What are plastic sinks? A comprehensive framework and functional typology

The global ocean receives an estimated 15 teragrams of plastic annually, yet less than 0.2% of this mass is detected floating at the surface. This imbalance, known as the "missing plastic paradox," indicates the presence of diverse plastic sinks, environments and processes that accumulate and retain plastic debris temporarily or permanently. This paper integrates geomorphological, ecological, and anthropogenic perspectives to develop a comprehensive classification of plastic sinks in coastal and marine environments. Four functional categories are identified: functionally permanent sinks (e.g., deep-sea sediments, mangrove peat) where burial and diagenetic conditions promote long-term sequestration potential under current low-energy depositional regimes; semi-permanent sinks (e.g., coral reefs, backshores, driftwood accumulations) that retain plastics through structural or biophysical trapping; transient sinks (e.g., garbage patches, litter windrows, the water column, estuaries) that act as short-lived reservoirs where net positive plastic storage (ΔS > 0) is maintained over hours to months; and anthropogenic sinks (e.g., coastal landfills, port basins, dumps) that function as engineered or accidental retention systems. Two short-term but critical mechanisms are emphasized: litter windrows, submesoscale convergence lines that concentrate floating plastics prior to deposition or sinking, and plastic litter blooms, extreme-runoff events that deliver rapid pulses of debris from land to coastal systems. These mechanisms show that plastic retention operates across natural and human domains and spans timescales from hours to centuries, shaping the fate of plastics in the ocean.