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 transuranic elements, primarily uranium and thorium. Lead isotope geochemistry is useful for providing isotopic dates on a variety of materials. Because the lead isotopes are created by decay of different transuranic elements, the ratios of the four lead isotopes to one another can be very useful in tracking the source of melts in igneous rocks, the source of sediments and even the origin of people via isotopic fingerprinting of their teeth, skin and bones. It has been used to date ice cores from the Arctic shelf, and provides information on the source of atmospheric lead pollution. Lead–lead isotopes has been