Seamounts grow through volcanic activity to great heights on the sea floor over millions of years. They are found in all of the oceans and reach heights of 1000 to 4000 metres. Smaller seamounts are also called knolls.
A coating on the rocksCobalt crusts are rock-hard, metallic layers that form on the flanks of submarine volcanoes, called seamounts. Similar to manganese nodules, these crusts form over millions of years as metal compounds in the water are precipitated.
As with manganese nodules, deposition occurs very slowly. Crusts grow 1 to 5 millimetres per million years, which is even slower than nodules. Depending upon the concentration of metal compounds in the sea water, crusts with different thicknesses have formed in different ocean regions. On some seamounts they are only 2 centimetres thick, while in the richest areas thicknesses can be up to 26 centimetres. Because the cobalt crusts are firmly attached to the rocky substrate, they cannot simply be picked up from the bottom like manganese nodules. They will have to be laboriously separated and removed from the underlying rocks.
It has been estimated that there are over 33,000 seamounts worldwide. The exact number is not known. Around 57 per cent are located in the Pacific. The Pacific is thus the most important cobalt crust region in the world. The western Pacific is of particular interest. The world’s oldest seamounts were formed here during the Jurassic period around 150 million years ago. Accordingly, many metallic compounds were deposited here over a long period of time to form comparatively thick crusts. This area, around 3000 kilometres southwest of Japan, is called the Prime Crust Zone (PCZ). The amount of crust in the PCZ is estimated to total 7.5 billion tonnes.
- 2.18 > Manganese nodules and cobalt crusts contain primarily manganese and iron. Because iron is plentiful in land deposits, it is not a key factor in marine mining. For the other elements making up lower weight per cents of the deposits, however, there are great differences to occurrences on land. In the manganese nodules nickel and copper predominate, while in cobalt crusts cobalt, nickel and rare earth elements are more significant.
A metal-rich crustLike manganese nodules, cobalt crusts also represent a very large metal resource in the sea. As the name suggests, the crusts contain a relatively large amount of cobalt compared to deposits on land and to manganese nodules. The largest share of metals in the cobalt crusts, however, consists of manganese and iron. The crusts are often more precisely referred to as “cobalt-rich ferromanganese crusts”. Tellurium is also comparatively abundant in cobalt crusts. Tellurium is necessary particularly for the production of highly efficient thin-film photovoltaic cells.
In absolute terms the crusts of the Prime Crust Zone do not contain as much manganese as the manganese nodules of the Clarion-Clipperton Zone. However, the quantities of manganese in the PCZ are still almost 3 times greater than the economically minable amounts on land today. Furthermore, in the southern area of the PCZ, comparatively high contents of rare earth elements are found in the crusts.
- 2.19 > Cobalt crusts occur in different ocean regions than manganese nod-ules. Each of these resources has its own especially abundant regions. The most important cobalt crust area is the Prime Crust Zone (PCZ) in the western Pacific. The area of greatest manganese nodule concentration is the Clarion-Clipperton Zone (CCZ).
- 2.20 > Cobalt crusts are especially abundant in the western Pacific within a region the size of Europe, called the Prime Crust Zone (PCZ). When compared to deposits on land and to the manganese nodule area of the Clarion-Clipperton Zone (CCZ), it is notable that the occurrence of cobalt and tellurium in particular are comparatively large in the PCZ, with amounts exceeding both the land deposits and those in the CCZ.
Strong currents around seamountsCobalt crusts form on all exposed rock surfaces on undersea rises, particularly on seamounts and knolls. Seamounts act somewhat like gigantic stirring rods in the sea to produce large eddies. Nutrients or other materials that rain down from the sea surface or that are transported by ocean currents are often trapped by these eddies at the seamounts. These can include metallic compounds that are deposited on the rocks. An important precondition for the formation of cobalt crusts is that the rock and the growing crusts remain free from sediments. This condition is met at the seamounts and other elevated areas. Currents carry the fine sediments away and keep the rocks and crusts exposed.
Cobalt crusts are found at water depths from 600 to 7000 metres. Studies at seamounts have shown that the thickest crusts and those richest in resources are located on the upper areas of the seamount slopes, where currents are most active. On the average these lie in water depths of 800 to 2500 metres, near the oxygen minimum zone. Analyses also show that the crusts between 800 and 2200 metres have the highest cobalt contents. Researchers do not know the reason for this. Like a sponge, or the activated charcoal used as a filter in aquariums, cobalt crusts are very porous. Thanks to the many micrometre-sized pores, the crusts have a large internal surface area. In the same way that pollutants are trapped in the pores of an activated charcoal filter, metal compounds are deposited on the large surface areas of the crusts. Because the dissolved metals occur at very low concentrations in sea water, growth of the crusts requires very long periods of time. The crusts are mainly formed through the deposition of iron oxide-hydroxide [FeO(OH)] and manganese oxide (vernadite, MnO₂). The other metals are deposited with the iron oxide-hydroxide and vernadite on the crust surfaces rather like hitchhikers. The reason is that, in the ocean, various metal ions attach themselves to the iron oxide-hydroxide and vernadite molecules in the water. Iron oxide-hydroxide is slightly positively charged and thus attracts negatively charged ions such as molybdate (MoO42-). Vernadite, on the other hand, is slightly negatively charged and attracts positively charged ions such as cobalt (Co2+), copper (Cu2+) or nickel (Ni2+). Incidentally, most of the metal ions contained in sea water originate from land. Over time they are washed out of the rocks and transported by rivers to the oceans. Iron and manganese, however, usually enter the ocean through volcanic sources on the sea floor called hydrothermal seeps.
Crust mining in sovereign territory?Manganese nodules and cobalt crusts are of equal interest for future marine mining because they contain traces of many industrially important metals that, because of the immense tonnage of the deposits, are of economic interest. But there are important differences with regard to the exploration and future mining of the crusts. One of these, for example, is the legal situation. In contrast to manganese nodules, most of the richest crust occurrences are not found in the international waters of the high seas but in the Exclusive Economic Zones (EEZs) of various island nations. Thus the International Seabed Authority (ISA) will not be responsible for determining the conditions for future mining there. Rather, the respective local governments will have jurisdiction. However, to date no country has presented concrete plans.
For those crust deposits in international waters, on the other hand, a binding system of regulations has recently been established. In July 2012 the ISA adopted internationally binding regulations for the exploration of such crust occurrences in regions of the high seas. It is true that China, Japan and the Russian Federation at that time had already submitted working plans to the ISA for future exploration in the international waters of the western Pacific, but the council and the assembly of the seabed authority first have to approve these. The working plans specify what basic information the countries want to collect in the upcoming years, including taking samples from the sea floor and analyses of the crusts, depth measurements or studies of faunal assemblages.
Problematic thickness measurementsThe exploration of cobalt crusts is also fundamentally different from the manganese nodule situation in some technical aspects. Manganese nodules can be brought quickly and easily on board with a box corer, similar to a backhoe, and then sampled to measure the metal content, for example. Furthermore, the nodules are relatively evenly distributed over the sea floor. This allows relatively straightforward assessment of the deposits by photos and video recordings, particularly with respect to the size of the nodules. Sampling and measurements of the thickness of cobalt crusts, however, are much more difficult because rock boulders have to be torn or drilled out. Local thickness differences are poorly constrained and the spot sampling is extremely time-consuming and expensive.
- 2.21 > Many metal ions end up in the cobalt crusts as “hitchhikers”. In the water, the metal ions attach themselves to iron oxide-hydroxide and vernadite molecules, and are then deposited onto the porous surfaces with them.
- Instruments that could be pulled through the water near the bottom to accurately measure the crust thickness while passing over would be much more efficient. This would allow large areas to be studied in a relatively short time. Scientists are therefore working to refine high-resolution acoustic instruments. These send sound waves into the sea floor and record the reflected signals, then calculate the layered structure of the subbottom. This kind of apparatus is standard technology in exploration for other resources on the sea floor. However, instruments precise enough to measure cobalt crust thicknesses to the nearest centimetre and to distinguish them from the underlying rocks are not yet available.
- An alternative method of assessment might be gamma-ray detectors, which are already being used today for measuring rock layers on land.
Many rocks contain radionuclides, which are unstable atoms that can decay and emit radioactive waves, or gamma rays. The detectors can measure these rays. Because radionuclides are present in different combinations or numbers in every rock unit or stratum, different rocks can be distinguished from one another based on their gamma-ray patterns. The crusts and the underlying volcanic rocks of the seamounts are significantly different in their mixture of radionuclides. Because this method is very precise, the thickness of cobalt crusts can be quite accurately assessed. As yet, however, there are no appropriate detectors available for routine use in the deep sea.
- 2.22 > Visually unimpressive, but extremely attractive for mining and metal companies: cobalt crusts on the sea floor.
Little more than conceptual studiesIt is also still not clear how the crusts can be mined at all in large volumes in the future. So far only conceptual plans have been presented and laboratory experiments carried out. The concepts being worked on by engineers include caterpillar-like vehicles that peel the crusts away from the stone with a kind of chisel, and pump them to the ship at the surface through special hoses. Specialists estimate that more than 1 million tonnes of cobalt crust material will have to be extracted to make the mining economical. Presumably, this can only be achieved if the crusts have a thickness of at least 4 centimetres. The caterpillars need the capacity to handle this. Moreover, they will also have to be able to work on the rough terrain of the seamount flanks.
For mining the cobalt crusts – and likewise for the manganese nodules – the transport of minerals from the sea floor to the ship also remains a challenge. Pumps and valves must be extremely resistant to wear in order to withstand the high demands on equipment. Engineers are presently testing the durability of hoses and pump prototypes using glass marbles, gravel and rubble. But it will be at least 5 years before a prototype of a conveyer system with caterpillar vehicle, pump technology and riser string can be realized.
Species-rich seamountsWith regard to protecting the environment, it is actually fortunate that the technical solutions for economical mining are not yet available, because it is not yet known to what extent the mining of cobalt crusts will damage deep-sea habitats. So far only a few hundred seamounts around the world have been thoroughly investigated by marine biologists. Many marine regions along with their seamounts have not been investigated at all with respect to their biology. Biologists therefore deem it necessary to investigate additional areas and biological communities on seamounts before mining of the crusts begins. The later it begins the more time they will have for this.
It is known that the species assemblages of seamounts vary significantly from one marine region to another. Like mountains on land that, depending on their geographical position and height, provide different habitats for different species, the species composition and diversity on seamounts also varies. In the past it was assumed that many endemic species occurred here. More recent studies have not been able to verify this presumption. Seamounts are also important for free-swimming organisms. This is probably related to the special current conditions here. The circling currents, for one, tend to keep nutrients near the seamounts. Secondly, nutrient-rich water is upwelled at seamounts from nearbottom currents, which leads to increased plankton growth. Because of this abundant supply of food at seamounts, both sharks and tuna are known to visit them frequently, for example in the southwest Pacific. These seamount areas are thus also very important for tuna fishing. In light of the estimated total number of at least 33,000 seamounts around the world, the knowledge we have about them is still fairly limited because few have been thoroughly investigated. In order to at least roughly estimate the diversity of the deep sea and how strongly deep-sea habitats around the world differ from each other, the GOODS Report (Global Open Oceans and Deep Sea-habitats) on the worldwide marine and deep-sea habitats was commissioned by UNESCO and published in 2009. This report divides the ocean into different bioregions. Depth was also especially considered. The report defined 14 bioregions within the depth range between 800 and 2500 metres, which is where the thickest and richest crusts also occur. The classification is based on biological information from deep-sea expeditions as well as oceanographic parameters, including carbon, salt and oxygen content, and the temperatures at certain depths. The structure of the sea floor, or topography, was also considered. This provided a distinction between flat deep-sea areas, hydrothermal seeps and seamounts. This classification system is still very rough, as the authors of the study admit, but the GOODS Report helps to predict which habitats can be expected in which marine regions.
- 2.23 > At a depth of 2,669 m, black coral, primnoid coral, and feather stars cover a part of the Davidson Seamount off the coast of California.
- Many animal species that live on or near seamounts are characterized by extremely slow growth rates and by producing relatively few offspring. The cold-water corals, for example, which live in the deep sea, can live for hundreds or even up to 1000 years. Some deep-sea fish also live to be more than 100 years old. They do not become sexually mature until around 25 years of age and only produce a few eggs at a time. Relatively large numbers of such species are often found at seamounts. Because they produce low numbers of offspring, they are particularly endangered by fishing or destruction of their habitats. If the adult animals die there may not be enough offspring to revive stocks.
Studies off Australia and New Zealand have shown that fauna at seamounts recover very slowly from intervention. For example, it has been shown that in areas where trawl nets have been used, even after an interlude of 10 to 30 years of inactivity, the fauna were significantly less species-rich than areas that had not been damaged by the trawl fisheries.
Scarcely studied – life on the cobalt crustsTo date only a few expeditions have been carried out with the explicit goal of investigating the habitats of cobalt crusts. Studies carried out by Japan between 1987 and 1999 in cooperation with SOPAC member states (Secretariat of the Pacific Community Applied Geoscience and Technology Division) are one example. The aim of these expeditions was to study the habitats at locations of various mineral resources in the ocean – the cobalt crusts, manganese nodules and massive sulphides – in the Exclusive Economic Zones of the island nations Kiribati, the Marshall Islands, Micronesia, Samoa and Tuvalu.
Thousands of underwater photographs were taken in order to identify the presence of organisms. Although the areas photographed, at 0.35 to 2 hectares, were relatively small, the researchers discovered a great diversity of organisms. In the megafaunal size class (larger than 2 centimetres), many attached, or sessile, species such as corals and sponges were identified. Sea pens and delicate colonies of small polyps were also described. The seamounts are characterized by a rocky substrate and strong currents, and these kinds of organisms are well adapted to these conditions. They are all filter feeders, and sieve food particles out of the water. Seamounts are an ideal habitat for them because the ocean currents provide them with abundant food. In addition, the photographs revealed crabs, starfish, sea cucumbers and squid, as well as xenophyophores, one-celled animals several centimetres in size belonging to a family that are usually less than one millimetre in diameter.
- 2.24 > Xenophyophores are poorly studied one-celled organisms that live in the deep sea, often on the slopes of seamounts. This specimen is 20 centimetres in size.
Assessing the impacts of miningScientists call for more detailed studies of the habitats on seamounts with abundant cobalt crust deposits before submarine mining can even begin. This applies particularly to the island nations in the southwest Pacific, whose territorial waters contain the richest crusts. After the joint studies with Japan, SOPAC members are now carrying out further research at seamounts that have been too poorly studied so far. Because cobalt crusts are limited to undersea rises, their extraction will be on a smaller scale compared to manganese nodules. The sediment cloud produced would also be significantly smaller than in the mining of manganese nodules, because no soft sediment would be stirred up. The details of cobalt crust mining impacts for the future are still unknown. According to experts, the following problems can be expected, which are very similar to those for manganese nodule mining:
- The machines used to strip the crusts would stir up rocks and particulates. Although these particle clouds would not be as large as in manganese nodule mining, there is still the fundamental hazard of a drifting cloud that would harm other habitats.
- In the harvested area all sessile organisms, the predominating groups of organisms on the cobalt crusts, would be destroyed.
- The use of harvesting machines and the pumping and cleaning of crust material would create noise and vibrations, which would disturb and drive away dolphins and whales.
- Waste water accumulated during harvesting of the crusts would be discharged back into the ocean. This would also produce a sediment cloud.
- The lights on the ships and harvesting machines could disturb marine birds and mammals as well as fish.
- The disposal of everyday ship refuse would pollute the ocean.
- 2.25 > In cross section the black cobalt crust, several centimetres thick, is easy to recognize against the light- coloured volcanic rock. This rock comes from the Louisville seamount chain in the southwest Pacific, which comprises over 70 seamounts.
- Proponents of mining stress that manganese nodules and cobalt crusts are present as thin layers lying directly on the sea floor or on the flanks of seamounts. In contrast to ore deposits on land, they are thus a two-dimensional resource that can theoretically be extracted with relatively little effort. On land, on the other hand, ores are extracted from mines or gigantic open pits, in which machines dig more than 100 metres deep into the earth. For production of these three-dimensional reserves, millions of tonnes of earth (overburden) have to be removed and transported before the actual ore can be extracted. This destroys entire regions and causes people to lose their homelands. Marine mining, however, would be a comparatively small intervention because only the surface of the sea floor or the seamount would be removed. There would be no need for infrastructures like streets or tunnels. There would also be no overburden piles. Due to the paucity of marine biological studies, the advantages and disadvantages of marine mining can hardly be evaluated at present. It is still unknown to what extent the mining would change life in the sea and what the eventual consequences would be for people and fisheries. These open questions can only be answered through continued intensive research and the necessary financial support for appropriate expeditions. Some researchers, sceptical biologists in particular, call for the harvesting of large experimental areas in pilot projects before industrial mining may commence, in order to be able to assess the impact of a large-scale mining operation. Ministries of research or the Euro-pean Union, for example, could provide financial support for such a large-scale test mine.