Radiogenic Isotopes in Weathering and Hydrology

There are a small group of elements that display variations in their isotopic composition, resulting from radioactive decay within minerals over geological timescales. These isotopic variations provide natural fingerprints of rock–water interactions and have been widely utilized in studies of weathering and hydrology. The isotopic systems that have been applied in such studies are dictated by the limited number of radioactive parent–daughter nuclide pairs with half-lives and isotopic abundances that result in measurable differences in daughter isotope ratios among common rocks and minerals. Prior to their application to studies of weathering and hydrology, each of these isotopic systems was utilized in geochronology and petrology. As in the case of their original introduction into geochronology and petrology, isotopic systems with the highest concentrations of daughter isotopes in common rocks and minerals and systems with the largest observed isotopic variations were introduced first and have made the largest impact on our understanding of weathering and hydrologic processes. Although radiogenic isotopes have helped elucidate many important aspects of weathering and hydrology, it is important to note that in almost every case that will be discussed in this chapter, our fundamental understanding of these topics came from studies of variations in the concentrations of major cations and anions. This chapter is a “tools chapter” and thus it will highlight applications of radiogenic isotopes that have added additional insight into a wide spectrum of research areas that are summarized in almost all of the other chapters of this volume. The first applications of.