Lead and lead isotopes in the North Pacific: mid-depth maxima and deep water anthropogenic source

Lead (Pb) concentrations and isotopic ratio in seawater samples that were collected in a section from 7 °S to 30 °N along 158 °W in the central Pacific ocean were determined using Mg(OH)2 coprecipitation ICPMS. Surface water Pb concentrations decrease by 30-50% since 1980¡¦s, probably due to the phasing-out of leaded gasoline in Japan. There is pronounced mid-depth maxima for Pb at ~ 600 m in subtropical oceans which coincides with salinity minima and the maxima of CFC, tritium Pu, Cs, Co, Bi and Fe, indicating the influence by the advection of the North Pacific intermediate water. 206Pb/207Pb ratios at 30¢XN are relatively constant in the upper 500 m, increase with increasing depth below 500 m and are lower than those for the Pleistocene sediments and Mn nodule, suggesting that anthropogenic Pb may have contaminated some of oldest waters of the world ocean, possibly via Pb regeneration during microbial degradation of sinking organic debris.