Underwater avalanches can transport microplastics from land into the deep ocean, new research has found.
Over 10 million tons of plastic pollution ends up in the world’s oceans each year, and it’s thought that around 99% is stored in the deep sea.
However, it was previously not known how plastic pollution gets there. The new research, published in Environmental Science & Technology, found that microplastics can be moved by sediment flows, which can travel thousands of kilometers across the seafloor.
It was discovered during a collaborative research project between the Universities of Manchester, Utrecht, and Durham, and the National Oceanography Centre. Scientists looked at how these flows, the largest on earth, are responsible for sorting different types of microplastics – burying some, and moving others vast distances across the sea floor.
For their experiments, scientists mixed sand with microplastic fibres and fragments inside a tank that was designed to simulate real-world sediment flows.
Concentrations of microplastic fragments were concentrated in the lower parts of the flow while microplastic fibres were distributed throughout the flow and settled more slowly.
The larger surface to volume ratio of fibres is thought to be the reason they are more evenly distributed. The high concentration of microplastic fibres in sand layers at the base of the flow is thought to be because they get more easily trapped by sand particles.
The scientists say the distribution of different types of plastic on the seafloor is important because the size and type of plastic particle determines how toxins build up the surface, as well as how likely it is the plastic will enter the gut of any animal that eats it, and what animal may eat it.
Dr Ian Kane said: ‘This is in contrast to what we have seen in rivers, where floods flush out microplastics; the high sediment load in these deep ocean currents causes fibres to be trapped on the seafloor, as sediment settles out of the flows.’