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
Hydrogen isotope biogeochemistry samples with more precision. Hydrogen isotope applications quickly emerged in petroleum geochemistry by measuring oil, paleoclimatology by observing lipid biomarkers, and ecology by constructing trophic dynamics. Modern advances are currently underway in the clumped isotope composition of methane after development of the carbonate thermometer. Precise measurements are also enabling focus on microbial biosynthetic pathways involving hydrogen. Ecologists studying trophic levels are especially interested in compound specific measurements for construction of past diets and tracing predator-prey relationships. Highly advanced machines are now promising position specific hydrogen isotope analysis of biomolecules and natural gases. All isotopes of a chemical element contain the