Radiogenic Isotopes as Tracers: A History of Applications From Planetary Differentiation to Pedogenesis.

Radiogenic isotope geochemistry takes advantage of the natural radioactive decay of a “parent” isotope to a “daughter” isotope of a different element, which creates variations in the isotopic composition of the daughter element. Since the 1940s, radiogenic isotopes have been successfully used for a wide range of applications, including determining the age of the solar system, working out Earth's internal structure, detailed geochronology and calibration of the geologic time scale, tracking anthropogenic and natural dust sources, quantifying nutrient cycling in soils, and many others. Much of the early work with radiogenic isotopes was focused on geochronology, including dating of meteorites and igneous rocks on Earth. However, pioneers in the field quickly found that radiogenic isotope “signatures” were valuable as well in tracking the provenance of dust, sediments, and ground and surface waters. Novel and useful applications of radiogenic isotopes were more often than not driven by advances in the mass spectrometer techniques and instrumentation used to measure them. The application of radiogenic isotopes to pedologic problems is a more recent phenomenon, having taken hold in the last ~25 years. In particular, isotopes of strontium (Sr) have been used to track exchange of cations in the soil-vegetation-atmosphere system. Applications include integrated watershed studies, cation cycling in climate and time sequences, tracking weathering vs. dust addition to soils, and nutrient uptake by vegetation. The use of other isotope systems such as neodymium (Nd), lead (Pb), and the short-lived uranium (U) decay series open up additional applications such as quantifying pollution inputs to soil systems and determining the timing of pedogenesis. Selected applications of radiogenic isotopes to pedogenic processes will be discussed to illustrate the utility and potential of these systems in soil cycling studies.