Tracing multi-anthropogenic inputs in intertidal mudflat sediments using lead and zinc isotopes

Intertidal mudflat sediments are valuable archives of long-term contamination in tropical estuaries, yet isotopic approaches remain underexplored in these environments. In this study, we present new Pb (²⁰⁸Pb/²⁰⁶Pb, ²⁰⁶Pb/²⁰⁷Pb) and Zn (δ⁶⁶Zn) isotopic records integrated with previously published Pb and Zn concentration data from a sediment core collected in the Santos-Cubatão Estuarine System (southeastern Brazil), reconstructing the history of contamination in one of the most industrialized coastal areas in Latin America. Pb isotopic ratios clearly separate deeper pre-industrial sediments with geogenic signatures from upper layers influenced by radiogenic Pb inputs associated with emissions from a steel plant. Zn isotopic compositions reveal temporal shifts in contamination: heavier δ⁶⁶Zn values correspond to high-temperature industrial processes, whereas lighter values after the mid-1980s reflect increasing contributions from diffuse urban effluents following the implementation of environmental controls. Importantly, isotopic data capture early anthropogenic signals of contamination in sediment layers where elemental concentrations alone still resembled geogenic background conditions. These findings demonstrate that Pb and Zn isotopes provide sensitive tracers of shifting contamination sources, enabling a refined reconstruction of pollution history in mudflats beyond what concentration data can reveal. This study underscores the potential of isotope geochemistry to improve legacy pollution assessments in estuarine systems worldwide.