Transport and Settling of Microplastics in Turbidity Currents

Although tens of millions of tons of plastic waste are released into the ocean each year, less than 300 kilotons remain on or near the ocean surface. This is particularly puzzling because more than half of plastics that are produced are buoyant in sea water. One mechanism that can result in buoyant plastic settling is the process of biofouling in which microbes and other organic material can accumulate on the plastics rendering them more dense. Less studied is the accumulation of inorganic material on the plastics. For example, clay has recently been shown to attach to plastics, particularly in the presence of surfactants. Here we report on laboratory experiments showing that plastic particles which are less dense than fresh water can settle due to the accumulation of glass spheres ("sand") on their surface. This process is shown to occur dynamically as sand and plastic particles mix turbulently during the impulsive release of a turbidity current, which can carry some of the plastic particles to depth along with the settling sand. [This work reports on experiments performed by Woods Hole Oceanographic Institute (WHOI) Geophysical Fluid Dynamics (GFD) Fellow Quentin Kriaa during the WHOI GFD Summer Program 2023, co-supervised by Claudia Cenedese and Jim McElwaine.]