Vertical concentration profiles of lead in the Central Pacific at 15°N and 20°S

Abstract

Concentrations of lead were measured in a surface transect and at two vertical profile stations (15°N and 20°S) in the Central Pacific. These measurements complement similar measurements made earlier in the North Pacific at 33°N and in the Northwest Atlantic at 34°N, as well as recent measurements of eolian lead input fluxes near each of these locations. The new transect of surface water concentrations of lead corroborates previous measurements, which decrease from 13 ng/kg at 30°N to 4 ng/kg at 17°S in the Central Pacific. This transect gradient is shown to overlie a similar geographic gradient of subsurface maximum concentrations of lead in the three Pacific vertical profile stations, decreasing from 14 ng/kg at 33°N to 11 ng/kg at 14°N to 2.5 ng/kg at 20°S. Lead concentrations at each of those locations exhibit maxima at 400 m, decreasing concentrations to 2500 m and approximately concentrations of 0.8–1.1 ng/kg below that depth. The subsurface maximum at the northwest Atlantic profile station (36 ng/kg at 34°N) is also congruent with surface water lead concentrations which decrease from 806 ng/kg to 32 ng/kg in an offshore transect from Rhode Island to 34°N, 66°W, and the shape of the Atlantic profile is congruent with those in the Pacific. There is a positive correlation between the magnitudes of eolian lead input fluxes and the magnitudes of the upper water maxima in lead concentration profiles at corresponding locations as follows: South Pacific easterlies 3 ng/cm^2 yr vs. 2.5 ng/kg; North Pacific easterlies 6 ng/cm^2 yr vs. 11 ng/kg; North Pacific westerlies 50 ng/cm^2 yr vs. 14 ng/kg; and North Atlantic westerlies 170 ng/cm^2 yr vs. 36 ng/kg. This relationship enables one to view the anthropogenic perturbations of the marine lead cycle on a global scale, since the industrial origin of eolian and seawater lead has been established by correlations between geographic patterns of industrial lead emissions to the atmosphere and isotopic ratios of industrial leads and by geographic patterns of Pb/silicate-dust ratios and lead isotopic ratios in ocean surface waters. These new data coupled with earlier biogeochemical data indicate that surface water concentrations of lead in the North Pacific and North Atlantic are now conservatively estimated to be 8 to 20-fold greater and those in the South Pacific are 2-fold greater than natural concentrations because of industrial emissions of lead to the atmosphere.

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