Microplastic particles on the surface of the ocean are generally thought to sink directly to the bottom of the ocean, and 99 per cent of missing marine plastics may reach the deep sea, but the exact location is still unclear. In a recent issue of Science, researchers from the University of Manchester, the National Oceanographic Centre in the United Kingdom, the University of Bremen in Germany, the University of Ifremer in France and Durham University in the United Kingdom used high-resolution sediment data and digital models taken from the Lenian Sea (off the west coast of Italy) to finally identify the location and content of the missing 99% of marine plastics (tiny plastic fragments and fibers).
Dr Ian Kane, lead author of the study, from the University of Manchester, said: “Almost everyone has heard of the infamous marine litter belt floating plastic, but we are shocked by the high concentrations of microplastics found on the deep seabed, which are not evenly distributed in the ocean, but rather in certain areas as powerful ocean currents are distributed. “
Microplastics: PM2.5 in the Sea
As early as 2004, Thompson of Plymouth University in the United Kingdom and others published a paper on plastic fragments in marine water and sediments, the first to propose the concept of “microplastics”.
It is reported that “microplastic” refers to the diameter of less than 5mm plastic debris and particles, also known as “PM2.5 in the sea.” Because the “microplastic” particles are small in diameter and small in size, it means that there is a higher surface area (the surface area than the surface area refers to the surface area of the unit mass of porous solids), the larger the surface area, the stronger the ability to absorb pollutants. Therefore, compared with non-degradable “white pollution” plastics, “microplastics” are more harmful to the environment.
It is well known that “microplastics” carrying industrial and domestic wastewater can enter the ocean from rivers, and ocean currents control the distribution of plastic on the surface of the world’s oceans, creating a patch of floating waste on the surface of the ocean. However, the concentration of plastic on the surface of the ocean accounts for only 1% of the global marine plastic. Most of the plastic sinks into the deep sea. For now, the location of the plastics at the bottom of the deep sea is still unclear.
Finally, a team of researchers from the University of Manchester in the United Kingdom, the National Oceanographic Centre in the United Kingdom, the University of Bremen in Germany, the University of Ifremer in France and the University of Durham in the United Kingdom collected sediment samples from the sea floor of the Dilnian Sea (part of the Mediterranean Sea) and combined them with calibrated deep ocean current models and detailed seabed mapping. In the laboratory, microplastics are isolated from the sediments, counted under a microscope, and then further analyzed with infrared spectra to determine the type of plastic. Using this information, the team was able to show how ocean currents control the distribution of plastic particles on the ocean floor.
It was found that the ocean currents controlled the flow of these “microplastics”, which carried microplastics to the undersea canyons and then transported them over the seafloor through “bottom currents”, eventually depositing these fine particles and accumulating large amounts of sediment. The researchers call these sediments “microplastic hotspots.” These hot spots appear to be deep-sea “garbage belts”.
In addition, these deep ocean currents carry oxygenated water and nutrients, which means that the “microplastic hot spots” of the seafloor can also accommodate important ecosystems that can consume or absorb these microplastics.
1.9 million microplastics per square metre
It turns out that microplastics are concentrated in the depths of 600 to 900 meters of the ocean, where the ocean currents interact most with the sea floor. They either settle slowly or are quickly transported deep into the underwater canyonby by the occasional murky current, a powerful underwater avalanche. Once microplastics enter the deep sea, slow-flowing ocean currents near the bottom of the ocean promote the flow of microplastics, which can prioritize the concentration of fibers and debris in large amounts of sediment, which in turn creates microplastic hot spots in deep-sea sediments.
It has been found that almost all seafloor samples contain microplastics, most of which are fibers. These undersea microplastics are mainly made up of fibers from textiles and clothing. These contaminants are not effectively filtered in sewage treatment plants and can easily enter rivers and oceans, while the highest concentrations of microplastics can accumulate on contoured rocks. Importantly, given that ocean currents provide oxygen and nutrition for deep-sea organisms, similarly, these toxic microplastics may enter biodiversity hotspots, increasing the chances of deep-sea organismin ingestion.
Microplastics are transported to the ocean by rivers containing industrial and domestic wastewater, then by powerful sediments (turbid streams) to undersea canyons, where they are transported through “bottom currents” and deposited in sediments
In addition, the researchers found that microplastics in the microplastic hot spots produced by ocean currents were also frighteningly high, reaching up to 1.9 million microplastics per square metre, the highest level ever reported in the global seabed environment.
Study author Dr Mike Clare, of the National Oceanography Centre, said: “Our study shows that detailed studies of ocean currents on the ocean floor could help us find microplastic transport routes in the deep ocean, which could help us find lost microplastics. Such an outcome highlights the need to use policy interventions to limit future plastic flows into the natural environment and minimize impacts on marine ecosystems. “
Ocean bottoms control the fate of microplastics in the deep sea (Source: Science)
Dr Florian Bohr, of the Department of Geosciences at Durham University in the UK, said: “Unfortunately, microplastics have become a new type of sediment particle that is distributed on the sea floor along with sand, soil and nutrients. As a result, our research shows that sediment transport processes such as ocean currents can concentrate plastic particles in certain locations on the seabed. “