Plumeworld ocean and snowball Earth

Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers, and policymakers. Join us as we explore the stories behind the science. I'm Paul Gabrielson. Recorded in the Dola Stone Rocks of South China, is evidence of a time when Earth's climate looked vastly different than today. During a period called Snowball Earth, the entire planet was covered in ice. In a recent PNAS study, Shu-hai Shao of Virginia Tech and colleagues studied stable isotopes of lithium in Dola stones to learn how snowball earth ended and what happened when the meltwater from continental glaciers met the highly saline water of the oceans. Shu-hai introduced us to Snowball Earth, when did it occur and what was Earth-like during that time? For much of the Earth history, we have a poor ice cap on both the North Pole and the South Pole, but for some parts of the Earth history, there was an air ice. So there's a spectrum of climatic conditions from very cold to very hot environment on the Earth. But from about 720 years ago to about 635 years ago, the Earth experienced a lot of the global ice age. Made the Earth look like today's Europa, the entire surface ocean, was frozen to perhaps half mile in depth. So that's an extreme limit of climate change. Almost a quarter of the ocean water was frozen during this global ice age that geologists call snowball earth because of the space that it would look like a snowball. There's a hypothesis about the end of the snowball earth period called the Plume World Hypothesis. What does this hypothesis say? Basically, during the snowball earth, the consumption of CO2 almost stopped, but the supply of CO2 into the atmosphere continued. So much CO2 in the atmosphere that it cancels or overcomes the reflectiveness of the ice so that the greenhouse gas began to melt the ice. And once that started to happen, it happens very rapidly because of a positive feedback. The more ice you melt, the Earth becomes less reflective and absorbs more heat. So the melting accelerates more and more. So this de-glycation or de-frosting of the Earth happened in a very, very rapid fashion. Now one of the important predictions of this scenario is that once you melt a lot of ice, you can generate continental glaciers. This melt water is lighter in density. So it's going to stay on the surface ocean and stay closer to the continents. Why would expect to see a plume of melt water in a coastal area or close to a continent at a regional or global scale? And this plume is stable for tens of thousands of years. How did you test this plume world hypothesis using lithium isotopes in rocks? Lithium is the third element in a periodic table and it has two stable isotopes. Lithium-6, which is the minor isotope and the lighter isotope because it has three neutrons and three protons in a nucleus. So the mass number is 6. And the other isotope is the heavier isotope or the major isotope is more abundant. Lithium-7, which has three protons but four neutrons. So 3+4=7 is mass number. Now these two isotopes chemically, they're very similar, but one is heavier than the other. So when chemical reaction happens, it fractionates. In other words, sometimes some reaction prefers one isotope over another. And this is a very nice property that we can take advantage of during the reaction of melt water with the continent, the melt water tends to concentrate the heavy isotope. So that lithium isotope signature of the melt water is different from the lithium isotope signature of the sea water that is aged and seasoned during millions of years of snowboard Earth. Now imagine in the end of the snowboard Earth a boom of melt water forms and stay on the surface of the water and closer to the continent for tens of thousands of years, we should be able to see that gradient from near shore to offshore. If you're closer to the melt water, your isotopic signature should be heavier because the melt water tends to concentrate the heavy isotope relative to the sea water that is isotopically different. So our prediction is that there's a gradient in isotopic composition when you walk from the near shore environment to the more offshore environment. What is dollar stone? Why did you look for lithium isotopes in that particular rock? Now one of the nice thing about the snowboard Earth is when ants or the glacier deposit stops but in need of tree after the glacier deposit you have a lay or a bed of rock called dollar stone which is made of calcium and magnesium carbonate. This is precipitated or deposited during the end of the snowboard Earth. So it's very nice that this dollar stone, what is known as a capital stone, can't record the de-calaciation of the snowboard Earth. So what we need to do is to sample the capital stone at different localities that we called a transact from near shore to offshore environments so that we can test the hypothesis of the lithium isotope signature. What did you find? We found exactly what we expected. When we analyzed the lithium seven, the lithium six ratio, we did see the ratio is high in the dollar stone precipitated in near shore environment and it's lower in dollar stone precipitated in more offshore environment. And this gradient, we called a two ant-membron mixing pattern. In other words, if you're closer to the meltwater, you'll see more meltwater signature if you're closer to the seawater and more offshore environment. You see more of the seawater signature in terms of lithium isotope ratios. What story does this study tell about the end of snowball Earth? The snowboard Earth informs us about the limit of climate change and help us to contextualize global warming and climate change that is happening today and in the future. Some of the process that happened during the snowboard Earth might also applicable to other icy water, Europa and other extraterrestrial bodies. It also informs us about the resilience of life. We know that there was life before the snowboard Earth and after snowboard Earth, which means that life, the biosphere is very resilient despite this dramatic, climatic change that happened in a geological past. What are the caveats or limitations of the study? When we analyze rocks to understand what is the lithium isotope of the water from which rocks precipitate, we need to understand the chemical process and the isotopic discrimination during these chemical reactions because it's very difficult to precipitate the dollar might, the mineral that makes dollar stone at room temperature in the laboratory environment. That discrimination of isotope is poorly quantified. So to backtrack what is the isotopic composition of the seawater of the meltwater is tricky. So the absolute fractionation is still a matter of uncertainty but in our study we are more concerned about the gradient. So in a sense we sort of avoid this problem to some degree. One of the caveats is we need to better understand or quantify the fractionation. The other implication of this study is that we should use multiple proxies to further test its eye parts. The lithium isotope is relatively new proxy and we should use other for example, joint emisotope proxy, magnesium isotope proxy. The hypothesis will also predict a near-shore, offshore gradient in those proxies. And what we need to do is to take samples from different environments that were deposited in the melt water and the seawater distance. see whether we can fight this same or similar, neo-shore, or actual gradient.