Concealed history of microplastic pollution in the Msimbazi estuary, Dar es Salaam, Tanzania

This study reconstructed the historical deposition of microplastics in sediments of the Msimbazi Estuary, Tanzania, using a radiometrically dated sediment core. Sediment chronology established using ²¹⁰Pb and the Constant Initial Concentration (CIC) model provided an approximately 83-year depositional record. The ²¹⁰Pb age model suggested marked temporal variability in sedimentation rates, with peaks around 1961, 1998, 2013, and 2015, potentially reflecting hydrological extremes and catchment disturbances. Anthropogenic microparticles were detected throughout most of the sediment core, with concentrations ranging from below detection limit (BDL) to 34 items kg⁻¹ dry weight (mean 8.5 ± 3.2 items kg⁻¹), and their abundance generally increased in sediments deposited after the 1960s. Fibers (67.7%) and fragments (28.5%) were the dominant particle types, while particles80% of all recovered anthropogenic microparticles. ATR-FTIR analysis showed that the recovered anthropogenic microparticles comprised natural fibers (linen, 40%; cotton, 25%) and synthetic materials, with approximately 35% confirmed as true microplastics, including polyester, polyethylene, cellulose acetate, and cigarette-filter-related particles. A one-way PERMANOVA indicated significant differences in microparticle composition between shallow (0–50 cm) and deeper (>50 cm) sediment layers (p < 0.05). These findings indicate that the Msimbazi Estuary has served as a long-term sink for anthropogenic microparticles and provide the first radiometrically dated reconstruction of historical microplastic deposition in a tropical urban estuary in Tanzania.