Two heavy machines for sample collection on the South Polar Sea's floor are being prepared on the working deck. The gravity corer (Schwerelot) is a long metal pipe, containing a plastic pipe. It can reach up to a length of 20 meters. It is slammed onto the sea floor under extreme pressure by more than 1.5 tonnes of lead weight. The sediment remains stuck in the inner plastic pipe, which is then transported on board the Polarstern.
The second machine is the MUC (Multicorer), is made up of many small gravity corers with a radius of approximately 2 meters, arranged in a circular configuration. Each can take up to 60cm of sediment from the ocean floor back up to the surface.
The research vessel comes to a stop in order to begin sample collection with the MUC. The machine was lowered to a depth of 704 metres with an on-board winch. Manfred Hagemann, the winch controller, can monitor the progress of the winching process via multiple cameras placed around the winch stands. He needs to be especially vigilant during the recovery of the steel cables (weighing more than a ton), and needs to make sure that they are being wound correctly, and not being squashed by the supporting wheels. If this does occur, then he must immediately stop the winding. However, luckily it all runs as planned and after more than an hour of tension, the researchers can breathe a sigh of relief: The first 30 centimeters of sediment have been extracted.
Sediment sampling with the gravity corer
At the same time, the deck personnel are preparing the second piece of equipment, the gravity corer (schwerelot). This is a much more complicated procedure and requires a lot more experience to operate. Norbert Lensch from the Alfred Wegener Institute, the technical leader, has worked with this piece of equipment for more than 24 years.
Gravity corer just before use
The first gravity core will be taken with a length of 5 metres. In a sturdy cage, clamped by a powerful arm on the deck of the Polarstern, the machine is lowered over the railing of the working deck and positioned 90° to the water surface. Then the gravity corer is fastened onto the winch hook. The machine is allowed to then free-fall into the water, the arm is folded away so as not to interfere with the movement of the steel cable. The corer remains on the sea floor for about 15 minutes, and bores 5 meters into the sediment with enormous pressure. It is then brought up again to the water surface. On board everyone is eagerly awaiting the results of the collection process: 4.63 metres of sediment sample from a gravity core of 5.0 metres. A great success for the project members on board!
The multicorer is brought back on board.
Inside the steel pipe, one finds the plastic pipe containing the core: the core is then divided into one-meter long segments, and exactly marked and numbered before being safely sealed. It is absolutely crucial to be able to refer to the depth that each segment was taken, and in which order they were taken within the core. Various experiments are then carried out on the cores.
Everybody helps when cores are transported into the laboratory.
These cores are very important to the researchers, as different sediment layers can be connected to their respective environmental conditions. So-called drop stones can be found at various locations within the samples. These occur when rocks or stones that were once enclosed in glacial rivers fall into the sea and sink to the bottom due to glacier breakup or moving icebergs. The positions where drop stones are found give researchers further insight into the freezing and thawing history of the region.
The one-metre long pieces are labeled.
The core-measuring bank is a special instrument that allows the slowly extracted cores to be connected to different parameters. These describe the physical characteristics of the sediment based on composition parameters. Conclusions can be made about the environmental conditions occurring at the time. The bank provides information not only about the freezing history of the region but also on the marine microorganism composition of the area. The next gravity core will be 15-20 meters long, and will bring many new results and information to the water surface from this relatively unknown part of the world.
Photos: Michael Trapp / Alfred Wegener Institute
Text: Michael Trapp
Written: Monday, February 26, 2007