Dredging: Dragging along the seabed to collect rocks

During the Arctic Ocean 2016 research cruise, we have been dredging to collect samples of rocks from the bedrock.

Cleaning the rocks from the dredge from mud (photo courtesy of Thomas Funck)

Studying these rocks allow scientists to get a better view of the origin of the seafloor and the transportation of rocks and gravel. Furthermore, the data is essential for determining if certain seafloor features are an extension of the Canadian continental shelf, which Canada is very interested in for their UNCLOS claim (more about this, and the background for the research cruise here).

Dredging is done by lowering a dredge bag with winch to the sea floor. The dredge bag is a metal net with a “mouth” with teeth, like in the photo below. The teeth pull off pieces of rock, and the rock pieces go into the dredge bag.

The dredge bag after coming up, being emptied of 700 kg (!) of rocks (photo courtesy of Thomas Funck)

The first procedure of dredging is determining the location. The area of interest is determined in advance, by looking for steep slopes of subsea ridges on the rough seabed map (as I have mentioned before, the seafloor of the world’s oceans and especially in the Arctic is not well mapped). When we get to the location, the seafloor is mapped in detail with Oden’s Kongsberg EM122 multibeam echo sounder, by sailing over the area in low speed. The mapping data look like in the photo below, which is one of the actual sites we dredged during this cruise. I think it looks really beautiful!

What the detailed mapping of a feature on the seafloor with the multibeam looks like. We dredged this site during the Arctic Ocean 2016 research cruise. The colour scale illustrate the depth, and the red tracks are the potential dredge tracks determined in advance.

There are two main reasons for dredging on a steep slope. A slope of more than 25⁰ have a low chance of containing drop stones, stones that have been transported from other places by glaciers. Drop stones do not represent the bedrock at the location, so you do not want them in the dredge. Furthermore, sediments will not settle on steep slopes. Thus, by dredging a steep slope you maximise the chance of your dredge bag being filled with stones who represent the actual bedrock, and not drop stones or clay sediments (but we do collect sediments also).

Successful dredging actually depend on the ice drift. Dragging the dredge bag by sailing with the engines will produce too much stress on the cable, so we have to be drifting. Therefore, the ice drift have to be in the right direction and speed. We usually test the ice drift by “parking” in an ice floe and seeing how the ship is moved.

When the dredge bag is back up to the surface, and hopefully full with hundreds of kilos of rock, the rocks are cleaned, photographed and packed up to send to researchers.

During the Arctic Ocean 2016 cruise we performed three dredging operations, and collected a total of almost 1000 kg of rock!

Trine Andreasen (University of Aarhus) logging positions and weights of the dredge during operations.

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