Hydrogen isotope biogeochemistry reactors in 1988, but nuclear weapons testing in the 1950s added large spikes of radioactive elements to the atmosphere, especially radiocarbon and tritium. This complicated measurements for geologists using radiometric dating of carbon. However, some oceanographers benefited from the tritium increase, utilizing the signal in the water to trace physical mixing of water masses. In biogeochemistry, scientists focused primarily on the stable isotope of deuterium as a tracer for environmental processes, especially the water cycle. The American geochemist Harmon Craig, once a graduate student of Urey's, discovered the relationship between rainwater's hydrogen and oxygen isotope ratios. The linear correlation between
Isotope geochemistry successfully used in forensic science to fingerprint bullets, because each batch of ammunition has its own peculiar Pb/Pb vs Pb/Pb ratio. Samarium–neodymium is an isotope system which can be utilised to provide a date as well as isotopic fingerprints of geological materials, and various other materials including archaeological finds (pots, ceramics). Sm decays to produce Nd with a half life of 1.06x10 years. Dating is achieved usually by trying to produce an isochron of several minerals within a rock specimen. The initial Nd/Nd ratio is determined. This initial ratio is modelled relative to CHUR - the Chondritic Uniform Reservoir -