Identifying the Origins of PET Nanoplastics in the Abyssal South Atlantic Using Backtracking Lagrangian Simulations with Fragmentation

During an expedition in January 2020, nanoplastics were sampled at a depth of -5170m over Cape Basin in the South Atlantic Ocean. Using photo-induced force microscopy, it was identified that these were polyethylene terephthalate (PET) particles with an approximate diameter of 50 nm at different stages of degradation. Using a state-of-the-art Lagrangian 3D simulation that includes an idealized fragmentation scheme, we backtracked virtual particles from the sampling location to establish the possible origins of the PET nanoplastics. We performed several simulations with different fragmenting timescales, for 13 years from the date of sampling, using the GLO12V3R1 hydrodynamic data from Copernicus Marine Environment Monitoring Service. We found that the nanoparticles (smaller than 1μm) could not have fragmented into nanoplastics at the surface and reached the sampling location within 13 years. The most likely scenario is that these particles started to sink from the surface as microplastics (larger than 1μm) and then fragmented in the water column far from the surface. This suggests that the fragmentation happened through processes like slow thermal oxidative degradation or hydrolysis and not photodegradation. We also found that less than 0.2% of the virtual particles came from the coast and that the fragmentation timescale prescribed to the modelled particles, affects how they drift in the ocean by controlling the time they drift at different depths. This study contributes to understanding the fate and sources of nanoplastics suspended deep in the ocean.