A critical assessment of copper and zinc isotopes as tracers for antifouling paint contamination in coastal sediments.

Antifouling paints (AFPs) are a major source of copper (Cu) and zinc (Zn) to coastal environments yet tracing their contribution to sediment metal contamination remains challenging due to overlapping sources and complex biogeochemical processing. We combined field observations, sediment core analysis, and in situ exposure experiments to evaluate the potential of Cu and Zn stable isotopes as tracers of AFP-derived contamination in a tropical marina (Bracuhy Marina, Brazil). Elemental data from a geochemically constrained sediment core reveal pronounced Cu and Zn enrichment following marina operation, superimposed on a natural baseline defined by rare earth element systematics and regional reference materials. In contrast, Cu and Zn isotopic compositions display substantial variability and do not follow simple binary mixing between AFPs and natural sediment sources. In situ experiments document significant isotopic shifts during metal release from paints and transfer across the paint-water-sediment interface. Zinc isotopes exhibit preferential retention of lighter isotopes in sediments, whereas Cu isotopes show greater isotopic variability, reflecting the influence of sedimentary biogeochemical processes. The isotopic signatures observed in sediments are consistent with a combination of adsorption, mineral interactions, redox-sensitive reactions, and post-depositional remobilization, although the relative importance of these mechanisms could not be directly constrained. Consequently, sediments exhibiting strong metal enrichment may retain isotopic compositions overlapping with the natural variability range, indicating a partial decoupling between metal concentrations and isotope signatures. These results demonstrate that Cu and Zn isotopes provide valuable insights into metal cycling in marina environments, but their application as quantitative tracers of AFP-derived contamination requires explicit consideration of isotope fractionation and post-depositional processes. A compartment-specific and process-oriented framework is therefore necessary for the robust interpretation of isotopic signatures in AFP-impacted coastal systems.