Morphology-Dependent Transport Potential and Sedimentary Distribution of Microplastics in an Abyssal Pacific Basin

Microplastic (MP) transport through the deep ocean remains poorly constrained, particularly regarding particle morphology and water-column biogeochemistry. We quantified MP abundance concentrations in the water column, expressed as items/L, and sedimentary MP inventories, expressed as items/kg dry weight (dw), in two full-depth profiles to 5650 m and characterized MP inventories in surface sediments from 33 stations in the western Pacific Pigafetta Basin. MPs occurred throughout the water column (0.7 ± 0.4–7.9 ± 2.8 items/L; mean, 3.0 ± 1.5 items/L) and generally decreased from euphotic to abyssal and bottom waters. Apparent oxygen utilization maxima and enhanced nutrient regeneration occurred at 800–1000 m, whereas MP abundance was negatively correlated with apparent oxygen utilization and potential density. This pattern is consistent with changes in MP-associated carrier phases and suspension states rather than local accumulation within the remineralization maximum. Fibers and fragments dominated the water column, while surface sediments contained relatively more fibers and smaller particles, indicating compartment-level differences in morphology and size. Sediment abundance ranged from 6.5 to 180.1 items/kg dw. A conservative, Fourier-transform infrared spectroscopy (FTIR)-constrained, morphology-resolved free-particle settling model estimated a mean sinking velocity of 17.6 m/d and a modelled free-particle vertical transport potential of 2.82 × 104 items/m2/d, with non-fibers contributing 87%. This transport potential is distinct from depositional flux and burial flux, neither of which was directly measured in this study. These modelled values represent transport potential under specified free-particle assumptions and are not direct measurements of depositional flux, burial, or long-term sequestration. The results identify morphology-dependent settling and carrier-phase transformation as processes that may influence the distribution of MPs between the water column and surface sediments.