Marine microplastics are widely observed across the global ocean, yet their source-to-sink fate remains difficult to resolve because anthropogenic supply, physical redistribution, vertical removal and sampling heterogeneity are tightly confounded. Here we develop a regime-resolved process-identifiability analysis to determine where microplastic patterns reflect separable mechanisms and where they remain observationally non-identifiable. Rather than fitting a single global distribution model, we decompose microplastic occurrence and accumulation intensity into source supply, hydrographic background, surface convergence, vertical redistribution and mesoscale-retention components, while explicitly accounting for seasonality, spatial autocorrelation and cruise-level observation structure.

We show that the dominant uncertainty in marine microplastic fate is not simply abundance, but mechanistic separability. Coastal and semi-enclosed waters exhibit source saturation and strong observation dependence, limiting inference on short-term physical redistribution. Gyre systems show transferable first-order physical controls, consistent with large-scale convergence and mixed-layer dilution, whereas higher-order process coupling improves local explanation but weakens spatial transferability. Frontal and boundary-current regions display the strongest identifiable physical coupling, where convergence, deformation and eddy retention jointly organize accumulation hotspots. These findings reveal that global microplastic fate cannot be represented by a universal driver hierarchy. Instead, source-to-sink mechanisms are regime-dependent, unevenly identifiable and constrained by observation structure. Our framework advances marine microplastic research from distribution mapping toward transferable mechanistic diagnosis of transport, retention and removal pathways.