Natural radionuclides: Tracers for transport processes in the oceanRates of many transport and reaction processes in the ocean can be evaluated with the natural clocks provided by the radioactive decay series of threeprimordial radionuclides present in the earth's crust: 232Th, 238U and 235U (detailed review in Ivanovich and Harmon, 1992). In all three decay series,isotopes of relatively soluble elements like U, Ra and Rn, decay to isotopes of highly particle-reactive elements (Th, Pa, Po, Pb), and vice versa. Theresulting disequilibria in the decay chains can be interpreted as a measure of transport rates of the particulate and liquid phases relative to each other.