[M-IS02] Ocean plastics, an earth science perspective

[MIS02-01] Strong and weak settling processes transporting small microplastics into the intermediate layer of the North Pacific

★Invited Papers

*Atsuhiko Isobe¹, Mao Kuroda¹ (1.Kyushu University, Research Institute for Applied Mechanics)

Keywords:small micropastics, Intermediate layers in North Pacific

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 were 6910 ± 2620 particles m^–3. Concentrations were high in isopycnal layers between potential densities of 23σ_θ and 25σ_θ and also rapidly increased below the North Pacific Intermediate Water (NPIW) with the salinity minimum around the 26.6–27.0σ_θ 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 strong settling via biological processes from the surface euphotic layer to deep layers that never outcrop at the sea surface, such as the NPIW. Meanwhile, the weak settling of SMPs of which density become close to neutral causes the other pathway, that is, the along-isopycnal subduction from isopycnal layers outcropping at the sea surface to intermediate layers. Therefore, the global inventory of weakly settling near-neutral SMPs is expected to be high in intermediate layers.