Isotopic Equilibration between Dissolved and Suspended Particulate Lead in the Atlantic-Ocean - Evidence from Pb-210 and Stable Pb Isotopes

Decreased consumption of leaded gasoline in the United States over the past two decades has drastically altered the flux and isotopic composition of Pb entering the western North Atlantic from the atmosphere. Here we exploit the resulting temporal changes in the distribution and isotopic composition of oceanic Pb to investigate interactions between dissolved and particulate Pb in the oceanic water column. Measurements of dissolved Pb isotopic composition on samples collected in 1987 in the upper water column near Bermuda demonstrate that surface water Pb-206/Pb-207 decreased from approximately 1.203 to approximately 1.192 since 1983 and that a measurable change propagated to 300-500 m since the 1984 profile of show and Boyle (1988), The first accurate measurements of suspended particulate Pb in an open ocean profile show concentrations of 1-3 pmol/L, equal to 2-4% of total Pb. Vertical profiles of (1) the stable lead isotopic composition and (2) the ratio of total Pb to Pb-210 in suspended particles closely track contemporaneous depth variations in these ratios for dissolved Pb throughout the upper 2000 m of the Sargasso Sea near Bermuda. Thus suspended particles reach isotopic equilibrium with ambient seawater Pb on a time scale which is shorter than their residence time with respect to vertical removal, in agreement with equilibrium scavenging hypotheses based on interpretations of Th isotope distributions. A simple flux model suggests that the effect of deep ocean scavenging processes on the flux and isotopic composition of Pb sinking on large particles was minor throughout the preanthropogenic and most of the anthropogenic era but has become important as surface inputs decrease to pre-leaded gasoline levels and may exceed the contribution of surface-derived Pb flux in the next decade.