JC010: Mud volcanoes and submarine canyons


 JC10



Cruise diary


Day 12: Monday 2 July 2007
Position: 47°50.28N 10°12.57W (Google Earth reference: 47 50’28”N 10 12’57”W)
Weather: sunny


Tina writes:

"For the past two days we have been in transit, as the scientific equipment cannot be deployed in transit the captain used this time to drill the ships crew in emergency procedure.  So at 16:15hrs the General Muster rang and we all had to go to an assembly point.  Part of the practice was a demonstration on how to put on the immersion suit and guess who they asked to put the suit on!

Tina pulls the suit out of its bag...

...putting it on...

...et voila! The latest look in sea survival gear!

It took me until after supper to recover and then it was off to the excitement of a piston core.  As I had never seen a piston core before I was really looking forward to it…….

First they got out the biggest crane on board, and we are talking really big. 
They need this as the piston core can be up to 21m long.

 

This is a picture of the piston core (long white object) lying alongside the ship on its cradle.

The piston core is lowered into position and then freed from its cradle using the crane.  When it has been freed it can be lowered into the sea.

The piston core is used to take long sediment samples from the sea floor. This time the core we collected was 10m long.  Inside the tube is a device that acts a bit like a syringe, creating a vacuum and pulling up the core while the tube its self is pushing into the sediment.  This method avoids ‘squashing’ the surface of the sediment when the corer first reaches the ground.

After we collected the core we had to slice it into sections.  Tomorrow we will be looking at what we collected.  As this is looking at the geology of the seabed (what the seabed is made from) and not the biology (living organisms) it is not so important that the core is looked at immediately, and we finished with the cores at 12 midnight.

Here are the cores waiting to be studied.  The sediment in the cores could be 18 thousand years old.  When we look at what is in the cores the geologists may be able to work out what happened in the region in the past 18 thousand years!"


Richard writes:

"Tina has given you a visual introduction to piston coring. Now the “Science Bit”!

With push cores and megacores where the core is forced into the seabed so trapping the sediment inside, piston coring works by sucking sediment into the core barrel. Aside from Tina’s introduction, there is already lots about how piston coring works in the blog for Wednesday 26 April 2006 of the CD179 cruise and you can also find out more on the page about sub-seafloor sampling.

 

Benefits of piston coring:

  • You can gather completely undisturbed sediment (no additional compression occurs)
  • You can go very deep in the sediment (up to 20m if desired…and lucky)

 

Why we don’t use it all the time.

The piston core is not used all the time because it is very time consuming – in preparation, deployment and recovery, but also in planning as a lot of pre-analysis of the canyon floor is required to help judge where to place it. For the samples that were needed this time, it was necessary to take core samples from the levees (the banks of sediment overspill on the canyon floor). This is because the bottom of the canyon sees much more sediment flow which can also erode the seabed thereby creating a poor geological record of past events (they may simply have been eroded away). However, on the levee the chances of recording an event as a sediment layer (all stack up on top of one another) as they spill out of the channel is much higher.

 

Why we used the piston core this time and what we were looking for

As I have already mentioned, megacores tend to be used more often. These are preferred by biologists, as they are only really interested in recent and current events. Geologists, however, are much more interested in how we have arrived at this point, and this is why they want to go deep in the sediment, where the events of previous years have been trapped (or even previous 100,000's of years). Basically, the further down in the sediment you go, the further back in time you ‘travel’. This is because as new sediment is added to the top, the older sediment becomes buried beneath. In some ways it is similar to an archaeologist digging for ancient settlements which have, through time, become buried too.

At present, the canyon gets limited sediment input. During the last ice age, the sea levels were up to 150m lower than today, bringing the coast nearer to the head of the canyon. A glacier surged down the Irish Sea and a large river system (River Thames and Rhine combined) flowed through the English channel. These supplied sediment to this canyon and the French canyons, all of which avalanched down the system.

The object of this piston core is to find out more about the last 10,000 years, after the ice melted, but also during the ice age when we think thee canyon was much more active...."


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