Isotopic evidence for the source of lead in the North Pacific abyssal water

Abstract

The absence of accurate measurements of lead (Pb) isotopic composition in the pristine North Pacific abyssal water has made it difficult to assess the relative importance of what are believed to be the two major Pb sources: the natural Pb introduced during preindustrial time and recent anthropogenic Pb resulting from leaded gasoline combustion and high temperature industrial activities. Here we report a vertical profile of seawater 206Pb/207Pb ratio and a meridional section of Pb concentration in the North Pacific Ocean. We observe two to three fold increases in Pb concentration along the deep-water flow path and a deep-water [superscript 206]Pb/[superscript 207]Pb ratio (~1.188) substantially lower than the pre-industrial value (~1.210). These data suggest that anthropogenic Pb has invaded the North Pacific abyssal water and become the predominant Pb source. A simple model calculation based on these data indicates that the anthropogenic Pb is transported to the deep ocean by sinking particles and that this Pb vertical flux has a [superscript 206]Pb/[superscript 207]Pb ratio that decreased during the past two centuries.