Models of the distribution of 210Pb in a section across the North Equatorial Atlantic Ocean

The deficiency of 210Pb relative to 226Ra in the world's deep ocean is well documented, and the overall residence time of 210Pb has been calculated to be about 15 to 100 years. It has been assumed, generally, that the removal mechanism is one of in situ adsorption on settling particles, but Bacon et al. (1976) suggested that a boundary scavenging process with diffusive and advective fluxes of 210Pb from the interior ocean may be a significant factor. In this contribution researchers examine some two-dimensional models of 210Pb in a section across the North Atlantic Ocean at 15N. Researchers show that satisfactory fits to the 210Pb distribution may be obtained with a model that includes both boundary scavenging and in situ adsorption with a constant adsorption rate constant or with a model that has no boundary scavenging but allows the in situ adsorption rate constant to be a function of distance from the boundary. Researchers have calculated the interior and boundary fluxes of 210Pb required of each model and suggest that the boundary fluxes are of a magnitude to be compatible with Fe and Mn redox cycling in slope and shelf sediments but that the variations in the in situ adsorption flux are difficult to explain in terms of the known distributions of fine particles. Researchers suggest that the boundary scavenging process is a function of the flux of Fe+2 and Mn+2 from anoxic sediments accompanied by rapid oxidation and precipitation in the boundary mixed layers.The boundary fluxes calculated for model amount to 2 to 8 dpm cm-2 yr-1 for slope and shelf sediments but only about 0.04 dpm cm-2 yr-1 for the deep ocean bottom boundary. The bottom boundary flux in the deep ocean is small and would not be detectable above the large particle flux of about 0l7 dpm cm-2 yr-1 contributed by biological removal from the ocean surface