Fundamentals of Biology I: Introduction PDF

Fundamentals of Biology I Part 1: Introduction 2nd October 2023 Prof. Derek Vance (D-ERDW) Planetary setting for the development of life Notes to read with the slides after the lecture The lecture will be in English. Because of this, and since we will cover a lot of material in this lecture – some o...

Fundamentals of Biology I Part 1: Introduction 2nd October 2023 Prof. Derek Vance (D-ERDW) Planetary setting for the development of life Notes to read with the slides after the lecture The lecture will be in English. Because of this, and since we will cover a lot of material in this lecture – some of it I appreciate not so familiar to biology students – I have prepared these notes to aid your understanding. They are designed to be read while looking at the slides after the lecture. I will show some complicated diagrams: but in all cases the point I want to make from these diagrams is really simple and easy to grasp. I have tried, in these notes, to indicate in bold italics, some of the concepts and ideas I would like you to know about and understand. Sometimes I use rather complex diagrams to convey important information, but I don’t expect you to understand everything on the slide. Where this is the case, I will try to indicate this in the lecture, but especially here, with red bold italics. 1. The timescale of Earth Slide 4: The Earth, and the Solar System containing it, is approximately 4.57 billion years (sometimes Byr or Gyr – or even Ga) old. A bit later I will introduce – very briefly and in overview only – how we know that. Some other ways of saying this: The Earth is: 4,570,000,000 years old 4.57x109 years old 4.57 Gyr or 4.57 Ga (Giga anna) (Giga is the prefix for 109) Some key events in the history of life on Earth shown here: This slide suggests that cellular life may have appeared on Earth around 4 Gyr ago, but actually this is very uncertain. The earliest life was microbial for sure, and prokaryotic. Eukaryotes probably evolved 1.5-2 Gyr ago – this is a bit more certain. We have had complex multicellular animal life for about the past 0.5 Gyr ago – this is really certain. The earliest fossils of modern humans – Homo sapiens - yet discovered are not more than 0.0004 Gyr old. Another important piece of information on this slide: molecular O2 in the atmosphere For about half of that 4.57 billion years the Earth’s atmosphere contained no oxygen – no molecular O2 . We will also look briefly later at how we know this. Today, the ocean gets its oxygen through chemical equilibration of the surface ocean with the O2rich atmosphere: An anoxic (no oxygen atmosphere) means an anoxic ocean. At about 2.4-2.3 Gyr ago there occurred a major change. From this point onwards the atmosphere, and the ocean, started to build up O2, but at first only slowly. There seems to have been another major step at around 0.5 Gyr ago, when atmospheric O2 levels may have risen to levels close to those today – 21% of the atmosphere. This evolution is just one example of the many two-way interactions between the “abiotic” planet Earth and the life on it. Complex life – at least as we know it on this planet – is impossible without the O2 that we and other animals breathe in and react with food to provide our energy. Moreover, life outside the ocean is probably impossible without an ozone (O3) shield in the upper atmosphere, to absorb dangerous incoming UV light from the Sun. Ozone is made from O2, so without O2 there would be little ozone. So complex life on the modern planet, and particularly sub-aerial life, depend on there being O2 in the atmosphere. BUT, it is also true that there would be no O2 in the Earth’s atmosphere without life. The only source of O2 to the modern surface Earth is oxygenic (oxygenproducing) photosynthesis, the process by which many autotrophs (the vascular plants that are everywhere around you, but also unicellular eukaryotic algae and some prokaryotic bacteria) split water to reduce oxidised carbon and make organic carbon, making molecular O2 as a waste product. Note that today photosynthesis is about evenly split between the land and the sea: about half of it occurs in the oceans and is done by single-celled microbes. Slides 5-6: Earth scientists, or geologists as they used to be called, study the Earth. Some study the hot interior of the planet, but the part of Earth Science that is most relevant here is surface earth science – the discipline that encompasses the history of the surface planet, including life on that surface planet. How do we know anything about this? It’s not as if anyone has lived through it and recorded that history. What we have are sedimentary rocks, often though not always deposited as sediments at the bottom of the ocean. A particularly useful kind of sedimentary rock is a “chemical sediment” – one made up of crystals of minerals that precipitated from seawater. Limestone is one such rock – made up mostly of the mineral calcite (CaCO3). But this mineral calcite is not pure – it contains cations other than Ca. To some extent, how much impurity and what kind is determined by the chemistry of the oceans from which the mineral precipitated. We turn this around: we can learn something about past ocean chemistry by looking in detail at the chemistry of minerals like calcite. Another thing that limestones and other sedimentary rocks also contain, of course, is fossils: remnants of the life that lived in the oceans when the sediment – later to become a rock – was deposited. We are often making an important assumption when we do this kind of thing. We are assuming that when we see carbonate (calcite-bearing) sediments being formed in today’s ocean that the same processes led to the formation of similar sediments in the past ocean. This is an example of the principle of uniformitarianism: “the present is the key to the past”. But how do we know which bit of the past a particular rock is telling us about? A random limestone rock we find and study might be 200 Myr old, it might be 500 Myr old. Without this more precise “age information” all we can say is that a given rock is telling us something about “the past Earth”. This is not very useful unless we know which bit of the past Earth - it is the recent past or the far distant past. We need to be able to date this rock – assign it an age so we can place it within the framework of geological history. Slide 7: Often, sedimentary rocks are lain down in a sequence – one on top of the other – in separate sheets that geologists call “beds”. If we know which way was once “up” we can at least assign relative ages within a sequence: this rock is younger than this one because it is on top. Most old sedimentary rocks, deposited in the oceans, are now seen on land. They have been moved there by tectonic upheavals at some time after they were deposited. Slide 7 shows an example from northern Italy. In the process of the tectonic upheaval these rocks are not lying in flat sheets as they would have been when deposited – they are tilted so that the original flat sheets are almost vertical. Because we know which rock is younger than the other, and because we can see what form of ammonite is present in each rock, we are looking at biological evolution in action. BUT how do we get numerical ages for these rocks - ages as numbers in millions of year? That kind of information would tell us exactly when these ammonites lived. But how do we know this? Slides 8-9: Well, it turns out that though many of the different beds in this sequence are limestone, there is also occasionally a volcanic layer. Somewhere close to this locality at this time there was one or more volcanoes erupting, spewing hot volcanic ash into the air for it to land and be deposited as different kinds of beds between the limestones. These beds contain a very important mineral, one that has been very important to dating many aspects of the history of the Earth. This mineral is called “zircon” – and it is common in volcanic rocks. Zircon (Zr2SiO4) crystallises at very high temperatures (about 700-800°C) in magmas. Like calcite it is not a pure mineral – minerals hardly ever are. The crucial thing about the mineral zircon is that the element uranium (U) can substitute for Zr in zircon. Slide 10: You do not need to understand the details of this slide at all – but I want you to see in outline how the timescale of Earth (including the age of the Earth itself) has been – and still is being – established. The only thing that I want you to take away from this slide is uranium has two isotopes that both decay radioactively to isotopes of lead (Pb) – and that this decay is very predictable. All atoms of uranium have 92 protons in the nucleus and 92 electrons. But there are two (main) different kinds of uranium atom – one with 143 neutrons and one with 146. This means they have different masses – 235 and 238 atomic mass units. They are also not stable – they decay radioactively but they do it very slowly. The probability that a 238U atom will decay in one year is 1.55125x10-10. This probability is called the decay constant (l). Another important number is the half-life: if I start with e.g. 100 atoms of 238U, how long will it take before half of them have decayed? In the case of 238U, this half-life (t1/2) is 4.47 Byr – close to the age of the Earth. Note that the decay constant is just that – a constant: 238U always decays at the same rate and it always produces an isotope of 206Pb. What this means is that a mineral like zircon, which incorporates U into its structure when it is made but virtually no Pb, will contain an amount of 206Pb now that is a function of the original U content and the age of the mineral. If we can get the zircons out of a rock and measure this amount, we can get its age. Slides 11-12: I am slowly getting to how we use this to get the age of the Earth as a whole. So there is a bit of a problem here. A very important feature of the Earth is that it is not just biologically alive, it is geologically alive. This is an important difference, for example, versus our planetary neighbour, Mars. All planets start off hot – through the conversion of the gravitational potential energy of infalling material as they grow AND because radioactive decay of elements like U don’t just produce new elements like Pb, it releases heat. The Earth is much bigger than Mars. Because of that it contained a lot of heat to begin with, it has more of the radioactive heat-producing elements, and it has cooled much more slowly. Some interior parts of the Earth are still so hot they are molten. This molten material convects, constantly transporting heat to the surface from which it is lost. You don’t notice this – but it is a long slow process that has been going on for 4.5 Byr. It is this energy, constantly being transported to the surface, that is the driving force for plate tectonics, earthquakes, volcanoes etc today. But another consequence is that the Earth is constantly being renewed. Rocks formed, say, 500 Myr ago will get remelted and the minerals in those rocks are destroyed and turned into new minerals. This resets radioactive clocks. There are – essentially – no parts of the Earth that we can analyse today and find a 4.57 Byr age for. The Earth as a whole is that old, but there is no part of it which will give that age if we analyse it. This is a problem in other ways. I said earlier that we need to read the rock record to find out what happened when in Earth history. But because tectonics on Earth is constantly destroying old rocks and making new ones out of the same material, we don’t have very many really old rocks left. We have no rocks at all from the first 0.5 Byr. We have very few from the first 1 Byr of Earth history. This is why it is very uncertain when life appeared on Earth: we have no physical evidence. If all the rocks there might have been from the first 0.5 Byr have been destroyed, so will the fossils. Slides 13-14: So what do we do to date the Earth? Mostly we need to look elsewhere in the Solar System to do this. I said earlier that Mars is now geologically dead, but Mars was geologically active for the first half of the age span of the Solar System, before it lost all its heat energy from the centre to the surface and hence to space. There are even smaller bodies, however, that stopped undergoing melting and the making of new minerals very early – some classes of meteorites. Meteorites mostly come from the asteroid belt between Mars and Jupiter but their orbits can be disturbed by the gravitational effects of the two nearby planets and they can leave the asteroid belt and some encounter Earth. We sometimes see them fall, we sometimes find them. Meteorites are the fragmented remains of one or more very small parent bodies (too small to be called a planet) that somehow got broken up. And the “smallness” is key: because of this they contain minerals and other objects that were formed very early in the Solar System and have not been destroyed since. Slides 15-17: It’s really important that you ignore the details on these slides. I wanted to show you that this is based on real data but we do not have time to go into the details at all. All I want you to see on slide 16 is that U-Pb isotopic data for the oldest entities ever found in meteorites give us an age of 4.567 Byr. And then on slide 17, though this is much harder, I want you to see that the best attempt we can make at dating the Earth as a whole suggests that it is just a little bit younger. Remember, that no part of the Earth is that old – some rocks were made yesterday, some billions of years ago. But if we could analyse the entire planet for its U-Pb isotopes we would get an age of about 4.56 Byr. 2. How and when did Earth acquire oceans and an atmosphere? Slide 19 What geochemists call the volatile elements – elements that are present on Earth predominantly in either a gaseous or liquid form - are essential to life. Life as we know it is impossible without the H and O that make water. Life as we know it is impossible without carbon. Phosphorous and nitrogen are essential to cells. How the Earth acquired these elements, and how they ended up at the surface of the Earth – in the oceans and atmosphere where life developed – is an important component of the planetary setting on which life developed. Here I ask two main questions: How did the Earth as a planet acquire its inventory of volatile elements? How and when did they accumulate at the Earth’s surface to form and ocean and atmosphere? Slide 21: It is generally accepted that both stars and planets form – accrete – via the gravitational collapse of interstellar clouds of gas and dust that first collapse into a rotating disk and then into discrete bodies with a star at the centre and planets around it. Until recently, we only know about the processes occurring in such environments from theories and models (going back to ideas put forward by Kant and Laplace in the 18th century), but more recently there have been observations of distant systems – other distant solar systems - which have confirmed, and also challenged, these models. Slides 22-23: Generally speaking, planetary scientists think about planet formation in terms of three stages. First note the timescale: 108 yrs is 100 Myr – the Earth is 4567 Myr old – all of this happened quickly relative to that timescale – very early in Earth and Solar System history. The term accretion refers to the process by which orbiting particles collide with each other, at first forming small bodies called planetesimals. This gives a size of “1km” for this first stage but there is of course a size range – and this is important because it controls the next stage – runaway growth – see slide 23. Basically, through a process that physicists call gravitational focusing, the bigger bodies get even bigger during the runaway growth stage because their greater gravitational fields mean they hoover up all the smaller bodies. Slide 23 is the result of a model calculation – you start with a load of small bodies that are all about the same size except for two that are substantially bigger. The axes don’t matter so much but for information: all planetary orbits around the Sun are elliptical, or eccentric (they are not circular but slightly squashed – they have one long axis and one short). The semi-major axis is the length of the longer axis. Here it is given in dimensions of astronomical units (A.U.). This is a common unit of length in astrophysics and is equal to 150 million km – the average distance between the Earth and the Sun. The y axis here shows how much the orbit deviates from a circle – how elliptical, how eccentric. An eccentricity of zero is a circular orbit. So we start at the top panel – with a range of sizes, a small range of semi-major axes and circular orbits. The model simulates the physics of what happens as these bodies orbit around the Sun, collide etc. We end up, 10,000 years later as these bodies move about, collide with each other, bounce off one another if they are small, stick to each other if the gravitational field of one is big enough. What we see is that the two bodies that were originally big have gotten much bigger – there is a positive gravitational feedback. Slides 24-25: There are a few important things to remember for the issue of how planets acquire volatiles: Earth-sized planets don’t accrete gas – they don’t have large enough gravitational fields – they only accrete solids. The gas giants – e.g. Jupiter – did accrete gas. They probably accreted solids at first but once they got to 10-15 Earth masses their gravitational fields were strong enough that they then accreted gas. In the early Solar System there was a strong temperature gradient. The Sun formed the hot centre – hot because gravitational collapse into a centre releases gravitational potential energy and then eventually because nuclear reactions started in the star at the centre. The outer Solar System, then as well as now, is really cold. This is important because it means that in the inner Solar System the volatile elements existed in the Solar nebula as gases – further out water, carbon dioxide, methane etc would have existed as solid ices. Planetary scientists often talk of the snow line – just like a snow line in the mountains. Inside the snow line temperatures are too high for volatile species to exist as solids, outside of it they are. The snowline in the early Solar System was beyond the position of Earth – further out from the Sun than Earth. So all the volatile elements in the region of Earth were in the form of gases, which are not accreted by Earth-sized planets. This is a problem – Earth DOES have an inventory of volatlile elements – like water. How much? Also not so easy to say. We know how much is in the oceans and atmosphere of Earth pretty well. But there is also some in hydrous minerals in the interior. Most people would put the water content of planet Earth as whole at about 0.1%. It doesn’t sound like much, but given the above explanation of slide 24 it ought to have none at all – it ought to be dry. Further out in the Solar System there are bodies that we would predict to have more water – meteorites and comets – but not the Earth. So – what’s wrong here? Why does the Earth that we predict to be dry in fact have water? Basically – in one sentence – it must somehow be that water-rich bodies further away from the Sun than Earth – beyond the snowline - collided with Earth and dumped their water there. If all these bodies and Earth had perfectly circular and uniform orbits around the Sun this could not have happened. They must have orbits that are irregular and elliptical – so that these bodies formed in the outer Solar System sometimes come closer into the Sun so that they collide with Earth and other inner Solar System bodies. Slide 26: For comets this is easy – comets have very highly elliptical orbits, such that we sometimes see comets pass across our skies that have orbits that also take them way out into the Kuiper belt (30-50 AU away from the Sun) and the Oort Cloud (100,000 AU away from the Sun). Could a collision between the early Earth and a big comet have led to Earth acquiring its volatile inventory? The general consensus right now is that the answer to this question is “no”. The reasons are complicated and you don’t need to understand them – but are to do with the fact that the hydrogen isotope composition (the ratio of the abundances of the two isotopes of hydrogen – 1H and 2 H (deuterium)) and the nitrogen isotope composition of Earth and comets are different. What the diagram on the right also shows is that the hydrogen isotope composition of some meteorites and Earth are similar. Slide 27-28: So the consensus right now – but this is still an open question – is that Earth’s water and other volatiles probably came from the collision of these water-rich types of meteorites with Earth early in its history. But – again - how could that happen? Again, if everything is in a perfectly circular orbit around the Sun, how could these things out here ever collide with the Earth? The answer that is currently the most popular – and again without going into details – is that the growth of the larger planets further out, like Jupiter, disturbed the orbits of the meteorites in the asteroid belt into very non-circular orbits. Slide 28 shows a summary of the model simulation in the animation in Slide 27, starting at top left with a load of small bodies with different water contents given by the colours. Also in this model Jupiter is growing at about 5AU. The model shows what happens as Jupiter grows – the huge gravitational effect of the growing gas giant starts to disturb the orbit of the smaller bodies just inside it, out of perfectly circular orbits. By 200 Myr at bottom right, there are hardly any small bodies left – they have all been accreted to bigger planets. And we have a few bigger planets in the inner Solar System that now have some water (as shown by the colour of the outer ring). This COULD make an Earth with about the right amount of water. Slide 29-31: OK, now, finally in the three stages of planetary accretion we have stage 3. By now, in the inner Solar System, we have only a few quite large planets. All the small bodies have been accreted to bigger bodies. The final stage – which models also predict – is one where – at least sometimes – two of these larger bodies collide with each other. These are now really huge and cataclysmic collisions, and there is a LOT of evidence now that the Earth suffered one of these at the end of its accretion. Specifically, there is evidence that the protoEarth collided with another body about the size of present-day Mars (the body has been called Theia). This was such a powerful and energetic collision that (a) the Earth probably melted and (b) a bit of the Earth and the colliding planet separated from Earth and became the Moon. Most people would put the age of this collision at about 4.51 Byr ago. So this collision was clearly very important to how Earth turned out – it gave us the Moon for a start. But the fact that the Earth was largely molten - a phenomenon that has become known as a magma ocean - following it is also extremely important. One thing that happened as a result is that metal-rich blobs started separating out and, because they were heavier than the rest, started to sink to the middle to form the Earth’s Fe-rich core, eventually giving the Earth a magnetic dynamo and a magnetic field. This sounds as far away from biology as one could be, but without a magnetic field that deflects all kinds of incoming Solar and cosmic radiation, the Earth probably would not have life. Another thing, even more important, is that it is probably during this period when the Earth was largely molten that the first oceans and atmosphere began to form. Just as volcanic magmas today have volatiles like water and carbon compounds dissolved in them, so did this magma ocean. And at the lower pressures at the surface these magmas degassed – the gases in them formed a separate physical phase and rose to form a volatile-rich phase at the top (kinda like bubbles of CO2 in fizzy water rise to the surface and join the gas phase at the top). Slide 31 (left) shows evidence – which we will not explain in detail here – that there was probably an ocean on Earth at least by 4.4 Gyr ago. How persistent that ocean was is unknown – i.e. was it a long-lasting ocean or did other large collisions simply blast it away, vapourise it? What we DO know is that the bombardment of Earth by large bodies from outside continued to be very heavy – probably decaying exponentially with time as Earth aged. Of course, infall of extra-terrestrial material continues today. It is happening all the time but we barely notice – material from dust size to substantial meteorites. The meteorite impact that killed the dinosaurs was for sure not the only one to have happened much more recently in Earth history. We don’t know this from studying Earth: any sign of meteorite impacts on Earth this long ago have long been eradicated because the Earth is so tectonically active. The Moon isn’t – and we know a bit about the cratering history of the Moon. The Earth and the Moon are close enough to each other that for sure the bombardment history of the Earth would closely match that of the Moon. Slide 32: Summary of early history of Solar System and Earth with timescale. 3. The long-term habitability of Earth Slide 34: Life –again, at least for our sample of one planet where we know something about it – requires a long, long time to develop. On Earth, complex multicellular animal life took 4 billion years to show up. It took more than 4.5 billion years for intelligent life to appear. This means that life seems to require a relatively stable environment for an awfully long time in order to develop. That is not to say that the surface environment of the Earth did not become unpleasant for life for short periods of time – it did. During something called the Permo-Triassic mass extinction, about 252 Myr ago, something happened – almost certainly a cataclysmic volcanic eruption – that wiped out 96% of all marine species, and about 70% of all terrestrial vertebrate species existing at that time. About 66 Myr ago another catastrophic event – in this case almost certainly a massive meteorite impact – wiped out about 75% of all the plant and animal species then existing – including the dinosaurs. We appear to have had about five – the above are the two biggest – of these mass extinction events since animals first evolved about 540-580 Myr ago. We are almost certainly in a sixth right now. These big extinction events have various causes – supervolcanoes, meteorite impacts, very cold periods (glaciations), us. BUT, what is also clear is that life has never been completely wiped out and had to start over. And, crucially, the Earth has the capacity to recover from these events on geologically short timescales (even us, though it will occur on geological, not human, timescales). Slide 35: So we need to take a look at how the surface environment of the Earth is regulated. You all know, for sure, that the source of virtually all the energy that powers everything at the surface of the Earth – life included – is the Sun. This diagram shows a number of important things: We are fairly sure, from astrophysical modelling, that the Sun was weaker early in Earth history – early during the life of the Sun itself. In fact, what the thick black line shows is that if we normalise the power of the Sun to its power today (so that today = 1), then the Sun was only about 0.7 times as strong, 70% the strength it is today, when the Earth formed. But even today, if we did not have a natural greenhouse, the surface temperature of the Earth, on average, would be -18°C (255K). The fact that it is actually 15°C (this is a global and annual average) is due to the small amount of greenhouse gas in our atmosphere, mostly CO2. The fraction of the total atmosphere that was CO2 in the pre-Industrial period was 0.00028: for every million molecules of air, 280 are CO2 (280 parts per million, ppm). This very small amount of CO2 is crucial to the surface conditions on our planet. In August 2020 this fraction was 0.000412, by the way. Back through time, because of the Faint Young Sun, the temperature at the surface of Earth would be even lower without a natural greenhouse effect (lower end of the blue shaded region) – as low as - 40°C at the formation of the Earth. With today’s greenhouse it would have been about -20°C (upper end of blue shaded region). BUT we know that there are marine sedimentary rocks that are at least as old as about 3.5 Gyr. In other words, we had a liquid ocean at least at 3.5 Byr ago – almost certainly earlier. So the surface temperature of the Earth MUST have been higher than 0°C, the freezing point of water. This problem has been called the Faint Young Sun Paradox. The easiest – perhaps the only – way to solve it is to invoke a stronger greenhouse back through time – more greenhouse gas in the atmosphere. Slide 36: All this, and the fact that the rock record appears to be telling us that, in general, the surface temperature of the Earth has mostly stayed within relatively narrow upper and lower bounds, seems to require that there is some sort of regulation going on. Yes, things have occasionally – but apparently for rather short periods of time – have gone wrong (e.g. mass extinctions) – but they don’t go crazy. Why might this be? Perhaps later in the course you will hear more about how life itself regulates the surface conditions on Earth. But, even before life evolved there was almost certainly another regulator, one that is still very important today. So a key aspect of the surface of the Earth is the natural greenhouse effect, and this mostly comes from the molecule CO2 in the atmosphere. CO2 at the surface of the Earth, ultimately, comes from the interior. Volcanic magmas have a lot of CO2 dissolved in them, they come to the surface and the CO2 they contain degasses into the ocean and atmosphere. If this was all that was happening, the CO2 would build up in the atmosphere, we would have more and more greenhouse gas and it would get hotter and hotter. But something is also removing it and this something, on an Earth before life evolved, is chemical weathering. CO2, when it dissolves in water like rainwater produces an acid. This acid dissolves rock, in a process we call chemical weathering. The exact chemical reaction results in the CO2 being converted to carbonate or bicarbonate ion – CO32- or HCO3- - and this is washed into the ocean in solution in rivers. Also in solution in rivers are cations – like Ca2+ and Mg2+ - from the dissolution of the rocks. In the oceans these things react – for example, Ca and CO3 – to precipitate out as a carbonate mineral – calcite – limestone. The net effect of all this, then, is to remove CO2 from the atmosphere (in rain) and to convert it into a form where it precipitates as a solid in the ocean. But the crucial thing about this whole process is that the more CO2 there is in the atmosphere the more acidic rain is, the higher the surface temperature of the Earth (more greenhouse gas), and the more active the hydrological cycle is at the surface of the Earth. This all makes the process of chemical weathering faster and this, crucially, leads to a negative feedback. Slide 37: So, imagine, that for some reason, there was a catastrophically huge amount of volcanism, like we think there was at the Permo-Triassic boundary 252 Myr ago and leading to the largest mass extinction we know about. Because of this volcanism, very large amounts of CO2 – as well as other nasty and sometimes lethal gases like hydrogen sulphide (H2S) – would build up in the atmosphere. This would lead to extreme greenhouse warming BUT also faster rates of chemical weathering. This would – given time – take all the CO2 back out again. The Earth has the capacity to fix its own environment - to recover from such catastrophic events. It is really important that you appreciate the timescale here. Chemical weathering is a very slow process. Even if it becomes an order of magnitude faster due to higher temperatures etc …… it is still a very slow process. The recovery takes hundreds of thousands to millions of years. This is geologically short – but don’t be fooled into thinking that this will get is out of our current anthropogenically-enhanced greenhouse on anything like the timescale we need it to. Finally, the text on the left: this regulator must be operating all the time, even under conditions more normal than the Permo-Triassic. To take an extreme situation: if we were to stop volcanism tomorrow, chemical weathering would empty the oceans and atmosphere of all its carbon in less than a million years. If we were to double the rate of volcanism and keep chemical weathering the same, we would double atmospheric CO2 in less than a million years. A million years may seem like a long time to you but it is geologically short. And we have, for sure, had variations in the rate of volcanism through time – but the greenhouse effect, the CO2 content of the atmosphere, and the surface temperature of the Earth that it controls have not changed drastically. This negative feedback – the tight coupling of surface conditions and chemical weathering - must be really important and strong. Slides 38-39: Just to close this section, let me quickly tell you about an example of a positive feedback that we are pretty sure occurred on our neighbouring planet – Venus – to illustrate just how easily things can go badly wrong and how positive feedbacks can lead them to go catastrophically wrong. We are fairly sure that, early in its history, Venus suffered such an event, called a runaway greenhouse. So Venus is a bit closer to the Sun than Earth and, as a result, must have been a bit hotter at the beginning – even with a simple atmosphere involving no greenhouse (which this slide assumes as a starting point). Because Venus was just that little bit hotter, though, more water was converted from the liquid form into the gaseous form in the atmosphere. Now, as you probably know, water vapour is itself a very powerful greenhouse gas. The more water vapour you put into the Venusian atmosphere, the hotter it gets, the more water vapour the Venusian atmosphere can hold, making it hotter still ….. etc. Earth and Mars did not do this – their temperatures go through the liquid field on the water phase diagram (the diagram that gives the pressures and temperatures where water is stable as liquid, gas or ice). A consequence of this – that requires just one more bit of explanation – is that Venus now has hardly any water – it is a dry planet. This is because the molecule H2O is photolyzed – it is broken down by incoming UV light from the Sun - in the upper atmosphere. When that happens the very light hydrogen atom simply escapes the gravitational field of the planet and is lost to space. Venus has lost nearly all its water because all the water has been in the atmosphere where it can be photolyzed and then the H lost to space. So Venus doesn’t have – has never had – oceans. This is also important. Despite the fact that Venus no longer has any water in its atmosphere – it has lost all the greenhouse gas that caused the runaway greenhouse in the first place – all the carbon on Venus is in the atmosphere as CO2. There are no oceans, and no carbonate sediments forming in those oceans to put it anywhere else. In fact, the partial pressure of CO2 in the Venusian atmosphere is 90 bars – there are 90 times more molecules of CO2 in the Venusian atmosphere than there are molecules of all gases (N2, O2 etc) in the entire atmosphere of the Earth. Venus still has a big natural greenhouse but today it is caused by CO2 – in the early history of the planet it was caused by H2O. Slide 40: So to sum up a bit: the fact that we have a pleasant environment on the surface of the Earth is no accident – there is a very real and scientifically sensible reason. An ocean is essential. Today’s oceans contain 40 times more carbon than our atmosphere – in solution in different form but still. If all this carbon was in the atmosphere as CO2 instead, the greenhouse effect would be extreme. But you also need continents – these are where chemical weathering happen and where the CO2 is removed in rain/chemical weathering. Without continents containing rocks that can be chemically weathered the negative feedback cannot happen. And then the final point leads me into the final section here. It matters whether the atmosphere is oxidising. If it is then carbon in that atmosphere will be in the form of CO2. If it is a very reducing atmosphere the carbon will make a different gas – like methane (CH4). 4. A hot topic: life and the redox state of the early Earth atmosphere Slide 42: The precise chemical makeup of the early Earth atmosphere both had profound impacts on the development of life and was profoundly impacted by it. One key aspect of this issue is the “redox state” of the early atmosphere: in the simplest terms, how rich in hydrogen it was versus oxygen. For sure, for the first half of Earth history the atmosphere contained no O2 – the molecule O2. Now our atmosphere has 21% O2. Later, below, we go into what caused that change. An atmosphere with no molecular O2 is a “reduced atmosphere”. But a key question for the origin of life is “how reduced?” For example, did the atmosphere contain enough hydrogen so that there was enough not only to combine with the oxygen, carbon and nitrogen to make water, methane and ammonia, but was there even so much that there was some left over to make H2 on its own? Slide 43: This question is crucial to some theories about the origin of life. For example, the Oparin-Haldane hypothesis (early 20th century) and the Miller-Urey experimental tests of that hypothesis that began in the 1950s. The hypothesis is that electrical energy from lightning, as well as energy from UV light from the very active young, could have provided the energy required to make inorganic molecules like methane, ammonia, H2 etc combine to make amino acids. The Miller-Urey experiment suggested it could happen – providing the building blocks for RNA – but that it would require the earliest Earth atmosphere to be strongly reducing. Slide 44-45: The problem is that models of the atmosphere produced by the degassing of a magma ocean is not strongly – but weakly - reducing. It for sure contains no O2 – but it is dominated by oxidised or neutral forms of carbon and nitrogen – CO2 and N2 – not the reduced forms that Miller-Urey synthesis seems to require. Slide 46: A key aspect of the models that simulate the post-moon-forming impact atmosphere – in equilibrium with a magma ocean – is that the models of the magma ocean also predict the separation of metallic Fe-rich blobs that sink to make the Earth’s core. Age constraints – again from methods of radiometric isotope geochemistry that we won’t go into – suggest that core formation was really early after the impact and that the volatiles that degassed from the magma ocean to make the earliest atmosphere were not in equilibrium with metallic Fe (that had gone into the core) but with iron in the form of FeO (as most Fe is in the silicate part of the Earth above the core). This means, for example, that hydrogen in these degassed volatiles would be attached to oxygen to make water – not in the reduced form of H2 that would form in a gas phase in equilibrium with reductant metallic Fe. Slide 47: There may be a way around this – in ways that have come to the fore quite recently and that are still under investigation. It works like this – in outline and with only a little more detail than on the slides: A set of elements called the Platinum Group Elements (Ru, Rh, Pd, Os, Ir, Pt and Au) ought to be very scarce indeed in the silicate Earth (crust and mantle), because when the metallic Fe core separated they are predicted to go with the Fe. They actually are very scarce in the silicate Earth – but it turns out they are way more abundant than they should be. Physical models predict continued impacts after the moon-forming impact – smaller and many too small to re-melt a significant portion of the upper parts of the Earth – but still substantial. These probably brought the elements above – the core had already formed so they could not go in there. So they remained near the surface of the planet – and we observe the higher than expected abundance in the silicate Earth than expected as a result. These impacts would have brought more metallic Fe- that could also not go in the core if the impact did not melt the upper part of the Earth. Many of these impacts are predicted to have been energetic enough to vaporize any ocean that would have existed at the surface of the Earth. And from a chemical point of view, they could have converted water into hydrogen – see reactions on slide 47. It could be that this set of processes controlled the redox state of the earliest Earth atmosphere – not magma ocean degassing. If so, then maybe the atmosphere was reducing enough to permit MillerUrey type synthesis. Slides 48-50: So early in the lecture I emphasised a very important aspect of the chemistry of the atmosphere and the oceans: the surface Earth lacked molecular O2 in the first half of its history. Now we live with, relatively speaking, an O2-rich atmosphere. The first question I am going to ask here is: how do we know that? It turns out that there are multiple lines of evidence for a major change in the oxidation state of the surface Earth at around 2.2-2.3 Gyr – something we call the Great Oxidation Event (GOE). Some of these lines of evidence are rather complicated and I am not going to get into them – in fact I am only going to explain one of them and only in outline. Slide 51: One piece of evidence is shown here. This is a picture of a rock – up close at top left, in a geological outcrop at bottom left is very common early in Earth history. The rocks is called a Banded Iron Formation (BIF) and they are found wherever we find rocks that date to the first half of Earth history – for example in South Africa and in Australia. They are called a Banded Iron Formation because they have these alternating grey-white and red bands. The grey-white bands are made of silica (SiO2, quartz is a form of silica you might have heard of though the silica in BIFS in a different form) precipitated from a silica-rich ocean. But the important part for us today is the red layers – these are made of iron minerals. In fact, BIFS are mined as the source of nearly all the iron we use today. Slide 52: One important thing about BIFS is that we only find them up to about 1.9 Gyr ago – a bit more than the first half of Earth history. Why? So BIFS do not form today for one simple reason: there is not enough Fe dissolved in the ocean. The iron concentration in solution in today’s ocean is tiny, as is the concentration in rivers. This is a bit weird initially because Fe is one of the most abundant elements on Earth – in the rocks of the solid Earth. The reason we don’t find any in the oceans today – or in any other natural waters like rivers or lakes - is because the surface Earth is rich in O2. Iron is a transition metal and it has multiple valence states – it can be Fe2+ or Fe3+. The divalent ion forms when O2 is low – there is not enough oxidising power to strip off the third electron. The trivalent form is stable when O2 is plentiful. The other crucial aspect of Fe’s chemistry is that Fe2+ is very soluble in water. Fe3+ is not. So, today, when rocks weather at the surface Earth, the Fe released to solution in soils converts into Fe3+ and precipitates out – soils are very rich in Fe. But that also means that rivers carry very little Fe to the ocean, and in the ocean most of the Fe that does get to the ocean precipitates very quickly. As a result, because the surface Earth is rich in O2, today’s oceans are just way too poor in Fe to ever make a BIF. This was apparently not true in the first half of Earth history when BIFs were common – it must have been an anoxic Earth. To get to the quantitative statement that there was 100,000 times less O2 in the early Earth atmosphere than today is much more complicated – but you can see the principle. Slides 53-54: There is a LOT of debate about the exact causes of the GOE but one thing is certain: it could not have happened unless oxygenic photosynthesis was happening. The reaction here is a stupidly simple one for oxygenic photosynthesis – the metabolism by which all modern plants use light energy to split water and reduce carbon. The side-effect is that molecular O2 is produced. Who was doing this 2.5 Gyr ago? Not the plants you know well that now occupy the continents – they only appear about nine tenths the way through Earth history. It would have been done by bacteria – something similar to the modern cyanobacteria. We know this type of organism evolves relatively early in the history of life – but a crucial question that is really difficult to answer and has been incredibly controversial is how early? Slides 55-56: So one thing you can do – obviously – is look for fossils in old rocks. But remember that cyanobacteria are tiny – would they even be preserved as fossils? Remember also that the further you go back in time the smaller the rock record becomes. We don’t have many rocks left now of the right age – to preserve any fossils there might have been. Some scientists have pointed to structures like these – very finely layered or laminated rocks that have domes and hollows. These often have organic carbon remains between the layers- but that organic carbon is now very degraded – something called kerogen. These things look very like something called a stromatolite. Slide 48 shows a modern stromatolite – from western Australia. Today these stromatolites are found in slightly difficult environments – in water that is too salty, or not salty enough, or environments that are subject to extreme variations in temperature. They are built by microbes – they are also called microbial mats – which colonise a surface, maybe then get covered in a fine layer of sediment, they colonise again etc – building up these layers. On the early Earth – with fewer competitors or animals to eat them – they might have been more common. But going back to slide 47: are these structures we find in 3.5 Gyr old rocks the same thing as what we see today? Slide 57: Then people have found this sort of thing – note the scale – these are really tiny – tiny chains of what might once have been cells. When you look in detail with very precisely targeted chemical analyses, also rich in organic carbon. Are these cyanobacteria – like these modern versions on the right? Slide 58: The thing is, if these are cyanobacteria, and if these are oxygenic photosynthesisers, and they are really 3.5 Gyr old, why did the GOE happen more than 1 Byr later? Could it be, for example, that these stromatolites and possible cells are telling us that there was cellular life but that the bacteria were not oxygenic photosynthesisers. There are other ways to use light to make a living – you don’t have to split water. You can, for example, do one of these two things at the bottom right of this slide. You can use H2S as the reductant, you can use Fe2+ (another way of writing this is Fe(II)). Neither of these ways of using sunlight (denoted hv here) to reduce carbon and make cellular organic matter produce molecular O2.