Heterogeneous seafloor deposition of heavy microplastics in the North Pacific estimated over 65 years.

Marine plastic pollution has been a public concern for many decades; however, transport processes of heavy microplastics to the seafloor have long been overlooked given the difficulties in sampling and modeling. The distribution of heavy microplastic deposition on the seafloor in the North Pacific for 65 years since 1951, was estimated using a particle tracking model with 577,143,840 particles. The model revealed that 22 % of heavy microplastics were deposited over 100 km offshore from their release locations. Strong currents, including the Kuroshio Current and Equatorial Counter Current, advected the heavy microplastics offshore; however, the behaviors of different-sized microplastics with different sinking velocities made the seafloor deposition heterogeneous. The seafloor was separated into six (three) clusters based on the origins (composition of sinking velocity categories) of the deposited microplastics. Deposited microplastics showed a rapid increase since the 2000s even for the open ocean far from emission sources.