What are Polymetallic nodules?

The existence of polymetallic nodules has been known for over a century, since the late 1800s, when they were collected during HMS Challenger’s expedition of 1873–1876. They were found in an area called the Clarion Clipperton Zone, commonly known as the CCZ.

Polymetallic nodules, as their name indicates, are multi-metal deposits that lie on the sea floor; they are not attached to the seabed but lie on top of it. The nodules are potato-sized and look similar to lumps of coal.


Composition of polymetallic nodules 

Polymetallic nodules used to be referred to as ‘manganese nodules’, owing to the high concentration of manganese in them. The composition of nodules varies slightly depending on where they are found. On average, in the Clarion Clipperton Zone (CCZ), each polymetallic nodule is composed of 28% manganese, 1.3% nickel, 1.1% copper and 0.2% cobalt. It is estimated that the CCZ contains more manganese, nickel and cobalt than all the known land-based reserves combined.

In addition to these valuable base metals, nodules of the CCZ contain interesting amounts of molybdenum, titanium, lithium and rare earth elements. The remainder of the nodules is made up of sand, the waste of deep sea organisms and deep sea sediment.


Formation of polymetallic nodules 

Polymetallic nodules form very slowly around a central nucleus; this happens through the process of ‘accretion’, which is growth through the gradual accumulation of layers over a long period of time. If you look at a cross-section of a nodule under a microscope, you are able to see these layers as concentric circles that are not dissimilar to the growth rings of a tree trunk.

The nodule’s nucleus can be an old shark’s tooth or a bit of bone that has fallen to the ocean floor and in many cases it is a fragment of another nodule.


Occurrence and distribution of polymetallic nodules 

Polymetallic nodules can be found in the abyssal plains of all major oceans at water depths between 4,000 and 6,000 m. However, they are most abundant in the Central Pacific Ocean in an area known as the Clarion Clipperton Zone (“CCZ”). It is estimated there are 21 billion tonnes of manganese in the CCZ; that is more manganese, nickel and cobalt than all known reserves on land.