Lead isotopic compositions of hydrothermal sediments on the slope of Eolo seamount (Aeolian Arc): implications for anthropogenic lead impact on recent hydrothermal processes

Eolo Seamount is a submarine volcano located in the eastern part of the Aeolian volcanic arc. During an R/V Bannock cruise in 1977, a 327.5 cm long core was taken from an area of extensive hydrothermal mineralization. We investigated the Pb isotopic compositions of the hydrothermal sediments of this core in order to provide insight into Pb behavior in low-temperature hydrothermal settings. Eolo hydrothermal sediments exhibit a wide range of Pb isotopic ratios with 206Pb/204Pb ranging from 17.446 to 18.916, 207Pb/204Pb from 15.570 to 15.683, and 208Pb/204Pb from 37.420 to 38.975. The isotopic compositions at the low-radiogenic end are unusual and cannot be derived from the possible natural sources in the area. Mixing considerations indicate that up to 80 percent of the lead in these sediments is anthropogenic in origin. The highest proportion of anthropogenic Pb is found in a hydrothermal nontronite layer, formed by precipitating hydrothermal Fe-Si oxyhydroxides onto bacterial walls. The Pb isotopes indicate that a significant part of the lead in the hydrothermal precipitates was derived from leaded gasoline, which constrains the hydrothermal activity to a time period between the 1950s and the 1970s. The anthropogenic Pb was probably scavenged from the Mediterranean seawater by sinking hydrothermal plume particles that then became incorporated in the Eolo hydrothermal sediments. The lead isotopic systematics indicate that the anthropogenic Pb has swamped the natural Pb signal in the Mediterranean seawater at the time of the hydrothermal activity. Furthermore, these results imply that the hydrothermal plume particles did not carry a significant amount of Pb extracted from the underlying rocks, suggesting that the Pb budget in low-temperature systems is controlled by seawater composition, not by leaching from underlying rocks.