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Deep-sea currents are behind the ocean’s thickest piles of microplastics

Plastic is everywhere: high atop mountains, scattered through national parks, and floating in the Great Pacific garbage patch. Now, researchers have tracked our plastic waste in another place—the deep sea. To find out whether certain features on the sea floor had greater concentration of microplastics, researchers examined sediment samples from the Tyrrenhian Sea, off the coast of Italy.

They found an average of 41 pieces in each spoonful of sediment from continental shelves. That number dropped to just nine pieces deeper down on the continental slope. But when scientists sampled piles of sediment that build up in the deep ocean, adjacent to fast-flowing currents, they found 190 pieces of microplastics per spoonful of sediment, the highest concentration of microplastics from the sea floor to date, they report this month in Science.

That amount—which adds up to 1.9 million pieces of microplastic per square meter—is likely dumped by the fast-flowing currents, meaning deep-sea circulation plays a role in where microplastics are deposited. These currents also bring vital nutrients and oxygen to the sea floor, scientists say, suggesting microplastic hot spots could overlap with areas rich in biodiversity. As researchers embark on expeditions to survey different areas on the sea floor—like submarine canyons and fans—they may just find the next hot spot.