Microplastics are increasingly accumulating in marine sediments

From samples taken from the Mediterranean Sea, researchers find that the amount of microplastics accumulating on the seabed is increasing at the same rate as global plastic production. They also found that the particles, once deposited in the sediment, do not degrade.

The presence of microplastics in the oceans raises concerns about their damage to the functioning of these ecosystems. Their distribution in water is still a poorly understood phenomenon today, partly due to limitations associated with available analytical methods. Nevertheless, some studies show that the seabed is an important sink for these pollutants. Scientists from the Institute of Environmental Sciences and Technologies of the Autonomous University of Barcelona (ICTA-UAB) and Aalborg University in Denmark conducted a study on the presence of microplastics in marine sediments and their accumulation over time. It was published in the journal Environmental Science and Technology.

To carry out this study, the researchers used samples taken in November 2019 in the Ebro delta in southern Catalonia. This area of ​​the Mediterranean Sea was chosen because rivers are hotspots for a number of pollutants, including microplastics. In addition, the influx of sediments from the Ebro provides a higher sedimentation rate than in the open sea.

A total of five sediment cores were taken, and later only one, located at a depth of about one hundred meters, was the subject of in-depth analysis. Each layer was cut into slices to study the microplastic composition and also to observe their degradation. The researchers were able to date the samples with lead-210 to determine the chronology of each part of the sediment in order to understand what happened to these microplastics over time. Using state-of-the-art imaging techniques, particles ranging in size from 11 to 1,000 micrometers could be measured.

The first finding from this study shows that the amount of microplastics accumulating in sediments increased exponentially between 1965 and 2016 at the same rate as global plastic production. “In particular, the results show that the amount of plastic particles deposited on the sea floor has tripled since 2000, and the accumulation, far from decreasing, has not stopped growing, mimicking the production and global use of these materials. says ICTA-UAB researcher Laura Simon-Sánchez.

Three types of particles are strongly present on the seabed

A second lesson from this study concerns the degradation status of buried particles. Thanks to chemical analysis, scientists have found that microplastics, once trapped on the sea floor, no longer break down due to erosion, lack of oxygen or light and remain intact. “The decomposition process mainly occurs in beach sediments, on the sea surface or in the water column. ICTA-UAB Professor Patrizia Ziver explains. Once deposited, degradation is minimal, so the plastics of the 1960s remain at the bottom of the sea, leaving traces of human pollution there. »

ICTA-UAB researcher Michael Grelaud made this research possible “See how the accumulation of polyethylene and polypropylene particles in packaging, bottles and food films, as well as polyester from synthetic fibers in clothing, has increased since the 1980s, but especially in the last two decades. » The amount of these three types of particles reaches 1.5 milligrams per kilogram of accumulated sediment, polypropylene is the most, followed by polyethylene and polyester.

This research provides a better understanding of the transport of microparticles from the ocean surface to the seabed. This paves the way for other lines of research, particularly a better understanding of the effects of these microplastics on organisms that thrive on the seabed.

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