Members of the naturally occurring decay series are found throughout the world’s oceans, though in activities that vary by several orders of magnitude within the same decay series. They can be distinguished on the basis of their overall reactivity — e.g., adsorption or incorporation — with particles. Physical and biogeochemical processes in the water column, and close to the sediment-water interface, lead to fractionation of mother and daughter nuclides and hence create disequilibria in the decay series. These disequilibria have become powerful tools in the study of marine processes. In order to illustrate their use in marine sciences, three examples are presented for the Atlantic sector of the Southern Ocean. For the group of particle-reactive radionuclides, the distribution of the short-lived isotope
Th (half-life 24.1 days) is used as a measure of export fluxes from the photic layer.
Ac (half-lives 5.8 and 21.7 years, respectively), belong to the more soluble nuclides. In contrast to
Th, they are hardly found in the particulate fraction of a sea water sample but instead exist in the dissolved state.
Ra is indicative of shelf water input while
Ac is a tracer for upwelling deep water masses.