Consumption of alkyl leaded gasoline has decreased considerably in the United States over the last 15 years. In order to evaluate the chemical response of the ocean to decreased atmospheric deposition of anthropogenic lead, new 210Pb and stable Pb data have been obtained near Bermuda (June 1983 to December 1984). Concentrations of these tracers in the mixed layer are highly variable during the period of observations (+/-25%). From June through September, soluble eolian deposits accumulate in the shallow mixed layer because mixing and productivity are inhibited by the seasonal thermocline. In the fall, these enriched surface waters mix with upper thermocline waters, and surface Pb concentrations decrease. In the winter, storms from the east coast of North America cause local episodes of high Pb and 210Pb input into the ocean surface; the resulting areally-limited high Pb concentrations are dispersed within less than 2 weeks by vertical and horizontal mixing. Pb/210Pb ratios in surface waters are almost invariant over this period (averaging 669 pmol/dpm, ranging from 564 to 718 pmol/dpm).

Surface Pb/210Pb appears to have declined by a factor of 2 since 1979, similar to the 2.6-fold decreases in U.S. leaded gasoline consumption during this period. A substantial fraction (on the order of 10%) of gasoline Pb is transported into the ocean. Pb concentrations in the upper thermocline may have declined by about 15% from 1979 to 1984. The helium-tritium thermocline ventilation model of Jenkins (1980), coupled with boundary conditions appropriate for Pb and 210Pb, shows that the supply of Pb and 210Pb to the thermocline is dominated by lateral isopycnal ventilation. The shape of the vertical profile is dominated by this physical ventilation, near-surface biological removal processes, in-situ scavenging, and (for stable lead) time-dependent input. Vertical transport by particles is responsible for minimal transport of dissolved lead into the upper ocean. Similar processes are likely to account for the plutonium maximum in the thermocline of the North Pacific. Observations of Pb and other reactive species in winter surface outcrops of thermocline isopycnals would provide valuable constraints on chemical processes within the thermocline.