Microplastics (MPs) preserved in sediment cores provide valuable archives for reconstructing historical contamination and polymer weathering. However, chronologically constrained studies integrating MP abundance, characteristics, polymer composition, and degradation remain scarce in tropical coastal environments. Here, we present the first comprehensive study combining ²¹⁰Pb geochronology, MP characterization, and polymer degradation assessment in sediment cores. Eight sediment cores were collected along an inshore-to-offshore transect from Rayong, Thailand. Sediment chronologies were established using the Constant Rate of Supply (CRS) model, and oxidative degradation of PE and PP was evaluated using FTIR-derived oxidation indices. Sediment composition differed among cores (Kruskal–Wallis, p < 0.001; PERMANOVA, R² = 0.855, p = 0.001), reflecting a pronounced inshore-to-offshore gradient. Total ²¹⁰Pb activity also varied among cores (p <  0.001), reaching
2.79 ± 1.01 dpm g⁻¹ in the offshore core, whereas CRS-derived sediment accumulation rates ranged from 0.05 to 0.14 cm yr⁻¹ and reconstructed a return back ~136 years (~1889–2025). Total polymer abundance (including natural cellulosic polymers, semi-synthetic polymers, and microplastics) and only MP abundance (Wilcoxon, p < 0.001) decreased with depth (ρ ≈ −0.3, p < 0.001). Fibers, red particles, and the 1000 - < 3000 µm size class dominated the assemblage, whereas resin and polyester were the dominant MP polymer categories after natural and semi-synthetic polymers. FTIR analyses indicated oxidative weathering of both PE and PP; PP exhibited slightly higher depth-related trends than PE (p > 0.05). These findings demonstrate that dated sediment cores provide robust archives for reconstructing historical MP deposition and offer a framework for assessing long-term plastic accumulation and persistence in tropical coastal ecosystems.

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