[M-IS17] Ocean Plastics, an earth science perspective

[MIS17-06] Deposition rates of small microplastics for the last 70 years in Beppu Bay, southwest Japan

*Michinobu Kuwae¹, Takuma Masuhara², Hirofumi Hinata³ (1.Center for Marine Environmental Studies, Ehime University, 2.Faculty of Science, Ehime University, 3.Faculty of Engineering, Ehime University)

Keywords:small microplastic, deposition rates, Beppu Bay, oleo-extraction

A mass influx of mismanaged plastics into the ocean and the resultant degradation of marine ecosystems through the misfeeding of plastics for organisms are ongoing concerns. Laboratory-based studies show that microplastics (MP) of various sizes cause the impedance of feeding at concentrations exceeding 1000 mg/m³. Numerical experiments suggest that such concentrations will be reached in the surface layer in several areas of the North Pacific in 50 years (Isobe et al., 2019). However, ocean observations and numerical experiments to date have not studied microplastics smaller than 300 µm, which have been treated in laboratory experiments, and the current budget of small microplastics (SMP) in the ocean, including the amount of microplastics in each reservoir and fluxes between reservoirs, remains unknown. Therefore, numerical modeling for the long-term predictions of the SMP in the ocean still has not been made. Long-term records of SMP deposition rates on the seafloor, which is the final sink, could be reconstructed from marine sediments. Such records play an important role in the validation of the MP budget in numerical experiments and in the prediction of sMP abundance in the surface layer of the ocean.
Recently, we have attempted to estimate deposition rates of SMPs using marine sediments. A previous study (Hinata et al., 2022) has estimated deposition rates of MP larger than 300 µm in Beppu Bay over the past 70 years, but not analyzed sMP (<300 µm size fraction). For the upper 70 cm of the sediment core sample from Beppu Bay, we extracted SMPs by the oleo-extraction method (Corami et al., 2021), identified, and counted by micro-FTIR to determine the concentrations and deposition rates of SMPs over the past 70 years. Here we describe the characteristics of the temporal changes and discuss the usefulness of the oleo-extraction method in reconstructing historical SMP deposition using marine sediments.