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Emerging and legacy pollutants in abyssal sediments of the world’s largest enclosed waterbody
Summary
Sediment samples from the abyssal zones of the western Pacific revealed both legacy pollutants (PCBs, DDT) and microplastics at significant depths, showing that even the most remote deep-sea environments are contaminated by human-made chemicals. The co-occurrence suggests microplastics may serve as carriers transporting persistent organic pollutants to the deep ocean.
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 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), 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.