Hello, and welcome to Our Earth. It's Climate, History, and Processes. I'm David Schultz. In this lecture, I want to talk about the early evolution of the atmosphere, specifically, where did the ocean and the atmosphere come from? As we have talked about before, the repeated melting of the Earth. And the settling of the more gravitationally dense elements of the molten Earth, led to a differentiation of the internal structure between the crust, the mantle, and the core. As a result, the lighter elements, the gases were expelled as volatiles, and became the second atmosphere that the Earth had. The first one being, you know, the leftover hydrogen and helium from the formation of the Earth, itself. As these volatiles were expelled, we produced the second atmosphere that the Earth had. So, when we think about the evolution of the atmosphere, we want to ask the following questions. Nitrogen comprises 78% of the mass of the atmosphere, where did it come from? Where did the carbon dioxide come from? because, this is going to be important in regulating the climate on Earth. And then finally, where did the oxygen come from? Essential for life on Earth, but also a product of life on Earth. So, we want to look at that. And then finally, we want to address the question, why has the climate remained remarkably stable? The outgassing produced this second atmosphere. This was the nitrogen, the carbon dioxide, the water vapour, ammonia, the hydrogen sulphide, and sulphur dioxide that was being emitted from the Earth and volcanos. Now, as the planet cooled, the water vapour eventually, condensed to form the first oceans. And it took with it the soluble gases, such as the sulfur dioxide. This then left the more inert gases that were less likely to dissolve in the water, the nitrogen and the carbon dioxide to become dominant in the atmosphere. Now, some have suggested that nitrogen,also arrived by comets was released by denitrifying bacteria, or from the oxidation of ammonia. But however, it happened it became clear that, as we see, that nitrogen reached more or less its current concentration early in the lifetime. Now, the current concentration of Nitrogen in the atmosphere is 0.79 bar, where a bar is a measure of pressure, which is equivalent to the weight of the atmosphere above it. Now there was a study by Marty et al in 2013, that looked at fluid inclusions in hydrothermal quartz that had an age of between 3 and 3.5 billion years ago. And what these inclusions showed was isotopic ratios that of nitrogen. They were about the same, as we have at present. And also that the partial pressure of the nitrogen was between half a bar, and about one bar. So, again, evidence is suggesting that within say a billion or so years of the Earth's formation that we had the current amount of nitrogen in the atmosphere. So, where did the carbon dioxide come from? Well, today, much of the carbon in the earth's system is stored, as organic matter or limestone which is calcium carbonate. And clearly, these didn't exist in the early PreCambrian. What we're talking about here before there was any life. So, if all the carbon in these storehouses were converted to carbon dioxide, then we would see these concentrations would be 100, or maybe even 1000 times present levels in the atmosphere. And the present level in the atmosphere only four hundredths of a percent. Now some models of the early atmosphere have 10 million times the amount of carbon dioxide that is in the atmosphere now. So clearly, there's been a large change in where the carbon is stored. Initially, from the atmosphere, now being stored in the solid Earth as time has progressed. And this carbon has been changed through organic processes, deposited on the ocean floor, and then stored as rocks. Now, when did oxygen appear in the atmosphere? The earliest atmosphere would have not had much oxygen at all, and recall, that there's 21% of the Earth's atmosphere, today is composed of oxygen. So we're talking about a really big change in the concentration of oxygen in the atmosphere. Oxygen was not part of the initial outgassing of the Earth's interior, and the only really abundant source comes from the production of oxygen through photosynthesis. And we know from looking at other planets that they don't have oxygen in their atmospheres. So, again, further evidence that without life, you wouldn't have a lot of oxygen. Now, there is one abiotic source of oxygen on the Earth, and this is where you can photodissociate water vapor with shortwave radiation, ultraviolet radiation, to produce hydrogen and oxygen. But the calculations on this process would produce only very small amounts. Perhaps, only 10 to the minus 7th times the present amount, so not very much at all. So before there were biological processes on Earth, there wouldn't have been much oxygen at all. What we see in the fossil record is a rapid increase in the oxygen concentration, starting around 2.5 billion years ago. Now, this may have only raised the oxygen content up to 1% or 2%. But that was enough to change fundamentally, the nature of processes on the Earth, as we'll see. And, this is called, the Great Oxygenation Event. So, as more and more oxygen was added to the atmosphere, eventually we reached the current value of 21% oxygen that we see. Then the last question that I asked at the start, is why has Earth's climate been so stable? Well, we know that in order to have life, that you couldn't have fluctuated the Earth's temperature too wildly. It can't have frozen the Earth solid. And it couldn't have boiled all the water vapor off, into the atmosphere. So there is a range of temperature around which makes it possible for life to exist. We've mentioned, these higher carbon dioxide levels during the Proterozoic that would have implied a very strong greenhouse effect, and we know that, at the present, that we don't have as much carbon dioxide in the atmosphere, as we did back then. So without this strong greenhouse effect, then the Earth would have had to have cooled pretty substantially, but in fact, we don't see that. We see that the Earth seems to have maintained the average temperature of about 15 to 25 degrees for many hundreds of millions of years. So, more specifically, we know that the sun's brightness has also increased during the lifetime of the Earth. This is what happens in the typical evolution of stars, as they grow older they increase their output. The idea that the sun was producing less output early in the evolution, is called the Faint Young Sun Paradox. Now it's possible that this increased amount of CO2 was enough to offset the Faint Young Sun Paradox. Because if the Sun is stronger the ocean's warm, you get more evaporation increases the rainfall, and that removes the carbon dioxide from the atmosphere. There's also the possibility, that it could have been as much, as 100 parts per million of methane in the early atmosphere that could have offset the Faint Young Sun. Or, because there was less continental area, there was a lower albedo, and so less solar radiation is being received by the Earth now, because the continents are more highly reflective than the ocean, which is much darker. Then finally, maybe we have the evolution of the Sun wrong, and then our models for evolution of stars need to be corrected. So, to summarize today's lecture, we know that the early Archean atmosphere would have had very little oxygen, that nitrogen levels were about the same as present relatively soon after the formation of the Earth, say between 3 and 3.5 billion years. And we know that carbon dioxide and methane concentrations in the atmosphere could have been much higher, and that would have offset the Faint Young Sun Paradox.
