Can Deep-Sea Mining be Sustainable? Insights From a Sediment Redeposition Study

By Dr Silpaja Chandrasekar, PhDReviewed by Laura ThomsonMar 14 2025

A recent article in Nature Communications described a polymetallic nodule collector trial at 4500 m depth. A gravity current formed behind the collector, channeling sediment downslope, while bottom currents dispersed particles up to 4.5 km. Suspended particle concentrations near mining lanes were significantly higher but decreased with time and distance. Most of the plume stayed near the seafloor, with rapid particle flocculation and sediment redeposition observed—the photogrammetric analysis estimated sediment redeposition at approximately 3 cm next to mining lanes.

Related Work

Past work on deep-sea polymetallic nodule mining included oceanographic observations, sediment analyses, and numerical models of plume behavior. Previous benthic impact experiments and small-scale disturbance studies provided sediment aggregation and dispersion insights.

Monitoring platforms and sensor settings were tested during a dredge disturbance experiment in the Clarion-Clipperton Zone. Observations from these studies informed strategies for assessing plume dispersion and ecosystem disturbances. These efforts laid the groundwork for refining deep-sea mining impact assessments.

Seafloor Data Acquisition

A HUGIN 6000 autonomous underwater vehicle (AUV) was used to collect multibeam echosounder (MBES), sidescan sonar (SSS), optical backscatter (OBS) data, and seafloor images.

MBES and water column backscatter data were acquired using the Kongsberg EM2040 at 400 kHz, with settings detailed in the expedition report.

Data processing included sound velocity profile correction, roll bias correction, filtering, and tide adjustments in queries per second (QPS) Qimera v1.7.

Navigation data were refined using Kongsberg’s NavLab software, ensuring no absolute shift compared to ship-based MBES. The final bathymetric grid was projected in universal transverse mercator (UTM) zone 10N and exported as a Geographic Tagged Image File Format (GeoTIFF).

Seafloor images were taken using a CathX HD color still camera (12.5 MP) mounted on the AUV.

Color normalization was performed using GEOMAR’s Image Normalisation software to correct lighting inconsistencies.

Photogrammetric processing in Agisoft Metashape Pro v8.3 generated orthophotomosaics and digital elevation models (DEM). Large datasets were split into 20,000 image chunks for processing to ensure efficiency.

Mangan analyzer software was used to assess nodule coverage using a hierarchically growing hyperbolic self-organizing map (H2SOM) approach.

Optical backscatter sensors (OBS) were deployed on stationary seafloor platforms to measure sediment suspension. A slight offset in JFE data was observed in clear water and corrected using fluorescence and turbidity sensor (FLNTU) comparisons.

OBS calibration followed standard protocols using sediment suspensions in clear bottom water at 1.5 °C. These steps ensured accurate measurements of sediment concentration in the study area.

Deep-Sea Mining Impact

This study examined the environmental impact of deep-sea mining by analyzing the imprints left by a nodule collector on the seafloor.

The trial, conducted on April 19, 2021, involved operating the collector for 41.33 hours over a distance of 21.37 km², covering 171 lanes and mining an area of 0.034 km².

The displaced nodules (~660 tons) were deposited in piles at the end of each lane. The mining tracks were distributed across three strips with varying slopes, and side-scan sonar mapping revealed that approximately 0.08 km² of the seafloor was disturbed, with sediment erosion reaching about 5 cm.

Orthophotomosaics confirmed the physical alterations, including caterpillar track marks and sediment extrusion.

A major concern was the formation and dispersion of the benthic sediment plume. Optical backscatter sensors recorded a maximum concentration of 264 mg/L at 1 m altitude, which was significantly higher than the background levels but returned to normal after 14 hours.

The sediment plume traveled downslope as a gravity current, covering about 380 m eastward, forming ripples indicative of sediment mobilization.

Observations suggest that the movement of large plume aggregates, behaving like fine sand, contributed to these formations.

The study highlights the extent of physical and sedimentary disturbances caused by deep-sea mining and provides insight into its potential ecological impact.

The autonomous underwater vehicle (AUV) tracked the plume up to 4.5 km south at 5 m and 10 m altitudes, detecting 0.1 mg/L concentration after 35 hours.

The plume spread over 9 km², with denser lower plumes moving eastward and lighter upper plumes following bottom currents southeast. Acoustic doppler current profilers (ADCPs) recorded plumes above 30 m at 1800 m southeast, confirming suspended particles rise over time at lower concentrations.

Particle flocculation was evident as ADCPs at 300 kHz detected aggregated sediment, with median sizes increasing from 0.012 mm to 0.147 mm during plume episodes. Settling velocity experiments showed a rapid descent at ≈100 m per day, with 80% of aggregated particles settling within 30–45 minutes. The redeposition covered nodules within 100 m of the impact site, reducing seafloor ruggedness. An estimated 11.2–12.3% of discharged sediment passed through the lower 2 m, with an additional 0.81–1.02% reaching 5–6 m altitude, suggesting further dispersal beyond observed limits.

Conclusion

The deep-sea trial showed that mining-induced plumes were largely confined to the seafloor, with sediment dispersion controlled by bottom currents.

Suspended particle concentrations decreased rapidly with distance and time, aided by fast flocculation and redeposition. Sediment accumulation near mining lanes reached approximately 3 cm, while erosion extended at least 5 cm.

These findings highlight the localized impact of polymetallic nodule collection on deep-sea sediment dynamics.

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