In a swath of seafloor measuring about the size of a coffee table, scientists have revealed that nearly 2 million tiny pieces of fragmented plastic can accumulate at some of the deepest ocean depths in what is now the highest level of microplastic ever recorded from the bottom of the ocean.
Deep-ocean currents have the ability to accumulate and transport 1.9 million pieces of plastic per 1 square meter (about 11 square feet) of the seafloor, new research suggests. Such “microplastic deposits” may be the deep ocean equivalent of “garbage patches” formed by surface currents around the planet.
"Almost everybody has heard of the infamous ocean 'garbage patches' of floating plastic, but we were shocked at the high concentrations of microplastics we found in the deep-seafloor,” said lead author Dr Ian Kane of The University of Manchester in a statement.
"We discovered that microplastics are not uniformly distributed across the study area; instead they are distributed by powerful seafloor currents which concentrate them in certain areas."
More than 10 million tons of plastic waste is estimated to enter the ocean every year, 99 percent of which is thought to accumulate in the deep ocean. To determine the extent of this accumulation, an international team of scientists collected sediment samples from the Tyrrhenian Sea, a part of the Mediterranean Sea located off of the Italian coast, at depths between 600 and 900 meters (2,000 and 3,000 feet) where currents have the greatest interaction with the seafloor. Microplastics were separated from sediment in the lab and then counted under a microscope using infrared spectroscopy to determine individual plastic types. Measurements of microplastic were then combined with models of deep ocean currents and mapping of the seafloor to determine where large deposits might accumulate.
Microplastics are known to pervade the seafloor, but the process by which they get there has remained largely unknown. However, it appears that deep-sea currents act as conveyer belts that transport tiny pieces of plastic across the seafloor, according to findings published in Science.
Nearly every sample analyzed was found to contain microplastics, chiefly those made up of fibers from textiles and clothing that are not filtered out through wastewater treatment plants. Once these plastic fragments enter the ocean, they settle slowly or can be transported through turbidity currents that act as underwater avalanches, pushing large volumes of water down submarine canyons to the seafloor. Once there, bottom currents can further move plastic around the ocean, which presents a potentially larger issue.
Deep-sea currents bring oxygenated water and nutrients from the upper parts of the water column to hard-to-reach areas in the deep ocean that are often important ecosystems for poorly understood or little-documented organisms.
"Our study has shown how detailed studies of seafloor currents can help us to connect microplastic transport pathways in the deep-sea and find the 'missing' microplastics. The results highlight the need for policy interventions to limit the future flow of plastics into natural environments and minimise impacts on ocean ecosystems,” said study co-lead Dr Mike Clare of the National Oceanography Centre.
Microplastics have become a “new type of sediment particles” due to their high prevalence in mixture with sand, mud, and other nutrients that allow them to be transported across the seafloor to form large sediment deposits known as contourite drifts. Understanding how microplastics move throughout the marine environment can help scientists to predict the locations of other potential hotspots and their impact on life in our oceans.