Photo/Illutration Microplastic sampling sites are shown in this map with red circles. The three sites on the east are on an abyssal plain, whereas the two in the center are located along a trench. (Partial and enlarged view of a figure provided by the Japan Agency for Marine-Earth Science and Technology)

Massive quantities of microplastics lie on the deep ocean floor 500 kilometers off the Boso Peninsula in Chiba Prefecture, the highest level ever found on the seafloor anywhere, a group of Japanese scientists has discovered.

The researchers from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and other institutions said the abundance of microplastics in sediments in that area far exceeds the figures found in other studies conducted overseas.

They used the Shinkai 6500 manned research submersible and equipment that can dive to great depths to sample seafloor sediments at seven locations in 2019.

The sites were around 1,000 meters deep beneath Sagami Bay, more than 9,000 meters deep along a trench off the Boso Peninsula, and nearly 6,000 meters deep on an abyssal plain located farther offshore.

Analysis of fine grains contained in the uppermost 1 centimeter of surface mud showed that microplastics abounded, in particular, on the abyssal plain, where there were about 600 pieces on average per gram of sediments in dry weight.

That was more than 10 times the quantity found beneath Sagami Bay and along the trench.

The level exceeded, at all sampling locations, the corresponding figure from a site at the bottom of the Mediterranean, the largest ever found on the deep seafloor. One of the abyssal plain stations produced 260 times the count at the Mediterranean site.

Microplastics generally refer to grains measuring 5 millimeters or less in size.

The researchers said that there is a likely route for fine particles, carried by the Kuroshio Current, to accumulate off the Boso Peninsula. 

Differences in material types and shapes of the microplastics collected at the different sites have offered clues to their transport pathways.

The microplastic deposits beneath Sagami Bay and along the trench contained diverse materials, and many were found to have relatively elongated shapes. They bore a resemblance to microplastics found in Tokyo Bay and the Sagamigawa river.

They likely initially flowed from land areas into the ocean and sank in Sagami Bay, following which they were swirled up by submarine landslides caused by earthquakes. They were then transported across the deep-sea topography along the Sagami Trough seabed depression to areas around the sampling sites along the trench, the scientists said.

By contrast, polyethylene and polyamide accounted for the most microplastic deposits on the abyssal plain. The deposits resemble the composition found in seawater of the Kuroshio Current.

An extension of the Kuroshio Current forms eddies in that abyssal plain area, where plastic bags and other plastic litter have also been found in bulk. Fine grains that were borne by the Kuroshio Current likely sank there, the researchers explained.

“We hope to conduct studies in upstream areas, around Okinawa Prefecture and off the main island of Shikoku, to understand the transport pathways of the microplastics,” said Masashi Tsuchiya, a JAMSTEC researcher who was on the study group. “I hope that will help take countermeasures at their emission sources.”

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