Title: Monitoring benthic plumes, sediment redeposition and sea oor imprints caused by deep-sea polymetallic nodule mining

Journal & Year: Nature Communications, 2025

Report Length: 16 pages

BLUF: This 2025 study in Nature Communications presents the most comprehensive in situ monitoring to date of benthic plume behavior, sediment redeposition, and physical seafloor disturbance caused by a polymetallic nodule collector trial in the Clarion-Clipperton Zone (CCZ), revealing critical insights for future environmental impact assessments and regulatory frameworks under development by the International Seabed Authority (ISA).

This paper, authored by an international team of marine scientists led by Iason-Zois Gazis and collaborators across European marine research institutes, documents an unprecedented real-world observation of the environmental footprint from pre-prototype nodule mining.

The trial featured a Global Sea Mineral Resources (GSR) hydraulic collector (Patania II) operating at ~4,500 m depth in the eastern CCZ.

The goal was to quantify plume formation, sediment resettlement, and physical seafloor impacts using a dense sensor network and autonomous robotic platforms.

The monitoring effort was part of MiningImpact 2, a European Joint Programming Initiative project.

Unlike previous laboratory or theoretical studies, this paper offers the first real-time, high-resolution empirical data from an actual deep-sea mining test under operational conditions with independent scientific oversight.

The collector used in the trial featured caterpillar tracks and a water-jet-based suction head, which mobilized both nodules and the upper 5 cm of biologically active sediment. Discharge occurred 2.5 - 3.2 m above the seabed, releasing 12 ± 3 kg of sediment per second.

Monitoring included:

  • 44 sensors on 23 fixed platforms placed 50 - 1,800 m from the mining site.
  • Two ROVs for visual observation and equipment deployment.
  • One AUV equipped with MBES, SSS, optical backscatter (OBS), and still cameras to observe plume dispersion in three dimensions.

All instruments were calibrated using CCZ-native sediments and water, and OBS readings were rigorously translated into sediment mass concentrations (mg/L).

Key findings: 

1. Gravity-Driven Plume Dynamics

A benthic gravity current was generated behind the collector and remained largely confined to <5 m above the seabed within the first 500 m. It was funneled downslope through steeper terrain, creating ripple marks and observable lateral expansion only after being blocked by a 6 m-high sill.

At 50 m distance, sediment concentrations reached 264 mg/L; a 10,000-fold increase over background levels (~0.02 mg/L). These levels dropped to baseline within 14 hours.

At 1,800 m, concentrations were still elevated (3.9 mg/L) but orders of magnitude lower, confirming a power-law decay in intensity.

In other words, Think of it like an underwater dust cloud stirred up by a bulldozer rolling across the seafloor. This cloud of sediment mostly hugged the ocean floor, flowing downhill like a muddy avalanche. It was intense near the machine — about 10,000 times more sediment than usual — but faded quickly. After 14 hours and 1.8 kilometers, it thinned out dramatically.

2. Vertical and Lateral Dispersion

  • Most sediment remained within 1 - 5 m of the seabed.
  • Only trace concentrations (0.06 - 0.1 mg/L) were observed at 30 - 50 m altitude and 4.5 km distance after 35 hours.
  • No sediment signal was detected upslope or perpendicular to prevailing current flow, confirming directional confinement.

3. Rapid Flocculation & Sediment Resettlement

Particle sizes increased from ~12 μm to 147 μm in the plume. These aggregated flocs had settling velocities up to 100 m/day, suggesting residence times of 30 - 45 minutes in the lower plume and explaining rapid redeposition near the source.

The collector operated for ~41 hours, covering 21.37 km and removing nodules across 171 parallel lanes (50 m long × 4 m wide), totaling 0.034 km² mined. Additional disturbance from turning maneuvers increased the impacted area to 0.08 km².

Using mm-resolution photogrammetry, researchers detected:

  • A minimum erosion depth of 5 cm per lane.
  • Sideways sediment extrusion from caterpillar tracks.
  • Sediment blanketing of unmined areas up to 3 cm thick across 6 km² — entirely obscuring nodules in areas <500 m from the mining lanes.

Three distinct sediment redeposition zones were classified:

  • Thick Coverage: Complete nodule burial with reduced seafloor roughness.
  • Medium Coverage: Partial burial with nodules still identifiable.Faint Coverage: Powder-like layer reducing optical contrast but not fully covering nodules.

These changes have profound implications for benthic ecosystem structure and the interpretation of post-mining recovery trajectories.

Assuming uniform erosion of 5 cm across 8,500 m of mining lanes, the study estimates 1700 m³ of mobilized sediment.

Redistribution models suggest most redeposition occurred within 6 km² of the impact site, although transport beyond that range cannot be ruled out.

Slope played a major role in plume directionality. On flat terrain, gravity currents were weaker and plume extent more localized.

This insight suggests a practical environmental mitigation strategy: prioritize mining in flatter areas to reduce the reach of sediment redeposition.

Sensor and monitoring effectiveness:

  • OBS and ADCP networks delivered reliable concentration and flow data.
  • MBES water column imaging successfully tracked the plume within roughly 700 m of the collector,  but not beyond due to low particle concentrations.
  • AUV surveys mapped vertical and lateral plume dispersion and seafloor changes with exceptional detail.

OBS (Optical Backscatter Sensor): Measures how much light is scattered by particles in the water to estimate sediment concentration.

ADCP (Acoustic Doppler Current Profiler): Uses sound waves to measure the speed and direction of water currents at different depths.

MBES (Multibeam Echosounder): Emits multiple sonar beams to map seafloor topography and detect particles in the water column.

AUV (Autonomous Underwater Vehicle): A self-guided robot that collects sonar, camera, and sensor data while surveying large seafloor areas.

Despite the technological success, authors caution that real-time environmental response remains limited by processing capacity and data volume. They advocate for future inclusion of high-performance computing on mining vessels, enabling near-real-time plume modeling and adaptive management.