The Evolution of Anthropogenic Lead in the Ocean, 1976-2000

Edward A. Boyle

Department of Earth, Atmospheric, and Planetary Sciences

Massachusetts Institute of Technology

Cambridge MA 02139 USA

The evolution of anthropogenic Pb in the ocean has now beendirectly observed for nearly 25 years, beginning with the pioneering NorthPacific (1976) and North Atlantic (1979) deep-sea profiles by Schaule andPatterson. Schaule and Patterson demonstrated that Pb concentrations werehigh in the upper ocean and low in the deep sea, with North Atlantic Pb >North Pacific Pb > South Pacific Pb. They argued that this pattern wasample demonstration of the anthropogenic influence on oceanic Pb becausethe enrichments corresponded to the respective eolian fluxes and to theupwind consumption of leaded gasoline. The ongoing response of the ocean todecreases in the utilization of leaded gasoline by North America, westernEurope, and Japan now provides overwhelming confirmation of theanthropogenic contribution to oceanic Pb. In the surface waters of thenorth Atlantic Ocean near Bermuda, Pb has decreased by fivefold, from ~160pmol/kg in 1979 to ~30 pmol/kg at the end of 1999, and significantdecreases in Pb are observed in subsurface waters down to ~1500 m depth. Inthe eastern north Atlantic south of the Azores, surface Pb has decreasedfrom ~130 pmol/kg (1981) and ~100 pmol/kg (1989) to ~40 pmol (1999), withsubsurface decreases observed down to ~1200 m. Mid-depth (~700m) waters nowhave higher Pb concentrations than above or below because of the lag due tothe decadal ventilation rates. In the north Pacific Ocean near Hawaii,surface water Pb has decreased from ~65 pmol/kg in 1976 to ~30 pmol/kg(North Atlantic and North Pacific surface waters now have comparable levelsof lead). It is likely that Asian industrial sources now play a largerrelative role in Pb emissions to the Pacific ocean compared to leadedgasoline.

Information from total Pb concentration is supplemented by limiteddata on Pb isotope ratios, which allow some source discrimination (e.g.,European vs. U.S. leaded gasoline). Shen and Boyle (1988), Sherrell et al.(1992) Veron et al. (1993, 1999), Hamelin et al. (1997) and Alleman et al.(1999) have established that Pb in the North Atlantic surface watersdominantly derived from the U.S. as would be expected from the 3-4 folddominance of U.S. leaded gasoline utilization relative to western Europeand the prevailing wind directions. Recent developments in multiplecollector plasma mass spectrometry will transform this difficultmeasurement into a tool that can be more widely applied in the future.

Coral Pb and Pb isotope observations of Shen and Boyle (1987)extend the perspective back to the turn of the 20th century, whenindustrial Pb emissions (smelting, coal burning, and other high temperatureprocesses) began their rise until the 1940's, when they were overtaken byPb gasoline emissions as the dominant source. The decreases in Pb gasolineemissions over the past two decades have probably restored industrial Pbemissions as the dominant source of Pb to the ocean.