Written by Jakob Lang

What do geoscientists do on a ship and how do they get their precious rocks after all? What is a dredge and how are the rocks processed on board? A short introduction into seafloor sampling.

Here on board R/V Sonne we can’t just take hammer and compass, walk out into the field, search for a nice outcrop and measure and collect interesting rocks. The objects of desire rest several 1000´s of meters below the sea surface. There we have pressure of 100 Bar per 1000m depth. In comparison: A normal car wheel has between 2 and 3 Bar. Thus, we have to use different methods to take our samples. We use echo-sounder, data processing programs and the heart of our research: The chainbag dredge. The dredge will be dragged over the seafloor on a winch. The teeth of the claw “bite“ rocks out of the seafloor which then will be caught within the chainbag and brought on deck.

For us not to fish in cloudy conditions we look at the seafloor with a multibeam echo-sounder. It sends out sound signals in the style of a fan towards the seafloor, which will then be reflected by solid surfaces and send back to the ship. Those signals will then form a picture, which shows the seafloor topography based on a colour chart. Out of this data, other data processing tools can build a 3D-model of the surface below. This is filled with information regarding steepness of the slope, location and direction of under-water-structures on which we can search for suitable tracks for dredging.

Thanks to the extensive steering elements of R/V Sonne and the know-how of the nautical officers on board it’s possible to steer the ship precisely to a certain point, keep it there even without an anchor and move it precisely along the track, even back- or sidewards. When a desired location is reached, the dredge needs to be put into water. Scientists and crew work hand in hand to bring the device safely over board. The teamwork of deck’s crew, navigators, winch-operator and scientists is crucial for this and works excellently during our cruise. As soon as the dredge makes ground contact, the ship will be moved along the track. During this movement cable will be laid out regularly in order to keep the dredge on the seafloor. When the end of the track is reached, the ship stops and the dredge will be slowly dragged over ground. Thanks to the DSHIP program it’s possible for us to take a look at the rope-tension and get a hint whether the dredge bites some rocks out of the seafloor.

Even though we give our best during selection and processing of a dredge track every now and then we get an empty dredge back. However, whenever we made a good catch, the initial sample processing on board can begin. In order to get a first look into the rocks the samples will be cut into halves and looked at, based on that they can be sorted into respective groups of samples. Afterwards each sample will be looked at and described through a microscope. They get a unique sample number and will then be bagged for transport. Further processing and analysis will be done at the institutes at home.

We hope you got a first look into the “geology team’s“ work on board. Look forward towards the next blog entry of SO307!