Uranium-Thorium Radionuclides in Ocean Profiles

Natural radioactivity in the environment originates from two sources. First, primordial radionuclides which were incorporated into the Earth at the time of its formation are still present in it because of their long half-lives. 238U, 235U, 232Th and their decay series (Fig. 1), 40K, 87Rb and 187Re are examples of this category. Second, cosmic ray-produced isotopes which are generated continuously in the atmosphere and earth's crust through interactions of cosmic rays with their constituents. 3H, 14C and 10Be are some of the isotopes belonging to this group. The distribution of all these isotopes in the oceans is governed by their supply, radioactive decay, water mixing and their biogeochemical reactivity (the tendency to participate in biological and chemical processes) in sea water. Water circulation plays a dominant role in the dispersion of isotopes which are biogeochemically ‘passive’ (eg,3H, Rn), whereas biological uptake and release, solute–particle interactions and chemical scavenging exert major control in the distribution of biogeochemically ‘active’ elements (eg, C, Si, Th, Pb, Po). Systematic study of the isotopes of these two groups in the sea can yield important information on the physical and biogeochemical processes occurring in sea water.