Mic drop: Dropstones are, in fact, polymetallic nodules

In search of oil seeps and to understand regional stra­tigraphy, VBER and TGS conducted two seafloor sam­pling surveys on Greenland’s north-eastern continental shelf over a decade ago. They retrieved samples through dredging and coring along the continental slope. Among the sample material were a multitude of ‘dropstones’, rocks carried out to sea by icebergs, or so was the interpretation at the time.

Dropstones were not the focus of the expedition, so about half of them were thrown back overboard, and the other half were filed away. Years later, Ebbe Hartz, a ge­ologist at Aker BP, had a light bulb moment. Based on his understanding of the region’s oceanography and min­eral-forming processes, he believed that the dropstones might in fact be polymetallic nodules. When the archive was checked, there indeed turned out to be nodules present and a project was launched to study them in detail. Twen­ty-four nodules from three different locations were selected for analysis.

The research revealed two separate groups of nodules, each with its own distinct appearance, chemistry, origin and age. Both nodule types were found at each sample location. The first, and most prevalent group, displays well-developed, thin concentric banding around a nucle­us. These nodules are interpreted to be of hydrogenetic or­igin: they formed over millions of years by precipitation of metals directly from seawater. Age dating indicates that growth began around 13 – 14 M yrs ago. This co­incides with the opening and deepening of the Fram Strait between Greenland and Svalbard and the estab­lishment of deepwater exchange between the Arctic and Atlantic Ocean.

The nodules thus point to a direct link between ocean circulation and metal supply to the Greenland shelf. Arctic water rich in dissolved metals began flowing south while oxygen-rich water from the North Atlantic flowed in the opposite direction. This favoured the necessary oxidising conditions and precipitation of dissolved iron and man­ganese on the seabed. Meltwater from the Greenland Ice Sheet may have provided additional metal input and influ­enced growth.

Nodules from the second group are fragments of larger, tabular nodules. They have an extremely high iron con­tent, around 60 wt%, suggesting a hydrothermal origin. Age dating shows that they are much younger than the hydrogenetic nodules, around 0.3 – 0.4 M yrs, and have grown much faster. Although the origin of this nodule type is a bit of a mystery, mineralisation is thought to be the result of weak and diffuse hydrothermal circulation near the seabed.

There it is, the discovery of the first polymetallic nod­ules in the North Atlantic! An accidental find that almost went unnoticed.