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Microplastics are an emerging hazard in the marine environment, and considered to eventually sink into sediments. An investigation into the long-term variation of microplastic accumulation in sediment cores is essential for understanding the historical trend of this contamination and its response to human activities. In this study, the multidecadal changes of microplastic abundances in two sediment cores from the inner shelf of the East China Sea (ECS) were revealed by two methods, i.e., a visual enumeration method based on scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and a quantitative method based on microplastic-derived carbon (MPC) abundances. The features of microplastics were determined via SEM-EDS and micro-Fourier transform infrared spectroscopy (μ-FTIR). The results reveal a multidecadal increasing trend of microplastic accumulation in the coastal sediments of the ECS since the 1960s, which may be jointly governed by the release of plastic wastes and oceanographic dynamics. Meanwhile, the breakpoint of the exponential growth of microplastics in the ECS occurs in 2000 AD, which well matches the rapid increasing of plastic production and consumption in China. Further, based on the MPC contents in sediments, the influence of microplastics on the quantitative evaluation of carbon storage in the ECS has been examined for the first time, revealing an insignificant (<2% before 2014 AD) but potentially-increasing (6.8% by 2025 AD) contribution of microplastics to carbon burial. Our results may provide the important data for evaluating and mitigating the impact of microplastics on the marine environment. Image 1 • A multidecadal increasing trend of microplastic accumulation was revealed in sediment cores in the ECS since the 1960s. • High abundances of microplastics were detected in the form of fragments with the prevalent size of <100 μm. • Microplastic accumulation in sediments in the ECS is associated with Chinese plastic production and consumption. • Microplastics have made an insignificant but potentially-increasing contribution to sedimentary carbon storage in the ECS. [ABSTRACT FROM AUTHOR]