Transient tracers change over time, such as radioactive material (Tritium and Cesium-137) and chemical concentrations (CFCs and SF6), which are used to date water masses and can also track mixing. In the mid-1900s, Nuclear weapons testing and chemical production released tons of compounds that are not naturally found in the environment. While extremely unfortunate, scientists were able to use the concentrations of anthropogenic compounds and half-lives of radioactive material to determine how old a water body is. The Fukushima nuclear disaster was really well studied by oceanographers, who tracked the radioactive material spread throughout the Pacific Ocean, and used that to better understand ocean currents and mixing patterns.