The radiometric dating method uses mass spectrometry to make extremely sensitive measurements of minute amounts of uranium-series elements naturally present in ancient polar ice. Researchers determine the age of the ice by comparing concentrations of daughter uranium-series isotopes to parent isotopes in the sample. The quantities of natural radioactive elements researchers measure are in the femtogram, or one quadrillionth of a gram, scale. The uranium-series elements are uranium, radium, thorium and protactinium.
Using the new method, Steven Goldstein, Michael Murrell and Andrew Nunn of the Laboratory's Chemical Science and Technology Division have refined previous age estimates for ice samples taken from Allan Hills, Antarctica and are currently studying samples from the Summit region of central Greenland.
"Our primary goal is to develop a more ideal method for determining the age of ice," says Goldstein. "Paleoclimatologists and others interested in determining the age of ice usually either count the visible bands or layers in the ice, or use carbon-14 dating methods. While both methods are fairly accurate, each has limitations. Band counting can't really account for any missing sections in the ice column and carbon-14 is generally useful for dating back only about 40,000 years. Our method could be more widely applied than counting banding and works on a time scale well beyond that of carbon-14 dating."
The team's uranium-series dating results for the samples from Allan Hills suggest a far younger age for the ice than previously thought. Earlier published data placing the Allan Hills ice's age at roughly 325,000 years was based on measurements made using alpha spectrometry. Los Alamos researchers estimate that the actual age is probably less than 100,000 years. Goldstein and his colleagues are quick to point out, however, that more samples and studies are needed to substantiate this determination.
The new dating technology also may someday help settle an apparent disparity in paleoclimate research between continental and marine records detailing glacial-interglacial climate change. The marine coral record of sea-level fluctuations indicates an age of 122,000 to 130,000 years for the onset of the last interglacial period. On the other hand, a Great Basin calcite vein record of continental climate indicates the onset occurred earlier, about 140,000 years ago. The uranium-series dating method for polar ice cores could be used to address this issue, helping to pinpoint the timing and origin of past natural variability in climate and atmospheric carbon dioxide. The development of the dating technique takes advantage of Los Alamos's scientific expertise in detecting and measuring very minute amounts of radioactive materials in the natural environment.
Allan Hills is a 12-mile-long group of hills located near McMurdo Station along the coast of the Ross Sea in Antarctica. Named for Prof. R.S. Allan of the University of Canterbury in New Zealand, Allan Hills is known for its abundance of old meteorites and as the source of ALH 84001 the controversial meteorite that produced speculation about possible life on Mars.
The ice cores from Greenland used in the study were from the base of a core made by the Greenland Ice Sheet Project Two. The Greenland Ice Sheet Project successfully completed drilling 3 kilometers through the Greenland Ice Sheet in 1993, making it the deepest ice core record in the world.
The Los Alamos research was presented recently at the Spring Meeting of the American Geophysical Union in Washington, D.C.
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