²¹⁰Pb/²²⁶Ra disequilibrium in the Santa Barbara Basin

Abstract

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 > 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.