Secrets of Earth's climate in six-million-year-old ice

[Music] 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. Ice cores, drilled through thick polar ice, are a window through time. The ice and the trapped bubbles within it allow us to take direct samples of water and air from a very different time in Earth's history. In a recent PNAS study, Sarah Shackleton of the Woods Hole Oceanographic Institution and colleagues sampled an ice core from the Allen Hills Blue Ice Area in Antarctica. The core contained ice and air from around 6 million years ago, far older than samples from previous ice cores. The results extend farther back in time the direct record of climate in that region, documenting long-term variations. Sarah, what kind of information can we get from Antarctic ice cores? So ice cores are similar in many ways to marine sediment cores, in that both accumulate layers over time of sediment, in this case, our sediment is ice, and those layers record information about past conditions. So one of the most common things that is measured on an ice core is the isotopic composition of the ice itself that gives you information about temperature. You can measure impurities in the ice, which can give you information, for instance, about atmospheric aerosol loading from dust or volcanic eruptions. But one of the most unique aspects of ice is basically that it traps past atmospheric air samples in the form of these tiny air bubbles, that allows us to evaluate how our past atmosphere has changed over time, including how greenhouse gas concentrations have changed. How far back in time did previous ice cores reach? So the oldest continuous ice core that's been drilled and analyzed goes back about 800,000 years, but our European colleagues recently finished drilling a core. They believe goes back to 1.2 million years. So here with work at the Allen Hills, we're basically going to a place where ice is discontinuous. It's out of stratigraphic order, but we can access much further back into the past compared to these continuous records. For a typical ice core, the deeper you drill, the older the ice gets, and it has this smooth increase in the age with depth, where we are basically the ice has been transported about tens of kilometers from where it was initially deposited. It's undergone complex flow, and that means that you can have ice that's folded, thinned in some areas, and just is a bit more complicated to interpret. What's unique about the Allen Hills Blue Ice Area? Blue ice areas in general are relatively unique aspects of the Antarctic ice sheet. They make up about 1% of the area, and these are sites where wind blows away any local snow that's been deposited, and it exposes older glacial ice at the surface. Not all of these blue ice areas have very, very old ice, so one of the things that we think is relatively unique about the Allen Hills is that it's a site where basically the ice stagnates, so the horizontal and vertical velocities go to zero, and we think that's one of the conditions that allow us for such old ice to be trapped there. I've done three field seasons there. Hopefully there will be more field seasons in the future. It's a very beautiful site. We are surrounded by topography mountains, so that's quite beautiful. It's a bit tough to do field work out there. Winds are one of the precursors for these blue ice areas, and slow flow of ice, and between the two of that means really windy and very cold conditions. It's a cool place to do field work. We're still learning a lot about the ice itself and exactly what it records. This work is associated with the center for oldest ice exploration, or cold eggs, which is an NSF-funded science and technology center, and it involves a bunch of different institutions. How do we know how old the air in the ice is? The method that we used to date this core is actually pretty simple in a way. It's almost surprising that it works, but basically we date this ice by measuring the amount of argon 40 in it. Argon 40 is an isotope of argon. It's the most common one in our atmosphere. Its concentration has been constantly increasing in our atmosphere with time. We have potassium in our crust and mantle, and one of its isotopes potassium 40 is radioactive, and it decays into argon 40. Then that argon 40 eventually gets introduced into our atmosphere through processes like weathering and volcanic degassing. We have this gradual accumulation of argon 40 in the atmosphere. This accumulation rate was determined by Michael Bender and his colleagues back in 2008. They measured the argon 40 concentrations in well-dated ice cores. If you know the accumulation rate of argon 40 in the atmosphere over time and the amount in our modern atmosphere, you can just measure the deficit in an ice sample to date it. What does this core tell us about changes in climate over the last six million years? So in addition to dating this ice, we measure the isotopic composition of the ice itself, which gives us an indication of the site temperature. From this we see a pretty steady cooling over the past six million years. This is consistent with existing helioclimate records, but these are really the first direct samples of anardic ice sheet telling the story going back so far. It also shows somewhat stronger cooling than temperature records from lower latitudes, which is consistent with our understanding of a polar amplification. So basically stronger temperature changes that higher latitudes. Tell us about the ice at the base. What's different about it and what might it signify? So this sample we actually could tell immediately that it was quite different from the rest of the ice. First off, because it's just very dirty, it was very filled with sediment. So as soon as it came up, we could kind of just tell that this had a different quality than the glacial ice just above it. And it also doesn't have any gas in it, which was pretty disappointed in one sense because then we didn't have a technique to date it. We did analyze the water isotopic composition. That suggested that it was deposited when the climate was quite a bit warmer than six million years ago, so about five degrees Celsius warmer. And the fact that there's no gas in it probably indicates that it melted and refroze at some point. So it's a very different sample than a typical glacial ice sample. It's still a bit of a mystery exactly how it was formed and exactly what it represents, but we think it was probably deposited before the growth of the ice sheet at this location. What kinds of research can we do now with this data set? So now that we have the age of this ice, our team has been making a bunch of other measurements on the ice, and of things like greenhouse gas concentrations, dust, other compositions of the gas, like the isotopic composition of atmospheric oxygen, other noble gas measurements that can tell us about global ocean temperatures. So there's plenty of Earthcoming work on these cores that will hopefully be out soon. What are the caveats or limitations of this study? One of the main things is that these records are still quite new, and they're more complicated to interpret than the continuous ice cores that we're used to working with. So with how highly compressed the ice is, especially the oldest ice, we're also probably averaging over glacial and interglacial cycles. So we're currently unable to study how the climate evolved across glacial and interglacial periods. That's definitely an limitation of the samples. And exactly what these records capture in terms of exactly how smooth or exactly how much we're averaging over glacial versus interglacial conditions is still outstanding question. I think we're making progress on these things, but it's definitely still new territory, and we're still trying to unravel these cores. Thanks for tuning into science sessions. You can subscribe to science sessions on iTunes, Spotify, or wherever you get your podcasts. If you like this episode, please consider leaving a review and helping us spread the word.