[MUSIC] Hi, my name is Svend Stouge, I am an associate to the Natural History Museum in Copenhagen, which is part of the University of Copenhagen in Denmark. I would like to welcome you to this lecture on Snowball Earth. And the Snowball Earth hypothesis. So what is it? You have to imagine several 100 million years ago, the Earth looked absolutely white. Not blue as it does today, but like a light reflective star when you observe it from space. The entire Earth was covered by ice, and the ice reflected nearly all solar radiation back to space. We name this type of global glaciation Snowball Earth. Although there have been many glacial events recorded in the history of Earth, two major periods of low-latitude glaciation - both periods occurred in the Proterozoic, and they appeared to be correlated with significant changes in the evolution of life and the atmospheric oxygen level. The Snowball Earth was responsible for both widespread extinction and macro evolution of living things. Snowball Earth hypothesis describes the coldest global climate that you can imagine. That is, Earth was completely covered by glacial ice from North Pole to South Pole. And the global mean temperature would be about minus 50 degrees Celsius. The average equatorial temperature would be about minus 20 degrees, which is roughly similar to the present Antarctica. The freeze would alternate with hot-to-green house global warming. Although the severity of these historical glaciation is debated, theoretical "hard Snowball" conditions are associated with the nearly complete shutdown of the hydrological cycle. Now, if you look at the timing of the Snowball Earth events, we have, the Snowball Earth were not frequent. But as mentioned before, during Proterozoic time, Earth experienced two intervals with two or perhaps more episodes of low latitude glaciation, which probably are Snowball Earth events. The oldest cold period lasted from 2,400 million years to 2,200 million years ago, which is nearly half the age of Earth. Between these 2,200 million years, and up to 740 million years, there was little or no evidence of glaciations whatsoever. The youngest cold period lasted from 740 million years to about 580 million years. It was characterized by two late Paleoproterozoic, and Neoproterozoic glaciations. These were named Sturtian and Marinoan. And the oxygen curve, if you look at that, the rocks recorded indicates that the atmosphere and ocean were oxygen poor until shortly before the onset of a Paleoproterozoic snowball at 2,400 million years ago. The old Snowball Earth events are associated in time with one, the first rise of "free" molecular oxygen, we named GOE, meaning the great oxygenation event. The free oxygen was probably also connected to the release of hydrogen peroxide that was stored in the ice, and released directly into the ocean and atmosphere when the ice was melting. Hydrogen peroxide can easily be preserved in ice, as it involves slightly lower freezing point, which is minus 1 degrees Celsius than water. And you may remember that hydrogen peroxide is part of our ozone layer so this is important. The global oxidation event in the aftermath of the oldest Snowball Earth is recorded in iron and manganese oxides. For example, the Kalahari manganese field. This is a great mining area and source for iron. The major impact of these two old events on life history is related to a turnover from prokaryote cells that is those without any nucleus, to eukaryote cells (with nucleus), or the rise of Eukarya and Fungae. The start of Marinoan glaciation, which is from 740 to 665 million years. These are the youngest and best known Snowball Earth glaciations. Associated with these we also have a second large oxygen - a rise of free oxygen, and we name that NOE, a noe, which refers to a neoproterozoic oxygenation event. The major impact of these two younger events, is that they are related to the rise of multicellular organisms. That means metazoans or simply animals, or the Ediacara fauna that followed just right after the last Snowball Earth event, and followed again by the Cambrian explosion. The history on the ideas about the snowball Earth is a long event, Global glaciation on Earth, as such, is not a new idea. It is already recognized that about 200 years ago was the first, should we say idea came up and in the history of science, but it was not until about the last 50 years that global picture came really into serious consideration. The ideas in new times rest upon the plate tectonics theory and was developed in the 60s and also on the palaeomagnetic technique. So, palaeogeography and palaeogeographical models, that means with distribution of continents, and the creation of super continents and the breakdown of super continents are important agents in the development of a snowball Earth hypothesis. In the newer history of the idea, especially three persons should be mentioned in the development of this hypothesis. Each of them had their own significant role in the Snowball Earth hypothesis. The Snowball Earth hypothesis was proposed to explain the wide distribution of cryogenian glaciations, but it was Kershwin and Hoffman that did this together. And then was Brian Harland, and he was now dead in 2003. And he started the whole idea. He already in 1964 proposed that the late Precambrian glacial deposits were widely distributed all over the globe. Therefore, we were talking about the global glaciation, extending from the poles to the low latitude areas, that means for tropics and all the way down to the sea level. However, many members of the scientific community objected to this idea. It was considered unlikely and possibly also a bit too wild. If not, it was immpossible, they said. So, this first approach or try, did fade out because of the criticism, and further 20 years went on before something really serious happened. One of the problems for Harland was that his data were plotted on the globe with the distribution of the continent as they are today, and thus did not reflect the geography of the past. The task he could not do at that time. He didn't have the technique. Then the second one is Joe Kirschvink. He is a multidisciplinary scientist doing biology, palaeomagnetist and he also doing geophysics. He is associated with the California Institute of Technology. He did in 1982, he wrote a little note where he coined this event to, and the name, he coined the name Snowball Earth to this late Proterozoic low latitude global glaciation. Actually interesting was that originally Joe Kirschvink, his purpose was to disprove the Snowball Earth idea. But he changed his mind completely and instead converted to the idea. Joe Kirschvink combined climate physics, geochemistry and geology to a model, and this became the much stronger idea and many opponents were still there but did not have the same negative response and effect as it happened for Brian Harland. So Joe Kirschvink related the formation of banded iron formations to glacial events, and he recognized that the accumulation of carbon dioxide in the atmosphere from volcanic outgassing did lead to an ultra-greenhouse effect. So although Joe Kirschvink did have better evidence, the skeptics were still many and strong, but this time the idea did not fade away. And this was also not the least because of the work and efforts by Paul F Hoffman, who is now professor at Harvard University. He was originally a field geologist and working in many places, especially in Canada, and he observed and used all his evidence from the Neoproterozoic rocks and combined and elaborated these with the ideas promoted by Harland and Kirschvink. And he became very famous for this Snowball Earth hypothesis. So he coined his field observation of "cap carbonates" based on more than 40 years of studies in especially in northern Namibia to a Snowball Earth hypothesis. In 1998, he and his co-author, Davis Schrag published the Snowball Earth hypothesis and today, the topic is still hot, but also controversial. Now, so what is the evidence? What is it you see to support for these interpretation when you go out as a geologist? The answer is, the rocks and the sediments are the clue. The first evidence is the heterogeneous poorly sorted sedimentary rocks, we call them Diamictite. They are characteristic sediments. They look like conglomerates composed of various types of clasts in the matrix. These kinds of sediments are actually interpretated as being glacial deposits or what are called tills. So it's these types of sediments that had this global distribution in the late Precambrian. Another evidence of a second evidence in the rocks is striations on the blocks. The striations on the blocks within the diamictite, these are considered to be characteristics for and formed by moving ice. And the third evidence is the single outsized blocks. These blocks are found in fine-grained laminated marine sediments. And they are considered as exotic to the deposited area. So these are interpretated as dropstones, that melted from floating icebergs and fell down to the undisturbed sea bottom. I'll just show you here a good field example called Ella ÿ where you actually see the glacial deposit, how they look like when you're in a landscape. And you can see this photo shows two horizons of glacial deposits composed of diamictite. The two diamictite horizons are considered to represent the Sturtian and the younger Marinoan glaciations. Now one of the important factors is our geochemical side of a global cycle, the carbon cycle, and with the sources that sink from carbon dioxide, which is a well-known greenhouse gas as you probably know. Rain and weathering captures CO2 and forms carbon carbonate, which is also limestone, and is deposited as carbonate or limestone in the sea or in the ocean. In this way the CO2, which is as I said an active greenhouse gas, is taken away from the atmosphere and the climate in this way remains stable or a temperate to a cool-house state is sort of fixed. If this geochemical carbon cycle in the Snowball Earth's getting closed, as you see Snowball Earth then, the the carbon dioxide is closed off. And the reason for this is that the ice is covering the sea, and that prohibits the communication between the ocean and the atmosphere. So therefore, the carbon dioxide builds up in the atmosphere because, even though you think that everything is cooling down, volcanics they don't care about ice, they are still moving on. So they emit, sending out all the gasses, and that includes a high amount of CO2s. That means that while we have this cold weather, you have no rain, so you are just building up a high amount of CO2. We have another factor that we should consider, which is carbon isotope ratios. There are two stable isotopes in seawater. So there's carbon 12 and carbon 13. Carbon 12 is common, whereas carbon 13 is rare. It makes only up about 1% of all carbon of the carbon isotopes, but you can measure it. In the biochemical processes, where you have photosynthesis which is the most important, they prefer the lighter carbon 12 isotope. This means that bacteria/algae tend to be slightly depleted in carbon 13, relative to abundance found in primary volcanic sources of the Earth's carbon. Therefore an ocean with photosynthetic life, will have a lower carbon 12 and carbon 13 ratio within the organic remains, and a lower ratio in corresponding ocean water. That means that organic components of lithified sediment will remain slightly but measurable depleted of carbon 13. And this technique and these measurements were used to demonstrate the Snowball Earth events. During the episode of Snowball Earth there is a rapid and extreme negative excursion in the ratio of carbon 13 and carbon 12. This is the deep freeze that killed off almost, or nearly all of photosynthetic life. So therefore there is no, should we say, removing of carbon 12. Close analysis of the timing of the carbon 13 "spikes" expressed as negative values in deposits across the globe allows the recognition of two, possibly three, glacial events in the late Neoproterozoic. Another factor, or evidence we use a lot is banded iron formations. They are also known as BIFs. They are colorful sedimentary rocks, a dark layered iron oxide minerals such as hematite and iron-poor chert, which is mainly a very fine-grained quartz. Now, in the presence of oxygen iron naturally rusts, and becomes insoluble in water. The banded iron formations are commonly very old, and their deposition is often related to sea water with low content of oxygen. In anoxic water, in contrast, the iron can be dissolved and preserved as a solution in the oceans. The atmosphere could not have contained much oxygen because oxygen and seawater exchange easily. However, oxidation in the Earth's atmosphere began in the early part of the Proterozoic era and when the dissolved iron in the ocean came in contact with photosynthetically blue-green bacteria, which produced oxygen, then oxygen and iron precipitated out as insoluble iron oxide. The iron bands were produced at the tipping point between an anoxic and oxygenated ocean. Today, the atmosphere is oxygen rich, about 21% by volume. And in contact with the ocean and is therefore not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formation that were deposited after the Paleoproterozoic, that means after 1.8 billion years ago, are the associated one with Cryogenian glacial deposits. For such iron rich rocks to be deposited there would have to be anoxia in the ocean, so that much dissolved iron as iron oxide could accumulate before it met an oxydant that could precipitate it as a ferric oxide. For the ocean to become anoxic, it must have limited gas exchange with the oxygenated atmosphere. Proponents of the Snowball Earth hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in the ocean sealed by the sea ice. Another difficult or different part is what we call cap carbonates. Cap carbonates are continuous layers of chemically precipitated limestone, which is calcium carbonate, and/or dolostone, this is calcium magnesium carbonate that sharply overlie Neoprotozoic glacial deposits. They have for long been considered as a strange system, because they are usually formed, they say, in warm water and they overlie directly cool water glacial deposits. They are typically three to ten meters thick and occur on platforms, shelves, and slopes world-wide. Even in regions otherwise lacking carbonate strata which suggests that their deposition is of a result of a profound change in the ocean chemistry. The cap carbonate is a post- glacial deposit related to the fast meltdown of a Snowball Earth. The isotope delta C carbon isotope, signature of the cap carbonate is near minus 5 per mil, consistent with the value of the mantle. And such a low value is usually taken to signify the absence of life since photosynthesis usually acts to raise this value. The mechanism involved in formation of cap carbonates is explained by the fast melting of the Snowball Earth in a hot house setting. Water would dissolve the abundant CO2 from the atmosphere to form carbonic acid, which would fall down as acid rain. This would weather exposed silicate and carbonate rock, including readily attacked glacial debris, releasing large amounts of calcium, which when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock. If you look at the influence of the plate tectonics and the super continents of a Wilson cycle, then it was at that time, especially at the late Precambrian glaciation that the Rodinia super continent, and the continental distribution has a great impact. Especially where continents should have a tropical distribution, which is considered necessary to allow for initiation of a Snowball Earth. First of all, the tropical continents are more reflective than open ocean, and so absorb less of the sun's heat. Most absorption of the solar energy on the Earth today occurs in tropical oceans. [MUSIC]
