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What if the “big ocean trash patches” were just the tip of the iceberg? While more than ten million tons of plastic waste enters the sea each year, we actually only see 1%, the portion that floats on the ocean surface. What happens to the 99% lost has not been clear for a while.
Plastic debris gradually breaks down into smaller and smaller pieces in the ocean until they form particles less than 5 mm, known as microplastics. Our new research shows that powerful currents sweep these microplastics along the seafloor in large “drifts,” which concentrate them in staggering amounts. We found up to 1.9 million pieces of microplastic in a 5 cm thick layer that covers just one square meter, the highest levels of microplastics ever recorded on the ocean floor.
While microplastics have been found on the seafloor around the world, scientists were unsure how they got there and how they spread. We think that microplastics would separate according to their size or density, similar to natural sediment. But plastics are different: some float, but more than half of them sink.
Read: [Chemical recycling could be the solution to plastic pollution]
Plastics that once floated can sink as they become coated with algae, or bond with other sticky minerals and organic matter. Recent research has shown that rivers also transport microplastics to the ocean, and laboratory experiments revealed that giant avalanches of underwater sediment can transport these tiny particles along deepwater canyons to greater depths.
We have now discovered how a global network of deepwater currents transports microplastics, creating plastic hot spots within vast streams of sediment. When strolling in these streams, microplastics can accumulate where marine life abounds.
From the bedrooms to the sea floor.
We examined an area of the Mediterranean off the western coast of Italy, known as the Tyrrhenian Sea, and studied bottom currents that flow near the seabed. These currents are driven by differences in water salinity and temperature as part of an ocean circulation system that spans the world. Seabed sediment currents can be many kilometers in diameter and hundreds of meters high, forming where these currents lose their strength.
We analyzed sediment samples from the seabed taken at depths of several hundred meters. To avoid disturbing the surface layer of sediment, we use samples taken with box cores, which are like large cookie cutters. In the laboratory, we separate the microplastics from the sediment and count them under microscopes, analyzing them using infrared spectroscopy to discover what types of plastic polymers there were.
Most of the microplastics found on the seafloor are clothing and textile fibers. These are particularly insidious, as they can be eaten and absorbed by organisms. Although microplastics alone are often non-toxic, studies show that the accumulation of toxins on their surfaces can harm organisms if ingested.
These deep ocean currents also carry water and oxygenated nutrients, which means that the seafloor hot spots where microplastics accumulate can also host important ecosystems like deep-sea coral reefs that have evolved to depend on these flows, but now they are receiving large amounts of microplastics. .
What was once a hidden problem has now been discovered: Natural currents and the flow of plastic debris into the ocean are turning parts of the seafloor into microplastic deposits. The cheap plastic products we take for granted eventually end up somewhere. Clothes that can only last for weeks in your wardrobe linger for decades or centuries on the seafloor, which can harm the unique and little-understood creatures that live there.
Ian Kane, University of Manchester Geology Reader, and Michael Clare, Principal Investigator of Marine Geoscience at the National Center for Oceanography, publishes a new publication from The Conversation under a Creative Commons license. Read the original article.
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