Tritium biological, chemical, and physical paths throughout the world oceans because of its evolving distribution. Tritium has thus been used as a tool to examine ocean circulation and ventilation and, for such purposes, is usually measured in Tritium Units where 1 TU is defined as the ratio of 1 tritium atom to 10 hydrogen atoms. As noted earlier, nuclear weapons testing, primarily in the high-latitude regions of the Northern Hemisphere, throughout the late 1950s and early 1960s introduced large amounts of tritium into the atmosphere, especially the stratosphere. Before these nuclear tests, there were only about 3 to 4 kilograms of
Isotope geochemistry usually is given as a multiple of the present atmospheric ratio (Ra). Common values for R/Ra: He/He isotope chemistry is being used to date groundwaters, estimate groundwater flow rates, track water pollution, and provide insights into hydrothermal processes, igneous geology and ore genesis. U-series isotopes are unique amongst radiogenic isotopes because, being in the U-series decay chains, they are both radiogenic and radioactive. Because their abundances are normally quoted as activity ratios rather than atomic ratios, they are best considered separately from the other radiogenic isotope systems. Uranium is well mixed in the ocean, and its decay produces Pa and