The study, conducted by researchers from the University of Aberdeen and the James Hutton Institute and published in Nature, found that pollution was pervasive and intense within the Mariana and the similarly deep Kermadec Trench.
The pollutant in question is called a persistent organic pollutant (POP), organic compounds that do not biodegrade. The environmental devastation these compounds can cause, as they are nearly impossible to remove once they are introduced to an environment, is significant. In addition, POPs have been linked to disruptions in the endocrine and reproductive processes of animals.
POPs do not degrade, and so when dumped in the ocean they would accumulate inside animals. Those animals would then sink into deep trenches like the Mariana when they died, at which point their bodies would be eaten by scavengers.
As a result, POPs are now heavily present in the bodies of the Mariana Trench's crustaceans. The report claims the pollution levels to be on par with the dumping grounds used by Chinese plastics manufacturers.
"In creatures that live in shallower waters, exposure to POPs can reduce reproductive success and thus population growth. It's hard to study deeper animals alive under controlled conditions but can assume the pollutants have a similar effect," wrote study lead Alan Jamieson in an opinion piece on The Conversation.
"The reality is that the deep sea just isn't that remote, and the great depth and pressures are only an imaginary defence against the effects of what we do 'up here'. The bottom line is that the deep sea – most of planet Earth – is anything but exempt from the consequences of what happens above it, and it's about time we appreciated that."
In recent decades, governments had begun to realize the dangers of POPs. Polychlorinated biphenyl (PCB), a POP used for things like coolant and transformer fluid, was banned worldwide in the 70's when it was found to cause birth defects and other health problems.
In 2001, the Stockholm Convention was drafted which eliminated POP production in most of the world. Only a small handful of countries have not ratified the treaty, including Turkmenistan, Uzbekistan, South Sudan, and the United States.
Another major risk to ocean life is plastic pollution, brought on by plastic microbeads present in soaps and shampoos. They go down the drain and end up in the oceans, where they present a danger to marine life.
"It seems that once again, we have a shocking example of our own stupidity, as people gradually realise that plastic microbeads are, funnily enough, made of plastic, and that stuff that goes down the sink doesn't magically disappear into another dimension," wrote Jamieson.
The United Kingdom passed a ban on plastic microbeads in some products in late 2016. New Zealand passed a full ban in January 2017.