TOKYO -- As many as 7,000-plus pieces of plastic trash per square kilometer have been found in nearly 6,000-meter-deep seabed areas off the Boso Peninsula of Chiba Prefecture in east Japan, according to a team of scientists from Japan and Europe.
The joint team consisting of researchers at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), based in Yokosuka, Kanagawa Prefecture, and other organizations have confirmed that the areas are an accumulation site for plastic trash. The average density of plastic debris distribution was as much as 4,561 pieces per square kilometer.
More than 10 million metric tons of plastic trash is thought to flow into seas around the world annually, 440,000 tons of which float on the sea surface, and it is unknown where most of it goes. In the seas around Japan, as there are large current vortexes off the Shikoku region in west Japan and the Boso Peninsula, plastic trash carried by the Japan Current from East Asia had been thought to get caught in the vortexes and accumulate on the seabed in these two areas, but the real situation had been unclear.
Using the manned deep submergence research vehicle Shinkai 6500, the team conducted research right under the current vortex off the Boso Peninsula and the deep seabed in Sagami Bay off Kanagawa Prefecture, also in east Japan, between August and September 2019. They counted the amount of debris that was more than 5 centimeters in diameter and appeared to be plastic using visual observations and photos.
As a result, a large amount of plastic trash was confirmed in three deep seafloor locations between 5,707 meters and 5,813 meters below the sea surface, in areas 480 to 520 kilometers off the Boso Peninsula. The seabed at a depth of 5,718 meters, some 480 kilometers offshore, had the largest amount of such trash, whose distribution density was estimated at 7,021 pieces per square kilometer.
At least 80% of trash found on the seafloor was disposable plastic such as food packages, including an almost undamaged bag for a hamburger patty with a legible manufacturing year as 1984. It remained almost undamaged apparently because ultraviolet rays, which help deteriorate plastic, do not reach the deep sea and temperatures are very low there.
Meanwhile, the density of plastic trash at a 1,400-meter-deep section of seabed, some 20 kilometers offshore in Sagami Bay, was estimated as 1,950 pieces per square kilometer, lower than that off the Boso Peninsula despite the location being closer to Tokyo.
Team member Ryota Nakajima, a vice chief researcher at JAMSTEC, said, "Floating trash is merely the tip of the iceberg, and it keeps accumulating in the deep sea. In other waters, there have been cases in which trash gets tangled in deep sea coral and chemicals in plastics were detected in the bodies of creatures. If the amount of plastic trash increases in the future, it is feared that it will have harmful effects on deep sea creatures."
The research results were published in the British journal Marine Pollution Bulletin.
(Japanese original by Mayumi Nobuta, Science & Environment News Department)