2I0Pb and 210Po in the equatorial Pacific and the Bering Sea: the effects of biological productivity and boundary scavenging

Abstract

New data are presented for 210Po and 210Pb concentrations in seawater of the equatorial Pacific and the Bering Sea. Their implications are discussed in terms of productivity and boundary scavenging. The 210Pb concentrations in the equatorial waters are high at the surface (0.10.15 d.p.m. 1 −1, approximately twice the 226Ra activity) with significant 210Po deficits in the upper 1000 m water column ranging from 2.6 to 4.3 d.p.m. cm −2. Clearly 210Po is preferentially removed relative to 210Pb from the surface waters. Along the Tokyo-San Diego-Panama transect in the midNorth Pacific, the removal rate of 210Po is correlated with the chlorophyll-a content in the surface waters, suggesting that scavenging of 210Po to particles is virtually controlled by phytoplankton in the open ocean. In the deep water, 210 Pb is always deficient relative to 226Ra. Box-model 210Pb residence times, based on mean 210Pb/ 226Ra 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 210Pb because of the high particle flux regime of the equatorial zone. The deep-sea 210Pb 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 210Pb 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 210Pb residence times available in the literature and the new data presented here suggests that the lateral transport of 210Pb via isopycnal mixing followed by scavenging at the sediment-water interface is a major control on the 210Pb- 226Ra disequilibrium in the deep sea. The surface ocean productivity may be of secondary importance in this context.