Ferromanganese nodules
# Ferromanganese nodules

Ferromanganese nodules are mineral concretions composed of silicates and insoluble iron and manganese oxides that form on the ocean seafloor and terrestrial soils. The formation mechanism involves a series of redox oscillations driven by both abiotic and biotic processes. As a byproduct of pedogenesis, the specific composition of a ferromanganese nodule depends on the composition of the surrounding soil. The formation mechanisms and composition of the nodules allow for couplings with biogeochemical cycles beyond iron and manganese. The high relative abundance of nickel, copper, manganese, and other rare metals in nodules has increased interest in their use as a mining resource.

## Composition

In both marine and terrestrial environments, ferromanganese nodules are composed primarily of iron and manganese oxide concretions supported by an Aluminosilicate matrix and surrounding a nucleus. Typically terrestrial nodules are more enriched in iron, while marine nodules tend to have higher manganese to iron ratios, depending on the formation mechanism and surrounding sedimentary composition. Regardless of where they form, the nodules are characterized by enrichment in iron, manganese, heavy metals, and rare earth element content when compared to the Earth's crust and surrounding sediment. However, organically-bound elements in the surrounding environment are not readily incorporated into nodules.

### Marine

The size of marine ferromanganese nodules can range from a diameter of 1-15 cm, surrounding a nucleus. The nucleus itself can be made from a variety of small objects in the surrounding environment, including fragments from previously broken down nodules, rock fragments, or sunken biogenic matter. Total nodule composition varies based on the formation mechanism, broadly broken down into two major categories: hydrogenetic and diagenetic. Hydrogenetic nodules have a higher iron and cobalt enrichment with manganese to iron ratios less than 2.5, while diagenetic nodules are more enriched with manganese, nickel, and copper with manganese to iron ratios typically between 2.5 to 5 but upwards to 30+ in sub-oxic conditions. The parent mineral for hydrogenetic nodules is vernadite and buserite for diagenetic nodules. The majority of observed nodules are a mixture of hydrogenetic and diagenetic regions of growth, preserving the changes in formation mechanisms over time. Generally, diagenetic layers are found on the bottom where the nodule is either buried in or touching the sea floor sediment and hydrogenetic layers are found towards the top where it's exposed to the above water column. Nodule layers are discontinuous and vary in thickness on micro to nanometer scale with those composed of higher manganese content typically brighter and those with higher iron content dark and dull.

### Terrestrial

Terrestrial ferromanganese nodules form in a variety of soil types, including but not limited to ultisols, vertisols, inceptisols, alfisols, and mollisols. Similar to the marine nodules, concretion layers are defined based on iron and manganese content as well as their combination. High iron content nodules appear a red or brown color, while high manganese content appears black or grey. The dominant metal oxide is related to the elements enriched in the nodule. In manganese-dominated nodules, enriched elements include barium, strontium, nickel, cobalt, copper, cadmium, lead, and zinc. In contrast, iron-dominated nodules are enriched in vanadium, phosphorus, arsenic, and chromium.

## Formation

### Marine

Marine ferromanganese nodules form from the precipitation of primarily iron, manganese, nickel, copper, cobalt, and zinc around the nucleus. The mechanism is defined based on the source of the precipitation. Precipitation sourced from the above water column is referred to as hydrogenetic, while precipitation from the sediment pore water is diagenetic. Nodule growth occurs more readily in oxygenated environments with relatively low sedimentation rates that provide adequate levels of labile organic matter to fuel precipitation. When sedimentation rates are too high, nodules can be completely covered in sediments, lowering the local oxygen levels and preventing precipitation. Growth rates for nodules are a current topic for research complicated by the irregular and discontinuous nature of their formation, but average rates have been calculated using radiometric dating. In general hydrogenetic nodules grow slower than diagenetic at approximately 2-5 mm per million years versus 10 mm per million years. The formation of polynodules from multiple nodules growing together is possible and hypothesized to be facilitated by deposited encrusting organisms.

### Terrestrial

Formation of terrestrial ferromanganese nodules involves the accumulation of iron and manganese oxides followed by repeated redox cycles of reductive dissolution and oxidative precipitation. The oscillating redox cycle is controlled by pH, microbial activity, organic matter concentration, groundwater level, soil saturation, and redox potential. Anthropogenic activity could influence these cycles through increased nutrient loading via fertilizers. Assessment of the changing paleoclimate conditions during soil evolution can be explored by analyzing the nodule's concretion structure when combined with dating techniques. Manganese layers typically form at higher redox potentials compared to iron layers, but a period of rapid increase in redox potential can form a mixed layer. As the nodules are formed, trace elements including but not limited to nickel, cobalt, copper, and zinc are incorporated. Trace metals composition is a product of three processes: uptake of parent material in surrounding soil, accumulation of the products of microbial iron or manganese-reducing bacteria, and complexation on the nodule's surface.

## Impact on nutrient cycles

Ferromanganese nodules are highly redox active, allowing for interaction with biogeochemical cycles primarily as an electron acceptor. Notably, terrestrial nodules uptake and trap nitrogen, phosphorus, and organic carbon. The higher rate of organic carbon uptake allows nodules to enhance a soil's ability to sequester carbon, creating a net sink. Phosphorus concentration in the nodules ranges from 2.5 to 7 times the value of the surrounding soil matrix. Microbes in the soil can utilize the nutrient enrichment on the surface of nodules coupled with their redox potential to fuel their metabolic pathways and release the once immobile phosphorus. Along with nutrients, ferromanganese nodules can sequester toxic heavy metals (lead, copper, zinc, cobalt, nickel, and cadmium) from the soil, improving its quality. However, similar to the release of phosphorus by microbes, reductive dissolution of the nodules would release these heavy metals back into the soil.

## Potential as a rare metal resource

The high natural abundance of nickel, copper, cobalt, zinc, iron, and manganese in ferromanganese nodules has promoted research into their use as a rare metal resource. The Clarion-Clipperton Zone in the northeastern Pacific Ocean has been observed as an area containing the highest concentration of resource-grade nodules. A bulk weight greater than 3% for nickel, copper, and cobalt is required to be considered resource-grade. Nodule formation in oxic waters at or below the carbonate compensation depth produces the most desirable rare metal ratio in hydrogenic nodules. As the grade of ores from terrestrial mines has decreased over time, ferromanganese nodules may offer a way to meet the growing global demand for rare metals. However, the low estimated growth rate of hydrogenic nodules of about 2-5 mm per million years categorizes them as a non-renewable resource.

