Evaluating the accuracy of core incubations to determine benthic fluxes of 222Rn, 228Ra, and 224Ra, measured in core incubations

Profiles of radioisotopes in bottom waters are often used to estimate vertical transport. However, horizontal transport often plays a role and may be difficult to evaluate. A boundary condition useful for constraining transport rates is to establish the isotope flux from benthic sediments. We tested a new method to estimate benthic fluxes of isotopes by incubating sediment cores from San Pedro Basin and measuring the changes in concentrations of 222Rn, 228Ra and 224Ra in the overlying water, over a six to seven day incubation period. These results have been compared to fluxes established by applying diffusion-reaction models to the sediment column, based on measuring isotope emanation from sediments, adsorption coefficients, and profiles of the parent isotopes in sediments. There was internal consistency (± 30%) between the core incubation measurement and the radon emanation flux calculated from diffusion-reaction models. The 224Ra flux measurements have been complicated by minor disturbances of cores during collection and the removal of its 228Th parent from core top water during incubation. The 228Ra measurements from San Pedro cores are still in progress, as they require a six month ingrowth period. Cores obtained from Cascadia Basin have also been incubated for two to four days. Incubation fluxes of 222Rn are consistent with the standing crop observed in bottom waters. Incubation fluxes of 228Ra are consistent with predictions from modeling solid phase profiles of radioisotopes in sediments.