The reliability of microplastics (MPs) as chronostratigraphic markers in highly disturbed estuarine environments remains debated. This study investigates the vertical distribution, composition, and weathering of MPs in a sediment core (0–240 cm) from the Santos–Cubatão Estuarine System (SCES), a heavily industrialized tropical estuary in southeastern Brazil. MP analysis (µ-FTIR and SEM-EDS) was integrated with geochemical proxies (Hg, Cd, δ15N) and radiocarbon dating to reconstruct recent depositional history. The core exhibits a mean MP abundance of 3097 ± 451 items.kg−1, dominated by secondary fragments and fibers, primarily polyethylene, polystyrene, and polypropylene. The highest concentrations occur in the upper 35 cm, where MPs are positively correlated with industrial metal contaminants (Spearman's r > 0.69). MPs are also consistently detected in deeper layers (40–200 cm) that exhibit pronounced radiocarbon age inversions. In this mixing zone, MPs are interpreted as tracers of post-1950 final sediment deposition, indicating the most recent depositional overprint on older sedimentary matrices. Below 200 cm, the absence of MPs and the return of Hg and Cd to baseline levels indicate pre-industrial background conditions. These findings support a signal consistent with early 1960s industrial expansion. MPs therefore represent a valuable complementary proxy for reconstructing recent depositional histories in hydrodynamically complex, industrialized estuaries, supporting their potential as temporal markers in anthropogenically altered environments where traditional geochronological tools are limited.
