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
Marine isotope stage stages 1–16 the SPECMAP figures are within 5 kya of the figures given here. All figures up to MIS 21 are taken from Aitken & Stokes, Table 1.4, except for the sub-stages of MIS 5, which are from Wright's Table 1.1. Some older stages, in mya (millions of years ago): Marine isotope stage Marine isotope stages (MIS), marine oxygen-isotope stages, or oxygen isotope stages (OIS), are alternating warm and cool periods in the Earth's paleoclimate, deduced from oxygen isotope data reflecting changes in temperature derived from data from deep sea core samples. Working backwards from the present, which is MIS