Radiogenic isotopes: systematics and applications to earth surface processes and chemical stratigraphy

Radiogenic isotopes have wide application to chemical stratigraphy, geochronology, provenance studies, and studies of temporal changes in Earth surface processes. This paper briefly reviews the principles of radiogenic isotope geochemistry and the distribution of a number of elements of interest in the environment, and then uses this information to explore the range of applications to chemical stratigraphy and other fundamental subjects of sedimentary geology. Many of these applications center on the reconstruction of secular variations in paleoseawater isotope signals. These “seawater isotope curves” can be used for stratigraphic correlation and to assess changes through Earth history in processes such as tectonic uplift, climate change, biogeochemical cycling, ocean circulation, crustal growth, and surface water evolution. Understanding the modern biogeochemical cycle of the element/isotope system of interest is essential for gaining the maximum information regarding the operation of ancient cycles. The main sources of uncertainty for reconstructing seawater isotope curves involve diagenetic alteration, sample impurities, and errors in age assignments. Rigorous sample selection criteria and pretreatment methods are keys to reconstructing the most accurate seawater isotope curves. Applications are discussed for the Rb–Sr, Sm–Nd, La–Ce, Lu–Hf, Re–Os, U–Th–Pb, K–Ar, and cosmogenic isotope systems.