ABSTRACT

“Settling model” is proposed as a removal mechanism of chemical elements in the ocean on the basis of the behavior of insoluble radionuclides in seawater. In the settling model more than 20 elements such as Fe and Th exist substantially in one particulate phase which is settling with a mean velocity of 2×10^-4 cm/sec by forming large particles with organic material. There is much favorable evidence for the settling model such as the coincidence of the residence time of these insoluble elements, specific activity of settling material similar to that of particles suspended in seawater, nearly the same proportion, about 10%, of particulate fraction (> 0.45μm) for all the insoluble elements when subsurface water is filtered, etc. The settling model is applied to Th-234 in the surface water and the vertical eddy diffusivity of the water is calculated to be about 30cm²/sec.