Clues emerge in 'missing' ocean plastics conundrum

Of the 4-12 million tons of plastic that enter the oceans each year, just 250,00 tons —less than one percent—stays on the surface

It's a puzzle that has perplexed scientists for years: humanity dumps millions of tonnes of plastics into the world's oceans annually, yet only a tiny fraction remains visible on the surface.

Now an international team of researchers believe they may be closer to determining where Earth's "missing plastics" end up, using an unprecedented global effort to track and draw down one of the most polluting materials ever invented.

As images of plastic-clogged beaches and swirling gyres of detritus bobbing on the high seas are prompting governments and cities to curb their throwaway culture, a growing body of evidence suggests a deeper problem of plastic permeating all ocean depths.

Of the between 4-12 million tonnes that enter the oceans each year, just 250 thousand tonnes are thought to stay at the surface. Overall, more than 99 percent of plastics dumped at sea over several decades are currently unaccounted for.

As plastics degrade through erosion, UV light and microbial decay, their density changes, putting them at the mercy of ocean currents—and, once they get pulled lower in the water, much harder for experts to track.

"It's quite difficult to decide where it all is because there are so many processes at work," Alethea Mountford, from Newcastle University's School of Natural and Environmental Sciences, told AFP.

"Even plastic at the surface can sink down and go back up again—it's moving between different possible sinks in different areas of the ocean at any time."

In a potential breakthrough, Mountford used a computer model of ocean currents for plastics of three different densities to project where most of the world's fragments collect once they start to sink.

The model showed significant build ups at depths varying thousands of metres in the Mediterranean Sea, Indian Ocean and the waters surrounding Southeast Asia.

Much of the plastic ends up on the seabed—as researchers outlined earlier this year in a separate study that found microplastic fibres in the guts of tiny shrimp that live at the bottom of the Mariana Trench—the deepest place on Earth.

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