[MUSIC] Hello and welcome to the Origins course, at University of Copenhagen. This is the third lecture, on the history of oxygen. In my first lecture, I told you how oxygen is produced on Earth and what makes oxygen accumulate in the atmosphere. In the second lecture, I sketched out the history of oxygen over the past four billion years and presented some of the evidence. In this lecture, I will elaborate on how we think atmospheric oxygen is regulated and how oxygen may have played a role in the evolution of animal life. I will both discuss how oxygen may have affected animal evolution and how plants and animals may have affected oxygen levels on Earth. Overall, the largest organisms on Earth have become bigger in concert with increasing atmospheric oxygen levels. This is shown in this figure. It appears that increases of atmospheric oxygen, broadly coincide with some of the major steps in the evolution of life. For example, the first protists are found sometime after the Great Oxygenation Event. The Neoprotozoic appearance of metazoans broadly coincides with a transition to more oxygenated oceans. Yet, correlation does not mean causation. There's a great risk for misinterpretation when looking at correlations over the longest time scales, and we should look a bit closer. We must also have an explanation of why oxygen would have caused organisms to grow bigger, or contrary, why larger animals would have caused oxygen levels to increase. I will spend this lecture to discuss some of those possible relationships. But first, you should take a closer look at correlations in the past 500 million years. It turns out that we are actually not quite sure how atmospheric oxygen has evolved in detail. Some of the apparent correlations between evolutionary steps and the history of life and changes in the oxygen levels may not even be real. In this figure, you can see the predicted atmospheric oxygen curves from two distinct, but reasonable, Earth system models. Both of these models fit the same geochemical records, which are the records of how carbon, sulfur, and strontium isotope compositions of seawater have changed over time. However, the two models are based on different assumptions about how oxygen is regulated on Earth. I will return to the assumptions a little later, but let's first take a look at some of the consequences, and how oxygen is thought to have affected animal evolution. In the GEOCARB world, the atmospheric oxygen level has been maintained relatively close to modern day level over the past 500 million years, except for a period of very high levels in the Carboniferous. The predicted oxygen levels in the Carboniferous are strikingly high. It was 50% higher than today. The higher oxygen levels are associated with extreme fire danger. Yet, the climate was hot and humid in the Carboniferous, and this may have prevented spontaneous forest fires. The high oxygen levels should also have influenced animal life. For example, the extreme oxygen levels may have allowed for the evolution of gigantic insects. Insects breathe passively by allowing oxygen to diffuse into their bodies. This means there will be more oxygen diffusing into an insect in the Carboniferous than there would be today. This could support a larger body tissue. In fact, the largest insects ever to have existed on Earth are from this time. With a wingspan of 65 centimeters, meganeura was ten times bigger than modern dragonflies. You can see it in this figure. Meganeura was a predator that fed on insects. Its size would have been a great advantage, because it could eat its smaller relatives but likely not get eaten by them. In the COPSE world, oxygen levels were also high in the Carboniferous, but forest fires kept oxygen from increasing very much above the modern level. In both COPSE and GEOCARB worlds, there was a major increase in atmospheric oxygen levels as a result of the invasion of vascular plants on land. Vascular plants have deep roots and they enhance weathering processes on land. Also, vascular plants produce nutrient-poor lignin that preserves well in the rock record and leaves more nutrient for marine primary productivity. All of these effects should ultimately enhance marine organic carbon burial. As I mentioned in my first lecture, it is organic carbon burial over many million-year time scales that sources oxygen into Earth's surface environments. This is true in both models. And the oxygen rise appears to be a robust feature. Perhaps the most striking difference between the two models is that COPSE predicts markedly lower oxygen levels in the first few 100 million years of animal evolution, before land plants and forest fires. Actually, the atmospheric oxygen level was so low in the Cambrian and Ordovician, that the deep ocean would have been anoxic. In fact, the molybdenum isotope data that I mentioned in my second lecture exactly agrees with this. Ocean anoxia was more widespread in the Paleozoic, relative to today. Thus, the molybdenum data is more consistent with the COPSE world than with GEOCARB. However, we should not forget that oceanic and atmospheric oxygenation is not always correlated, because global oceanic, oxygen levels also depend on nutrient input to the oceans. The gigantic insects in the Carboniferous serve as one example of how animal evolution may have responded to increasing oxygen levels in the environment. Other ideas include the emergence of large predatory fish with an agile lifestyle. They have a high metabolic oxygen demand. Predatory fish grew dramatically in size in the Devonian at a time when atmospheric oxygen levels began to increase. Another idea is that higher atmospheric oxygen levels drove the diversification of birds and mammals 100 to 65 million years ago. Birds and mammals have high metabolic oxygen needs, at least during part of their life cycle. The GEOCARB model predicts a rise of atmospheric oxygen at this time increasing towards the modern day level. Another link between oxygen and animal evolution is the potential killings of marine fauna when oxygen levels decline and anoxia expands in the oceans. Paleontologists have compiled an enormous database, with all the animal species in the fossil record. From this, it is possible to estimate the rate and magnitude of animal extinctions and the rate of originations through time. There are five most fatal extinction events in the Phanerozoic. Four of them are claimed to be associated with ocean anoxia. The last one occurred 65 million years ago and included the extinction of the dinosaurs. The largest animal extinction event took place in the late Permian, with the demise of 90% of all marine species. The Late Permian extinction event coincides with an expansion of anoxic and sulfidic waters in the oceans. Because all animals require oxygen, it seems plausible that animal species and even entire groups of animals, were killed during this event. We can only imagine what happened, perhaps marine animals died by suffocation or by hydrogen sulfide poisoning. It seems that anoxia had been an effective kill mechanism. This is also our experience today. In some coastal regions, where oxygen depletion occurs, fish and other animals are the first species to disappear. Well, I have given you examples of how oxygen may affect life. Now it's time to see how geo-biologists think life may have affected oxygen. To understand this, you will need to know about the possible feedbacks in the Earth system. You can basically think of this, as the fundamental laws in Earth science. Yet, we do not know what those laws are. We're still in a stage where we have to generate hypotheses. The COPSE and GEOCARB models predict different oxygen curves because they make different assumptions about how atmospheric oxygen is regulated on Earth. Basically, we know from the existence of animals and from the continuous record of charcoal that oxygen levels have stayed above 70% PAL and probably below 150% present day atmospheric levels, for the past 400 million years. All models must meet those requirements. Yet, it is actually not straightforward to balance oxygen sources and sinks this closely. One must make assumptions for what had prevented oxygen levels from a steady increase and what prevented oxygen from disappearing entirely. In the GEOCARB model, oxygen is actually not directly regulated, it is indirectly regulated because the oxygen source, organic carbon burial, is strongly regulated. In GEOCARB, the organic carbon buried in marine sediments is rapidly recycled, so when it's buried in rocks, any oxygen released to the atmosphere will soon get removed, when those rocks are exposed to weathering again. This feedback is perfectly reasonable from a geological perspective, because most organic carbon burial occurs on the continental shelves, and the continental shelves do not sink into the mantle. Over 10 million-year time scales, the organic carbon buried on the continental shelves are uplifted by tectonic processes, eroded and weathered. Therefore, this memory effect prevents oxygen rise to infinity and oxygen decline to zero. We can draw this, in a feedback diagram. Here, a solid arrow between two parameters implies a positive cause-effect relationship, and a dashed arrow means that there's a negative effect. In all models it is organic carbon burial that drives atmospheric oxygen levels up. As I said, in the GEOCARB model, there's also a negative effect between organic carbon burial and atmospheric oxygen, because organic carbon will accumulate in these young sediments that will soon get exposed by tectonic uplift and again consume oxygen when weathered. In the COPSE model, oxygen is directly regulated and there are two mechanisms to regulate atmospheric oxygen levels. You see that in this figure. There is a positive feedback between organic carbon burial and atmospheric oxygen. The frequency of forest wildfires, increases dramatically with higher atmospheric oxygen, and fire is thought to limit vegetation. There's also a relatively weak negative feedback between oxygen and land vegetation, because photosynthesis in plants is more productive when the waste product, oxygen, is low in abundance compared to CO2 that they use for metabolism. Plants exert primary control over weathering of rocks on land. This also means that plants control phosphorous delivery to the oceans. Phosphorous is a limiting nutrient for marine primary production. With more phosphorous source into the ocean, the more new organic matter can be produced in the oceans. And finally, it is the primary productivity, that drives organic carbon burial and oxygen release. Overall, this makes a negative feedback loop. This is also known as a stabilizing feedback loop, because whenever any of the involved parameters is subject to change, then the sequence of feedbacks will always counteract that change. In COPSE world, it is the fire feedback and how land vegetation controlled phosphorous weathering that stabilizes oxygen levels at a relatively high level. At lower oxygen levels where things do not burn, there's another feedback loop in action. Basically, oxygen is stabilized by the effect anoxia in the oceans has on marine phosphorous availability. Again, phosphorus is the nutrient that limits primary productivity and ultimately organic carbon burial. The reason why anoxia enhances marine phosphorus availability is that phosphorus removal from the oceans is efficient in oxic settings, but not in anoxic and sulfidic settings. The reason for this, is that phosphorous is removed on iron oxides and these will re-dissolve and liberate phosphorous back into the water column in euxinic settings. So, the COPSE feedback loop at low atmospheric oxygen looks like this. Let's see what happens if anoxia expands in the ocean. More anoxia would limit phosphorous removal on iron oxides and promote more phosphorous recycling in the oceans. This means more phosphorus is available from marine primary production. With more marine primary production, there is more organic carbon burial. After millions of years this will lead to an increase of atmospheric oxygen. By Henry's law, more atmospheric oxygen means more dissolved oxygen in the ocean, and the anoxic zones in the oceans, will shrink. Therefore in COPSE there's another negative feedback loop that acts to stabilize atmospheric oxygen, at low oxygen levels. There is also a positive sub-loop in the COPSE model. This is because, as primary production increases, it is not only enhancing organic carbon burial, but also more organic matter will sink into the water column. This will consume more oxygen as it gets respired by aerobic respiration and marine anoxia will expand even further. In this case, this is a positive feedback loop. It will cause the involved parameters to run away in the same direction, as they initially change. However, the positive sub-loop, operates quickly, whereas the negative feedback loop operates slower and prevents the system from run away on longer time scales. You have seen feedback loops and how we think atmospheric oxygen is regulated on Earth. I have given you examples of how oxygen affected animal evolution both during oxygen rise and during periods of crisis when oceans became more anoxic. Now I will introduce you to the last and most thought-provoking concept. Could it be that life actually caused oxygen to rise? Actually the COPSE model already gives you an example of this. In the COPSE world, it's land plants that both cause atmospheric oxygen levels to increase and prevents it from increasing too much. Therefore, the COPSE model is a good example of plant life causing oxygen to rise. Now I will introduce you to some of the ongoing research in this field of science. ???? Could it be that animals increase organic carbon burial and cause oxygen levels to increase on Earth? Maybe, one suggestion is that the emergence of zooplankton enhanced the sinking of organic matter in the oceans. This is because zooplankton like these crustaceans here, they produce faecal pellets and this makes organic matter sink through the water column much quicker than, for example phytoplankton. Unfortunately, zooplankton leaves hardly any fossil record and we do not know when exactly they first evolved on Earth. However, there are some reasons to believe that the Neoproterozoic oceans were rich in smaller chunks of organic carbon, that would have stayed buoyant in the water column. This was no longer the case in the Cambrian. There's evidence that the organic matter from the algae in the photic zone was destroyed and never preserved in sediments older than 590 million years. But these algal remains are common in sediments younger than 530 million years. This could be explained if the algal remains were efficiently packed into faecal pellets and quickly transported out of the water column and preserved in the Phanerozoic sediments. In the COPSE world, we can see what the effect of faecal pellets would be. Indeed, it seems that faecal pellets would cause ocean oxygenation. Before zooplankton emerged in the oceans, there were no faecal pellets, and the sinking rates of organic matter must have been slow. The oceans contained more dissolved organic carbon. The dissolved organic carbon was used for aerobic respiration and removed oxygen from the water column. Once zooplankton emerged and produced faecal pellets, then that dissolved organic carbon in the pelagic zone was decreased, and this organic matter would have been available for oxygen removal. With less oxygen consumed, the oceans would become more oxygenated. This would, in turn, expand the habitats for animals in the oceans and establish an accelerating feedback loop. We may think that animals made oceans clearer and more oxygenated. For a period, the enhanced removal of organic matter would also have enhanced organic carbon burial and contributed to a rise in atmospheric oxygen, but this effect would be short-lived since global production of organic carbon has not increased. In fact, with smaller anoxic zones there will be less primary production after the emergence of zooplankton. Therefore, animals might have caused ocean oxygenation, but probably not atmospheric oxygenation. I think this is a mind-boggling idea. What we're saying is that emergence of animals with guts basically improved conditions for other complex organisms. It seems that complex organisms evolved through a self-cascading process on Earth. There are many other ways that animals could have changed the ecosystem and affected the global biogeochemical cycles, but all of these ideas still need to be tested. It is possible that geology played a significant role on kickstarting the Cambrian explosion. With this, I hope to have convinced you that the rise of oxygen in the atmosphere and oceans is probably among the most important events in Earth's history. Oxygen has influenced the evolution of life, and to a great extent, life has played a role in shaping and regulating oxygen evolution on Earth. Thank you. [MUSIC]
