon-12, the lighter sort of gas.
AF: Okay, and that’s how these plants are, they don’t have built in mass spectrometers to separate the stuff out.
NL: No, they don’t know the difference between carbon-12 and carbon-13. Actually, a really good example is when you actually have plants. Plants will have—they have lower isotopic, they have lower carbon-13 values than the atmosphere because of their selectivity rate. They actually prefer the lighter carbon, this 12C carbon. The whole plant ecosystem has lighter carbon than, on average, the whole atmospheric system. And usually that plant’s C is the atmospheric carbon dioxide. But if you have plants and, actually, any organisms living in a closed canopy forest, that carbon dioxide of that closed canopy forest is so influenced by the plants itself, the carbon dioxide in the understory low carbon-13. The plants that end up metabolizing that low carbon-13 have even lower delta-13 or carbon-13 ratios. Basically they don’t—they basically eat whatever you feed it, and if they have plenty of CO2 to choose from, no matter what that starting CO2 is, no matter if that starting CO2 is quite depleted in carbon-13, those plants are going to prefer any—those plants are going to be even lower than the initial value. So it’s all just sort of relative.
AF: And that concludes this first half. Stay tuned for the second half, where we talk about Mars and Titan carbon isotopic ratios, which will be available next Tuesday, May 2nd. In the meantime, keep listening and have a great day.
End of podcast:
365 Days of Astronomy
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