Applications of PB-210/RA-226 and PO-210/PB-210 disequilibria in the study of marine geochemical processes

The distribution of 210Pb and 210Po in dissolved (less than 0.4 micron) and particulate (greater than 0.4 micron) phases was measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported 210Pb activities, maintained by flux from the atmosphere, were present in the surface mixed layer and penetrated into the thermocline to depths of about 500 m. Dissolved 210Po was ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter was enriched in 210Po, with 210Po/210Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2-y residence time for 210Pb and a 0.6-y residence time for 210Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 y). At depths of 100 to 300 m, 210Po maxima occurred and unsupported 210Po was frequently present. Calculations indicate that at least 50 percent of the 210Po removed from the mixed layer is re-cycled within the thermocline. Similar calculations for 210Pb suggest much lower re-cycling efficiencies