Identifying the origins of nanoplastics in the abyssal South Atlantic using backtracking Lagrangian simulations with fragmentation

During an expedition in January 2019, nanoplastics were sampled at a depth of −5,170 m over Cape Basin, in the South Atlantic Ocean. Using photo-induced force microscopy, it was suggested that these were polyethylene terephthalate (PET-like) particles with various sizes down to 100 nm, at different stages of degradation. By using a state-of-the-art Lagrangian 3D model, which includes fragmentation, we backtracked virtual particles to map the origin of the PET nanoplastics sampled at this location. Fragmentation processes are crucial to understanding the origin of nanoplastics (and microplastics) because they allow for detecting when and where particles become so small that they transition to a colloidal state, in which the buoyant force becomes negligible. We found that it is very unlikely that the nanoplastic particles entered the ocean as nanoplastics and then drifted to the sampling location. We also found that the fragmentation scheme, particularly the fragmentation timescale prescribed to the modeled particles, affects how they drift in the ocean by the velocity with which they sink. This study contributes to understanding the fate and sources of nanoplastics in the deep ocean and the development of 3D backtracking simulations for source attribution of ocean plastic.