A few of you have asked about our Piston core and how it works and more importantly how do we get such a large corer back onboard!
I have been thinking about writing a comprehensive document on this for a while so I will try my hardest to explain how it all fits together and how we get a core from the seabed. I'll also try and throw in some history...
Right, piston coring is one of the more common sea floor sampling methods that we use. The piston corer was invented in 1947 by Professor Borje Kullenberg to enable the Swedish Deep Sea Expedition to collect long samples of sediment, up to 24 meters long, from beneath the sea floor.
Traditionally, we used Gravity Corers which were tubes with big heavy lead weights attached to them and driven into the seabed. Kullenberg made modifications to this design, adding an internal piston that helps scientists gather even longer mud samples. Piston corers, like gravity corer, are generally used in areas with soft sediment, such as clay. A gravity corer is just a weighted pipe that is allowed to free fall into the water. Piston corers have a piston mechanism that is triggered when the corer hits the bottom. The piston helps to avoid disturbing the sediment.
Above: A traditional gravity core
onboard RRS James Cook during JC015
So how does it capture the mud? Well, if you have ever poked a straw into a drink, put your finger over the top of the straw, and then extracted the straw with the fluid trapped inside, you have an idea of how the piston corer works. Like the straw, the corer plunges into the seafloor and collects mud in its hollow open pipe. A seal on the bottom of the device (core catcher) will retain the sediment sample during retrieval.
This piston core acquires samples that are virtually undisturbed, making this tool and modified versions of it a very important tool in modern sea floor sampling
The Parts of A Piston Corer
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Above: Author's diagram of our piston core. Thanks to the team for helping me with this!
Pic 5: The top of the piston core. The wire connected to the piston goes into the hole. The trigger arm sits on the metal cross bar.
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Pic 1: At the end of the Core Barrel is the core cutter
Pic 2: Inside the barrel just above the core cutter is the "core catcher". The core catcher is made up of lots of "fingers" as the barrel drives into the mud these "fingers" are pushed up. When the barrel is lifted they spring back stopping the core from falling from the barrel.
Pic 3: A collection of core barrels and plastic liners onboard RRS James Cook. The plastic tubes sit inside the solid metal core barrels.
Pic 4: The piston which sits inside the plastic lining.
Left (Pic 6): The trigger arm. Just visible on the left of the image is the hole which clamps onto the cross bar in Pic 5. When pressure is released from the arm this mechanism springs open releasing the piston core towards the seabed
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Above: Our piston core is winched over the side of the RRS James Cook just after a sampling run. Notice the long coring tube where the sea floor sediment is collected and the big heavy the weight located on top of the tube |
Above: The trigger arm with no tension - showing the mechanism in the "open" position just before attaching to the Piston Core
Above: The trigger arm with weight attached - showing the mechanism in the "closed" position |
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| Pic 1: The piston core is lowered to the seabed with the trigger core dangling freely below it |
Pic 2: When the trigger core strikes the seabed, tension is removed from the trigger line causing the trigger arm to lift and the mechanism to open - releasing the piston core |
Pic 3: With the trigger mechanism open - the piston core is released and gravity causes the big heavy weight to fall towards the seabed |
Pic 4: The core penetrates the seabed and is driven deep by the weight pulling the sediment into the barrel.. The piston remains level with the seabed - thus preserving the core structure. |
Pic 5: The piston core is pulled out of the seabed. The core Catcher stops the core from falling out of the core barrel as it is recovered |
Recovering the Piston Corer
To get the massive corer back onboard so that the cores can be removed the RRS James Cook is a cumbersome operation, but our lads get it done in no time. In order to get the piston core back onboard a special frame is attached to the side of the ship
Before we can start to recover the piston core the both the trigger mechanism and the smaller trigger core must be removed.
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| Above: The trigger core is brought onboard |
Above: The trigger core is then removed |
Above: Then the trigger mechanism is removed |
The piston core is then winched up to deck level and slotted into a special holder where it is detached from the long restrictive wire used to raise and lower it to the seabed.
With the wire removed the piston core is then lifted by the ship's crane into the special frame attached to the ship.
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| Above: The piston core is placed in a special holder attached to the side of the ship |
Above: With the core in the holder, the tension is removed from th wire which is then detached |
Above: The whole piston core is then craned into the recovery frame |
Once the piston core is in the recovery frame the whole core is secured to stop it falling out. The recovery frame is pivoted allowing it to rotate. A winch at one end then pulls the bottom end of the frame until it is horizontal with the ship
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Above: A pivot at one end of the recovery frame
Above: A winch at the other end of the recovery frame |
Above: The recovery frame in the horizontal position ready for the core barrels to be removed.
The core barrels are then detached from the big core weight and the plastic tubes containing the cores are removed.
Above: The core barrel is removed from the piston corer
Above: The plastic liner containing the core is then removed from inside the core barrel
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| Above: Cutting a core sample into a mangeable size |
Above: Cutting a core sample into a mangeable size |
 
Above: My lovely assistants with a manageable cores - usually we cut the cores to 1.5m in length
These small core segments are then cut in half lengthwise so the sample can be examined and subsampled.
Above: A Core sample is cut in half prior to examination
The cores are examined......
Above: Scientists examining core samples and taking samples
Above: A core sample showing the distinct layers
But why do we get cores?
By studying sediment, our scientists can learn about ocean circulation, climate, the formation of ore deposits, the movement of oceanic plates, salinity of water, and the stability of the seafloor for oil drilling and exploration. Sediment cores allow scientists to see the presence or absence of specific fossils that may indicate climate patterns at times in the past, such as during the ice ages. Some scientists refer to the cores as time capsules, because the information they contain can span the past hundreds of thousands and even millions of years. Scientists may then use this information to improve understanding of the climate system and predict patterns and events in the future.
Above: Scientists examining a core sample
Above: Scientists examining a core sample
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