manganese nodules

manganese nodules When HMS Challenger made its epochal voyage of oceanographic research and discovery under the auspices of the Royal Society and the British Admiralty, scientists aboard made many unusual discoveries. One of the least expected discoveries was the presence of manganese-rich nodules sitting on the sea bed in the deepest parts of the ocean. During the voyage, from December 1872 to May 1876, 133 sites were sampled by dredging the sea floor. Many of the dredge hauls brought up curious black nodules that ranged from pea-sized to grapefruit-sized, but averaged about 3 cm in diameter. The Challenger scientists analysed the nodules and found them to be composed of the oxides of manganese and iron, and also to contain small amounts of copper, nickel, cobalt, and other minerals.

Manganese nodules were treated as little more than scientific curiosities until the period of intense oceanographic research that started in the years following the Second World War. Ocean-going geologists confirmed the Challenger observations and quickly discovered that vast areas of the sea floor are literally peppered with nodules, some of which have combined contents of copper, cobalt, and nickel of several per cent by weight. It was soon realized that the metals in nodules are a potentially vast mineral resource, and by the mid-1960s commercial interest in the nodules had developed. Several ways of mining the nodules were devised, and ways of processing them in order to recover their metal contents were developed. Commercial attention given to the economic potential of manganese nodules in the 1960s and 1970s raised government concerns in many countries and was a major factor in starting the discussions that led to the 1982 Convention of the Law of the Sea. The Law calls for an equal sharing of all marine mineral resources among both developed and developing nations. Commercial mining and processing of nodules has still not commenced (1999), but many experts anticipate that it will not be far into the twenty-first century before mining does start, most likely in the Pacific Ocean.

Although the nodules are generally called manganese nodules, following the terminology introduced by the Challenger scientists, it would be more accurate to describe them as ferromanganese nodules because they contain both manganese and iron compounds. Some of the iron is present as the mineral goethite, but much is amorphous ferric hydroxide. The principal manganese compound is δ-MnO₂, but the manganese minerals todorokite and birnessite are also commonly present, together with amorphous manganese dioxide. Copper, cobalt, and nickel are not present in the nodules as separate minerals. In part they are incorporated in the manganese minerals, and in part they are absorbed on the surface of the amorphous manganese dioxide. Internally, the nodules are layered, like the layers of an onion, and at the centre of each nodule is a small nucleus of foreign matter around which growth occurred. The nuclei are varied and include hardened fragments of deep-sea clay, diatoms, radiolaria, chips of basalt, sharks' teeth, and even whale ear-bones. Nodular growth is thought to be very slow. Radiometric dating of growth bands is not precise but different methods give concordant ages suggesting growth rates of a few millimetres in a million years. The slow growth rate is thought to enhance the absorption of heavy metals like copper and cobalt from sea water.

Closely related in origin to manganese nodules are the ferromanganese incrustations found on exposed rocks of the mid-ocean ridge, seamounts, and other places in the ocean where bare rock is exposed. Up to 20 cm thick, the encrustations are similar in composition to nodules, and like the nodules they sometimes contain potentially valuable concentrations of copper, nickel, and cobalt.

The compositions of both nodules and encrustations vary widely from place to place. The highest concentrations of copper, nickel, and cobalt seem to occur in the regions of the ocean where the rate of sedimentation is least and the nodule growth rate is slowest. These conditions occur on the abyssal plains of the central Pacific Ocean where the underlying sediments are red clays.

The origin of the nodules and the encrustations has given rise to much debate. Although the involvement of microscopic organisms in the precipitations of the manganese and iron-rich layers has sometimes been suggested, no clear proof of biological mediation has yet been demonstrated. Most scientists who have studied nodules now support an inorganic origin for them, with oxidation of iron and manganese in solution as the cause of precipitation.

There are two principal ways by which manganese, iron, and the other metals found in nodules reach the sea: by weathering of terrestrial rocks followed by river transport to the sea, and by deep-sea volcanism, principally along the mid-ocean ridges. The pathways followed by the metals once they enter the sea are less certain. Many nodules contain evidence of dissolution on the under surface where the nodule was in contact with sediment. This indicates that nodules may encounter reducing conditions in the sediments. Under such circumstances, iron and manganese would be reduced and go into solution. From such evidence it is hypothesized that the sediment pile is the main reservoir of manganese and iron and that there is a slow, upward flux, through the sediments, of dissolved manganese and iron. On reaching the sea floor the dissolved species are oxidized and precipitated on growing nodules.

Brian J. Skinner