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

Abstract

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. The ²¹⁰Pb 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.