Transport and Fluxes of Microplastics to Deep-Sea Sediments via Turbidity Currents through the Congo Canyon

Plastic pollution is a growing global concern, with significant implications for marine ecosystems. While microplastics (<5 mm) are abundant in shallow marine environments, their transport pathways and fluxes to the deep sea remain poorly understood. Submarine canyons, such as the Congo Canyon off West Africa, act as major conduits for sediment and associated pollutants, including plastics, to the deep-sea environment. These canyons are frequently flushed by fast gravity-driven sediment flows called turbidity currents capable of transporting vast quantities of material over distances of >1,000 km. These are the longest sediment flows yet measured in action on Earth, and they eroded and carried a mass of terrestrial organic carbon similar to that buried each year in the global oceans. However, despite their significance in natural particle transport, it remains unclear how efficiently they carry anthropogenic particles, such as microplastics, to the deep sea.

This study presents the first dataset that directly measures microplastics transported by turbidity currents. A sediment trap moored 156 km offshore in the Congo Canyon, at a water depth of 2,172 m, captured sediments from eight (0.5-1 m/s) turbidity current events occurring between September and December 2019. Microplastics were extracted and analyzed for their number, size, shape, and polymer composition using Laser Direct Infrared (LDIR) imaging. Microplastic flux estimates were calculated to quantify the transport capability of these flows.

The results demonstrate that turbidity currents are highly efficient in transporting microplastics, with concentrations reaching up to 13,266 particles per kg of sediment. PET (polyethylene terephthalate) and rubber were the most abundant polymer types, likely due to their higher density and resistance to degradation. Variability in microplastic abundance across different flow events appears to be influenced by differences in sediment sources and flow dynamics. Annual fluxes of microplastics transported through the Congo Canyon are estimated to be approximately 50,000 kg, underscoring the significant role of turbidity currents in redistributing microplastics on the deep seafloor. These microplastics may accumulate in canyon floors and distal lobes, forming potential sinks.

This research provides critical insights into the mechanisms governing the deep-sea transport of microplastics and highlights the importance of submarine canyons in global plastic pollution dynamics.