Microplastics in the Great Lakes: Becoming Benthic

From the dump of the Great Pacific to the interior rivers, plastics are among the most common contaminants on the planet. Microplastics – plastic particles smaller than five millimeters – are particularly common. As they accumulate in the Earth's waters, microplastics also become a permanent part of the sedimentary layers of the planet.

Today, Patricia Corcoran, a sedimentary petrologist, and her students at the University of Western Ontario are studying the behavior of microplastics as a geological phenomenon.

What are the main sources of microplastics in Great Lakes sediments? What factors influence their distribution and where do they focus? To explore these issues and highlight the potential impacts of microplastics on animals, the Corcoran team analyzed sediment samples off and off Lakes Huron, Ontario, Erie and St. Clair, as well as their tributaries. Abundances reached 4,270 microplastic particles per kilogram of sediment in dry weight in lake sediments and up to 2,444 microplastic particles per kilogram in river sediments.

The team found that the more organic debris in the sample, the more microplastics there were. Benthic microplastics – those incorporated into lake bottom sediments – were also more abundant near densely populated areas, which are also associated with locations in the plastics industry.

Surprisingly, all the plastic fibers found in the benthic samples were not plastic at all. "When we analyzed the fibers chemically, only 33% was made of plastic, the other materials, such as dyed cotton or cellulose," explains Corcoran. "So we can not assume that every fiber we see under a microscope is plastic."

The Corcoran team also sampled pellets (microplastics of the size of a lens) from 66 beaches on the five Great Lakes. They found a total of 12,974 shots on 660 square meters of beach, about the equivalent of an eighth of the surface of a football pitch.

With the exception of the two beaches containing the most pellets, they found little relationship between population or industry density and the number of pellets, says Corcoran. Instead, the pellets were more concentrated near the tributaries. "In other words," she says, "rivers and streams are the main pathways for pellets to reach lakes."

The burial of microplastics in lake and river sediments is only one of the ways in which Corcoran has explored how plastics are becoming an integral part of the future rock record. of the earth. She also investigated anthropogenic rocks on a Hawaiian beach, which she and her colleagues called "plastiglomerate".