Krypton Helps Accurately Date Ancient Antarctic Ice
April Flowers for redOrbit.com – Your Universe Online
Climatologists use deep-drilled ice core samples in order to understand the climate changes our planet has endured during and between the Ice Ages. A team of scientists has recently developed a new technique, described in the Proceedings of the National Academy of Sciences, to confirm the age of a 120,000-year-old sample of Antarctic Ice.
Scientists will be able to use the new dating technique to identify ice that is much older, allowing them to reconstruct climate more accurately, much farther into the planet’s past. The team hopes this will lead to a deeper understanding of the mechanisms that underlie the shift into and out of ice ages.
The new method, a radiometric-Krypton-dating technique, was used on ice from Antarctica’s Taylor Glacier to provide an accurate means of confirming the age of such samples. The team asserts that the new method is the most precise dating tool for ancient ice currently in use.
“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 Christo Buizert, a postdoctoral researcher at Oregon State University, in a recent statement. “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.”
During the Middle Pleistocene transition (MPT) — which took place between 1,250,000 years ago to 700,000 years ago — there was a shift in the frequency of ice ages, so being able to reconstruct the Earth’s climate back to 1.5 million years ago is very important. Scientists believe that the Earth shifted in and out of ice ages every 100,000 years, give or take, during the last 800,000. Evidence exists, however, that prior to that time, a shift took place every 40,000 years.
“Why was there a transition from a 40,000-year cycle to a 100,000-year cycle?” asked Bulzert. “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.”
Laymen are more aware of carbon-14 dating, which measures the decay of a radioactive isotope that has a constant and well-known decay rate, then compares this rate to a stable isotope. Krypton dating uses this same technique, but that isn’t the end. Krypton is a Noble gas. Noble gases have the maximum number of electrons possible in their outer shell, making them extremely stable and nearly impossible to have chemical reactions. Krypton, specifically, has a half-life of approximately 230,000 years. Carbon-14 dating isn’t very effective on ice because any carbon-14 found in the ice was produced inside the ice by cosmic rays.
Also produced by cosmic rays, Krypton is formed outside the ice then trapped in air bubbles within the Antarctic ice. Krypton-81 is a radioactive isotope that decays very slowly, while Krypton-83 does not decay at all. The age of the ice is determined by comparing the proportion of stable-to-radioactive isotopes.
Researchers have wanted to use Krypton dating for more than 40 years, but Krypton-81 atoms are few and far between and very difficult to count. In 2011, a new detector technology made it possible for Krypton-81 dating to be made available to the earth sciences community. The Atom Trap Trace Analysis (ATTA) is the name of the new atom counter developed by a team of nuclear physicists led by Zheng-Tian Lu at Argonne National Laboratory.
At Taylor Glacier, the research team melted several 660 pound chunks of ice — obtained with the newly developed Blue Ice Drill — to release the air trapped in the bubbles. The air was then stored in flasks. The team isolated the Krypton from the air and sent it to Argonne National Laboratory. Argonne scientists first used Krypton dating over 10 years ago on very old groundwater, and have refined the technique using ATTA in the time since.
“The atom trap is so sensitive that it can capture and count individual atoms and Argonne currently has the only instrument in the world with that capability,” said Buizert. “The only problem is that there isn’t a lot of Krypton in the air, and thus there isn’t much in the ice, either. That’s why we need such large samples to melt down.”
Lu says that the refining of ATTA has allowed them to be able to date samples as small as 90-175 pounds and they plan to be able to date ice samples as small as 45 pounds within the next year.
The isotope ratio between Krypton-81 to Krypton-83 in the Taylor Glacier samples allowed the team to date the ice cores at 120,000 years old. The team validated their results by comparing the results to well-dated ice core measurements of atmospheric methane and oxygen from that same period.
The team plans to continue their research by identifying the oldest ice in the Antarctic, which might prove to be quite challenging.
“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. “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.”
Old ice is also exposed at the edges of an ice field, Brook explained.
“The international scientific community is really interested in exploring for old ice in both types of places and this new dating will really help,” Brook said. “There are places where meteorites originating from Mars have been pushed out by glaciers and collect at the margins. Some have been on Earth for a million years or more, so the ice in these spots may be that old as well.”