Bomb Tritium in the Pacific Ocean

During 1959--1973 bomb-produced tritium was monitored in surface seawater from 15 stations in the Pacific Ocean. Some 500 tritium determinations were carried out. Between 1965 and 1972 a series of subsurface samples from 49 locations in the Pacific Ocean was measured, and tritium concentrations as a function of depth were determined. The determinations had a standard error of +-4% and an uncertainty in the zero value of about 0.17 tritium unit. The most significant oceanographic conclusions that must be drawn from these data can be summarized in the following way: (1) Lateral flow along contours of constant density is the major factor controlling the tritium distribution in the surface and subsurface waters of the Pacific Ocean. Input of tritium into the water below the mixed layer occurs between 45 degree and 30 degree N and south of 60 degree S. The sinking of water in the mid-Pacific at 30 degree N and its flow toward the equator was previously recognized from the presence of a salinity maximum below the thermocline. The southward component of the rate of this flow is estimated to be 1.3+-0.5 cm/s. In many other areas the tritium content of water under the mixed layer is undoubtedly also due to local sinking of surface water and subsequent lateral subsurface flow. (2) An upper limit for the coefficient of vertical eddy diffusion can be calculated. This upper limit of 0.3 cm2/s is well below estimates derived by other methods, but it agrees well with the upper limit of 0.2 cm2/s derived from tritium measurements in the Sargasso Sea. It appears that transport across lines of constant density is much slower than previously assumed. (3) The mean lifetime of water in the mixed layer of the North Pacific Ocean varies greatly with geographical location. North of 10 degree N the average residence time is between 9 and 15 years, depending on the assumed rate of the molecular exchange of water between atmosphere and ocean