Spatial and temporal evolution of lead isotope ratios in the North Atlantic Ocean between 1981 and 1989

Weiss, D., Boyle, E.A., Wu, J., Chavagnac, V., Michel, A. and Reuer, M.K. (2003) Spatial and temporal evolution of lead isotope ratios in the North Atlantic Ocean between 1981 and 1989. Journal of Geophysical Research, 108 (C10), art no. 3306. (doi:10.1029/2000JC000762).

Abstract

Lead concentrations and isotope ratios were measured in North Atlantic surface water samples collected in 1981 (29°–79°N, 6°E–49°W) and in 1989 (23°–39°N, 29°–68°W). In the early 1980s, 206Pb/207Pb ratios in the North African Basin averaged 1.193 ± 0.005 (1? ). Similar radiogenic ratios within the level of analytical precision (average 0.29%) were found in the Labrador and Iceland Basins (1.198 ± 0.006) and in the Norwegian Sea (1.196 ± 0.008). These radiogenic mixed layer signatures along with atmospheric global lead emission patterns suggest that most North Atlantic lead in the early 1980s was derived from North American leaded gasoline. Samples in the East Iberian Basin near Portugal and France showed lower 206Pb/207Pb ratios, between 1.167 and 1.182, indicating a significant influence of less radiogenic atmospheric lead transported from Europe and possibly the influence of the Rio Tinto acid mine drainage very close to shore in the Gulf of Cadiz. [Pb] across the entire North Atlantic Basin ranged between 54 and 145 pmol/kg, with the lowest values (54–74 pmol/kg) found at high latitudes (>65°N). In the late 1980s, surface waters in the western subtropical North Atlantic (North American Basin/Sargasso Sea, >47°W) and in the eastern subtropical North Atlantic (North African Basin/Central Iberian Basin, <45°W) showed very similar 206Pb/207Pb signatures with little zonal variation, ranging from 1.177 to 1.192. Lead concentrations ranged between 47 and 137 pmol/kg, increasing slightly from west to east. South of 25°N in the equatorial North Atlantic, crossing the subtropical/tropical surface water boundary, the 206Pb/207Pb seawater signatures were significantly less radiogenic (1.170–1.175) and concentrations were lower (51 pmol/kg). This difference suggests a relative increase in the atmospheric lead supply from the western Mediterranean/North African continent via Trade Easterlies and illustrates the effective barrier between the subtropical/tropical surface water exchanges. Triple-isotope plots (206Pb, 207Pb, and 208Pb) suggest that most of the lead can be accounted for by wet aerosol deposition derived from the adjacent landmasses of America to the west (transported via the North American Westerlies) and from Europe to the east (transported via the European Easterlies) and probably by some advected surface waters from the Sargasso Sea. The 1989 triple-isotope plot suggests, however, a third lead source in the subtropical western North Atlantic, possibly leaded gasoline from Mexico. Gasoline lead emission patterns as well as atmospheric lead isotope signatures confirm that gasoline was the main pollutant source in the early 1980s but suggest that contributions from high-temperature industrial processes (coal combustion, steel manufacture, waste incineration) have been increasing in the late 1980s. From isotopic mass balance estimates, lead inputs to the 1980s North Atlantic were dominated by North American sources (>53%). These elemental and isotopic results demonstrate the strongly variable isotopic and elemental signatures of North American and European lead throughout the North Atlantic Ocean, frequently dominated by high 206Pb/207Pb and [Pb] North American signatures throughout the subtropical North Atlantic gyre.

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