LAUSR

Nowadays, people have known more about the distribution features on the surface of beaches. However, the understanding on the microplastics (Mps) distribution in the vertical direction on beaches is still lacking. This study analysed the vertical distribution of Mps from five beaches, with one long core [Shilaoren Beach (C1), 8 m] and four short cores [Aoshan Beach (C2); Liuqinghewan Beach (C3); Bathing Beach No. 3 (C4); and Golden Beach (C5); approximately 70 cm]. An increasing trend of Mps from bottom to top layers were observed with an average of 16.6 ± 4.8 Mps/25 g d.w. in C1. Conversely, a fluctuating trend was found in C2-C5, with an average Mps abundance on the 70 cm depth to be 6.7, 7.1, 11.1 and 7.0 Mps/25 g d.w., respectively. These Mps were mainly comprised of fibres (>98%) and few were fragments. Mps were not uniformly distributed within all cores, and this variation was due to changes in Mps sources, hydrodynamics and sedimentation process in the beach environment. Although the vertical accumulation of Mps is not as stable as the other sedimentary environments, it still presents the increasing trend in the 8-m core from the bottom to the top, which is consistent with the plastic production history in China since 1950. Therefore, the 8-m core has been deposited on the beach in the recent 70 years. Thus, this study provides a valuable example for tracing the sedimentation history on the beach, which can help in understanding the sediment deposit and transport processes with time on beaches.