The processes controlling the fate of particle-reactive U/Th decay series isotopes in the oceans must be fully understood in order to ascertain their usefulness as tracers for particle dynamics. Thorium kinetic and equilibrium behavior appears to be the consequence of a combination of physical (particle- particle interactions) and chemical (adsorption) processes. A colloidal pumping hypothesis stating that Th isotopes are transferred from the dissolved state to macroparticles via a colloidal will be tested. Research suggests that Th "scavenging" rates most likely represent the rate of coagulation of submicron particles with a large-particle pool rather than Th sorption itself. Laboratory experiments will use inorganic and organic model particles in order to further test and simulate the proposed coupling of sorption and coagulation processes. Field experiments will test the oceanic scavenging model under experimentally constrained conditions to better understand the applicability of results from laboratory studies to natural environments.
