Short-lived U/Th Series Radionuclides in the Ocean: Tracers for Scavenging Rates, Export Fluxes and Particle Dynamics

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Abstract
The uranium and thorium decay series have numerous radionuclides with half-lives ranging from fractions of a second to a few years. The former include many of the isotopes of radon, polonium, bismuth, lead and thallium and these are generally too short to show appreciable disequilibrium with respect to their precursors in the decay series. Others, however, have half-lives that make them useful in tracing a variety of oceanic processes that occur on short time scales. This group includes isotopes of thorium: 234Th (half-life = 24.1 d), 227Th (18.6 d), 228Th (1.9 y), radium: 224Ra (3.64 d), 223Ra (11.1 d), radon: 222Rn (3.8 d) and polonium: 210Po (138 d). Of these, the Th isotopes and 210Po are particle-reactive, while the radium isotopes and radon tend to remain in solution. As a consequence, Th and Po are useful in quantifying the rate of scavenging from solution onto particles and the processes governing the dynamics of particles in the ocean, while Ra and Rn are appropriate tracers for fluid processes such as advection and diffusion (Cochran 1992). Other chapters in this volume highlight recent developments in the use of short-lived Ra isotopes and Rn as tracers of estuarine mixing and submarine groundwater discharge (Swarzenski et al. 2003; Porcelli and Swarzenski 2003). Likewise, the oceanic chemistries of the longer-lived particle reactive radionuclides in the U/Th series, such as 230Th, 231Pa and 210Pb are treated elsewhere in this volume (Henderson and Anderson 2003). Accordingly our emphasis here is with recent developments in the application of the short-lived Th isotopes (234Th, 227Th, 228Th) and 210Po as tracers for particle-associated processes. These radionuclides have in common the fact that each is produced by a longer lived parent which is stably dissolved …