super(210)Pb and super(210)Po in the equatorial Pacific and the Bering Sea: the effects of biological productivity and boundary scavenging

New data are presented for super(210)Po and super(210)Pb concentrations in seawater of the equatorial Pacific and the Bering Sea. Their implications are discussed in terms of productivity and boundary scavenging. The super(210)Pb concentrations in the equatorial waters are high at the surface (0.1-0.15 d.p.m. l super(-1), approximately twice the super(226)Ra activity) with significant super(210)Po deficits in the upper 1000 m water column ranging from 2.6 to 4.3 d.p.m. cm super(-2). Clearly super(210)Po is preferentially removed relative to super(210)Pb from the surface waters. Along the Tokyo-San Diego-Panama transect in the mid-North Pacific, the removal rate of super(210)Po is correlated with the chlorophyll-a content in the surface waters, suggesting that scavenging of super(210)Po to particles is virtually controlled by phytoplankton in the open ocean. In the deep water, super(210)Pb is always deficient relative to super(226)Ra. Box-model super(210)Pb residence times, based on mean super(210)Pb/ super(226)Ra activity ratios, are approximately 90 years at the open-ocean equatorial sites. These are significantly shorter than those of the North Pacific gyre (greater than 200 years). The short residence times can be ascribed to the intensified scavenging of super(210)Pb because of the high particle flux regime of the equatorial zone. The deep-sea super(210)Pb residence time becomes significantly shorter as the western topographic boundary is approached, and is only 8 years in the Bismarck Sea. This tendency cannot be ascribed to the difference in the particle flux in the equatorial zone. It seems that super(210)Pb is actively taken up from the deep water by sediments at or near the bottom interface presumably in association with manganese redox cycling. The Bering Sea data support this mechanism. Compilation of deep-sea super(210)Pb residence times available in the literature and the new data presented here suggests that the lateral transport of super(210)Pb via isopycnal mixing followed by scavenging at the sediment-water interface is a major control on the super(210)Pb- super(226)Ra disequilibrium in the deep sea. The surface ocean productivity may be of secondary importance in this context.