Sedimentary geochemistry of 224Ra

Marine sediments are important nutrient and trace metal sources to water column. The export of materials from sediments to water column has significant impact on the environment. Therefore, it is important to understand material transport and controls on sediment processes. ^ 224Ra can be used as a tracer for porewater and particle transport in ocean bottom, saltmarsh and tidal flat sediments. 224 Ra can provide information on the effects of adsorption-desorption reactions and exchange between sediments and water. The commonly used tracers, such as 222Rn, 210Pb and 234Th do not have these capabilities. The 224Ra measurement method (Rama et al., 1987) for dissolved 224Ra was improved by controlling water content. A sensitive and rapid method was developed which measures the adsorbed 224Ra. These methods enable the direct calculation of Kp in the sediments. ^ A diagenetic model considering adsorption capability of sediments for radium, bioturbation processes and adsorption-desorption reactions was established to describe the distribution and transport of 224Ra in sediments and 224Ra fluxes from sediments. The model indicates that the particle bioturbation and desorption (PBAD) flux driven by bioturbation of particles and release of adsorbed 224Ra at the sediment water interface is often more important than porewater advective and/or diffusive flux. The distribution coefficient and the particle-mixing rate in the topmost sedimentary layer control the PBAD flux. ^ Cores from a saltmarsh, a tidal flat and Long Island Sound bottom were measured for dissolved and adsorbed 224Ra and leachable Mn and Fe, Ba, Zn, Ni and Pb. With a least-squares fitting process, porewater advection, biodiffusion and particle mixing rates were extracted. The calculation of 224Ra, Ba, Zn, Ni, and Pb fluxes demonstrated that the particle bioturbation and release flux is the most important flux. The results of this study (1) explain lower fluxes calculated with dissolved concentrations of trace metals vs. higher measured fluxes with a flux-chamber and (2) provide a means to more accurately predict site specific metal fluxes through the quantification of sediment dynamics with 224Ra. ^ This study defines the coupled importance of adsorption-desorption reactions an bioturbation in the sediments and will thus likely see broad applications in sedimentary geochemistry.