Long-term mixing of water bodies is currently being studied using anthropogenic tracers (i.e. contaminants). During the past several decades, the surface ocean has been 'tagged' with substances that previously did not exist in the ocean in significant amounts. CFCs (chlorofluorocarbons) started to be commercially produced in the 1930s and 1940s for use as a refrigerant and in manufacturing processes. Their concentration in the atmosphere has gradually built up, and from here they have entered the oceans where they remain chemically stable. They can thus act as an indicator of the age of a water mass. Tritium, mostly from bomb testing, is used in a similar way. The substances are carried with surface water as it moves and changes into sub-surface water masses.
For example, North Atlantic Deep Water (NADW) is one of the major deep water masses of the world ocean. It has an important role to play in heat and CO2 budgets and so affects global climate. It forms from surface and near surface waters, and so NADW formed in the last three to four decades will contain tritium and CFCs. Recent work following these anthropogenic tracers has shown the path taken by recently formed NADW in the Deep Western Boundary Current (Smethie, 1993).
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