Lead isotopes in deep sea sediments reveal alteration of the isotopic composition signal in the water column at highly productive sites in the South Atlantic Ocean

Abstract. Lead (Pb) isotope fingerprints in marine sediments are frequently used as tracers of natural or anthropogenic Pb sources. This assumes that Pb isotope ratios are not altered within the water column or sediments. Recent studies suggest reverse scavenging of Pb isotopes by biogenic particles during sinking through the water column, but it remains unclear if and how this affects the Pb isotope signal in deep sea sediments. This study examines Pb isotope records (²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb) in pelagic sediments of a highly productive area in the South Atlantic Ocean and their relation to primary productivity as reflected by the silicon (Si) and total organic carbon (TOC) contents. Three cores were extracted at three different stations northwest and southeast of South Georgia and at different water depths (St9: 3796 m, St15: 8066 m, and St16: 5002 m). Downcore Pb and total organic carbon concentrations ranged from 4 to 12 µg g^-1 and from 0.17 to 0.89 % respectively, with highest Pb concentrations in the core taken at the deepest water depth. Lead isotope signatures reflected Patagonian dust as the major Pb source. Lead isotope ratios in the sediment cores were generally larger than those of dissolved Pb in the water phase observed in previous studies, indicating a separation between Pb isotope pools in the water column. Distinct variations in Pb isotope ratios correlated with the content of Si and TOC, indicating an influence of primary productivity on Pb scavenging in the water column. Correlations of Pb concentrations and isotope ratios with TOC content were most pronounced at the deepest sampling station, but were less distinct at other stations presumably due to dilution by biogenic Si. Besides implications on the biogeochemical Pb marine cycle, our findings question the use of Pb isotope signatures in deep sea sediments for source tracing. Future studies combining water phase, particulate matter and sediment Pb isotope analyses should address how signatures of Pb isotope ratios are altered in the water phase during sinking and investigate fractionation processes during Pb burial in deep sea sediments.