On the abyssal plains, at depths between 3,000 and 6,000 meters, polymetallic nodules are scattered across millions of square kilometers, much like potatoes in a field. These mineral ores are formed over millions of years from metals dissolved in the ocean water or released during microbial degradation of organic material in the sediments. As global demand for critical metals, such as nickel, cobalt, and copper, grows, so too does the pressure to exploit these resources economically.

Due to the extreme conditions in the deep sea, its ecosystems and high biodiversity (made up mostly of small organisms living in the sediment) are particularly sensitive to disturbances. Since 2015, the European JPI Oceans project MiningImpact, coordinated by the GEOMAR Helmholtz Center for Ocean Research Kiel, has been investigating the potential environmental impacts of deep-sea mining.

Previous analyses of decade-old disturbance traces in the Clarion-Clipperton Zone and the Peru Basin indicate that mining will cause long-term damage: Biodiversity and essential ecosystem functions will be affected for many centuries.

A major but poorly understood risk is the spread of suspended sediment plumes generated during mining operations. To better understand this process, the scientists closely monitored the test of a remotely operated pre-prototype nodule collector developed by the Belgian ISA contractor Global Sea Mineral Resources.

The study, now published in Nature Communications, provides the first detailed data on the far-field spatial footprint of mining-induced plume dispersion and redeposition beyond the mining area itself.

“While the main sediment fraction resettles within a few hundred meters from the source, we could detect small changes in sediment concentration up to 4.5 kilometers away,” says lead author Iason-Zois Gazis, a researcher in the DeepSea Monitoring Group at GEOMAR.

Monitoring a mining-induced sediment plume in 4,500 meters depth

On 19 April 2021, a nodule collector was deployed for 41 hours at a depth of 4,500 meters. During this time, the vehicle traveled approximately 20 kilometers and covered an area of 34,000 square meters (roughly the size of five football pitches). The sediment plume generated by the vehicle was measured using numerous calibrated sensors mounted on stationary platforms placed on the seafloor, as well as remotely operated and autonomous underwater vehicles.

The study found that a flow of dense suspended particles (a gravity current) developed behind the collector, traveling downslope through steeper sections of the seabed for up to 500 meters. Subsequently, the further spread of the sediment plume was driven by natural near-bottom currents.

Near the mining site, sediment concentrations were up to 10,000 times higher than under natural conditions, and returned to normal levels after 14 hours. Most suspended particles remained within 5 meters above the seafloor, resettling relatively quickly, aided by particle flocculation. A low-concentration plume of fine sediment particles left the monitored area at 4.5 kilometers distance.

Using high-resolution 3D mapping of the seafloor, the researchers mapped the mining imprints with millimeter-resolution and estimated the amount of sediment removed in the mining area and subsequently redeposited on the seabed. In the mined areas, nodules were removed with at least the top 5 centimeters of the seafloor. Meanwhile, the redeposited layer reached a thickness of about 3 centimeters, completely covering the nodule habitat in the close vicinity (up to ~100 m distance), and thinning out with increasing distance from the mining area.

The study provides valuable information for the ongoing development of international regulations by the International Seabed Authority (ISA), including state-of-the-art technologies and strategies for the monitoring of potential future deep-sea mining operations. MiningImpact researchers are continuing their analyses of the environmental impacts, and the results of this study help to accurately link physical impact types with ecological effects.