<SUP>210</SUP>Pb/<SUP>226</SUP>Ra disequilibrium in the Santa Barbara Basin

The vertical distributions of 210Pb and 226Ra in the Santa Barbara Basin have been measured. The 210Pb/226Ra activity ratio is close to unity in surface water, but ranges from 0.2 to 0.6 in deep water with a mean value of 0.3 (d &gt; 250m), suggesting rapid removal of 210Pb from the water column. The 210Pb concentrations in the particulate phase at different water depths indicate that the removal of 210Pb is due to adsorption on settling particles. It is estimated that the particulate 210Pb contributes about 50-70% of the total 210Pb measured on unfiltered water samples of the Santa Barbara Basin. The fate of 210Pb (and Pb) in the water column is thus strongly controlled by the settling particles, which have a mean residence time of one year or less in the basin. Material balance calculation for 210Pb in the basin suggests that there is an external source supplying about 70-80% of the 210Pb observed in particulate material or sediments. This excess 210Pb is most likely provided by particles entering the basin loaded already with 210Pb