Krypton-85 in the Ocean

Abstract

Abstract A method is described to determine for the first time the 85 Kr concentration in seawater. Samples of 220 l of seawater are degassed aboard a research vessel. The krypton contained in the gas sample is isolated in the laboratory and its 8s Kr activity is measured in a gas counter. The background of the lowel-level counting-array is 0.026 cpm; surface water samples yield a netto count-rate of 0.13 cpm. The reproducibility of the method was checked by gas standards (krypton-air mixtures) and moreover by three surface water samples; both sets agree within ± 1 σ, or ± 5%. The mean value of three samples of great depths (1600, 2000, 2000 m) is 5% of the surface water concentration. This figure represents the possible contamination of the samples by atmospheric 85 Kr during the various preparation stages (blank value). Two profiles have been measured from the North Atlantic (about 40°N, 18°W, November 1972). A strong decrease of the 85 Kr concentration is found in the upper 1000 m. In a simple one-dimensional vertical diffusion-advection model the profiles can be simulated with a coefficient of apparent vertical eddy diffusion of 5 cm 2 / sec in the depth range of 300 to 1000 m (main thermocline). A possible application of oceanic 85 Kr data is the combination of depth profiles with those of other tracers, in particular 3 H. Another application of the 85 Kr method is the dating of groundwaters younger than 20 years.