Wisconsin Public Television Unversity Place  Astrology And The Search For Life Beyond Earth  Clark Johnson  2/27/13 UP842  cc >> Welcome, everyone, to Wednesday Nite at the Lab. I'm Tom Zinnen. I work here at the UW Madison Biotechnology Center. I also work for UW Extension Cooperative Extension, and on behalf of those folks and our other organizers, Wisconsin Public Television, the Wisconsin Alumni Association, and the Science Alliance here at UW Madison, thanks for coming to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year, and I'm happy to say that tonight is the start of our eighth year of doing this 50 times a year. So thanks again for coming out. This is your chance every week to experience science as exploring the unknown here within this community of researchers. Tonight, I'm delighted to be able to introduce to you Clark Johnson. He's a professor of geology. He was born in San Francisco and studied geology at UC Davis, and then he got his PhD from Stanford, and then he must have fallen and hit his head because he came here to Madison, because if you've lived in San Francisco, Davis and Stanford/Palo Alto, they all have palm trees. 

[LAUGHTER] He is a volcanologist as well as an astrobiologist. He got his undergraduate degree in geology the same month that Mount St Helens went up, which is May of 1981. And if you've never gone to Mount St Helens, I highly recommend it. It's an awesome place and the original meaning of awesome. Tonight, Clark is going to talk with us about the astrobiology program here at UW Madison which is funded by NASA, and I'm looking forward to this. When I was started in biology and took a biology class in 1972, the biosphere was about 10 meters thick. It was the soil and down a little bit and up into the air a little bit. We had, humans had no idea these things way down at the hot vents in the ocean. We had no idea that there could be microbes floating around at 10,000, 20,000, 30,000 feet in the air. It's an amazing thing, and to think that there are living bacteria in rocks meters and meters, if not kilometers, into the Earth is amazing. 

I hope to hear more about this tonight. Please join me in welcoming Clark Johnson to Wednesday Nite at the Lab. [APPLAUSE] >> Thank you. What I'd like to do tonight is give you a little feel for what astrobiology is. It's kind of an odd phrase, a relatively young discipline, highly interdisciplinary, and then talk to you a little bit about some of the research that we're doing specifically at Wisconsin. We're part of the NASA Astrobiology Institute which funds 14 teams, we're one of them, that are spread across the United States, and people are pursuing various things from NASA missions to searching for exoplanets to understanding the origin of the ribosome and RNA to all sorts of things geological and deep time, which is what I'm going to focus on. Being a geologist, we have that long time component, and we use the Earth as our natural laboratory because we can freely walk on it a little bit easier than Mars. Although, certainly the Curiosity rover as part of the Mars Science Lab mission on NASA now is doing some fantastic things, and what we do has implications for what one might look for in terms of future missions to Mars. But the deep time component is really one of the only ways we can learn what the early life record was on a planetary body, such as Earth. It's our only example where we have a clear idea of that early in its history. 

But because a place like Mars probably had a very similar history early in the past, similar to Earth, we use that as a reference frame. So, I'm going to just talk, give you a little bit of an outline here. Talk about what astrobiology is, talk about the specific component that we focus on in our team and that is what we call biosignatures and define what that is. I want to set the stage a little bit too and give you a little feel for the challenges that early life had on Earth in terms of establishing itself, and then I'll spend most of the time on the rock record of early life. We'll talk about fossilized microbes. We'll talk about microbial communities. We'll talk about one specific example, one that's actually quite pertinent to the upper Midwest, and that's something called banded iron formations that some of you may know about. And then we'll talk about some of the tools we use that are a little bit more specialized such as isotopic fingerprints. So, the study of life in the universe basically falls under this discipline called astrobiology which is relatively new. The NASA Astrobiology Institute started in 1996, largely spurred by a paper that was published by Dave McKay and others in Science that proposed that a meteorite from Mars contained evidence of fossilized life on Mars. 

Dave McKay died last week, unfortunately. He had had some heart problems, but he really can be credited with starting people to think about this in detail, although the field of exobiology had been going on since the 1960s somewhat related. Now, that evidence for life in that Martian meteorite has largely been discounted, but that doesn't mean that we shouldn't be looking. And so the NASA Astrobiology Institute funds us. We're starting the second five years of our grant. We had the first five years we finished last year. The next five years is a $9 million five-year grant, so we're very happy about that. There's a couple of international journals in astrobiology, and then this is our website right here, if I can find it, but maybe I'll give up. It's the Astrobiology Research Consortium. So there are three big questions as posed by the International Astrobiological Society. 

How does life begin and evolve? Does life exist elsewhere in the universe? What is the future of life on Earth and beyond? Really big picture questions. I like to think of astrobiology as a multi-generational effort. Our team, in five years, is not going to solve all these big picture issues, but we're going to set the stage for things that go on for decades that include future missions and things like that. So they're pretty fundamental questions which means they're pretty exciting. It also means that one has to work in an interdisciplinary fashion. When I started working with biologists back 15 years ago now, I quickly came to this agreement with them when we were starting to work on microbial experiments and doing isotopic measurements, which is my expertise, and then the culture experiments, which is the biology component, and I said, well, if you don't laugh at my stupid questions about biology, I won't laugh at you stupid questions about geology. And you really need to take that approach. I say you have to be comfortable with being temporarily ignorant when you're venturing into new fields. But it's quite rewarding because I think there's great insights when working between traditional barriers, things that people often miss. Okay, so this question of evidence for life, and our team really is known as the biosignature team in the Astrobiology Institute, and what are these biosignatures? And so I'll just start with a few really simply examples. 

We make a lot of biosignatures on the Earth, and here's just one example. It doesn't have to be something living, although we are biosignatures in this room. It could be a biosignature on another planet. We certainly left those on other planets. When we look into the old rock record, it could be a big dinosaur bone or it could be something a little bit more subtle such as dinosaur tracks. Now, one of the big questions we have is, are there biosignatures on Mars? The Mars Curiosity rover is capable of measuring organic carbon compounds, but it's a little bit of a long shot for that because of the extreme degradation from UV radiation near the surface. To really look for a life detection mission, that's a little bit farther on. But, in terms of characterizing the early environments of Mars, it's outstanding, the Curiosity rover. So, we're putting biosignatures on Mars now. That actually is a real problem. 

NASA tries to sterilize these spacecrafts, but it probably is true that we are bringing organic compounds. We're certainly leaving biosignatures in terms of debris on there. Now, if we find that on Mars and we didn't put it there, that would be a really amazing biosignature. [LAUGHTER] If we found something like that, that would also be an amazing biosignature. But the expectation is that it's going to be very subtle because I'll show you in a minute that on Earth, microbial life dominated the planet for several billion years. Advanced life or complex life is not expected. It's something that may be relatively rare on a planet. So, let's talk about the comparison between these two. Prior to about 4.2 billion years ago or so, both Earth and Mars likely had water. I don't have time to go into the evidence for that. 

The evidence in terms of Mars is based on looking at older terrains and looking for evidence for flowing water. The evidence for old water on Earth is a little bit harder to find and has been teased out by isotopic studies of very old mineral grains. So if we had water, and the image on the right there is Mars just filling it up with some level of water on it, and you recognize the image on the left, the right is Mars, what about life, what about organic carbon? Well, there's several problems for establishing life for the inner plants, be they Mars or Earth, and one of them is the habitable zone in terms of solar radiation and distance relative to the size of the star which is roughly about where Earth and Mars orbit, Mars is on the extreme outer limit of that, is actually where very little of the organic and volatile components of the solar nebula condensed. Most of those, as you know, are in the outer planets, places like Jupiter or Neptune and Uranus. So, that was strike number one in terms of creating an environment for life on Earth. Another big one that you may not be aware of is that there was a cleaning up of the solar system that has been dated by determining the ages of impact melts on Apollo lunar landing sties. That's the histogram on the right. And those peak at about 3.9 billion years old. That's really important. Without the Apollo missions and returning those samples and doing high precision geochronology, we would not have know about this impact event. 

That has very important implications for life because there's an argument that if life started in this 4.2 billion year old wet world that I mentioned earlier, but then we add this 3.9 billion year old late heavy bombardment, maybe life underwent some attempts but it was completely sterilized, and it wasn't until we ceased that heavy bombardment at about 3.9 billion years old that you see that curve starting to climb up to actually when we have success for life. For those of you who are biologists, it also has possible implications for when the last common ancestor is in our current phylogenetic tree of life. So the 16S, 18S RNA tree of life, the last common ancestor, that red box in the bottom there, could be no older, perhaps, if the Earth was completely sterilized, and this is all hypothetical guessing but it is a possibility, if life was sterilized at 3.9 billion years old. So, it was a tough road for the first half a billion years. That early life, a lot of people argue, was not the kind of microbial life that we might find up out in Lake Mendota or out in the oceans today, but something more akin to marine hydrothermal vent communities. Something that is chemolithotrophic, something based on the chemistry reactions that life can get an energy yield out of. In the modern world, modern hydrothermal vent systems at these mid-ocean ridges, a little cartoon there in the left, many of you know has produced an enormous amount of insights into a non-photosynthetic-based ecosystem. Maybe that was the type of ecosystems in the early Earth. There's phylogenetic evidence for that. Again, a little bit different tree of life here showing bacteria, archaea, and eukaryotes, and everything in that shaded area near the root of the tree are hyperthermophiles or chemolithotrophs. 

The chemical reactions that occur at mid-ocean ridge spreading centers, the process of what we call serpentinization, makes organic carbon, makes hydrogen, and that may give us some basis for supporting an early hyperthermophile high temperature type of life. But there certainly was a driving force to go toward photosynthetic life, and the table, I like this little table from a textbook that I've used that simply shows the power of energy sources on the Earth. And if you look at the top one there, solar radiation is 360. Whereas geothermal flow, like at mid-ocean ridges, is many orders of magnitude lower. Five orders of magnitude lower. So if light could be utilized, if the cellular machinery could be constructed to have photosynthesis, there's a huge energy yield. Much, much less geothermal energy on the Earth than there is incoming sunlight. But early photosynthesis was probably not oxygenic as we know it today, not producing oxygen. And the key is probably the difference in the abundance of the electron donors and the cellular machinery it takes to break that up. And many people argue that important electron donors were not water, such as the top one, the more traditional one for oxygenic photosynthesis that we think about, but something like sulfide, which is the bottom curve there or the bottom equation, or ferrous iron. 

Ferrous iron was much more abundant in the oceans because we didn't have an oxygen atmosphere early in its history, so it was stable. Today, ferrous iron is readily oxidized to ferric iron. It rusts. But that wouldn't be the case in an early Earth, and so we have to think about things differently. So, for example, one way of coupling oxidation of FE2, ferrous iron, to ferric iron would be this equation down below which would be anoxygenic photosynthetic iron 2 oxidation. That's a big mouthful but probably an important early Earth metabolism. So what does the rock record show? How can we test some of these ideas and some of these possible types of metabolisms that could have existed on the early Earth? Well, let's review very briefly the geologic time scale. And I like this one because it has the very large subdivisions. The orange, the red part up at the top is the Hadean, the hell-like period. This was before the discovery of evidence for early liquid water on Mars or Earth, but it traditionally ends at about 3.8 billion years ago which is about where we get the first rocks that you can hold in your hand, not very trace small quantities of mineral grains. 

That middle period is the Archean. That second period in the blue is called the Protozoic, but the classic fossil record, that is, the hard-shelled fossil record that is well-preserved, really doesn't start until 540 million years ago which in Earth's history is relatively recent. Geologists through around millions of years. To me, 540 million years ago, oh that was just yesterday. I'm really interested in the early part of the Archean, for example. Some of you will recognize that fossil. That's the state of Wisconsin fossil, the trilobite. And the trilobite appears very abundantly in rocks around the Madison area in these 500 million year old samples. The problem is the vast majority of geologic time, the Proterozoic, the Archean, and probably some of the Hadean with that caveat of the late heavy bombardment messing things up, that was all microbial life. Simple, single-celled life. 

Somewhere in there we evolved from prokaryotes to eukaryotes, but we're still talking about single-celled life for most of Earth's history. So that is the footprint that we need to look for when we're looking at a place like Mars. That's the most likely life form, not the big dinosaur bones. It'd be great to find that or the rusted car. Now, there's a strong University of Wisconsin connection. The first clearly accepted microfossil discovery of Precambrian age, that is this period before the hard-shelled fossils, was discovered by UW professor Stan Tyler in 1953 up in the Gunflint iron formation which is exposed near Thunder Bay on Lake Superior. Prior to that, the phrase Precambrian paleontology was an oxymoron. It didn't mean anything because there was no paleontology in the Precambrian. So it was a stunning discovery right out of the University of Wisconsin. There's Stan Tyler in 1959 lecturing at a very famous rock outcrop on the Baraboo quartzite, which is up in the Baraboo hills. 

These are the photographs from his paper. A lot of people looked at those and they said that just looks like a bleb. You're telling me that's a microfossil? And it was a tough case to make, but it was the most reasonable case to date and very early on in the 1950s. So how do we interrogate these and really decide whether these are microfossils or some strange remnants of some carbon thing, something like that? Well, there's a variety of things, we can look at them. This is a very young one. That's really young, 850 million years old from Australia. That's an optical image. We can look at that in various fluorescence that are sensitive to different chemical compositions. We can look at them in a 3D image looking at things like Raman spectroscopy to look at the organic carbon structure. Here's another one. 

We can look at this in three dimensions. This is a little animation showing that these are actually not just flat blebs. They actually have three-dimensional shape to them. And for all intents and purposes, this looks like a sheath of cyanobacteria cells. This outcrop is pretty well accepted to be that. So they have three dimensions, we can image them with Raman spectroscopy and other methods, and then see, okay, this looks like it is organic carbon. But there is a problem. You have to realize that the geologic record as we go farther back in time is really, really scant. The Earth is a wonderful recycling machine. Or a terrible recycling machine if you're interested in early Earth, and why? It's because of the plant tectonic processes that are always recycling rocks. 

So, this is kind of a schematic plot of relative mass of surviving rocks, but I think it makes intuitive sense. As we go farther and farther back in time to the right, the record gets scantier and scantier. Moreover, the rock record of nice sedimentary rocks that could preserve these low temperature features like microfossils gets really rare. Most of these old rocks have be metamorphosed. They have been subject to the high temperatures and pressures, and those microfossils or organic carbon are blown away or destroyed. So our record for how we look at this, we have to be a little bit more clever than looking for microfossils, although that remains important. So the oldest fossils that have been proposed are those guys at 3.5 billion years old. I'll show you the locality in a minute. Now I also mentioned tests that one needs to do determining whether it's just a bleb or whether it's graphite or whether it might be organic carbon. And Raman spectroscopy is one component. 

These are various spectroscopic measurements on carbon. They're kind of oriented with the most well-preserved at the top. We see a very broad band of scattering, and that's due to the complexity of the hydrocarbon chains and the functional groups in the organic carbon. Down here, these very sharp peaks are inorganic carbonate minerals, and the old ones are kind of in between. So, again, this record gets a little bit scattered because we keep going back in time and the record gets poorly preserved. So let's take a look at where some of these proposed localities are. We'll go to what we call the Pilbara craton, a craton is simply and old, stable part of the continent. We'll go to northwest Australia, go to a place called Marble Bar which is known as Australia's hottest town because it's over 100 degrees Fahrenheit 154 days of the year, the longest record in Australia. Kind of nice to think about on a day like this when it's snowing like crazy. Here's downtown Marble Bar from a Google Earth image. 

There's some geologists from Madison in the inset. There is a bar at Marble Bar called the Ironclad Hotel, and if you go there, the proprietor, a woman, will tell you she has two rules: no fighting and no talking rocks because this place is inundated by geologists, and she gets sick of it. [LAUGHTER] The only little bit of green patch is right there by that little pin. There's a little caravan park that you can stay. The water supply is that little water supply tower right there at the bottom. Those are filled once per year during the rainy season, and if you come towards the end of that, the water is pretty green. I've been out there when you wished it was a little bit fresher. Anyway, that's the vegetation and beautiful area. Another Google Earth image. Lots of trees? No. 

Great for geology? Yep, because we can see everything. So Marble Bar is that pushpin to the far right, and this place called the shock locality is this pushpin down in this creek to the lower left. And that is where the oldest microfossil locality is proposed to be. Very famous, right? You wouldn't have noticed it. That came from this paper by Bill Schopf, paleontologist at UCLA. Here's part of our group doing more research on these rocks. And this is an optical image and a 3D-Raman image of some of this work that he proposes to reflect cyanobacteria, which is quite an amazing proposal. His evidence is in part based on the morphology. So these are modern cyanobacterial sheaths. Each individual cell, there are the fine small ones, this is a cross-section through the sheaths, and when you string many of them together, you get these cyanobacterial sheaths. 

By measuring the diameters of these and plotting them all up, all those bars are different species that Bill named at the Schopf locality there our near Marble Bar, and a lot of them fall in this cyanobacteria. Single-celled prokaryotes but of cyanobacteria size. It's consistent with but size and shape doesn't tell us metabolically what they were actually doing. Those are two different things. I wish I could show you dozens upon dozens of similar localities throughout the world of 3 billion year old age, but there are a depressingly few number of them. Okay, the organic carbon spectroscopy, which I've highlighted in that red box, you can see it's degraded. It doesn't have the nice, broad scattering peak to say, ah, yeah, that's organic carbon. We got that. That is a microfossil. Nor is it the very sharp peaks of the inorganic carbon. 

It's kind of in the middle. And that's pretty typical of this type of endeavor. Yeah, it's kind of in the middle. And so you start assembling multiple lines of evidence to give you confidence. All right, if we expand our footprint for life detection a little bit larger, let's go away from microfossils to actually looking at microbial communities, and this has been really, really successful. These are called stromatolites. These at layered microbial communities. They're very rare in the modern environments because if you have a layered microbial community, there's a lot of grazers, a lot of animals that are going to come over and eat it. There's one unique place, although there are a couple other places but this is the most famous one, where modern stromatolites occur, and this is a place called Shark Bay in Australia. It preserves these stromatolites because of restricted access to the Indian Ocean. 

This bay has become hypersaline. It keeps the animal population, the grazing population down. People have mapped out a variety of different types of stromatolites. The important thing to remember is that these form by photosynthetic community constantly growing up toward the light source. They become stabilized by incoming sediment that then are then pushed down deeper in the mat by growth of the microbial community. It's a distinct sedimentation process that's different than, let's say, having a stream deposit sediment on a little bump. And one of the ones that people have really focused on is the one on the right, the conical stromatolites, because there's not way that people have figured out that can form by a sedimentation. If you think how sedimentation would deal with a bump, it would tend to smooth out that bump with draped layers of sediment on there. Eventually it'd become flat, right? The conical stromatolites are constantly growing upward, and that's a morphology you'll want to keep a look at when I show you some pictures from the field. Here's some modern microbial maps. 

This is Shark Bay. Again, it's a hypersaline environment so the animal grazers, the snails, things like that are kept at bay. And we're going to go back to Australia. Yes. Are there only a few parts of the world where we can find these really old rocks undisturbed? Mm-hmm. Yep. So, from a reference frame from similar Google Earth image, there's the town of Marble Bar on the upper right, there's another pushpin, the Apex --, the Schopf locality for the microfossil, and that one is called the Trendall locality, this one in the lower left. And I've drawn arrows for that unit. The nice thing about this locality is that we can follow the geologic context for a hundred kilometers. We can see out toward the ocean; we can see going deeper and deeper in the water column; we can see onto the land. 

We know we're in a near shore environment. And that geologic context is really important because it gives us confidence that we actually are in the proper environment. Here's what those rocks look like in the field. Let's look at a couple of examples. So, these rocks are becoming younger up towards the top. You can see relatively flat layers. And then you start to see the build up of these conical stromatolites. And in fact, if you look at them carefully, you see that the conical stromatolites don't decrease in amplitude, like it would be if it was getting covered up with sediment. They actually increase. Here's a closeup of that. 

Again, those are equal layers. Who knows if they represent annual layers. People have speculate on that, but that is something that is growing outwardly out by photosynthesis. We don't know what their metabolism was. We don't even have any microfossils at this locality, but it's something, and these are features that clearly weren't being related to sedimentation. Like this one, this is called the Mickey Mouse ears, that's actively growing up, I think you can convince yourself of that, rather than something that's being covered up in a funny way. Okay, but we're still with morphology. We don't know what they were doing. I'll show you a couple other pictures in the field. This is looking in the bedding plane of those stromatolites. 

Here's a close up of that. Those conical stromatolites are coming out at you. I'll show you an oblique view right here. You can see there's some conical stromatolites. You can see some of them have had the tops broken off from erosion. And so you want to get this three-dimensional context in the field. You want to see you find that stromatolite horizon off in that direction that they disappear when I go into the deeper water parts of the system, and then they disappear when I go onto the land part, the geologic context. Now, these stromatolites, this is a plot of occurrence of Archean aged stromatolites, one of those early groups of times, and we've been talking about those in the Pilbara of Australia, there's about a half a dozen of them, of really, really old age. Not a lot. Why? Because the Earth is such a great recycling machine and destroying these old rocks. 

Plenty if you want to go down to 2.5 or 2.7 billion years old, but if you're interested in the earliest part of life history, that's incredibly young, right? Okay, a bigger footprint, a chemical footprint. I want to spend the rest of the time talking about this type of rock unit called banded iron formations. This is a histogram of age versus the size of these individual deposits in gigatons. The biggest ones occurred about 2.5 billion years old, and then there's a little scant evidence for that one marked Isua. That's in Greenland at 3.8 billion years old. Now, these are important. The highest concentration of iron per unit of rock is in the these banded iron formations. This is the largest iron formation mine in the world. It's the Hamersley mine. It exports 200 million tons of iron ore a year, most of that going straight up into Asia. 

Just to note, you see how big this is, there are non-stop, 24/7 freight trains just leaving, going off to Port Hedland to go on to tanker ships and to container ships in the Indian Ocean. It's just enormous. Similar age rocks are found in South Africa. We do an awful lot of work in South Africa as well. In fact, the South African rocks, even though they're a little bit smaller in deposits, are better preserved, less metamorphism. Here's the unit that actually gives most of the world's iron ore. It's called the Brockman iron formation, banded iron formation, or BIF. 2.5 billion years old. Here's a young one. This is the Gunflint. 

The same unit that's Stan Tyler discovered the microfossils in up near Thunder Bay in Canada. Here's a map of these iron formations in the Lake Superior region. Now, some of you probably know that this iron ore mining was really responsible for the location of the steel industries and the automobile industries in the upper Midwest. Most iron mining is now in Australia and South Africa. Although, of course, in the news the -- range is currently being considered for mining. Minnesota has a number of active mines in these deposits. Okay, let's talk about how these might form. There's a couple of oxidation and reduction reactions that we need to consider. All of you are familiar with oxidation of iron. Let's say I put an iron nail out on the street, and after a little while it will turn to rust. 

I can also run that in reverse by reducing that. So, here is that anaerobic photosynthetic iron 2 oxidation. That big mouthful of a metabolism there where we have ferrous iron, the reduced form, with CO2. We fix that into organic carbon, the CH2O plus ferric iron. So that's the oxidative direction. But I can essentially run that reaction in reverse. I can take that ferric oxide, that FEOH3, that's ferric iron as a hydroxide, the organic carbon, and I can reduce that to ferrous iron and release bicarbonate, that HCO3 or CO2, essentially the same thing. This turns out to produce diagnostic changes in the isotopic compositions of irons, and I'll define what that means in a minute, and the isotopes now, if I can find a fingerprint for this metabolism, now I'm starting to really nail down what this early life was doing. It gives me an insight into the processes that I can't determine just by the morphology. I can speculate that the stromatolites are photosynthetic, but I actually don't know how they're doing it just by looking at the shape. 

So, isotopes, everybody kind of remembers their introductory chemistry, I bet, where we have electrons, neutrons, and protons? The number of protons define what the element are, but I can't have varying amounts of neutrons. So for iron I have four combinations of different amounts of neutrons giving me four different masses. These are the stable isotopes of iron. I'm going to talk mostly about the two most abundant ones, iron 56, which is most abundant, and then the next one which is iron 54. So I'm going to show you 54/56 ratios. I'm going to cast that actually in terms of a delta value. If any of you have done things like carbon isotopes, it's like a delta C13 value. How do they fractionate? Well, I've got two little ball and stick models. On the right is a ferric iron molecule that's complex by six waters with the oxygens of those water vibrating like mad. The hydrogens are omitted. 

That is a very short bond length. The iron/oxygen bond length of ferric hydrated water is short relative to the ferrous iron. That probably makes intuitive sense to you, right? Ferrous iron has less of a positive charge so it's less attracted to the negatively charged oxygen. So the vibrational frequency of the ferrous iron are less than the vibrational frequencies of the ferric iron. Now, if I've got a variety of different masses of isotopes and I can substitute these in, who's going to lower the free energy of the system the most? It should make intuitive sense that the ferric iron will prefer the heavy isotope, lowering its free energy, in a relative sense, to the ferrous iron, which will, in a relative sense, prefer the light isotope. In terms of 56/54 ratios, the ferrous iron would be low, the ferric iron would be high. In terms of this nomenclature that we report the data in, which is just simply relative to a standard, the delta 56 value would be low for the ferrous iron, the delta 56 value for the ferric iron would be high. Ah, remember those two metabolisms that I had for the banded iron formations? The iron oxides on the far right produced by anoxygenic photosynthetic iron 2 oxidation should be high in delta 56 values. The low delta 56 value should be in the reductive phase. This is a test. 

This is something that we worked out through many laboratory experiments, both in abiologic and biologic systems, to get us this fingerprint, this biosignature. So let's look at this one again. Here we have photosynthetic iron oxidation to the right. At the top, running the reaction essentially backwards. Iron reduction running along the bottom. Now, these occur in two different locations. In the photic zone, in the shallow parts of the ancient oceans, I should have the top one running, the forward oxidative reaction. But in the sediment section where I have this organic carbon and ferric iron raining down onto the sea floor, that's where the reductive cycle should take over. The prediction then is I should see negative delta 56 values in iron 2 bearing minerals in these banded iron formations, things like magnetite and -- which are ferrous bearing minerals. This should be the dominate fingerprint because it's the last thing that the rock saw before lithified and being turned into solid rock. 

And the really cool thing is we saw it. So, I put it on the top there, that histogram of banded iron formation, the major ones being deposited about 2.5 billion years ago, that big part of the histogram. Here the delta 56 values are reference frame is zero in the horizontal line, most iron in the crust has a delta 56 value of zero, our references in igneous rocks. That's makes sense. And the excursion occurs in the delta 56 values for the banded iron formations right when we had the major deposition. It actually was a stunning discovery for us. When the banded iron formation deposition starts to disappear, so does the iron isotope signature. So we think this really nails the fact that microbial processes, the oxidative process in the photic zone, the reductive reverse process down in the sediment section, the last thing the sediment saw before being lithified, is actually how these rocks were formed and how they were deposited. And that is the fingerprint of microbial iron reduction as the last process that we saw. So, the biggest footprint for this, we would tag, we could go into the molecular phylogeny of the tree of life and find the iron reducers and say, okay, we think they were well established globally by 2.5 billion years ago. 

That's their greatest footprint. Then the signal goes away. Does that mean that microbial iron reducers disappeared? No. We can find them everywhere today. But their footprint, the signature that they left in the surface environments became highly restricted. Today we find microbial iron reduction in anoxic environments or right at the oxic/anoxic interface. Well, where that big excursion occurs is actually before the first documented rise of oxygen in the atmosphere. But that makes perfect sense because -- iron reduction is an anaerobic process. Once the rise of oxygen occurred, then the footprint or the fingerprint or the likelihood we would find this signal in the rock record decreases. Doesn't mean the metabolism disappeared. 

It means that its footprint or influence on the rock record decreased. Who was responsible for the rise of oxygen that occurred after 2.5 billion years ago? Oxygenic photosynthesis would be the most likely one. So now, and I don't have time to talk to you about the molecular data and the carbon isotope data for photosynthesis, but it fits in nicely that iron reduction, iron metabolism was cycling iron before oxygenic photosynthesis really kicked in and we saw the rise in oxygen. Those actually make sense in terms of the ecosystems that we would expect. Now, there's a very important implication. All of the world's iron used in today's economies are from these banded iron formations. And I just told you that the isotopic fractionation say that all of that was cycled by microbes. And, in fact, they were key to forming the things in the first place. So I think it's kind of amazing. All the metal we have, I got my iPhone and all this stuff that we use, this all came mostly from this mine, and we know that it was cycled by microbes 2.5-3 billion years ago. 

It's kind of a neat thing. But it also now gives us the context and the hunting license, if you will, for what we might look for on Mars. Mars is called the red planet. There's an awful lot of iron oxide on there, but we also know there's some reduced iron minerals on there. Can we determine the ages of those? Can we measure the isotopic compositions of those? Can we see if this metabolism was at about the same time on Mars or maybe even earlier if Mars evolved in a little bit faster manner and then shut down geologically because Mars did not have the plate tectonic record that we have on Earth. That system became much more dormant if it ever had plate tectonics. Now, we won't make these analyses on the Curiosity rover now. It's not equipped for that. These are actually pretty sophisticated, tough measurements to make. The mass spectrometer that I use in my lab is about the size of this front area. 

You're not going to shrink that down and put it on to a rover. You can't cheat the laws of physics that well, but in the long-term, we have sample return missions planned. So our team is basically focused on developing the interpretive context, not only to understand the evolution of life on Earth, using Earth because we can look at it easily, but then what is the interpretive framework for when we actually do get samples from Mars. So, the real world application of all of this that probably most people wouldn't expect is, ah, yeah, astrobiology helps me understand how the iron in my iPhone was formed, how all the iron we use in our every day life. Okay, so I wanted to leave plenty of time for discussion and questions. So the conclusions of what I want to just leave you with, just a taste of the kinds of things that we've been working on and what astrobiology wrestles with. It really was a challenge to get life started in the solar system. We had the issue of the habitable zone being not in the right place to where all the organic matter is and the water. We had the issue of the late heavy bombardment, things like that. Earliest life, as anybody guesses, it's deeply rooted in the phylogeny-based tree of life. 

Were likely hyperthermophiles but there really was a big energy drive to go to photosynthesis. The evidence from banded iron formations would suggest that the earliest form of photosynthesis probably used ferrous iron as the electron donor, anoxygenic photosynthetic iron 2 oxidation. And only later did we evolve to producing oxygen, oxygenic photosynthesis. That's relatively late in the planet's history, probably after 2.5 billion years old. It is hard to tease out the microbial record. We can look for microfossils. That's almost a shot in the dark. We can guess the rock units that we would look for these in, but whether they are preserved, finding whether they survived or whether we can interpret them unambiguously, that's tough. We want to know were they there, but we also want to know what were they doing? And morphology only gets us so far in that sense. Looking at things like isotopes as a tracer, we start to tease out a little bit different fingerprint, but the advantage of that is that it tells us something more about the metabolism. 

And an important role for iron, much more than today in the Earth's oceans, both in the oxidative and the reductive cycle, enough to make this isotopic fingerprint that we find it everywhere in these banded iron formations that we look at, that's a big fingerprint, tells us something about iron respiring bacteria about 2 to 3 billion years ago. That's a level of extensive iron cycling that we haven't seen today. Doesn't mean that the metabolisms disappeared. Just means their footprint on the modern world is much smaller. And, interestingly enough, it's also key to our high technology world. Everything that we use was cycled by microbes. Afterward, after all this iron was cycled, oxygenic photosynthesis began about 2.5 billion years ago. There's a lot of evidence that I didn't have time to talk about. It's based on molecular biomarkers, looking at specific organic compounds, looking at carbon isotopes, looking at microfossil and stromatolite records, and also looking for tracers for ambient oxygen, things that are mobile only in an oxic atmosphere. We've looked at that, and that also helps support that. 

But it's relatively young. 2.5 billion years. That's young to an astrobiologist interested in the earliest part of the Earth but still probably sounds old to most people. And that's all I have. Thank you. [APPLAUSE]