Centennial records of small microplastics in the East China Sea: Temporal evolution and implication for sediment...

Small microplastics are abundant in marine sediments but remain poorly resolved in long-term sedimentary records. Here, we used laser direct infrared imaging to quantify 20-500 μm microplastics in two dated sediment cores from the Yangtze River Estuary-East China Sea continuum. Particles <100 μm accounted for more than 90% of all detected microplastics in both cores, indicating that small microplastics dominate the sedimentary inventory and have likely been underestimated in previous records. The two cores showed distinct proximal-distal differences. The Zhejiang-Fujian coastal mud depocenter contained higher microplastic abundance than the Yangtze subaqueous delta, whereas polymer composition differed markedly, with polyurethane dominating the distal core and polyamide dominating the proximal core. These contrasts indicate that sedimentary microplastic burial is shaped by source-to-sink transport and depositional setting across the estuary-shelf continuum. The centennial records were divided into four stages: pre-1949, 1949-1980, 1980-2000, and post-2000. Stepwise increases and major inflection points broadly coincided with transitions in China's plastics industry, whereas pronounced post-2000 fluctuations likely reflect the basin-scale engineering activities and hydrodynamic disturbance. Although the first occurrence of microplastics is not a reliable isochronous marker, transitions in dominant polymer abundances provide useful complementary constraints for evaluating ²¹⁰Pb/¹³⁷Cs-based sediment chronologies. These results demonstrate that small microplastics provide sensitive sedimentary records of anthropogenic activity and can improve centennial-scale interpretation of microplastic burial on river-dominated continental margins.