Tracer applications of noble gas radionuclides in the geosciences

Noble gas radionuclides, including ⁸¹Kr (t_1/2 = 229,000 years), ⁸⁵Kr (t_1/2 = 10.8 years), and ³⁹Ar (t_1/2 = 269 years), possess nearly ideal chemical and physical properties for studies of earth and environmental processes. Recent advances in Atom Trap Trace Analysis (ATTA), a laser-based atom counting method, have enabled routine measurements of the radiokrypton isotopes, as well as the demonstration of the ability to measure ³⁹Ar in environmental samples. Here we provide an overview of the ATTA technique, and a survey of recent progress made in several laboratories worldwide. We review the application of noble gas radionuclides in the geosciences and discuss how ATTA can help advance these fields, specifically: determination of groundwater residence times using ⁸¹Kr, ⁸⁵Kr, and ³⁹Ar; dating old glacial ice using ⁸¹Kr; and an ³⁹Ar survey of the main water masses of the oceans, to study circulation pathways and estimate mean residence times. Other scientific questions involving a deeper circulation of fluids in the Earth's crust and mantle are also within the scope of future applications. We conclude that the geoscience community would greatly benefit from an ATTA facility dedicated to this field, with instrumentation for routine measurements, as well as for research on further development of ATTA methods.