Isotopic evidence of contaminant lead in the South Atlantic troposphere and surface waters

Abstract

Abstract The third Intergovernmental Oceanographic Commission (IOC) Baseline Contaminant cruise (May–June 1996) has established the first lead isotopic compositions in the surface water and the atmosphere of the Equatorial and South Atlantic Ocean. These ratios have evidenced both anthropogenic and natural origins of lead along the cruise transect (from 33°S to 10°N). The isotopic gradients tentatively have been, attributed to aeolian as well as surface-water advective inputs from a suite of rather local and remote sources to the Southern Hemisphere. Relatively low 206Pb/207Pb ratios (x±sd) were encountered within the South Equatorial Current between 17°S and 5°S (1.156±0.003). Those were bracketed by more radiogenic ratios at higher latitudes in the Southern Hemisphere (33°S to 23°S), within the Brazil Current and the Subtropical Gyre (1.163±0.003), and in the Northern Hemisphere (0° to 10°N) (1.165±0.005). The latter were comparable to ratios of surface water in the North Atlantic Equatorial Ocean (1.169±0.006), under a combined contaminant influence of both North American westerlies (1.19–1.20) and European easterlies (1.155–1.165). That predominance of contaminant lead contrasts with the measurable presence of natural lead in surface waters of the Equatorial Ocean, which are attributed to aeolian inputs of Saharan dust. The ratios of lead in surface waters at higher latitudes in the South Atlantic are characteristic of anthropogenic lead aerosols also detected in Antarctic ice, and could substantiate as well the hypothesized aerosol recycling of lead by sea-spray emission in the far Southern Hemisphere. The atmospheric lead isotopic compositions (206Pb/207Pb) in bulk depositions (1.171±0.006), precipitation (1.171±0.006) and aerosols (1.168±0.011) were, generally, more radiogenic than the surface waters (1.162±0.005). Beside a poor representation of a short term atmospheric sampling, this difference could reflect a recent evolution in atmospheric lead emissions, which have not yet been reflected in oceanic surface waters. It may also be due, in part, to advective lead inputs from surface oceanic circulation of the South Equatorial Current.