Persistent organic pollutant accumulation in Pacific abyssal plain sediments and biota: Implications for sources, transport, and deep-sea mining

Despite its vast size and ecological and economic importance, the deep sea is among the least understood ecosystems on Earth. While much remains to be discovered, researchers have established that the deep sea is being exposed to numerous anthropogenic factors including chemical pollution. Persistent organic pollutants (POPs), defined by their persistence in the environment, bioaccumulation, and toxicity, are continually discharged and transported into the deep sea despite efforts to ban or reduce their production under global and regional conventions. Here, we provide the first evidence of these POPs in sediment and biota in the Pacific abyssal plain, an area currently targeted for deep-sea mining. Sediment and fish tissue (Coryphaenoides sp., a deep-ocean predator and scavenger fish) collected from two sites in the eastern Clarion-Clipperton Zone of the Pacific abyssal plain were tested for three POPs: polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and per- and polyfluoroalkyl substances (PFAS). Similarities between the sites in PCB congener concentrations suggested that PCBs were distributed evenly over the region. Conversely, higher variability in PBDE congener concentrations and PFAS concentrations from the same sites suggested that these chemicals had more patchy distributions across the region. Sediment PFAS were especially variable, detected at a high concentration (1.5 ng/g wet weight) in only one of five samples but measured in all fish muscle samples at levels comparable to some freshwater ecosystems. Results suggest that raining particulates (e.g., particulate organic matter and microplastics) dominate vertical transport of PCBs, resulting in more uniform distribution, while episodic events such as carrion-fall and vertically migrating species may drive PFAS transport processes resulting in patchy spatial distributions and differences in scavenging biota versus sediment. Unintentional PCBs (e.g., pigment components) comprised a large proportion of total PCBs in sediment and muscle tissue, suggesting that unregulated PCB releases are accumulating in the deep Pacific Ocean.