Sequestration of plastics in sediments is considered the ultimate sink of marine plastic pollution that would justify unexpectedly low loads found in surface waters. Here, we demonstrate that mangroves, generally supporting high sediment accretion rates, efficiently sequester plastics in their sediments. To this end, we extracted microplastics from dated sediment cores of the Red Sea and Arabian Gulf mangrove (Avicennia marina) forests along the Saudi Arabian coast. We found that microplastics <0.5 mm dominated in mangrove sediments, helping explain their scarcity, in surface waters. We estimate that 50 ± 30 and 110 ± 80 metric tons of plastic may have been buried since the 1930s in mangrove sediments across the Red Sea and the Arabian Gulf, respectively. We observed an exponential increase in the plastic burial rate (8.5 ± 1.2% year−1) since the 1950s in line with the global plastic production increase, confirming mangrove sediments as long-term sinks for plastics.

Despite the high attention given to the amount of plastic debris floating in the open ocean, these only accounts for a small fraction of the plastic annually entering the marine environment. It is widely believed that most marine plastic sinks in the water column and ends up buried in sediments where it remains unless disturbed. While many case studies have assessed the stocks of plastic in marine sediments, from intertidal habitats to the deep ocean, none has attempted to synthetize the published data and calculate a global estimate. Here, we compiled a dataset of published stocks of plastic in marine sediments and used it to calculate the global stock to ultimately demonstrate that marine sediments are a major sink of plastic in the global ocean.

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