The sediment contains a chemistry that took millions of years to stabilize in the nearly freezing water of the abyssal plain, far below the surface of the Central Pacific. It’s not noticeable, it’s not spectacular, and it doesn’t look like what you see in pictures of marine life. It’s chemical—dissolved metals bound to organic molecules in the water that fill the voids between the grains of sediment, an equilibrium system that the organisms that live in and on that sediment have adapted to throughout geological time. In ways that the scientific community is just starting to comprehend, polymetallic nodule mining would penetrate that system and mix it.
Seawater that has seeped below is not the same as pore water, which is the water trapped within seafloor sediment. Over time, the breakdown of organic matter, the interactions between dissolved elements and minerals, and the gradual biological processes of the creatures residing within the sediment all contribute to the formation of its unique chemical character. At mining-relevant depths, copper concentrations in pore water can vary widely, from 3 to 96 nanomolar, and they frequently surpass those in the bottom water above. That’s not very concerning under typical, unaltered circumstances because the chemistry keeps it contained. Over 99 percent of that dissolved copper is bonded to naturally occurring organic ligands, which are molecules that hold copper in stable complexes and preserve free copper ions at quantities well below those that are harmful to the sediment’s fauna.
The crucial detail that makes the mining disturbance issue truly complex rather than merely concerning is the organic buffering system. There is copper. There are amounts of it that, if they were free, may be harmful. However, because there are enough ligands attaching it to support the load and more, it isn’t free. Because of the increased ligand capacity, even small amounts of extra copper released from disturbed soil can be absorbed into the same stable complexes without raising the concentrations of free ions to hazardous levels. Limited physical disruption may have a less harmful effect under that chemistry than a straightforward metal concentration value would originally indicate.
The issue is that commercial mining is neither gentle nor limited. The active upper sediment layer—the area where organic matter is most concentrated, pore water chemistry is most biologically active, and the ligand system performing the copper buffering is most dense—is stripped away by collector vehicles built to collect nodules from the seafloor as they traverse the abyssal plain. The chemical gradients that maintain the stability of pore water chemistry are altered when that layer is exposed to the bottom water above it. The equilibrium circumstances under which the organic ligand complexes were generated are altered by variations in temperature, pH, and oxygen levels. Data from disturbed sites, not simply undisturbed ones, are needed to answer the question of whether such systems remain stable when the conditions surrounding them change.
The disruption is transported beyond the direct path of the collector vehicle by the silt plume produced by mining operations. Trace metals, including copper, that were previously trapped in sediment are carried by fine particles that are resuspended from the seafloor and move with ambient currents, possibly over long distances. The metals carried by those particles may partially desorb and release into the water column in forms that the surrounding water’s ligand chemistry may or may not be able to buffer when they come into contact with different chemical conditions as they rise or travel, such as different oxygen levels or pH gradients. Field data from actual commercial-scale activities haven’t yet properly caught the spatial scale over which that chemistry plays out in a real plume event, affecting real creatures across a real area, although lab studies can approximate it.
The organic buffering system might be resilient enough to withstand the amounts of disturbance that economically feasible nodule mining would cause. When considered separately, the chemistry offers a solid foundation for cautious optimism on the acute toxicity issue. The longer-term trajectory—what happens to the microbial communities that produce the organic ligands after their habitat has been physically disturbed and whether those communities can reconstitute the buffering capacity after disturbance on a timeline that keeps the chemistry stable—is what the pore water copper research ignores. Abyssal ecosystems take a long time to recover. The nodules themselves regrow at a rate of one or two centimeters every million years. They are surrounded by chemistry that follows its own schedule, and no one has conducted the experiment long enough to determine what that schedule looks like after a mining operation.
