CD 157: Investigating the submarine canyons off Portugal


 


Why do we use different corers?


Above: seafloor mapping is used to choose coring targets

Each corer has a different purpose. The type that we use is decided by looking at the pattern of the sediments from previous samples, our expectations and a detailed sea floor mapping process. As it takes a long time to deploy them and move between sites, the decision is made by Phil Weaver, Principal Scientist and Doug Masson who is expert at interpreting sea floor data. Choose the wrong one and you could end up with a useless sample or a broken piece of equipment.

All of the corers are lowered at about 60 meters per minute to within 100-200m of the sea bed. We know this depth from the echo-sounder, the winch records the amount of wire that has been wound out [‘veered wire out’]. From here they are lowered more carefully. A monitor in the winch control (right) measures the strain on the wire in tonnes. A typical value would be 4 to 5 tonnes.

After the corer has penetrated the sediment the strain drops. This is the sign for the winch man to slowly haul on the rope. As it is pulled back the strain increases - up to about 7 tonnes so far on this trip. [It is possible for the longer corers to temporarily anchor the ship to the seabed as they cannot be pulled out directly!]

The Kasten Corer

The Kasten Corer is a stainless steel  box, rectangular in section, 15cm on each side and 2,3 or 4 metres long. It has a trap at the bottom and a weight at the top which pushes the box into the sediment. It is particularly useful for taking large samples of soft sediment. From the sample collected we take two tubes of mud along the length of the box and a section of thin slabs which are sent off for a whole range of tests. This corer is the muckiest when on deck and takes a bit of cleaning with the hose pipe.

The Piston Corer

This consists of a long tube which, on our trip may be up to 18m long and 150mm diameter. It has a much larger weight on top. The piston is a clever device which works like the plunger in a syringe. Its purpose is to minimise the compression of the layers as the corer is pushed through. We are trying a new version that has been borrowed from Dutch scientists.

It returns with a long cylindrical sample which is cut into 1.5m lengths, each is carefully labelled so that we know their order and the right way up.

In the wet lab we slice these in half lengthways. Russell describes the sediment pattern and Belinda is responsible for storing. One half is photographed and held as an ‘archive section’. The other half will be used for the tests done back at SOC. Both have to be stored at 4oC to preserve them.

The piston inside the Piston Corer is ‘triggered’ by a smaller corer which is attached to a trigger arm. As this hits the bottom it also takes a small core. This is particularly useful as it will be of the surface which is often blasted away by the large impact of the main corer.

The mega corer

We also use a mega corer. This takes very careful samples of the sea bed with the water immediately above it. Plastic tubes are slowly forced into the sediment and held in place by spring loaded lids top and bottom. This sample is useful for biological sampling of the organisms found in the top sediments and for a really accurate profile.

In summary, the piston corer takes long samples [up to 18m] but messes up the sediment at the sea bed. The mega corer takes excellent samples of the seabed. The Kasten corer takes large volume samples.

Have a look around the rest of the web site for more pictures of each corer type in action.


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© Challenger Division for Seafloor Processes
April 2004
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