Physical and Biological Impacts of Deep-Sea Mining are Still Apparent on the Seafloor after Four Decades
Source Article: Jones, D.O.B., Arias, M.B., Van Audenhaege, L. et al. Long-term impact and biological recovery in a deep-sea mining track. Nature 642, 112–118 (2025). https://doi.org/10.1038/s41586-025-08921-3
In 1873, the HMS Challenger was in the midst of the first global oceanographic expedition. As the ship crossed the North Atlantic, its dredges hauled some small, dense, black lumps onto the deck- the first discovery of polymetallic nodules. These “peculiar black oval bodies” are now known to exist in many locations along the deep ocean’s abyssal plains (3,000 – 6,000 meters deep). Nodules grow extremely slowly (1-10 cm per million years) as metal ions, primarily manganese and iron, dissolved in seawater are deposited on their surface. They also contain other metals like nickel, copper, cobalt, and rare earth elements, which are essential components of products ranging from cellphones to medical devices. These metals are also necessary for the development and expansion of clean technology, like renewable energies, that will be crucial for reducing greenhouse gas emissions and mitigating global climate change. Because of the highly lucrative composition of these nodules, many believe they should be mined from the deep ocean. However, to date, there have been no large-scale deep-sea mining campaigns.
The abyssal plains where nodules are typically found were long considered to be oceanic deserts, completely devoid of life. That perception changed as deep-sea exploration expanded following the groundbreaking Challenger expedition. The abyssal seafloor is full of life that has adapted to thrive in the deep’s harsh conditions; conditions that could be forever changed by mining activities. The problem is, we still don’t know exactly how mining will impact the communities of organisms living among these manganese nodules. So, should these manganese nodules be removed from the seafloor without fully understanding the potential ramifications for the life forms living among them, or should the seabed be left untouched, potentially delaying our ability to scale up renewable energy? Luckily, a mining test conducted over four decades ago has enabled a group of scientists to provide some much-needed information pertinent to this debate.
An environmental impact assessment 44 years in the making
In March of 1979, the Ocean Minerals Company carried out a deep-sea mining collector test in an area of the north Pacific Ocean known as the Clarion-Clipperton Zone (CCZ). The 4.5-million square kilometers of the CCZ contain an estimated 21 billion tons of manganese nodules, making it a key area of interest for mining. During the test, an experimental, self-propelled mining machine was lowered to the seafloor and collected an unknown quantity of nodules from an area of roughly 0.4 square kilometers over four days. In doing so, the large screws propelling the machine dug tracks up to nearly 3 feet deep in the seafloor on either side of the nodule collector path and sent clouds, or plumes, of sediment into the water column.
Scientists aboard the research ship RRS James Cook revisited the location of the mining test in 2023 to investigate the health of the ecosystems within the mining test’s collection tracks, propulsion tracks, and plume area, in addition to an undisturbed control site roughly two kilometers away. The scientists used sediment cores and the remotely operated vehicle ISIS to collect nodule, animal, and sediment samples; survey the impacted seafloor; and determine if the ecosystems had “recovered”. In this case, recovery is defined as a return to the original state of the ecosystem, assuming the control site is representative of the original ecosystem. The state of the control site was assessed on several parameters, including physical, chemical, and biological characteristics. Physically, the scientists were mostly interested in the shape of the seafloor around the disturbed area. Chemically, they measured the concentrations of organic carbon and nitrogen within the sediment. And for biology, they tested the composition of the animal community and the density, or number of animals per unit area. Broadly, animals were broken up into four size categories defined by the scientists: microbial (< 0.063 mm), meiofaunal (0.063 – 0.3 mm), macrofaunal (0.3-20 mm), and megafaunal (>20 mm). Community composition and density are important because animals may have returned to, or recolonized, the disturbed areas, but these areas may not be capable of supporting the same communities that existed there prior to the mining activity.
Physical changes to the seafloor caused by the propulsion screws and collector in 1979 were still very evident when the site was revisited in 2023. Images taken during the assessment clearly show deep grooves in the seafloor and, in the area affected by the plume of sediment kicked up by the collector, the displaced sediment had settled and partially buried the nodules present (up to around 1 centimeter). Regarding the chemical makeup of the sediment, the organic carbon and nitrogen content of the sediment was, on average, lower and much more variable where the nodules had been removed relative to the surrounding undisturbed areas. However, organic matter has accumulated in the depressions formed by the propulsion tracks. These results highlight the persistence of the physical and chemical changes to the seafloor resulting from deep-sea mining.
Life has returned to the mined seafloor, but the community has changed
Interestingly, the densities of the microbial communities in the disturbed and undisturbed areas were not significantly different from one another. Both the propulsion and collection tracks had been recolonized by, mostly mobile, mega and macrofauna in the 44 years between the 1979 test and 2023 assessment, meaning the disturbed areas were somewhat recolonized and capable of supporting larger animal life again after some time. However, the megafaunal communities in the mining tracks were only about one-third as dense as those in the undisturbed areas (0.1 vs 0.28-0.33 individuals per square meter), and the communities within the tracks were far less diverse than those in the undisturbed areas (35 vs 76 biological groups observed).
This study demonstrates that, even after four decades, the physical and biological environment of the abyssal plain has not completely recovered from a small-scale deep-sea mining operation. The mining tracks are still very much present, and while some large animals have been able to recolonize the disturbed areas, the composition and density of the reestablished megafaunal communities are significantly different from those that exist in the undisturbed areas of the CCZ. However, the extent of recovery appears to depend on the extent of disturbance. Compared to the mining tracks, the sediment plume has had little effect on the seafloor environment. The propulsion tracks, in particular, show the greatest change in community structure. The authors of this study attribute this phenomenon to the accumulation of organic matter in the depression caused by the propulsion screws, which has attracted mobile megafauna, while stationary megafauna from the undisturbed areas are not able to recolonize the disturbed areas as easily.
As deep-sea mining looks increasingly inevitable, minimizing the direct impact of the collector as it moves along the seafloor will be necessary to limit the ecological impact of this practice. Swarms of smaller, autonomous collectors, capable of selectively avoiding life on the sea floor, have been proposed as a more ecologically friendly option than large, hydraulic collectors that barrel along the seafloor, as used in this study. However, swarms are likely to incur much higher operating costs and may just transfer the disturbance from the seafloor to the water column, as they will need to make more frequent trips to the surface with the collected nodules. Overall, this study has highlighted the long-lasting impacts of deep-sea mining and demonstrated that further research is needed to assess the impact of deep-sea mining before large-scale operations begin.
Secondary Sources:
https://www.isa.org.jm/exploration-contracts
https://www.geomar.de/en/discover/marine-resources/manganese-nodules
https://deepseamining.ac/#gsc.tab=0
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