Approximately half the plastics that have ever been produced can float on the ocean’s surface because they are lighter than seawater (PlasticEurope 2015). However, they will not remain on the surface forever and, sooner or later, they will sink deeper into the ocean (UNEP and GRID-Arendal 2016).
Plastic bottles and bags will sink when they become heavier than seawater. One way this can happen is when they become fouled by a lot of marine creatures, such as the corn barnacle (Ye & Andrady 1991). Another way is when inorganic matter, such as grains of sand, enter the plastic and stays there (Pham et al. 2014). When microplastic is ingested by marine creatures, it may get transported with these from the ocean’s surface to the bottom of the ocean as well (Choy & Drazen 2013).
However, this time, we will focus only on the water movements that also carry debris into the depths. If the specific gravity of plastic debris is almost the same as that of seawater, these plastics will be transported to the depths of the ocean with the deep water circulation while maintaining a state of neutral buoyancy (neither floating nor sinking). If seawater sinks from the surface layer into the depths, the plastic waste will also be transported to the deep with this water movement (Tubau et al. 2015).
How is the seawater submerged itself? This phenomenon is driven by salinity and temperature variations in the water, and it occurs more at the mid to high latitude areas of the sea (Ivanov et al. 2004, Durrieu de Madron et al. 2005). For example, during winter, seawater decreases in temperature and increases in salinity due to being exposed to an icy, windy climate. When wind moves over the water, it produces a great deal of evaporation, leading to a decrease in temperature and resulting in an increase in the salinity of the seawater.
As the seawater cools down, it becomes heavier because its density increases. The seawater also becomes heavier when its salinity increases. When the surface layer of the water is denser and saltier than the lower layer, cascading occurs. This is like a waterfall, whereby heavier seawater will descend along the continental slope to a greater depth until its density becomes the same as the water around it (Shapiro et al. 2003, Durrieu de Madron et al. 2005).
Plastic waste will be conveyed into the abyss with this dense water flow (Tubau et al. 2015). In addition, the plastic waste that is conveyed to the deep sea floor will accumulate in a particular area, namely from the submarine continental shelf to the continental margin, especially the areas in the submarine canyons (Pham et al. 2014). A submarine canyon is a narrow steep-sided trench that has been eroded into a continental shelf or a continental slope by underwater currents.
Scientific investigation of a submarine canyon using trawl net samples has revealed that there is a vast amount of plastic debris originating from land that accumulates in these submarine canyons (Pham et al. 2014). Further research also shows that there is a good amount of plastic debris accumulating in oceanic trenches, which a long yet narrow hemispheric-scale topographical depressions in the sea floor (Shimanaga 2016).
There are two specific places in the world’s oceans where the seawater submerges from the ocean’s surface to several thousands of meters deep without stopping. These places are off the coast of Greenland in the North Atlantic Ocean and around the South Pole.
As discussed before, when the seawater sinks to the deep ocean, it needs to have a low temperature and high salinity. Because evaporative cooling is predominant in the areas around Greenland and the South Pole, the seawater here is very dense. As a result, large-scale seawater subduction occurs in these areas.
The dense water flows downhill to about several thousand meters deep in the ocean, and then it travels along the bottom of the world’s oceans as if on a global conveyer belt; eventually, this dense seawater will return to the surface again.
This large-scale deep ocean circulation is called thermohaline circulation. Some of the plastic particles and debris on the deep ocean floor may be carried by such large-scale seawater subduction (UNEP and GRID-Arendal 2016).
Once the dense seawater enters the thermohaline circulation, it takes several thousands of years to complete its process (the seawater moves to the deep, circulates around the world, and reaches the surface again) (UNEP and GRID-Arendal 2016).
Therefore, people living a thousand years in the future may see our plastic debris, possibly one of our most negative legacies.