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

Abstract

The distribution of Pb-210 and Po-210 in dissolved (<O.4 micron) and particulate (>0.4 micron) phases has been 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 Pb-210 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 Po-210 is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in Po-210, with Po-210/Pb-210 activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2-y residence time for Pb-210 and a 0.6-y residence time for Po-210 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 100-300 m, Po-210 maxima occur and unsupported Po-210 is frequently present. Calculations indicate that at least 50% of the Po-210 removed from the mixed layer is recycled within the thermocline. Similar calculations for Pb-210 suggest much lower recycling efficiencies. Comparison of the Pb-210 distribution with the reported distribution of Ra-226 at nearby GEOSECS. stations has confirmed the widespread existence of a Pb-210/Ra-226 disequilibrium in the deep sea. Vertical profiles of particulate Pb-210 were used to test the hypothesis that Pb-210 is removed from deepwater by in situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate Pb-210 concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities (>10 m/d) to account for the inferred Pb-210 flux. It is proposed that an additional sink for Pb-210 in the deep sea must be sought. Estimates of the dissolved Pb-210/Ra-226 activity ratio at depths greater than 1,000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of Pb-210 at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3-6 x 107 cm2/s. The more reactive element Po, on the other hand, shows evidence of rapid in situ scavenging. In filtered sea water, Po-210 is deficient, on the average, by ca. 10% relative to Pb-210; a corresponding enrichment is found in the particulate phase. Total inventories of Pb-210 and Po-210 over the entire water column, however, show no significant departure from secular equilibrium, and reliable estimates of particle sinking rates cannot be made.