Plastics on Sea Surface Settle Deep in Sea Floor, Controlled by Deep-Sea Circulation

Over 10 million tons of plastic make their way into the oceans globally per year, thanks to humans and their extraordinary ways of hampering the delicate balance that the environment rests upon. But, recently, the accumulation of plastic on surface of Earth’s oceans have raised serious concerns. What is being observed on the surface is only 1% of the total plastic that are dumped into the oceans, the rest 99% settles in the deep sea. The plastics on the oceans’ surface are distributed in a particular pattern by the ocean current, especially the ocean gyres (ocean gyres are large circular ocean currents formed by global wind pattern and the forces of Earth’s rotation). These gyres concentrate plastics into sort of hot spots that we now call the ‘garbage patches’.

Major portion of the plastics in the oceans are present as micro plastics, which are small fragments of and fibres originating as manufactured particles in synthetic textiles or breakdown products of larger plastic debris. These micro plastics can go deep into the sea bed and settle there. Are these micro plastics concentrating in the sea bed with a particular pattern and if so what drives that?

An international research project comprising research groups from many countries, recently has come out with some settling answers of these questions. Published in Science on April 30, the findings of the project say that micro plastics could be found in the sea bed with upto 1.9 million pieces in a thin layer that covers just 1 square kilometer, indeed a huge concentration of the tiny plastic particles in the sea bed.

The project comprised the University of Manchester (UK), National Oceanography Centre (UK), University of Bremen (Germany), IFREMER (France) and Durham University (UK).

The research shows that the deep ocean currents (the thermohaline driven currents) actually settle the micro plastics in the sea floor and also govern the distribution of it. The deep-sea currents also create micro plastic hot spots in the sea floor. These hot spots appear to be similar to the ‘garbage patches’, the plastic concentration hot spots on ocean surface.

Dr. Ian Kane of University of Manchester and the lead author of the study was quoted to have said “Almost everybody has heard of the infamous ocean 'garbage patches' of floating plastic, but we were shocked at the high concentrations of microplastics we found in the deep seafloor. We discovered that microplastics are not uniformly distributed across the study area; instead they are distributed by powerful seafloor currents which concentrate them in certain areas."

The plastics, in the oceans, either can settle down very slowly or they can be transported rapidly by deep water avalanches, which travel down the submarines to the deep sea floor. After reaching the sea floor, the plastics are settled by the continuous seafloor current or bottom currents. Due to these forces generated by different ocean currents, the plastics are concentrated in hot spots.

The deep ocean currents that settle the pattern of the micro plastics in the sea floor also carry oxygenated water and nutrients, that the ecosystem near the sea floor depend upon. This implies that the micro plastic hot spots could also be the ecological hotspots that may house crucial ocean biodiversity. The serious consequence of it is that the bio systems in these areas could end up consuming the plastics.

Dr. Mike Clare, National Oceanography Centre, and also a co-author of the paper said— “Our study has shown how detailed studies of seafloor currents can help us to connect microplastic transport pathways in the deep-sea and find the 'missing' microplastics. The results highlight the need for policy interventions to limit the future flow of plastics into natural environments and minimise impacts on ocean ecosystems”.