sion and we have a crude idea of the average rate of this process. 
Yet we know nothing of its vertical and horizontal variations or of 
the source of energy which drives it. A unique method of tagging 
which can be used to measure the rates of downward mixing from sur- 
face to intermediate depths has been provided, however, with the re- 
cent introduction of fission and nuclear products such as radiostron- 
tium and cesium, tritium and man-produced carbon-14 into the surface 
waters. In the deep and bottom waters, concentrations of the natural 
radioactive isotopes including radium-226, silicon-32 and cosmic ray 
produced carbon-14 provide nuclear "clocks" for the study of rates 
of advection and turbulent diffusion. 
We know that, ultimately, we must be able to predict how 
fast, and in what directions, a substance introduced at any depth 
and location in the sea will spread, over periods of hundreds to 
thousands of years. At the present time we are only beginning to 
understand the pattern of variation of these radioisotopes although 
precise analytical techniques for all of them exist. 
In addition, we need to know more about the return of nu- 
trients to the surface of the ocean since this is perhaps the single 
most important factor in determining the organic productivity of vari- 
Ous regions of the sea. The vertical distribution of many oceanic 
constituents is determined by interactions between physical processes 
which stir the water and biochemical ones which fix elements to, and 
release them from, particulate matter in the ocean. The particulate 
flux consists largely of the remains of organisms which live in the 
