Isotope tracers, long-term

Long-term tracers are those elements whose isotopic compositions provide information on the physical and chemical state of the oceans on timescales of several thousands of years to millions of years (My). For example, paleo-oceanographers aim to... 

One such proxy makes use of element ratios. For example, the ratio of Cd to Ca in foraminiferal tests is a proxy for the PO4 content of the past water mass in which the foraminifera formed and therefore provides information on the past thermohaline circulation. This tracer is explained in detail in the article on trace elements in foraminiferal tests. Similarly, the ratio of the intermediate uranium decay products Pa and °Th, measured in bulk sediment, may under certain conditions provide information on the advection of water masses in the overlying water column in the past see Cosmogenic Isotopes and Uranium-Thorium Series Isotopes in Ocean Profiles). Isotope ratios are ideally suited as long-term proxy tracers. Some of these isotope ratios are characteristic of certain seawater properties and... 

Table 1 Long-term isotope tracers currently in use...

Sr isotopes represent the best-studied long-term tracer, as well-dated carbonate sequences are readily available, the extraction and analysis is simple, and the long residence time ensures worldwide homogenization. Therefore, a single isotope evolution curve has... 

CFCs in the Ocean. Long-Term Tracer Changes. Oxygen Isotopes in the Ocean. Stable Carbon Isotope Variations in the Ocean. Tritium-Helium Dating. 

In 1923, Hevesy applied his radioactive tracer studies to the absorption of minerals by plants. It had long been assumed that plants, in contrast to animals, do not excrete the minerals they absorb. Hevesy demonstrated that solutions containing the lead isotope then termed thorium-B were absorbed by plants and caused them to retain some radioactivity. When unlabelled lead solution was subsequently used as nutrient, Hevesy found that the radioactive isotope gradually returned to the nutrient solution. Clearly, there was active metal exchange occurring at all times. 

---