Lasers Date Ancient Groundwater
Daniel S. Burgess
Using a laser-based isotopic analysis technique, geologists and physicists in the US, Switzerland and Egypt have determined that the water in Egypt's Nubian Aquifer is up to 1 million years old. The results reveal changes in the local climate over that time and suggest that the method is appropriate for dating other similarly ancient environmental samples.
Neil C. Sturchio, a professor of earth and environmental sciences at the University of Illinois at Chicago, said that dating groundwater is important for scientific and practical reasons. It reveals how temporal changes in climate are recorded in the chemical and isotopic composition of groundwater. And it enables scientists to establish groundwater velocities, which allow them to predict how environmental contaminants may spread through the water supply.
Figure 1. In atom-trap trace analysis, a DC discharge excites krypton-81 in the gas sample into a metastable level, and the excited atoms are subjected to transverse laser cooling, deceleration in a Zeeman slower and trapping by 811-nm radiation. An avalanche photodiode counts the number of trapped atoms by their fluorescence. Courtesy of Thomas O'Connor.
Dating groundwater that is 50,000 to 1 million years old, however, has presented problems. The available natural isotopic tracers with sufficiently long half-lives for the radiometric dating of such samples are the "cosmogenic" isotopes chlorine-36 and krypton-81, both of which are produced in the reaction of cosmic rays with atmospheric gases. After these isotopes enter the groundwater, their decay can be used to estimate its age.
The deposition rate of chlorine-36, Sturchio explained, varies with latitude, elevation and distance from the ocean, and both subsurface processes and cosmogenic ones produce it, introducing uncertainty into the interpretation of chlorine-36 measurements in terms of groundwater age. Krypton-81 is more evenly dispersed in the environment and is not generated in substantial quantities by subsurface processes, making it the better tracer for this application. But until this point, measuring krypton-81 has required large samples and particle accelerators.
Atom-trap trace analysis, a technique developed a few years ago by physicists at Argonne National Laboratory in Illinois, changes that. Using a tabletop laser setup, researchers can determine the abundance of krypton-81 with samples 10 times smaller and with instruments 100 times less expensive than are required in accelerator mass spectroscopy.
Figure 2. The setup is orders of magnitude smaller, simpler, more efficient and more economical than the apparatus required for the alternative technique, accelerator mass spectroscopy. Courtesy of George Joch.
Zheng-Tian Lu, a staff scientist at the laboratory, explained that atom-trap trace analysis is based on the laser cooling and trapping of neutral atoms, approaches developed over the past three decades. The krypton-81 in a sample of gas is excited into a metastable level, optically cooled, decelerated in a Zeeman slower and loaded into a magneto-optical trap (Figure 1). Other species, such as krypton-85, are deflected or pass through the trapping region. Under 811-nm illumination in the trap, the atoms of krypton-81 fluoresce, and an avalanche photodiode records this response, enabling the user to quantify the amount of the isotope in the sample.
The second-generation setup used in these groundwater dating experiments features external-cavity diode lasers built at the lab using 20- to 100-mW diode lasers from SDL Inc. and Toptica Inc., as well as a 500-mW master oscillator power amplifier diode laser system from Toptica. An EG&G avalanche photodiode serves as the photon counter.
Figure 3. The researchers collected water samples from wells in Egypt's Western Desert. The age of groundwater at the sites varied from 210,000 to 1 million years, consistent with predicted flow vectors in the aquifer. Courtesy of Zheng-Tian Lu.
In the work, the researchers collected tons of water from six wells in Egypt's Western Desert (Figure 3). They extracted on-site the gases dissolved in the water and shipped them to the University of Bern in Switzerland, where the krypton was isolated and spiked with a calibrated amount of krypton-85. The atom-trap trace analysis of the normalized krypton gas was performed at Argonne.
They found that the samples are between 210,000 and 1 million years old, consistent with predicted flow vectors through the aquifer from the southwest of Egypt toward the northeast.
Together with isotopic hydrogen and oxygen ratios, Sturchio said, the findings indicate that the region periodically varied between wet periods, in which moist air masses traveled across North Africa from the Atlantic, and dry periods, in which moisture came from the Mediterranean to the north.
Lu said that the team hopes to build a third generation of the system that will boost efficiency so that samples of only a few kilograms of water or ice will be required. Sturchio suggested that such a system might be used to date groundwater in the Yellowstone National Park geothermal system and in various locations in central North America, and to investigate glacial ice in Greenland and seawater circulation through the ocean floor.
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