Researchers from Newcastle University discovered polychlorinated biphenyls at quantities fifty times greater than those observed in crabs residing in severely contaminated river estuaries in China when they removed amphipods from the Mariana Trench in 2017 and analyzed their tissue chemically. These crustaceans contained PCBs from industrial activity that had been outlawed for decades in the majority of nations. They had sunk slowly thru eleven kilometers of water from the surface world, where they had come from manufacturing facilities, landfills, and the typical effluent of industrial civilization, to gather in the fat tissue of animals that lived in total darkness at the planet’s most remote location. There was no human interaction with the animals. Human chemistry was all over them.
This is what it means to be able to see human fingerprints from seven miles away. Although plastic garbage big enough to take pictures has been captured at hadal depths by submersible cameras, it is not visible in the sense that it might be seen with the unaided eye. Both chemically and biologically, it can be seen in the quantifiable presence of substances that are not found in nature, as well as in animals and sediments in locations that very few submersibles and no living human have ever visited.
Although the final concentration of these contaminants in deep-ocean ecosystems has only recently been described, the paths they take are well-documented. The plastic in the ocean does not remain at the surface forever. It breaks down into ever-tinier pieces until it reaches the microplastic scale, where the particles are thick enough to sink when they come into contact with marine organic matter and light enough to be carried by vertical mixing. Over the course of months and years, that sinking marine snow—the continuous fall of organic detritus from the sunlit surface—carries pollutants downward via the water column and concentrates them at deep due to the biological assimilation of creatures that eat it. What floats in from the nearby ocean floor is collected by the trench’s morphology, which functions as a funnel.
The xenophyophores that grow on the trench walls, the amphipods that dominate hadal scavenging communities, and the snailfish that have evolved to survive under pressures that would crush most materials are examples of species that are not adapted to chemical pollution. Cold, darkness, crushing pressure, and sporadic food availability were the main stressors in the environment in which they originated. Under those circumstances, the fat-soluble industrial chemicals building up in their tissue do not decompose. They remain. Additionally, elimination rates are essentially insignificant due to the animals’ sluggish metabolisms, which developed as an energy-saving tactic in a low-food environment. Without a way to remove it, the contamination accumulates throughout long lifespans.
In addition to the picture of chemical contamination, deep-sea mining causes another type of harm. The sediment plumes produced by commercial and test mining vehicles are carried by ambient currents and settle over areas that were not directly disturbed, traveling well beyond the equipment’ direct tracks. In addition to smothering filter-feeding organisms and burying the microhabitats that sustain benthic communities, this sediment occasionally transports trace metals from disturbed nodule fields into the water column, where they enter the food chain via different pathways than the industrial pollutants coming from above. Ecosystems that have been stable for geological timescales are under pressure due to the combined effect of chemical contamination raining down from the surface and physical disruption spreading outward from the seafloor. These ecosystems essentially have no capacity to recover on timescales relevant to human decision-making.
The statistics on abyssal recovery rates make it difficult to look at them without feeling uneasy. On timeframes of centuries to millennia, the deepest ocean communities undergo regeneration and change. At a rate of centimeters per million years, nodule fields regenerate. For longer than any present regulatory framework is intended to account for, contamination injected now will continue to be bioavailable to hadal fauna. International deep-ocean waters are governed by frameworks that precede the era of serious hadal research; they were negotiated at a time when the extent of human effect at those levels was not yet known, much less comprehended.
