[MUSIC] In the last episode, we tried to place constraints on when plate tectonics in continents originated on Earth. Now you might wonder, does it even matter when continents formed? If life originated in the oceans, which is very likely, then why do we need to know when the continents came? It's important, because continents, while perhaps not an original environment for life, are a primary control on another feature that is critical for the habitability of this planet, its surface temperature. Earth's surface seems to have remained relatively clement over the span of recorded geologic history, and that there has always been evidence of liquid water at least in localized locations on Earth's surface. And that means that surface temperatures have remained between the freezing point and the critical temperature of water for nearly all of Earth history. When you compare that to the temperatures of our nearest planetary neighbors, Mars having average surface temperatures of minus 55 degrees Celsius, and Venus averaging positive 460 degrees Celsius, and the moon having daily temperature fluctuations of more than 300 degrees. It is rather incredible that Earth's climate has remained so stable and within a temperature range so suitable for life. But how does Earth temperature depend on the presence of continents? Well, the answer begins with another energy source we mentioned in last episode, external energy that comes from solar radiation. Energy from the sun is what derives surface processes such as weathering, erosion, and biological activity. The amount of energy that the Earth receives from the sun is controlled by how much heat the sun emits, and how much of that heat is absorbed by the Earth's atmosphere as opposed to reflected back to space. The first part of that equation, the amount of heat that the sun emits is called the Sun's Luminosity. And that value has not remained constant over time leading to a phenomenon called The Faint Young Sun Paradox. Solar luminosity is a function of the rate at which nucleosynthesis occurs in the sun's core. As hydrogen combusts into helium, it releases heat that excites surrounding hydrogen atoms, increasing their collision rate so that they combust even faster. The increasing efficiency of the hydrogen to helium engine over time releases an increasing amount of energy, translating to increasing brightness, or heat radiance over time. The consequence is that when the Earth formed four and a half billion years ago, the sun was only about 70% as bright as it is today. If the other parts of the equation, the fraction of the sun's heat that is absorbed by the Earth, were to remain constant over time, the implication of the Faint Young Sun is that Earth was significantly colder at the beginning of Earth history by about 35 degrees Celsius, and has been slowly warming up since. Now, this is the paradox. We know Earth couldn't be frozen over for the first two billion years of Earth history, because we have records of life older than two billion years. And we have records of sediments that were deposited in liquid water that is older than two billion years. And we know from other proxies in the rock record, that the temperature didn't just steadily increase over all of Earth history. It has fluctuated between warmer periods and colder periods. The only way to solve the paradox, is if the other control on Earth's surface, the temperature, the amount of heat absorbed by the atmosphere has also fluctuated with time. So we need to determine what factors control this heat absorption parameter and how they might have changed over Earth history. Well in this modern world of significant climate change associated with human industrialism, you are probably familiar with the idea that green house gases, mainly CO2 and methane, are very important insulators of heat, because they let the suns short wave length radiation pass through the atmosphere to warm the Earth's surface. But they trap the long wavelength infrared rays that the planet radiates back to space, increasing the heat absorption and thus the temperature of Earth's surface. But there's another factor that also controls how much heat Earth absorbs. And that's called the albedo. Albedo is a measure of the reflectivity of a surface. That is, Earth's albedo describes how much of the sun's heat is absorbed by the Earth's surface, relative to how much is reflected back to space. And the concept is similar to why you wear a white shirt on a warm summer's day, instead of a black shirt. Dark colors absorb more heat warming you up, and light colors reflect heat better, keeping you cool. In terms of Earth's surface, the dark colors are the oceans and the light colors are the ice caps and continental crust. That means in order to figure out how much albedo has changed over Earth history, we need to know how much land there was and how much ocean. And we have already established that we're uncertain about the continental crust growth patterns over Earth history. We might have had all the continents we do now within the first few hundred million years of the Hadean, or their growth might have been a slow process gradually growing over time. But what about the oceans? Were they giant in the Hadean, big enough to submerge continents and cover the entire surface? And only becoming smaller by being slowly incorporated into the Earth through subduction? Or is the opposite true? Have oceans grown with time as water is slowly degassed from the mantle to the surface? Well, before we can begin to think about whether the oceans are coming or going. We first have to determine where water on Earth comes from. And actually our current best guess is from space. The space hypothesis actually comes from the noble gasses, neon, argon, and so forth. Early in the 20th century, a man named Francis Aston established the relative amounts of all the elements on Earth. He noticed a pattern. There's a correlation between the abundance of an element and its atomic number, with a light element being the most abundant, and heavy elements being the most rare. Noble gases, however, fall off this trend. They have much lower abundances on Earth than they are supposed to. This is a pattern that's not seen in the solar system's abundances of the elements, only on Earth. And the reason is related to the moon-forming impact. Noble gases, like water, are volatile species. That means they prefer to be in a liquid or gaseous state at relatively low temperatures. Which is why they are predominantly concentrated into the atmosphere and hydrosphere of a planet. It is very likely that the most volatile species of the earliest Earth were lost during early accretion, before the Earth's core had fully amalgamated and developed Earth's protective magnetic field. At this time, Earth's atmosphere was exposed to electromagnetic solar winds, which blew away whatever early atmosphere Earth and nearby proto-planets had, including the noble gasses and the nascent hydrosphere. That would mean that the water on Earth today had to arrive from faraway. Beyond the snow line, where temperatures in the solar system were low enough for water and other volatile compounds to condense into solids and thus be impervious to solar wind. Meteorites and comets, the ones that formed on the far side of the snow line and far away from the sun's electromagnetic winds are very rich in water. And during the first half billion or so years of Earth's history, there was a significant amount of impacts from meteorites and comets on Earth, because the solar system was so full of these smaller objects, and they were being gravitationally pulled into the larger planets. The amount of water coming in from space is now negligible, because we have so few impactors. But this early, heavy bombardment of impacts was significant enough to bring with it oceans and many other volatiles, in what is called the Late Veneer. So how much water did it bring? Well, on modern Earth, we have pretty good estimates of how much water is in each of the reservoirs that collectively make up the hydrosphere. The ocean is by far the largest of these reservoirs. And in comparison, the biosphere is essentially not negligible. But glaciers and surface waters and even continents and the mantle all hold a fair amount of water. In continents, and in the mantle, water is stored in the mineral structures of rock forming minerals. For example, this mineral, which is muscovite, has one water in every molecular unit of it's structure. The implication of these extra reservoirs, is that when there were no continents on Earth, and when there were no glaciers, all of this water would likely have been in the oceans. And if we assume that continents were not always present, and that glaciers are transient, we can simply measure the size of these reservoirs to calculate how much bigger the oceans might have been before such reservoirs existed. This is a model that shows how the mass of the oceans might have changed over Earth history. The lower limit of the blue field represents ocean volume change, if you began with no continents and no glaciers, and steadily accumulated them over geologic time. The upper limit of the blue field accounts for an additional component, the loss of hydrogen back to space. Now, Earth didn't always have an ozone layer. Because we didn't always have atmospheric oxygen. Oxygen began to appreciably accumulate in the atmosphere around 2.4 billion years ago. And before that time, some hydrogen atoms would disassociate from water molecules in the atmosphere and then become photolyzed and gravitationally escape to space. So that means the oceans may even have been larger than our base estimate. They might have been even larger if all the water in the mantle started at the surface because it arrived via meteors, and it's being slowly incorporated into the mantle via plate tectonics. So let's get back to our Faint Young Sun problem. We know that albedo has a significant control on Earth's temperature, and it's controlled by the relative abundances of light continents and dark oceans. And we are still uncertain about the details but most likely, over the Precambrian, there was a general decrease in the amount of oceans and a general increase in the amount of continents. So that, overall, Earth's albedo has been steadily increasing. Earth has been becoming subtly more reflective over time. In terms of Earth's surface temperature, the changing albedo may have balanced out the sun's luminosity a fair bit, maybe making it cool instead of freezing for most of the Precambrian. But the rock record doesn't tell us it was simply cool in the Precambrian and warmer in the Phanerozoic. In fact, several very specific suites of rock suggest that Earth experienced very cold periods where glaciers extended all the way to the tropics, followed by periods of extremely warm conditions. Now, these icehouse, hothouse transitions in the Precambrian are called snowball Earth events, and you will learn a lot more about them in upcoming lectures. But such events can't be explained with what we know so far of the Precambrian environment, a gradually increasing solar luminosity in a gradually increasing albedo. Instead, we need a mechanism that can cause brief and rapid perturbations in Earth's climate. And for that, we look to the final temperature control, the atmosphere. The modern atmosphere is made up mostly of nitrogen and oxygen, but it also has some important trace gases that act as greenhouse gases. That is, gases that trap some of the heat that has traveled from the sun to the Earth, and radiated back from the Earth to space. Most prominent and significant of these gases are CO2, methane, water vapor and ammonia. Now in the modern Earth, atmospheric ammonia and methane are very minor because there is free oxygen in the air. Reduced species like these quickly react with oxygen to form water and CO2 and various nitrous oxides. But, two significant features of the rock record tell us that in the Precambrian, oxygen levels were very low. The first of these is banded iron formations, or BIFs. BIFs are marine chemical sedimentary rocks that contain extremely high concentrations of iron. Mostly in the partly oxidized mineral magnetite, this black one here. They are often very large and laterally continuous deposits, in fact they are so big and so iron rich that they serve as a primary source of mined iron ore. Now in order for such large iron deposits to form in ancient oceans, sea water had to be very rich in iron. Oxidized iron is highly insoluble, meaning that nearly as soon as it forms, it is incorporated into a solid mineral like hematite or magnetite. For iron to accumulate its dissolved ions in the ocean, there had to be very little free oxygen available to react with the iron. These BIFs occur throughout the Precambrian, suggesting that there must have been low atmospheric oxygen for at least most of this time. But low enough for atmospheric methane and ammonia to be stable. Well, it's actually possible to put a quantitative number on how much oxygen there could have been in the early Precambrian using the different stable isotopes of the element sulfur. Now in the presence of free oxygen, atmospheric sulfur only exists in the form of sulfate. Any chemical reaction involving sulfate, the three sulfur isotopes, which is 32S, 33S, and 34S are always partitioned into or out of the sulfate molecule in a ratio determined by their relative masses. We call this mass-dependent fractionation. In a mass-dependent fractionation, this parameter cap delta 33S, always equals zero. So you can see in this diagram, that in the last 2.4 billion years, cap delta 33S, has only equaled zero. Meaning sulfate has been the only stable sulfur species in contact with the atmosphere. Which means there must be oxygen. But, before this time, cap delta 33S did not always equal zero. A process called mass-independent fractionation occurred, which means sulfur species other than sulfate, were stable at Earth's surface. To have mass-independent fractionation, you had to have atmospheric oxygen less than five times the modern levels. So this change in the sulfur isotope record is interpreted as this time when appreciable oxygen finally became present in the atmosphere, and it is known as the great oxidation event. Now with such low oxygen before this point, reduced species could exist in the atmosphere of the early Earth. Ammonia was probably still rare because it would quickly photolize to nitrogen and hydrogen gas due to high solar ultra violet radiation. However, methane could have been significantly more abundant than it is today. There are no proxies for measuring methane concentrations of ancient atmospheres, but it is plausible that it existed in high quantities because it wouldn't easily break down in the early anoxic atmosphere. And because there was a likely source for methane and methanogens, which are organisms that convert water and dissolved carbon dioxide into methane. Now the other gas that was likely more abundant is CO2. The earliest hypothesized solution to the Faint Young Sun paradox was actually increased greenhouse warming from higher concentrations of atmospheric carbon dioxide. The grey field in this figure is how much atmospheric CO2 is necessary to solve the Faint Young Sun paradox without the aid of any other factor like albedo or methane. But this requirement was likely not met. Ancient atmospheric CO2 concentrations can be determined by looking at different mineral concentrations in ancient soil horizons called paleosols. And these analyses estimate that CO2 is about ten to 60 times higher than modern day during the Precambrian. But not nearly as significant as the early Faint Young Sun solutions projected. But why was CO2 higher in the Precambrian? And why has it changed? Well, atmospheric CO2 is controlled by the carbon dioxide cycle. A very complex system with several sources, sinks, and feedbacks. But a primary source is volcanism, the result of the mantle releasing internally derived energy as well as magma and volatiles. And a sink is the aerial exposure of continental crust. CO2 reacts with silicate crust during weathering to form chemical sedimentary rocks called carbonates. Carbonates can be buried on platforms of continental crust, or they can be subducted back into the mantle. So, sorting out our answer to the long term CO2 cycle actually requires going full circle. Back to determining mantle temperatures, continental growth rates, as well as the onset of plate tectonics. In summary, we cannot fully understand the evolution of our planet by looking at just one aspect of it. Where we came from as living organisms is deeply rooted in the origin of the Earth. Which includes the arrival of oceans, the growth of continents, the onset of plate tectonics and the regulation of atmospheric gases. You will learn a little more about all of these things in upcoming classes that examine our more recent and much better preserved geologic history. I hope this week has given you a foundation for understanding the beginnings of Earth and the geologic concepts that shaped this planet. Enjoy your further exploration into the origins of all things. [MUSIC]
