Impacts of deep sea mining

Our knowledge of the ecosystems and biodiversity of the deep sea is still poor and we do not fully understand the role of the deep sea at an Earth system level. But we do know that the deep ocean plays a vital role in everything from driving thermohaline circulation to producing oxygen and sequestering anthropogenic carbon dioxide. These are services of a value to humanity which far exceeds any monetary worth available to the mining minority. The loss of these services could be catastrophic.

We do not know what level of damage the deep ocean can tolerate without losing its resilience and ability to provide vital functions. Moreover, mining will take place against a backdrop of multiple threats to the ocean from climate change, overfishing and pollution, and impacts are increasingly showing up in the deepest zones. We know that building ocean resilience is crucial, yet we have no evidence that such rebuilding in the deep ocean is possible.1

The specific impacts that can be anticipated around the mining sites include the obvious disturbances to or destruction of the benthic layer and the organisms living there, increased toxicity of the water column, and sediment plumes.

Plumes are caused when fine-particle tailings from mining are dumped back into the ocean, creating either “near-bottom” or “surface” plumes. Near-bottom plumes occur when the tailings are pumped back down to the mining site. The floating particles increase the cloudiness of the water and interfere with filter-feeding organisms and species. Surface plumes are released directly back into the ocean. Depending on the size of the particles and water currents, the plumes could spread over vast areas, impacting a wide range of species including migratory ones such as whales and sharks. There is a further concern that the plumes could impact zooplankton and light penetration, which would in turn damage the food web of the area2 and potentially carbon sequestration capacity.

All these mining activities will bring large quantities of particle-laden carbon dioxide and nutrient-rich cold water to the sea surface. These flows could also potentially be polluted, for example by hydrogen-sulphide. And where waters are released at the surface or in mid-water, a number of large-scale impacts may occur to alter pelagic and/or benthic ecosystems. If – as expected – some of the mined material will be processed at sea, this will lead to the deposition of tailings, potentially remobilizing toxic chemicals.3

The mining companies themselves acknowledge massive impact, with Nautilus Minerals saying that the potential environmental impacts include material and habitat “removal”, plume generation and water quality distress, and noise and vibration disturbances.4

Additional threats continue to emerge. In 2011, it was reported that high concentrations of REEs (in particular the rarer heavy REEs) can be found in the top few meters of deep sea clays, as well as in the nodules and cobalt crusts already being explored. The impacts of deep sea mud mining will probably exceed even those involved with polymetallic nodules, because they will penetrate deeper into the sediment and the operational and discharge plumes may be more severe. REE mining operations in muds could also include the application of weak acid during processing.Also unclear is how benefits derived from the common heritage of humankind will be shared fairly. Benefits from the deep sea go beyond the actual minerals to the much more complex issue of marine genetic resources, such as potential medical and commercial applications discovered in the deep ocean. As yet no plan exists to ensure that all humankind can benefit from such discoveries in addition to the potential mineral wealth, and mining may destroy valuable genetic resources before they have even been discovered.

Equally unresolved is how damage to the common heritage should be compensated and where liability and redress will rest. The risks and threats posed by mining are significant and could result in far-reaching damage to the seabed, water column, species existing within both, and the people dependent upon them.
Ultimately a critical question persists as to the cost versus benefit of deep sea mining. Is the potential benefit from mining worth the damage to ecosystems and the risks posed to wider ocean health, or to coastal and other communities dependent on ocean services?


1. See “Ecological restoration in the deep sea: Desiderata” (van Dover et al, Marine Policy, July 2013). 
2. Ahnert, A.; Borowski, C. (2000). “Environmental risk assessment of anthropogenic activity in the deep-sea”. Journal of Aquatic Ecosystem Stress and Recovery 7 (4): 299.
3. Van Dover, C.L. (2011). “Tighten regulations on deep-sea mining”. Nature, vol.470. 3 February 2011.