Settling and along-isopycnal subduction of small microplastics into intermediate layers over the North Pacific Ocean

A candidate of the fate of ocean plastics is fragmentation to small microplastics unmonitorable in the current observation framework. In the present study, the vertical distribution of small microplastics (S-MPs; 10 μm < sizes < 300 μm) and its relation to water masses were investigated by seawater samplings and hydrographic surveys from the sea surface to 1000-m depth in the North Pacific Ocean. The average ± standard deviation of S-MP concentrations in 12 layers of four stations were 6910 ± 2620 particles m^–3. The S-MP concentrations become high in isopycnal layers between potential densities of 23 σ_θ and 25σ_θ and again rapidly increased beneath the North Pacific Intermediate Water (NPIW) with the salinity minimum around 26.6–27.0σ_θ isopycnal layers. A simple model approach to reproduce the observed S-MP distribution suggested two pathways for S-MPs originally floating in surface convergence zones. One is the fast settling of S-MPs via biological processes from the surface euphotic layer to deep layers which are never outcropped at the sea surface like NPIW. Meanwhile, the weak settling of S-MPs of which density become close to neutral causes the other pathway, that is, the along-isopycnal subduction from isopycnal layers outcropped at the sea surface to intermediate layers. In fact, microplastic concentrations at the sea surface between 23σ_θ and 25σ_θ contour curves (archived in the AOMI database) become larger than those in the surrounding isopycnal layers in a similar fashion to the S-MP vertical profile. S-MPs are also abundant in the same surface area along with MPs, as is likely due to the surface convergence, so that the surface S-MPs are suggested to migrate below the sea surface by the along-isopycnal subduction. The global inventory of near-neutral S-MPs with weak settling is expected to be greater in intermediate layers, requiring carefully designed field surveys for both sampling and subsequent processing.