The new tool is a scientific technique called radiometric krypton dating, which recently allowed researchers to accurately determine the age of a 120,000-year-old Antarctic ice core sample.
Details of this finding were published by the Proceedings of the National Academies of Science (PNAS).
One of the most important steps for scientists investigating past climatic events is the ability to find polar ice samples that date back as far into time as possible. These ice samples contain frozen bubbles of ancient air that can be analyzed in a laboratory, allowing scientists to reconstruct Earth’s climate history.
“The oldest ice found in drilled cores is around 800,000 years old and with this new technique we think we can look in other regions and successfully date polar ice back as far as 1.5 million years,” said Oregon State University’s Christo Buizert, lead author of the PNAS paper. “That is very exciting because a lot of interesting things happened with the Earth’s climate prior to 800,000 years ago that we currently cannot study in the ice core record.”
Like the well-known carbon-14 dating system used to determine the age of organic materials such as wood, Krypton dating also measures the decay of a radioactive isotope which is known to have a constant and well-known decay rate, and compares it to a stable isotope – an isotope that doesn’t automatically undergo radioactive decay.
However, unlike carbon-14 dating, which was developed in the late 1940s, krypton is a noble gas that is stable, doesn’t interact chemically, and has a half-life of around 230,000 years. Scientists have found that the carbon dating system doesn’t work well on ice samples because the carbon-14 isotope is produced within the ice itself by cosmic rays and is only able to date material to about 50,000 years ago.
According to the researchers, Krypton is also produced by cosmic rays that bombard the Earth but is then stored within air bubbles that are trapped inside Antarctic ice. Krypton produces two isotopes that help scientists perform the dating process: one is a radioactive isotope called krypton-81, which has a very slow decay time, and the other is krypton-83, which is a stable isotope that does not decay. Scientists are able to determine the age of the ice by comparing the percentage of stable isotopes (krypton-83) to the radioactive isotopes (krypton-81).
The researchers said that the radio-krypton dating technique has be around for more than 40 years, but it wasn’t until 2011 when scientists at the Argonne National Laboratory in Chicago developed an innovative method, named the Atom Trap Trace Analysis (ATTA), that radiometric krypton dating of water and ice became possible.
For their ice core dating experiment, researchers melted several 300-kilogram lumps of ice that were retrieved from the Taylor Glacier in Antarctica, in order to release the air stored in the bubbles. The air from the ice bubbles was sent to scientists at the University of Bern, Switzerland, who isolated krypton from the air samples. The krypton was then sent to the Argonne National Laboratory for an Atom Trap Trace Analysis, which revealed that the glacier samples to be 120,000 years old.
With this technique to help them with their work, researchers say their new challenge is to find some of the oldest ice in Antarctica, something that isn’t as easy as it may sound.
“Most people assume that it’s a question of just drilling deeper for ice cores, but it’s not that simple,” said Edward Brook, an Oregon State University geologist and co-author on the study. “Very old ice probably exists in small isolated patches at the base of the ice sheet that have not yet been identified, but in many places it has probably melted and flowed out into the ocean.”
According to Buizert, it’s important that Earth’s climate be reconstructed as far back as 1.5 million years because it will help scientists learn more about a change in the number of ice ages that took place in what is called the Middle Pleistocene transition. While scientists think Earth has shifted in and out of ice ages every 100,000 years for the past 800,000 years, evidence suggests the planet entered and exited from ice ages much more frequently before that time – at every 40,000 years.
“Why was there a transition from a 40,000-year cycle to a 100,000-year cycle?” Buizert said. “Some people believe a change in the level of atmospheric carbon dioxide may have played a role. That is one reason we are so anxious to find ice that will take us back further in time so we can further extend data on past carbon dioxide levels and test this hypothesis.”