Accumulation of Th, Pb, U, and Ra in marine phytoplankton and its geochemical significance

The bioaccumulation of U, Th, Ra, and Pb in four diverse nanoplanktonic algal species and a picoplanktonic blue-green alga was determined with radiotracers. Among the nanoplankton, differences of l-2 orders of magnitude in volume/volume concentration factors (VCFs) were observed for a given nuclide, but larger differences were observed among the four nuclides, with VCF values of Th > Pb> Ra = U. The picoplankton cells, with greater surface : volume ratios, had significantly higher VCF values. The mean VCF values in the nanoplankton of Th and Pb were 1.5 × 105 and 3.6 × l04 in the light and 2.8 × l05 and 7.3 × l04 in the dark. The VCFs of Th and Pb in the picoplankton were both about 2 × 106, irrespective of light. Retention half-times of 228Th and 210Pb in fecal pellets of Artemia salina, fed radiolabeled diatoms, were 20-50 d, but > 120 d for 228Th at 4°C. The results suggest that sinking plankton and their debris could account for most of the natural series radionuclides sedimenting out of oceanic surface waters