Settling and Along-Isopycnal Subduction of Small Microplastics Into Subsurface Layers of the Western North Pacific Ocean

The vertical distribution of small microplastics (SMPs; 10-300 μm in size) and its relation to water masses were investigated through seawater sampling and hydrographic surveys from the sea surface to 1000 m in the North Pacific Ocean. The average ± standard deviation of SMP concentrations in 12 layers at four stations was 6910 ± 2620 particles m^-3. Concentrations were high in isopycnal layers between potential densities of 23 and 25σ_θ (100-300 m depths). Elevated concentrations were also frequently detected below the North Pacific Intermediate Water (NPIW), characterized by a salinity minimum around the 26.6-27.0σ_θ (approximately 600 m depth) isopycnal layers. A simple modeling approach to reproduce the observed SMP distribution suggested two pathways for SMPs floating in surface convergence zones. One pathway is the weak settling of SMPs of which the density becomes close to neutral, causing the along-isopycnal subduction from isopycnal layers outcropping at the sea surface to subsurface layers above the NPIW. Therefore, the global inventory of weakly settling near-neutral SMPs is expected to be high in the subsurface layers. Meanwhile, the strong settling via biological processes causes the other pathway from the surface euphotic layer to deep layers that never outcrop at the sea surface.