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Distribution and Transport of Fragmenting Microplastics in a 3D Global Eulerian Model
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Scientists built a new computer model showing that as ocean plastic breaks into smaller and smaller pieces, it doesn't just float on the surface, it sinks hundreds of meters deep and spreads out, with the tiniest fragments (smaller than a human red blood cell) building up in the centers of major ocean current systems. This matters because these ultra-small plastic particles are the ones most likely to enter marine food webs and eventually end up on our plates, so understanding where they accumulate helps researchers better predict human exposure risks down the line.
Fragmentation, the breakage of matter into smaller pieces, is an important mechanism responsible for generating microplastics (MPs). We present the first global three-dimensional Eulerian model that resolves fragmentation alongside MP transport. The evolution of particle size is modeled as a transfer from larger- to smaller-size bins, governed by a fragmentation kinetics framework. Relative to a reference simulation without fragmentation, two distinct effects are identified: (1) the surface concentration field of MPs becomes horizontally dispersed, and (2) MPs sink to depths of 500 m where the reference simulation shows negligible concentration. The vertical shift can be explained by the loss of buoyancy when particle size decreases, which facilitates horizontal sub-mixed layer transport once the particles sink below 100 m depth. Neutrally buoyant particles (with diameter d < 1 um) are continuously produced in the ocean by the fragmentation of larger particles and accumulate in the major oceanic gyres. Ultimately, the concentration of these neutrally buoyant MPs peaks at the gyre centers, a behavior that is not captured by prior models. Furthermore, the globally integrated size spectrum exhibits a steepening power-law slope over time that continues to evolve throughout our 25-year simulation. Comparisons with the AOMI Level-3wm observational dataset demonstrate a meaningful improvement in predictive skill relative to previous models: including fragmentation elevates the spatial correlation between modeled and observed surface concentrations from 45% to 58%.
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Distribution and Transport of Fragmenting Microplastics in a 3D Global Eulerian Model
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Scientists built a computer model showing that ocean plastic doesn't just float on the surface, as it breaks into smaller pieces, it loses buoyancy and sinks hundreds of feet deep, spreading far beyond where we can easily see or measure it. This matters because it means the tiniest plastic fragments (small enough to potentially enter our bodies through seafood) are likely accumulating in ocean currents and depths we haven't been tracking well, suggesting our current estimates of microplastic exposure may be incomplete.
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