Ferromanganese nodules

The size of marine ferromanganese nodules can range from a diameter of 1-15 cm, surrounding a nucleus.[2][3] 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.[2] Total nodule composition varies based on the formation mechanism, broadly broken down into two major categories: hydrogenetic and diagenetic.[3] 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.[2] The parent mineral for hydrogenetic nodules is vernadite and buserite for diagenetic nodules.[2] The majority of observed nodules are a mixture of hydrogenetic and diagenetic regions of growth, preserving the changes in formation mechanisms over time.[3] 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.[2] 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.[2]

Terrestrial ferromanganese nodules form in a variety of soil types, including but not limited to ultisols, vertisols, inceptisols, alfisols, and mollisols.[1] Similar to the marine nodules, concretion layers are defined based on iron and manganese content as well as their combination.[1] High iron content nodules appear a red or brown color, while high manganese content appears black or grey.[1] 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.[1] In contrast, iron-dominated nodules are enriched in vanadium, phosphorus, arsenic, and chromium.[1]

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.[2] Precipitation sourced from the above water column is referred to as hydrogenetic, while precipitation from the sediment pore water is diagenetic.[2][3] Nodule growth occurs more readily in oxygenated environments with relatively low sedimentation rates that provide adequate levels of labile organic matter to fuel precipitation.[2] When sedimentation rates are too high, nodules can be completely covered in sediments, lowering the local oxygen levels and preventing precipitation.[2] 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.[1][2] In general hydrogenetic nodules grow slower than diagenetic at approximately 2-5 mm per million years versus 10 mm per million years.[2] The formation of polynodules from multiple nodules growing together is possible and hypothesized to be facilitated by deposited encrusting organisms.[2]

Formation of terrestrial ferromanganese nodules involves the accumulation of iron and manganese oxides followed by repeated redox cycles of reductive dissolution and oxidative precipitation.[1] The oscillating redox cycle is controlled by pH, microbial activity, organic matter concentration, groundwater level, soil saturation, and redox potential.[1] 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.[1] 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.[1] As the nodules are formed, trace elements including but not limited to nickel, cobalt, copper, and zinc are incorporated.[1] 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.[1]