Seasonal Intrusion of Central South Atlantic Water (SACW) as a Vector of Lead Isotopic Signatures in Ilha Grande Bay, Brazil

This study investigates the hydrography and geochemical signature in Ilha Grande Bay (RJ, Brazil), focusing on the seasonal intrusion of South Atlantic Central Water (SACW) and its interaction with lead sources. CTD (Conductivity, Temperature, and Depth) data revealed the presence of SACW during the summer campaigns (Mangaratiba/2011 and Frade/2012), characterized by temperatures below 20 °C and salinity between 34.6 and 36. The intrusion is driven by northeasterly winds that favor coastal upwelling, establishing a classic thermohaline stratification. The winter campaigns did not detect SACW, confirming its seasonal nature. Isotopic analysis of Pb in sediments identified six Pb206/Pb207 intervals, indicating multiple sources, including natural contributions, industrial waste, and urban effluents. The Pb206/Pb207 ranges were defined based on cluster analysis and frequency histograms, which are common methods in isotopic provenance studies. An overlap between the most radiogenic isotopic signatures and the presence of SACW suggests that this water mass acts as a vector for transporting trace elements from the deep oceanic region to the coast. This study provides the first evidence that the South Atlantic Central Water (SACW) acts as a seasonal vector, importing a distinct radiogenic Pb isotopic signature onto the continental shelf of Ilha Grande Bay. By synoptically coupling physical water-mass analysis (CTD) with Pb isotopic tracers, we introduce a novel approach that successfully discriminates oceanic from anthropogenic Pb sources, offering a new framework for understanding contaminant transport in coastal areas influenced by boundary currents. It is concluded that the coastal dynamics in Ilha Grande Bay are governed by the seasonal interaction of coastal, continental, and oceanic waters, and that the integration of physical and geochemical data is crucial for understanding mixing processes and contaminant transport in this complex environment.