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

The distribution of 210Po and 210Pb (grand daughter–parent) in the dissolved 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 the 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 210Po profiles is that the dissolved activities in the surface waters are about one-third of the equilibrium concentrations, suggesting weeks to a month removal times of 210Po by adsorption on settling particles. The mean dissolved 210Po/210Pb activity ratios in the equatorial and South Atlantic are 0.27 and 0.39, respectively, representing a pronounced disequilibrium in surface waters. The box-model calculation yields a residence time of about 73 and 130 days for dissolved 210Po in the surface water at the equatorial and southern sites, respectively. The gross deficiency of 210Po extends throughout the upper 500 m; however, the 210Po residence time in the intermediate depths is longer than that in surface water. The observed 210Po–210Pb disequilibrium has been examined with respect to POC concentration, which yields a significant positive correlation (r2=0.61), suggesting that the 210Po deficiency is mainly associated with biological removal. 0Pb distribution shows distinctively higher concentrations in the equatorial surface waters (19–22 dpm/100 l), suggesting an enhanced atmospheric flux of 210Pb. The residence time of dissolved 210Pb (400–780 days), with respect to atmospheric input, is longer than dissolved 210Po and 234Th residence times, and also shows a latitudinal decrease toward the equator in the surface ocean. The shorter residence time in the surface equatorial ocean (0–20 m) seems associated with higher productivity and more rapid particulate scavenging.