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

[MIS02-P05] Reliable estimation of fine-sized microplastic deposition rates using marine sediment cores

*Michinobu Kuwae¹, Kazushi Kinugawa², Hinata Shingaki³, Miu Hiasa³, An Morimoto³, Hirofumi Hinata⁴ (1.Center for Marine Environmental Studies, Ehime University, 2.Super Science Course, Ehime University, 3.Faculty of Science, Ehime University, 4.Faculty of Engineering, Ehime University)

Keywords:microplastic, deposition rate, marine sediments, oil-freezing extraction, Seto Inland Sea, East China Sea

In laboratory experiments conducted for microplastic (MP) particle toxicity studies, most organisms exhibit particle toxicity at a concentration of 1000 mg/m³ in water (Isobe et al., 2019). It is predicted that by the 2060s, this concentration will be exceeded in some parts of the waters around Japan and the Pacific Ocean (Isobe et al., 2019). This may be a threshold which may mark the onset of full-scale marine ecosystem degradation on Earth. However, the majority of the particles used in these laboratory experiments are smaller than 300 µm, which is a size range that escapes conventional net sampling. Thus, the concentration and distribution of MPs in the ocean at sizes that induce particle toxicity, as well as their horizontal transport and removal processes onto the seafloor, remain unresolved. Predicting the future abundance of this particle size range is an urgent research priority.
Moreover, because the deposition rate of MPs of this size in marine sediments is unknown, the fate of 30% of the missing plastic from the ocean surface remains unexplained (Isobe et al., 2022). Meanwhile, the conventional heavy-liquid separation method used for MP extraction from sediments has limitations: it fails to remove diatom frustules and other particles that interfere as noise in microscopic FTIR analysis, and MPs co-precipitate with silt-size particles, preventing accurate measurements for concentration in sediment.
Here, we present an improved method for reliably estimating concentrations of sedimentary MPs by utilizing the lipid-freezing extraction method and the polyethylene marker-based quantification method. Additionally, we report recent annual deposition rates of small MPs in marine core samples collected from Beppu Bay, Osaka Bay, and the East China Sea.