CD179: Deep-sea biology of the Portguese canyons


Daily diary

Wednesday 26 April 2006

Sarah writes:

Well, today was Piston Core Day, a piece of equipment not yet used on this cruise. A Piston Core consists of a very heavy weight at the top of a 6-, 9- or 12-m metal pipe. Into the pipe is inserted a yellow 'drain pipe' of equal length. The Piston Core is used to take samples of the sea bed sediment, so that geologists can study patterns and layers in the sediment to see what kind of sedimentary regime has existed over the past 100,000s years.

Some of the work we are doing on this cruise with the Piston Cores is related to a big earthquake that occurred in 1755. The earthquake's epicentre was in the sea somewhere near to where we are working now. The earthquake shook Lisbon, which was then hit by a tsunami caused by the earthquake. This is how we know the year that the earthquake occurred in, because there are written documents detailing the devastation of Lisbon.

In recent years, scientists who have taken Piston Cores in the abyssal plains off the coast of Portugal have found evidence of an underwater landslide that turned into a large turbidity current around the time of the earthquake. This evidence takes the form of one very thick layer of sand and mud in the abyssal plain sediment cores; these layers are called turbidites because of the turbulent flow of sediment and water that caused them. The hypothesis regarding the source of these turbidites is that the 1755 earthquake caused an underwater sediment landslide that transformed into a turbidity current downslope, and probably ran down the canyons that we are studying on this cruise. This is a good hypothesis, but the problem is that scientists haven‚t found which canyon, and where in the canyon the sediments might have come from. It is because of these unknowns that we carried out TOBI surveys at the start of the cruise, in order to try to find possible scarp structures in the canyons, which might be where a landslide started. Such structures would help Raquel and Veerle decide where to carry out the Piston Cores so that sediment samples can be taken in locations that a landslide might have slid over. If a big hiatus, or missing layer, of sediment is found in the sediment cores, this would indicate that a landslide or period of erosion removed the normal layers of sediment that had been building up on the sea bed.

The first picture shows the Piston Core being deployed into the water. You can see the big weight at the top of the core barrel; this weighs 1,450 kg or 1.5 tonnes ˆ about the weight of a large car! There is a small 'trigger core' attached to the large weight. When the trigger core hits the seabed, the weight is released and drives the long white core barrel into the sediment. When it is pulled out of the sea bed by the ship's winch, the sediment stays in the yellow drain pipe and is brought up to the surface. Today our first Piston Core Deployment carried a 9 m pipe and went down to 3,550 m to the sea bed below.

Deployment of the piston corer...

...and recovery

Bringng the core on board is a tricky operation

The second picture shows the Piston Core being recovered, which is a very tricky procedure because it is so big and heavy. You can see Jez and Terry, two of the cruise engineers making sure that the barrel is placed correctly onto the cradle that holds the core. Once it is safely in the cradle, the barrel can be removed from the weight, and then the drain pipe can be removed.

We were very excited about getting our first Piston Core sample, and so we were then very disappointed to find that the core was completely empty from top to bottom! We could see that there was some very gritty sediment on the bottom of the barrel, and so it looks like the core hit a hard layer of coarse sediment and could not penetrate any further. This meant that no sediment could be collected in the core. Despite this setback, we decided to do another Piston Core, but at a different place on the seabed. The second time we used a 6 m barrel, in only 3,200 m of water. This time we were successful, and had a core full of mud! By the time the second core came up it was night time, and so we had to start work on the core in the dark.

As the yellow 'drain pipe' is removed from the barrel, it is cut into 1.5 m sections, you can see Veerle doing this in the photograph with John and Raquel holding the pipe. When the two parts have been separated you have to be quick to put caps over the cut ends so that the mud doesn't fall out. When the pipe has been cut up into sections, we then take them inside to then cut them into half lengthways. You can see Veerle and John doing this with the core splitter.

Cutting the core into sections

Splitting the core

Raquel makes careful notes

Amazing what a bit of deep-sea mud can tell us!

Once the cores have been halved, one half is placed straight away into storage boxes and placed into the core store in the hold of the ship. The other half is taken outside to be photographed and Raquel studies it for interesting features. Raquel also writes down details about the core, for example how long it is and whether there is already evidence of differences in the layers of sediment. Raquel is looking for evidence of the landslides down the canyons we are working on, but until Raquel can work on them further in the lab back at the National Oceanography Centre we can't tell for sure what is in these cores.

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April 2006
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