Sluggish Growth: Newsletter Challenge (September 2016)

This month's Challenge Question: Specs & Techs from IEEE Engineering360:

A researcher studying the velocities of various natural processes is impressed by the glacial pace of one in particular, taking place at a rate of approximately 317 am/s. Occurring at great depths on Earth, it involves the formation of materials of economic interest. What is the process?

And the answer is:

The formation of manganese nodules is one of the slowest processes on Earth, accumulating at a rate between 10 to 100 millimeters per million years. The nodules grow by the precipitation of metal compounds out of ocean water around a nucleus such as a piece of shell on the ocean floor. There are two methods by which this process commonly takes place: hydrogenous growth and diagenetic growth. Hydrogenous growth largely involves precipitation of the manganese oxide mineral vernadite that forms naturally in the ocean. Diagenetic growth is unique in that it involves ocean water that has been enriched with metal compounds gathered after reacting with sediments in the sea floor. Upon emerging from the sea floor, the metal – usually the manganese oxides todorokite and birnessite – contained in this pore water is deposited onto the growing nodules. Most manganese nodules are a mixture of the two processes.

Because of the slow timescale of their growth, manganese nodules tend to grow only in regions of relatively stable environmental conditions such as those provided by a plentiful flow of Antarctic bottom water. This flow clears out fine sediment that would otherwise cover up newly forming nodules, while also stimulating transport of oxygen rich water from the surface to the depths.

The nodules contain a variety of metals other than manganese, including iron, copper, nickel, cobalt, titanium, tellurium, thallium, zirconium, and rare earth elements. This elemental bounty has attracted the attention of mining interests in the past, although the investment required to carry out operations has so far deterred any significant mining projects.