Comparison of Pu and ¹³⁷Cs as tracers of soil and sediment transport in a terrestrial environment

Following atmospheric nuclear weapons testing in the 1950s and 1960s significant quantities of ¹³⁷Cs and ^239+240Pu were deposited worldwide. In recent decades, ¹³⁷Cs has been commonly used as a tracer of soil erosion and sedimentation, particularly in the Northern Hemisphere where atomic deposition was three times as great as in the Southern Hemisphere. The relatively short 30-year half-life of this isotope means that its sensitivity as a tracer is rapidly decreasing. In contrast, with half-lives of 24,110 and 6561 years, the sensitivity of the two plutonium isotopes remains essentially the same as when it was deposited. Here we use the technique of Accelerator Mass Spectrometry to demonstrate the potential of anthropogenic Pu as an alternative to ¹³⁷Cs as a tracer of soil transport in Australia. We measure an average ¹³⁷Cs/^239+240Pu activity ratio of 27.3 ± 1.5 and an average ²⁴⁰Pu/²³⁹Pu atom ratio of 0.149 ± 0.003, both slightly lower than the global average.