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

The third Intergovernmental Oceanographic Commission (IOC) Baseline Contami nant cruise (May-June 1996) has established the first lead isotopic composi tions in the surface water and the atmosphere of the Equatorial and South A tlantic Ocean. These ratios have evidenced both anthropogenic and natural o rigins of lead along the cruise transect (from 33 degreesS to 10 degreesN). The isotopic gradients tentatively have been, attributed to aeolian as wel l as surface-water advective inputs from a suite of rather local and remote sources to the Southern Hemisphere. Relatively low Pb-206/Pb-207 ratios (x +/- sd) were encountered within the South Equatorial Current between 17 de greesS and 5 degreesS (1.156 +/- 0.003). Those were bracketed by more radio genic ratios at higher latitudes in the Southern Hemisphere (33 degreesS to 23 degreesS), within the Brazil Current and the Subtropical Gyre (1.163 +/ - 0.003), and in the Northern Hemisphere (0 degrees to 10 degreesN) (1.165 +/- 0.005). The latter were comparable to ratios of surface water in the No rth Atlantic Equatorial Ocean (1.169 +/- 0.006), under a combined contamina nt influence of both North American westerlies (1.19-1.20) and European eas terlies (1.155-1.165). That predominance of contaminant lead contrasts with the measurable presence of natural lead in surface waters of the Equatoria l 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 ch aracteristic 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 (Pb-206/Pb-207) in bulk depositions (1.171 +/- 0.006) , precipitation (1.171 +/- 0.006) and aerosols (1.168 +/- 0.011) were, gene rally, more radiogenic than the surface waters (1.162 +/- 0.005). Beside a poor representation of a short term atmospheric sampling, this difference c ould reflect a recent evolution in atmospheric lead emissions, which have n ot yet been reflected in oceanic surface waters. It may also be due, in par t, to advective lead inputs from surface oceanic circulation of the South E quatorial Current. (C) 2001 Elsevier Science Ltd. All rights reserved.