Fangzhu Wu

• Since the invention of the first synthetic polymer in 1907, plastics have revolutionized industries but have also caused significant environmental challenges. Over 170 trillion plastic particles are estimated to float in the world's oceans. Once in the marine environment, these plastics fragment into smaller particles (microplastics, MPs, <5 mm) under environmental forces, contaminating ecosystems worldwide, including the remote Arctic. This thesis investigates the Norwegian Coastal Current (NCC), a key transport route for MPs from northern Europe to the Arctic Ocean. Using novel sampling devices and advanced micro-Fourier transform infrared microscopy (μFTIR), small MPs (SMPs, 11–300 μm) were analyzed in seawater and sediments, providing the first detailed assessment of their spatial and temporal distribution in the NCC. The results reveal the prevalence of SMPs from surface seawater to deep sediments, including layers deposited before the advent of plastics. Key findings include a relatively homogeneous horizontal distribution of SMPs in surface and subsurface seawater and significant variability in sediment concentrations (54–12491 MP kg⁻¹) across cores. SMP accumulation trends in post-1950 sediment layers varied, challenging their reliability as markers of the Anthropocene. A total of 21 polymer types were identified, with smaller size classes dominating, highlighting their ecological significance. Further analysis of data representativeness revealed significant variability in MP concentrations and polymer diversity across stations, emphasizing the need for standardized protocols to ensure reliable data. Despite being based on a single research cruise, this study provides a valuable snapshot of SMP distribution in the NCC. The findings critically evaluate current MP research practices and highlight the need for robust methodologies to improve the reliability of future studies.