The study suggests that this is over 35 times the amount of plastic thought to float on the surface.
The researchers say this work is the first global estimate of microplastics – pieces of plastic that have been eroded by the elements into tiny bits, smaller than 5 mm (0.19 in) – at the sea floor.
Using a robotic submarine, the team collected samples of deep-sea sediments up to 3,000 meters (9,800 feet) from six sites in the Great Australian Bight, 380 kilometers (236 miles) off the coast of South Australia.
An analysis of the 51 samples taken found an average of 1.26 microplastics per gram of sediment. This is 25 times more microplastics, the researchers say, compared to previous deep-sea studies.
“Even the deep ocean is vulnerable to the problem of plastic pollution,” said Justin Barrett of CSIRO’s Oceans and Atmosphere Organization, who led the study, in a statement.
“The plastic pollution that ends up in the ocean degrades and decomposes and ends up in the form of microplastics,” she said. “The results show that microplastics are already sinking to the ocean floor.”
“They were surprised to see large quantities of microplastics in such a remote location,” said Dr Dennis Hardesty, lead research scientist and co-author of the study.
Research shows that plastic can build up on the sea floor by sinking into the water column or from currents that transport microplastics down underwater grooves to the deep sea floor.
“Our research found that the depths of the ocean is a reservoir for microplastics,” said Hardesti. “By determining the location and quantity of the plastic particles, we get a better picture of the extent of the problem.”
Core samples were taken in March and April 2017 from depths ranging from 1,655 meters (5,400 ft) to 3,062 meters (10,000 ft). The researchers then expanded their data taken off the coast of Australia to obtain a global estimate of the weight of microplastic particles in deep-sea sediments and came to an estimate of 14.4 million metric tons. An estimate they say is conservative because the site was so remote and far from urban population centers.
While their findings show that there is an enormous amount of microplastics on the sea floor, the researchers say it is a fraction of the total amount of plastic that is dumped annually into the world’s seas.
Most of the plastic thrown into the ocean most likely ends up on coasts, the researchers say, rather than on the ocean surface or the ocean floor.
The results show that it is more urgent than ever to find effective solutions to stop plastic pollution before it reaches the oceans, and to stop plastic use in general, according to Hardesty.
“This will help guide waste management strategies, create behavioral change and opportunities to prevent plastic and other garbage from entering our environment,” she said. “Government, industry and society need to work together to significantly reduce the amount of garbage we see along our shores and in our oceans.”