Located about 200 miles southwest of Guam in the western Pacific, the Mariana Trench is so deep that the summit of Mount Everest would still be more than a mile underwater if it were thrown into it. It is, by most accounts, the most isolated place on Earth; it is separated from the surface by about 11 kilometers of frigid water, is lightless, and is under pressure that most materials cannot survive. It is like exploring a different planet, according to researchers who have sent sensors and submersibles inside it. They discovered something unsettlingly familiar in the tissues of the tiny, pallid crustaceans that were scavenge the trench floor. PCBs. In 1979, industrial chemicals were outlawed in the US. found in certain samples at amounts higher than those seen in China’s Pearl River Delta’s heavily developed coastal waters.
Although it works across scales that are challenging to keep in mind at the same time, the mechanism that placed them there is not complex. For the majority of the 20th century, industrial and agricultural sources have been releasing persistent organic pollutants into the ocean, including PCBs, flame retardants, plasticizers, and a variety of industrial chemicals that bind easily to organic matter and resist decomposition.
They are dispersed throughout the planet’s oceans by ocean currents. There, they attach themselves to the organic particles that create what oceanographers refer to as “marine snow,” which is the steady, slow-moving fall of microplastic fragments, feces, dead animals, and shed cells that sinks steadily from the sunlit surface zone toward the seafloor. The poisons are concentrated in the sediment layers that build up at depth when that material sinks.
The geometry of the trench takes care of the rest. A hadal trench’s V-shaped walls, which converge as they descend, function as a topographic funnel. At the bottom of the trench, material that is drifting down from the nearby ocean shelf is directed inward and downward, accumulating in concentrations that represent both local sinking and the combined input from a large surrounding area. The Mariana Trench is located in an area of the Pacific where pollutants from industrial sources in North America and Asia have been accumulating for many years. The trench floor is the final destination for all of the stuff.
The amphipods that inhabit those depths are designed to be scarce. The organisms that survive in hadal zones do so by effectively storing fat and metabolizing it slowly. Food arrives infrequently, such as a whale carcass sinking from above or a periodic pulse of organic material from a fertile surface season. They are extremely efficient accumulators of pollutants because of this adaptability. Because PCBs and related substances preferentially bind to fat tissue, they are lipophilic. Over the course of its life, an amphipod with significant lipid stores that lives in cold water and has a slow metabolic rate accumulates toxins because it lacks the biological capacity to digest and remove them. More is added with each meal of tainted carrion. As a result, tissue concentrations of industrial pollutants are orders of magnitude greater than those found in nearby water or sediment.
In scientific circles, the response to the 2017 paper that first made these findings generally known was more akin to dismal confirmation than shock. Based on their understanding of trench sediment chemistry and marine snow transfer, deep-sea ecology researchers had long hypothesized this trend. The final tenable reason to believe that human contamination had not reached the deep ocean was eliminated when it was found in amphipod tissue at amounts greater than those found in coastal industrial zones. It hadn’t. All it had done was absorb the contaminate gradually and store it effectively.
The implications for the underlying presumptions of deep-sea governance are what elevate this above an ecological footnote. Policies pertaining to ocean pollution have typically been based on an implicit paradigm that views the deep water as a remote, mostly passive recipient of surface activity; it is impacted, but not directly. The evidence from the Mariana Trench points to a system that is more like to a closed one, in which pollutants that are injected anywhere in the ocean eventually accumulate at the bottom. That is not the same as a large dumping site or a localized spill. The bill is due in the locations least able to handle it, which is a worldwide accounting issue.
