The Expanding Reach of Environmental Radiotracers – New Chronometers And More Sensitive Measurements

Abstract:

Radiotracers in the environment provide powerful tools for understanding environmental processes. Viewed as an age continuum, methods using shorter-lived radionuclides (<100 y) like ³H, ⁷Be, ⁸⁵Kr, ¹³⁴Cs, and ¹³⁷Cs generally rely on measurements of radioactive decay in samples. Methods using longer-lived radionuclides (>1,000 y) like ¹⁰Be, ¹⁴C, ³⁶Cl, and ⁸¹Kr generally rely on atom-counting measurements such as accelerator mass spectrometry. Significant challenges exist in the age range between 100 and 1,000 years where useful radiotracers are difficult to measure by either method and can have very low abundance. These challenges are being addressed with more sensitive measurements using both atom counting and radioactive decay, extending the reach of established radiotracers as well as adding new chronometers.

Improvements in atom-counting methods will be reviewed; the practicality of using ⁸¹Kr (abundance ~5×10^-13 in atmospheric krypton) for age-dating old aquifers has been established and current work focuses on improving sample utilization efficiency to allow smaller samples to be measured. Better efficiency also brings lower-abundance isotopes within reach, for example ³⁹Ar.

Improvements in radioactive decay counting will be reviewed; these take advantage of ultra-pure materials to achieve lower backgrounds and are adding new age-dating reach to the environmental science tool-set with intermediate half-life radionuclides, for example ³²Si for sediment cores. These methods also improve sensitivity for established radiotracers like ³H and will allow smaller samples to be measured, allowing specific processes to be traced. For example, using ³H as an indicator of carbon cycling through organic compounds in soil systems.

Progress in both atom counting and decay counting is expanding the use of ³⁹Ar for age-dating aquifers, measuring ocean mixing, and age-dating younger glacial ice. Argon-39 is a challenging intermediate-age radiotracer (269-year half-life) with low abundance (~8×10^-16 in atmospheric argon). New decay counting capabilities have expanded worldwide capacity for ³⁹Ar aquifer measurements, while reduced ion backgrounds and increased efficiency have allowed atom-trap trace analysis to support ocean circulation studies in the Atlantic and dating of European glacial ice.