Polymetallic Nodules: Batteries in Rocks

Polymetallic nodules, also known as manganese nodules, are mineral deposits found on the seafloor. They are composed of iron and manganese oxides and can contain other valuable metals such as nickel, copper, cobalt, and rare earth elements. These nodules form very slowly over millions of years through the accumulation of metals from seawater.

Polymetallic nodules are of interest due to their high concentrations of valuable metals, which are increasingly in demand for modern technologies like batteries, electronics, and renewable energy systems. However, their deep-sea location and the technical challenges involved in harvesting them sustainably and economically have made their exploitation a topic of ongoing research and debate.

These nodules are typically found on the abyssal plains of the ocean, particularly in the Clarion-Clipperton Zone (CCZ) in the Pacific Ocean, the Peru Basin, and the Indian Ocean. They lie partially buried in sediments at depths of 4,000 to 6,000 meters.

Left: Polymetallic nodules coat fields of the ocean floor. Right: Hematite concretions, The Grand Staircase–Escalante National Monument, Utah.

Polymetallic Nodules Formation

How do polymetallic nodules form?. The formation of polymetallic nodules is a slow and complex process that involves several factors:

1. Nucleation

The process begins with the nucleation, where small particles act as a core. These particles can be fragments of basalt, marine organism debris, or existing nodules. Around these cores, layers of metal oxides begin to precipitate.

2. Precipitation

The primary mechanism for the growth of nodules is the precipitation of metal oxides from seawater. There are two main sources for these metals:

Hydrogenous Precipitation: Metals precipitate directly from seawater. Manganese and iron hydroxides slowly accumulate around the nucleus.
Diagenetic Processes: Metals are remobilized from the sediments below the nodule and then precipitate on the nodule surface. This is a secondary process that enhances the growth of nodules.

3. Growth

The growth of polymetallic nodules is extremely slow, typically only a few millimeters per million years. The accumulation rates depend on various factors, including the availability of metal ions, the rate of sedimentation, and biological activity.

4. Biological Activity

Microorganisms play a significant role in the formation and growth of nodules. Bacteria and other microorganisms can influence the precipitation of manganese and iron oxides. For instance, some bacteria can oxidize manganese, facilitating its deposition on nodules.

These processes often occur concurrently, resulting in the slow growth of nodules over millions of years, with growth rates typically ranging from millimeters to a few centimeters per million years.

Polymetallic Nodules Sizes and Shapes

Polymetallic Nodules Sizes and Shapes from Clarion Clipperton Zone (CCZ).
Photos by: Kim, M.G, et al.

Sizes: Polymetallic nodules can range from tiny particles only visible under a microscope to large, hefty pellets exceeding 20 centimeters (8 inches) in diameter. Most commonly, you'll find them between 3 and 10 centimeters (1 and 4 inches) across, resembling a potato or hen's egg.

Shapes: While some are perfectly spherical, many are more irregular. Common shapes include: Spherical, Ellipsoidal, Oblate (flattened), Prolate (elongated), Discoidal (disc-shaped) influenced by the dynamics of the surrounding sediment and water movement.

Texture: The texture of nodules can vary from smooth to rough, depending on the rate of growth and the environmental conditions.

Polymetallic Nodules Composition

Polymetallic nodules are rich in a variety of metals, making them significant from an economic and industrial perspective. The primary components of these nodules include:

Manganese (Mn): The predominant metal, often making up about 20-30% of the nodules.
Iron (Fe): Constitutes approximately 6-15% of the nodules.
Nickel (Ni): Typically present at concentrations of about 1-2%.
Copper (Cu): Also found in concentrations around 1-2%.
Cobalt (Co): Usually present at about 0.2-1%.
Other Elements: Nodules can also contain trace amounts of other metals, including:

Titanium (Ti)
Rare Earth Elements (REEs)
Molybdenum (Mo)
Lithium (Li)

Map of polymetallic nodule deposits around the world.

Polymetallic Nodules Occurrence and Locations

Polymetallic nodules are found scattered across the seafloor in all major oceans, including the Pacific, Atlantic, Indian, and Arctic Oceans, and even some lakes. However, deposits with commercially interesting concentrations are more localized. Here are some of the key regions:

Clarion-Clipperton Fracture Zone (CCZ): This vast area in the central Pacific Ocean, roughly halfway between Hawaii and Mexico, holds the largest and most well-studied deposit of polymetallic nodules.
Peru Basin: Located off the coast of South America, the Peru Basin holds another significant deposit of polymetallic nodules.
Central Indian Ocean Basin: Nodule deposits extend across the seafloor of the central Indian Ocean.
Penrhyn Basin: Situated near the Cook Islands in the South Pacific, the Penrhyn Basin contains commercially viable concentrations of nodules.

Polymetallic nodules cover vast areas of the ocean floor, rich in minerals essential for electric vehicle batteries.

Importance of Polymetallic Nodules

Polymetallic nodules are important for several reasons:

Economic Value:

Polymetallic nodules contain significant amounts of valuable metals such as manganese, nickel, copper, and cobalt. These metals are essential for various industrial applications, including the production of stainless steel, batteries, and electronics.

With increasing demand for these metals and the depletion of terrestrial resources, polymetallic nodules represent a potentially vast and relatively untapped source of these materials.

Technological Advancements:

The extraction and processing of polymetallic nodules have the potential to drive technological innovation. Developing efficient and environmentally sustainable methods for deep-sea mining could lead to advances in underwater robotics, remote sensing, and resource management.

Strategic Resource:

Access to polymetallic nodules can enhance a country's resource security. Countries with the capability to mine these nodules can reduce their dependency on foreign sources of critical metals, which is strategically important for national security and economic stability.

Environmental Considerations:

While the potential benefits of mining polymetallic nodules are significant, there are also considerable environmental concerns. The deep-sea environment is delicate and not well understood, and mining activities could disrupt ecosystems, harm marine life, and cause sediment plumes that impact water quality.

Understanding and mitigating the environmental impacts of nodule mining is crucial for balancing resource extraction with marine conservation.

Scientific Research:

Studying polymetallic nodules provides valuable insights into deep-sea processes and the history of the Earth's oceans. The nodules and surrounding sediments can serve as records of past oceanographic conditions, contributing to our understanding of global climate change and geochemical cycles.

Underwater vehicle with scraping mechanism for collecting polymetallic nodules.

Polymetallic Nodule Collecting

How are they collected? Unlike land-based mining, which often involves digging or drilling, polymetallic nodules lie loosely on the seafloor. Collection systems use water jets to lift the nodules from the seabed and into the collection vehicle. The technology is still under development, but recent trials have shown promise with minimal impact on the top layer of sediment where most of the nodules reside.

Environmental Impacts of Mining Polymetallic Nodules

The potential environmental damage caused by the extraction of polymetallic nodules from the deep seabed.

The environmental concerns associated with polymetallic nodules mining. The potential environmental impacts of polymetallic nodule mining are significant and multifaceted, leading to considerable concern among scientists, environmentalists, and regulatory bodies. Key concerns include:

Habitat Destruction:

Benthic Ecosystems: Mining operations would disturb or destroy habitats on the ocean floor, affecting organisms that live in or on the seabed.
Biodiversity Loss: Many species that inhabit these areas are not well understood, and their loss could have unknown ecological consequences.

Sediment Plumes:

Resuspension: The process of extracting nodules would resuspend sediments, creating plumes that can spread over large areas.
Smothering: These plumes can smother marine life and habitats, affecting filter feeders and other organisms sensitive to changes in water clarity and quality.

Toxic Release:

Heavy Metals: Disturbing the seabed can release toxic heavy metals trapped in sediments, potentially contaminating the surrounding water.
Pollutants: Other pollutants, such as hydrocarbons and organic matter, may also be released, impacting marine life and food webs.

Noise and Light Pollution:

Machinery: The operation of mining equipment generates noise that can disrupt marine mammals and other sea life sensitive to sound.
Light: Artificial lighting from mining equipment can affect deep-sea organisms adapted to darkness.

Cumulative Impacts:

Multiple Operations: The cumulative effect of multiple mining operations in a single area could be greater than the sum of individual impacts, leading to large-scale environmental changes.

Recovery Uncertainty:

Slow Recovery: The deep-sea environment is characterized by slow biological and geological processes. Recovery from disturbances could take decades or even centuries.
Unknowns: Many aspects of deep-sea ecosystems are still poorly understood, making it difficult to predict the full extent of mining impacts.