The discharge of deep-sea tailings from the Gardanne alumina plant into the Cassidaigne Canyon (Mediterranean Sea) for nearly five decades generated extensive bauxite residue deposits. This study investigated whether these historical deposits act as a secondary source of dissolved trace metals to bottom waters following the cessation of solid discharge in 2015. Sediment cores collected at five stations in 2016 were analyzed for porewater and overlying-water concentrations of Al, trace elements and early diagenetic tracers (Mn, Fe, DOC). Stations located closest to the discharge area (U03, SR1), where residue thickness reached ~12 cm, showed porewater enrichments and significant upward diffusive Al fluxes (4.5 and 8.2 pmol m-2 s-1, respectively). Mn, Fe, Co, Ni, As and Pb exhibited similar upward fluxes. Porewater profiles further indicated that bauxite residues displaced the active early diagenetic zones downward, with enhanced Fe and Mn reduction occurring beneath the deposit layer. Overlying bottom waters at these stations displayed elevated Al concentrations (270-580 nmol L-1) compared to regional levels (50-150 nmol L-1). At station K (close to U3), persistent water Al concentration anomalies (330-500 nmol L-1) were observed between 2014 and 2023, extending from approximately 300-600 m depth down to the seafloor (~900 m). In contrast, the open-sea station (U11), characterized by a thin residue layer (~2 cm), showed limited diagenetic activity and no Al anomaly in bottom waters. Overall, historical residue deposits sustained sediment-water exchange of dissolved trace metals through early diagenesis remobilization, independently of active industrial inputs, with exchanges confined to the proximal section of the canyon.
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