Tracing anthropogenic Hg and Pb input using stable Hg and Pb isotope ratios in sediments of the central Portuguese Margin

Abstract

Three short marine sediment cores from the Cascais Submarine Canyon (CSC; cores 252-32 and 252-35) and the Estremadura Spur (core 252-16) on the central Portuguese Margin were analysed for Hg, Pb, Al, and Mn concentrations, and both Pb and Hg stable isotope compositions, in order to reconstruct trends in Hg and Pb sources and accumulation. Sediment ages were determined based on 210Pb measurements. The studied cores reveal increasing Hg and Pb concentration, independent of grain-size variations, since the middle of the 19th century towards the Present. Concomitantly, a decreasing trend of 206Pb/207Pb to less radiogenic values towards the surface was found, indicating increasing anthropogenic inputs. The lowest values of δ202Hg were generally observed in the older sediments characterized by low Hg and Pb concentrations, suggesting a low input of anthropogenic metals. Odd mass number (199Hg and 201Hg) Hg isotopes in sediment samples from core 252-16 were characterized by positive mass independent fractionation (MIF), while recent sediments from cores 252-32 and 252-35 did not reveal significant MIF, probably reflecting both the proximity to the source of anthropogenic Hg contamination (Tagus Estuary) and the importance of the CSC as a particle carrier. The multi-tracer approach, based on both stable Hg and Pb isotopic signatures, confirms anthropogenic Hg and Pb enrichment in recent marine sediments and also allows us to distinguish between areas dominated by detrital (e.g. CSC) versus hemipelagic (Estremadura Spur) sedimentation.