Determining coastal mixing rates using radium isotopes

Coastal waters contain elevated dissolved activities of four radium isotopes. These elevated activities arise through desorption of Ra from particle surfaces and the input of submarine groundwaters enriched in Ra. The input of Ra near the coast is balanced by a flux of each Ra isotope toward the open ocean. Two of the Ra isotopes decay almost completely before they reach the edge of the continental shelf; the other two decay hardly at all. These differences in decay rates provide a powerful constraint on models of water movement and mixing on the shelf. These models are used to assess the factors that control the export of radium. Understanding the factors that control the export of radium allows an assessment of physical processes that regulate fluxes of other dissolved constituents in the coastal ocean. In this paper I use observations that were made during vertically stratified conditions in the South Atlantic Bight. Offshore transects of the long-lived 226Ra and 228Ra indicate that eddy diffusion controls their distributions within 50 km of shore. The short-lived 223Ra and 224Ra distributions in this region yield an eddy diffusion coefficient of 360–420 m2 s−1. The offshore fluxes of 226Ra and 228Ra derived from their across-shelf activity gradients and the eddy diffusion coefficient require a substantial volume of groundwater discharge to balance Ra removal.