Subsequently, a massive effort was put in by India for exploration of polymetallic nodules in the Central Indian Ocean Basin (CIOB) by using a number of research vessels. This national programme (running into crores of rupees) is being funded by Department of Ocean Development, New Delhi.

To-date, India has surveyed an area of nearly 4 million sq km in the CIOB. This resulted in the identification of two mine sites, each 150,000 sq km area with equal commercial grade (Cu+Ni+Co wt%) and abundance (kg/sq m) of nodules. In 1984, India filed her claim with the Preparatory Commission (PRECOM) for the International Sea Bed Authority (ISBA). In 1987, India became the first country in the world to be allocated exclusive rights for further exploration.

What are polymetallic nodules and the criteria for their formation?

Polymetallic nodules are Fe-Mn oxide deposits, potato shape, porous, black earthy colour with size ranging from 2 to 10 cm in diameter.

Different shapes of polymetallic nodules .

Nodules occur at nearly 4 to 5 km depth in the deep oceans and they take one million year to grow to one millimeter.

Under the microscope, the cross section of a nodule shows
alternative layers of iron (dark colour) and manganese (light grey colour).

Under water camera, attached to various sampling devices, reveals a dense carpet of nodules overlying the sedimented seafloor.

Thick carpet of polymetallic nodules on the CIOB seafloor		Occurrence of polymetallic nodules in the Indian Ocean

In the Indian Ocean, nodules occur in different basins such as CIOB Wharton Basin, Crozet Basin, Madgascar Basin, Somali Basin, South Australian Basin and Arabian sea.

The prerequisite conditions to form the nodules are:

• Low sedimentation rate
• Availability of nucleus around which accretion of oxides takes place
• Oxidising environment
• Bottom currents of low velocity

How are nodules collected from the seafloor?

Boomerang grabs or free fall grabs are used for collection of the nodules. The recovery of nodules helps to estimate the abundance (kg.sq m). The abundance of nodules varies from traces up to 25 kg/sq m.

A free fall grab being lowered.

Associated nodules and sediments can be collected with grabs such as Petterson, Van Veen and Petterson. Bulk nodules for metallurgical purpose and rocks are recovered by Dredges.

Pettersson grab being lowered for sediment sampling

What is the chemical composition of nodules?

To determine the chemical composition of nodules the samples need to be dried, powdered in an agate mortar, digested in mixture of Hydroflouric, Perchloric and Nitric acids. The digested solutions are analysed by using various analytical instruments. The average composition of nodules from the CIOB is as follows. (Source: Jauhari & Pattan 2000).

Average Chemical Composition of Nodules

Which are the minerals present in nodules?

Todorokite and delta MnO₂ are the two main minerals present in the nodules. Generally , nodules rich in manganese have todorokite and those rich in iron have delta MnO₂.

How are polymetallic nodules formed?

There are three processes for the formation of nodules.

• Hydrogenous process whereby metals are supplied from the water column and these accrete on a suitable nuceli. Hydrogenous nodules have smooth surface texture and are rich in Fe, Co, Ti, P and Pb content. The Mn/Fe ratio of these nodules is ~1.
• Diagenetic process supplies metals from the underlying sediment through the pore water by remobilisation. Diagenetic nodules have rough surface texture and are rich in Mn, Cu, Ni and Zn content. The Mn/Fe ratio is more than 2.5.
• Mixed type which is a combination of hydrogenous and diagenetic types.

The following are some significant scientific results:

• Nodule grade (Cu+Ni+Co %) is inversely related to abundance (kg/sq m).
• Manganese and iron show inverse relation suggesting their different source.
• Nodules with 2 to 6 cm size have high Mn, Cu, Ni and Zn concentration.
• Nodules grow with 1-2 mm per million year.
• Rare earth elements are supplied to nodules in association with Fe, Ti and P from the seawater.

Nodules of the CIOB

The formation of ferromanganese nodules on the ocean floor requires a nucleus, low sedimentation rate, oxidising conditions and low velocity bottom currents.

Bottom topography also plays an important role in the distribution of manganese nodules. The highest manganese concentration are generally found in nodules collected from high relief areas like valleys, followed by hilltops and slopes. Nodule abundance is least in the plains but have the highest content of Mn, Cu, Ni.

In contrast, nodules from hilltop have lowest concentrations of these metals. The size of nodule ranges between 2 and 10 cm in diameter, with a majority of them lying between 2 and 4 cm size range. Nodules in the CIOB are associated with practically all types of sediment and the nodule abundance varies from traces to 20 kg/m.

Nodules from siliceous sediment are smaller in size, rough surface texture, with todorokite as a dominant mineral enriched in Mn, Cu, Ni and Zn suggesting their supply through early digenetic process.

Ferromanganese nodules from red clay area are enriched in Fe, Co, Ti and P suggesting their supply mainly by hydrogenetic process. Rare earth elements are highly enriched in these nodules (~800 ppm) and are generally carried by a single authigenic phase consisting of Fe-Ti-P suggestive of their supply by seawater.

Ferromanganese nodules from the CIOB consist of ~75 % Mn as Mn (IV) and traces of Mn (III) was detected by electron spin resonance spectra. These nodules are formed under less oxic conditions compared to Pacific Ocean nodules. Mossbauer spectra of nodules exhibit a well-resolved doublet suggesting presence of paramagnetic Fe (III).

Research from the project