Dating Water and Tracing Bones

Dating water and tracing bones to high precision will be more widely available for geological and biomedical applications thanks to state-of-the-art atom counting techniques.

In a pair of new papers, Zheng-Tian Lu of Argonne National Laboratory (lu@anl.gov) and his colleagues have demonstrated two new applications of Atom Trap Trace Analysis (ATTA; see Update 460), in which researchers trap desired isotopes with lasers and magnetic fields and then count them with laser techniques.

ATTA has now been used to count krypton-81 atoms in groundwater samples in the ancient waters of the Sahara. Kr-81 (half life=229,000 years) is a rare isotope produced by the cosmic rays in the atmosphere, and accompanies more common forms of atmospheric krypton.

Trapped in water underneath the Sahara, the abundance of Kr-81 relative to other Kr isotopes provides information on how long the water has been there. Extracting krypton from the Nubian aquifer in the western Sahara, and using the ATTA technique, the researchers found that the water's age ranges from 200,000 to a million years old, depending upon the sample location.

In another application, researchers used ATTA to count individual calcium-41 atoms released from the bones of a human subject. This isotope is injected into osteoporosis patients and subsequent measurements of its abundance can be used to monitor bone loss and retention rates. Until now, medical researchers had to rely upon particle accelerators to perform this task.

But the smaller and potentially cheaper ATTA is now precise enough to do the job, with the ability to detect one Ca-41 atom per 10⁸-10¹⁰ calcium atoms. With further increases in precision (in which one Ca-41 atom can be detected amidst 10¹⁵ other calcium atoms) the technique could be ideal for archaeological dating (half-life of Ca-41 = 103,000 year) of ancient bones ranging from 50,000 to 500,000 years old. (Sturchio et al., Geophysical Research Letters, 12 March 2004; and Moore et al., Physical Review Letters, upcoming article; see news release on GRL paper.)