Scientists say they have developed a means of accurately dating Earth's oldest and densest polar ice by analyzing the composition of krypton gas trapped within ancient air bubbles.
In a study published Monday in the journal PNAS, researchers used the new dating method, called Atom Trap Trace Analysis, or ATTA, to study ice recovered from the McMurdo Dry Valleys in Antarctica.
"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," lead study author Christo Buizert, an Oregon State University paleoclimatologist, said in a statement.
"That is very exciting because a lot of interesting things happened with the Earth's climate prior to 80,000 years ago that we currently cannot study in the ice core record," Buizert said.
Potential uses, Buizert and his colleagues say, are dating meteorites recovered in Antarctic ice, and studying the Earth's climate and its cycle of ice ages.
Krypton is a noble gas that is present in the atmosphere at extremely low levels, or about one part per million. In the upper atmosphere, exposure to cosmic rays can transform a stable krypton isotope into a slow-decaying radioactive isotope.
Scientists say that air bubbles in polar ice will contain some of these radioisotopes. By comparing the radioisotope's state of decay to stable krypton isotopes, researchers can determine how long the gas has been trapped in the ice. That's essentially how scientists use carbon-14 techniques to date ice, but that method is only accurate back to about 50,000 years.
But even though krypton can be used to date much older ice, its trickier to work with.
Because there's so little krypton in the air, you have to melt down a lot of ice to obtain sufficient samples. Also, you need a device that can count, or trap, individual atoms.
Fortunately, such a device was developed in 2011 by a team of nuclear physicists at Argonne National Laboratory in Illinois.
To test the atom trap on ancient ice, study authors obtained more than 600 pounds of ice chunks from Taylor Glacier, Antarctica, melted them down, trapped the escaped air in flasks and had it analyzed.
According to the authors' estimates, the ice was about 120,000 years old.
Study authors say they now want to find older ice.
"Most people assume that it's a question of just drilling deeper for ice cores, but it's not that simple," said study coauthor 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."
In some cases, the very old ice has been exposed at the edge of an ice field.
Scientists say they hope that as the dating method is refined, they can work with smaller amounts of ice.