228Ra and 228Th in coastal waters

Abstract

Typically, the waters above the continental shelf contain higher concentrations of 228 Ra but lower concentrations of 228 Th than do the surface waters of the open ocean. The vertical profiles of 228 Ra concentration in the shelf waters of the Middle Atlantic Bight suggest that the shelf sediments are the chief source of 228 Ra, but that the sediments of bays and estuaries are a significant secondary source. In the mid-shelf and outer shelf waters of the Bight, the 228 Ra concentration increases and the 228 Th/ 228 Ra activity ratio decreases from the region bordering Nova Scotia to the region uust north of Cape Hatteras. The mean residence time of thorium in the surface waters of the Bight, calculated from the 228 Th/ 228 Ra activity ratios, ranges from 4 months in the slope waters and outer shelf waters and 1·5 months in the inner shelf waters of the region bordering Nova Scotia to 1·5 months in the slope waters, 20 days in the outer shelf waters and 10 days in the inner shelf waters of the region just north of Cape Hatteras. The vertical profiles of 228 Th concentration in the surface mixed layer and thermocline of the open ocean qualitatively resemble the profiles of concentrations of nutrients, such as phosphate. The process most likely to produce the observed rapid transport of thorium out of the surface waters probably involves uptake of thorium by phytoplankton, ingestion of the phytoplankton by zooplankton, and inclusion of the thorium in rapidly settling fecal pellets.