Geochronological variations of PAHs in sediments from a preserved tropical estuary, Northeast Brazil

Historical PAHs contamination is well documented in urbanized estuaries; however, their dynamics in tropical ecosystems considered relatively preserved remain poorly investigated. This study reconstructs the historical deposition of PAHs in the Serinhaém Estuary, a tropical Environmental Protection Area in Northeastern Brazil, quantifying their concentrations, distinguishing their sources, and assessing the associated ecological risks. To achieve these objectives, five sediment cores were collected, dated using ²¹⁰Pb geochronology, and analyzed by gas chromatography. Diagnostic ratios and multivariate statistical analyses were applied to identify contaminant sources and distribution patterns. The sedimentation rate was approximately 2.5 ± 0.1 mm·year^-1. Results showed that cores C1 to C4 were dominated by high-molecular weight PAHs of predominantly pyrogenic origin, likely associated with regional biomass burning. In contrast, core C5, located downstream, displayed low concentrations with petrogenic signatures, suggesting inputs related to maritime activities. Core C3 exhibited the highest total concentration (Σ16PAHs = 1508 ng·g^-1), with a pronounced peak in the early 2000s. In this layer, compounds such as fluoranthene (154.36 ng.g^-1), benzo[a]anthracene (138.55 ng.g^-1), chrysene (138.91 ng.g^-1), benzo[a]pyrene (153.86 ng.g^-1), and dibenzo[a,h]anthracene (42.64 ng.g^-1) exceeded threshold effect levels (TEL), indicating potential ecological risk. Multivariate analyses confirmed both vertical and lateral heterogeneity, highlighting specific episodes of increased contaminant input. Although overall concentrations ranged from low to moderate, distinct contamination events were identified, reflecting contributions from both diffuse and localized sources. These findings demonstrate that even legally protected estuaries remain vulnerable to anthropogenic pressures. Reconstructing sedimentary records proves essential for understanding the history of contamination and underscores the need for continuous monitoring to support effective conservation strategies.