210Pb and 210Po results from F.S. "Meteor" cruise 32 in the North Atlantic

The distribution of 210Pb and 210 Po on dissolved (< 0.4 micron) and particulate (> 0.4 micron) phases has been measured at ten stations occupied during cruise 32 of F.S. "Meteor" in the tropical and eastern North Atlantic. Both radionuclides occur principally in the dissolved phase. Unsupported 210Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved 210Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in 210Po, with 210Po/210Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. At depths of 100-300 m, 210Po maxima occur, and unsupported 210Po is frequently present, an observation that suggests rapid re-cycling within the thermocline. Comparison of the 210Pb distributions with those reported for 226Ra at nearby GEOSECS stations confirms the widespread existence of a 210Pb/226Ra disequilibrium in the deep sea. Close to the bottom, profiles of 210Pb and 226Ra usually diverge, and 210Pb concentrations frequently decrease with depth, suggesting a sink 210 Pb near the seafloor. Particulate 210Pb concentrations ordinarily show little systematic variation with depth. At depths greater than 1000 m, dissolved 210Po activities are, on the average, less than those of 210Pb by 12%. A corresponding 210 Po enrichment in the particulate phase is found.