Transport and Burial of Microplastics by Turbidity Currents

Plastic pollution poses an increasing threat to marine ecosystems and human health. Microplastics, having sizes ranging from micrometers to millimeters, are known to be abundant on the ocean floor. However, the dynamics controlling their transport from the shelf to the deep ocean and their burial on the ocean floor remain largely unknown, particularly as the majority of produced plastics are less dense than water. Turbidity currents are thought to be one of the processes transporting microplastics, together with terrestrial sediments and organic carbon, to the deep ocean through submarine canyons. A combination of laboratory experiments and theoretical analysis investigated the dynamics controlling the transport and burial of buoyant microplastics by turbidity currents. When microplastics and surfactants were added to a turbidity current, the behavior was qualitatively similar. However, the averaged turbidity current propagation velocity was larger when buoyant microplastics are present, and a small amount of the buoyant microplastics was found on the bottom. The microplastics were found to have sediments attached to it, which we hypothesize caused it to sink to the bottom. Although the mechanisms causing the conglomeration of sediments and microplastics are still unknown, a hypothesis is that surface forces between the sediments and microplastic particles allow the two to stick together and form conglomerations. In addition, the conglomerations have a lower density than the sediments allowing the turbidity current to maintain a substantial velocity for longer times.