Mud, glorious mud!

Although we work on different topics and at different times of the day, the coring and CTD teams spend quite a lot of time together. Or rather, Maureen comes with us regularly to do the water sampling of our CTD, we go to their lab to get our fresh water, and we sit together for meals and in the evenings to enjoy a nice cup of tea. Yet, until yesterday morning, I had never taken the time to go and spend a moment in their lab to observe them work. But it is now done, and I even think I understand what they do! (if not, this article shall be corrected…)

Deploying the multicore

Deploying the multicore

It all starts with the collection of a sediment core. Three options are available aboard:

– the gravity core, basic, always the first one to be done, it gives a good idea of the first 4 m or so of the sediment layers;
– the multicore (pictured) is more relevant for the study of the interface between the ocean and the sea floor;
– the piston core goes much deeper into the sea floor, up to 9 m here, but is harder to retrieve and must be avoided when the ship drifts too much.

Once the cores are out of the water, they are walked from the back of the ship (or “aft deck”) where the coring is done to the front, next to our container, into the main lab. They stay here for a while in order to be at room temperature. Then they can go through Maziet’s machine!

The machine whose name I forgot, aka “Maziet’s machine

The machine whose name I forgot, aka “Maziet’s machine

This machine slowly pushes the core through four different instruments. It first gets bombarded by Gamma rays, whose penetration tells us how dense this part of the core is. Second instrument is a magnetometre, while the third measures the sound velocity through the section of the core (indicating its water content), and the fourth its conductivity. All of these give a first idea of what the inside of the core should look like, and in particular where there are abrupt changes in its properties (mirroring abrupt changes in the environment).

I am a bit biased towards the next step, probably because I was allowed to do it. Rhizons. I have no idea where this name comes from, but this is how you call the process of drilling a hole half-way through the core and putting a syringe in it to create a vacuum pump in order to collect water from the core. This water then joins the samples collected by the CTD for future analysis.

My own little rhizons

My own little rhizons

After that, it finally becomes muddy! The core goes back outside where it is sawed open! Well, the plastic is, whereas the core itself is delicately cut with a wire in order to minimise any sediment displacement. Once back in the lab, any bit of mud that has been displaced is removed with a cooking spatula (you know, the type for icing on cake?) and stored in a bag until we have our mud sculpting competition. Buttons are positioned every 10 cm along the half-core, and it gets photographed.

This is where the description work starts and my knowledge stops because we were all busy again. But I gathered that colours have to be identified using an international standardised palette (a bit like when choosing paint in a shop), and smears are made where there are interesting changes. Minerals and grain sizes are assessed, along with the potential species of foraminifera. I actually saw a cute transparent sea snail through Anne’s microscope!

More than 90 m of marine sediments = a lot of half-cores

More than 90 m of marine sediments = a lot of half-cores

And finally, the half-core is added to the stack of half-cores, that was moved a few hours after I took this photo to make room for more half-cores. When they are taken off the ship in October, some will be studied further, while others will be archived if whomever wants to study them in the future. In the meantime, I will go back to my underwater footages that have definitely improved, especially if you like explosions!

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