What if the "great ocean garbage patches" were just the tip of the iceberg? While more than 10 million tonnes of plastic waste enters the sea each year, we actually see just 1 percent of it – the portion that floats on the ocean surface. What happens to the missing 99 percent has been unclear for a while.
Plastic debris is gradually broken down into smaller and smaller fragments in the ocean, until it forms particles smaller than 5 mm, known as microplastics. Our new research shows that powerful currents sweep these microplastics along the seafloor into large "drifts", which concentrate them in astounding quantities.
We found up to 1.9 million pieces of microplastic in a 5 cm-thick layer covering just one square metre – the highest levels of microplastics yet recorded on the ocean floor.
While microplastics have been found on the seafloor worldwide, scientists weren't sure how they got there and how they spread. We thought that microplastics would separate out according to how big or dense they were, in a similar manner to natural sediment. But plastics are different – some float, but more than half of them sink.
Plastics which once floated can sink as they become coated in algae, or if bound up with other sticky minerals and organic matter.
Recent research has shown that rivers transport microplastics to the ocean too, and laboratory experiments revealed that giant underwater avalanches of sediment can transport these tiny particles along deep-sea canyons to greater depths.
We've now discovered how a global network of deep-sea currents transports microplastics, creating plastic hotspots within vast sediment drifts. By catching a ride on these currents, microplastics may be accumulating where deep-sea life is abundant.