Emerging and legacy pollutants in abyssal sediments of the world’s largest enclosed waterbody

This study provides the first integrated assessment of emerging (microplastics, MPs) and legacy (organochlorine pesticides, OCPs; polychlorinated biphenyls, PCBs; and polycyclic aromatic hydrocarbons, PAHs) contaminants in abyssal sediments ( > 700 m) of the southern Caspian Sea. Vertically resolved cores (0–6 cm) were analyzed to determine compound-specific distributions and persistence. MPs were found only in the surface layer (0–2 cm) at 16 items kg −1 dw, composed exclusively of polyethylene (PE) and polyethylene terephthalate (PET) fibers, indicating restricted downward transport. OCPs such as 4,4 ′ -DDT and Dieldrin declined sharply with depth, whereas transformation products (4,4 ′ -DDE) and recalcitrant compounds persisted. PCBs were confined to surface sediments, while total PAHs ( ∑ PAHs) increased with depth (49–68 ng g −1 ), reflecting stronger contributions from natural petrogenic sources associated with pre-industrial conditions. These results show that the southern Caspian abyss functions as a long-term sink for both emerging and legacy contaminants, governed by hydrographic isolation, weak bottom circulation, and sediment focusing. The co-occurrence of MPs and persistent hydrocarbons underscores the role of enclosed basins in global contaminant storage and long-term pollutant fate.