River floods in the Anthropocene impact sea-floor geochemistry, pollutants and bacterial communities in coastal systems

Abstract. This study examines the sedimentary and microbial responses offshore the Marche Region (Italy) to the September 2022 flood, one of the most severe recent hydrological events, which delivered large amounts of sediment and anthropogenic contaminants to the Adriatic Sea. We employed a multidisciplinary approach integrating sedimentology, geochemistry, organic matter analysis, pollutant assessments (Polycyclic Aromatic Hydrocarbons, PAHs and Poly- and Perfluorinated alkyl substances, PFASs), and benthic microbial community structure. Sediments collected five days post-event offshore six river mouths reveal that flood deposits, ranging from fine sand to coarse silt, were largely confined within the nearshore zone down to the 15 m isobath. This distribution reflects intense riverine inputs and a brief windstorm-enhanced coastal circulation that generated patchy, temporary sediment accumulations in the prodelta sector. Heavy metal concentrations remained below regulatory thresholds, whereas organic pollutants were heterogeneously distributed, with peaks offshore urban and industrial zones. PAH signatures indicate mixed pyrogenic and petrogenic sources, while next-generation PFASs (6:2FTS) showed localized but severe contamination linked to upstream industrial activities. Simultaneously, the flood introduced strong spatial heterogeneity in benthic bacterial communities, with sediment texture and organic matter content driving compositional shifts. Freshwater-associated taxa became prominent in offshore deposits, highlighting riverine sedimentary imprints. Despite the flood's magnitude onshore, its offshore sedimentary and ecological signatures were spatially limited and ephemeral. These findings underscore the ecological significance of episodic sediment and contaminant inputs, while highlighting the challenges in detecting such transient events in the marine stratigraphic record.