Po-210 and Pb-210 in the South-equatorial Atlantic: distribution and disequilibrium in the upper 500 m

The distribution of Po-210 and Pb-210 (grand daughter-parent) in the dissol ved and total (dissolved + particulate) phase of sea water samples has been measured in the upper 500 m at four stations along a western transect in t he South-equatorial Atlantic. Such a transect allows one to make inference about the particle-associated scavenging processes in the equatorial versus subtropical regions of the South Atlantic. A common feature of the Po-210 profiles is that the dissolved activities in the surface waters are about o ne-third of the equilibrium concentrations, suggesting weeks to a month rem oval times of Po-210 by adsorption on settling particles. The mean dissolve d Po-210/Pb-210 activity ratios in the equatorial and South Atlantic are 0. 27 and 0.39, respectively, representing a pronounced disequilibrium in surf ace waters. The box-model calculation yields a residence time of about 73 a nd 130 days for dissolved Po-210 in the surface water at the equatorial and southern sites, respectively. The gross deficiency of Po-210 extends throu ghout the upper 500 m; however, the Po-210 residence time in the intermedia te depths is longer than that in surface water. The observed Po-210-Pb-210 disequilibrium has been examined with respect to POC concentration, which y ields a significant positive correlation (r(2) = 0.61), suggesting that the Po-210 deficiency is mainly associated with biological removal. The Pb-210 distribution shows distinctively higher concentrations in the eq uatorial surface waters (19-22 dpm/1001), suggesting an enhanced atmospheri c flux of Pb-210. Th, residence time of dissolved Pb-210 (400-780 days), wi th respect to atmospheric input, is longer than dissolved Po-210 and Th-234 residence times, and also shows a latitudinal decrease toward the equator in the surface ocean. The shorter residence time in the surface equatorial ocean (0-20m) seems associated with higher productivity and more rapid part iculate scavenging. (C) 1999 Elsevier Science Ltd. All rights reserved.