Surprising underwater distribution of plastic fragments and fibres

Microplastic hotspots in underwater avalanches

Microplastics in a turbidity current
Microplastics in turbidity currents. Source: Pohl et al. 2020

The plastic soup, threatening ecosystems by floating on the surface the oceans, has been in the news often over the last few years. Also, we know that many plastics and microplastics that are found in this plastic soup originate from land, where urban areas pollute rivers. But what happens to the microplastics that do not float, and find their way into the sediment of the ocean floors? Researchers from Utrecht University, the University of Manchester and Durham University uncovered surprising processes of deposition and distribution of microplastics in underwater avalanches, so-called turbidity currents.

“What you have to realise is that about 95% of the plastic that goes into the oceans, is lying on the sea floor,” says Dr. Florian Pohl, sedimentology researcher at Utrecht and Durham University, and lead author of the study. “Not all plastics float, many have a density higher than water. We wanted to know what happens to those fragments.” Uncovering these processes can highlight important new research areas where microplastics may have a damaging effect on seafloor ecology.

Microplastic fragments and fibres used in the experiment
Mircoplastic fragments (a) and fibres (b) used in the experiment. Source: Pohl et al. 2020

100s of kilometres long avalanche

A gravity-driven turbidity current is a mixture of sediment and water that flushes sediment into deeper layers of the ocean. These underwater avalanches can be hundreds of kilometres long, and their effect on the deposition and distribution of microplastics can be vast. The researchers simulated this phenomenon in the lab with flume experiments (click here to see the experiment in action), and showed where microplastics – both fragments and fibres – ended up in the avalanche flow.

“Most of the fragments end up at the base of the flow as they are carried along the currents,” Pohl explains. This was expected, as fragments are heavy and flow with the sediment. More surprising was what happened to the plastic fibres.

Counterintuitive effect on fibres

Because the fibres are light, you would expect them to float and distribute themselves on top of the sediment as they eventually float down. But something counterintuitive happened there: “In these turbidity currents, it works differently. Because the fibres get trapped by the sand grains in these currents, they are pushed down into the sediment.” Therefore, fibres are enriched in the sediment deposits of these underwater avalanches. This process can result in the occurrence of hot-spots of microplastic fibres in seafloor sediments, in particular in submarine-canyon systems. These systems are the deep-sea equivalent of rivers on land, and pathways for turbidity currents across the seafloor.

The Monterey Canyon system
The Monterey Canyon system. Source: public domain

Due to the unique deposition behaviour of different microplastics, fragments are concentrated in the spots where the avalanche ends, whereas fibres are distributed relatively evenly over the whole flow. “With these results, it’s easier to identify new priority research sites on the seafloor such as the African Congo Canyon and the Monterey Canyon in California, and monitor these kinds of flows for microplastics,” Dr. Joris Eggenhuisen, assistant professor at UU and co-author of the study, explains. “This is exactly what I want to study at Durham University in the future,” Pohl adds.

It is difficult to say what the long-term ecological effects of these seafloor microplastics will be. “At the moment, the plastics behave in a way we can’t entirely explain with models, and we need fundamental studies on the mechanisms of transport and deposition of plastics in natural environments,” Eggenhuisen concludes. 

Publication: Florian Pohl, Joris T. Eggenhuisen, Ian A. Kane, and Michael A. Clare, 2020. Transport and Burial of Microplastics in Deep-Marine Sediments by Turbidity Currents. Environmental Science & Technology Article ASAP. DOI: 10.1021/acs.est.9b07527