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

New data are presented for Po-210 and Pb-210 concentrations in seawater of the equatorial Pacific and the Bering Sea. Their implications are discussed in terms of productivity and boundary scavenging. The Pb-210 concentrations in the equatorial waters are high at the surface (0.1-0.15 d.p.m.l(-1), approximately twice the Ra-226 activity) with significant Po-210 deficits in the upper 1000 m water column ranging from 2.6 to 4.3 d.p.m. cm(-2). Clearly Po-210 is preferentially removed relative to Pb-210 from the surface waters. Along the Tokyo-San Diego-Panama transect in the mid-North Pacific, the removal rate of Po-210 is correlated with the chlorophyll-a content in the surface waters, suggesting that scavenging of Po-210 to particles is virtually controlled by phytoplankton in the open ocean. In the deep water, Pb-210 is always deficient relative to Ra-226. Box-model Pb-210 residence times, based on mean Pb-210/Ra-226 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 Pb-210 because of the high particle flux regime of the equatorial zone. The deep-sea Pb-210 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 Aux in the equatorial zone. It seems that Pb-210 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 Pb-210 residence times available in the literature and the new mechanism. Compilation of deep-sea data presented here suggests that the lateral transport of Pb-210 via isopycnal mixing followed by scavenging at the sediment-water interface is a major control on the Pb-210-Ra-226 disequilibrium in the deep sea. The surface ocean productivity may be of secondary importance in this context. (C) 1998 Elsevier Science Ltd. All rights reserved.