Mud, here we come!

What is a gravity core and why do we use them?
 
We love mud!  Seafloor mud stores vast amounts of information about the present and the past and it’s the whole reason we are on this voyage.  We have two main ways of collecting it from the depths of the sea; the multicorer and the gravity core.  The multicorer collects eight short tubes of seafloor surface mud, but we need to take a longer core to get a glimpse of the past that is hidden deeper in the sediment.  This is where the gravity corer shines! 
 
The gravity corer is just what it sounds like, it uses gravity to plunge a long PVC tube into the seafloor to recover sediments that were buried long ago.  The length of gravity core tube that we can use depends on its overall weight when filled with sediment and the grade of wire cable used to pull the tube out of the sediment.  With the type of gravity corer we have, the wire we are using, and the type and thickness of sediment we are encountering, we decided to use a 10 ft (~3 m) tube on the gravity corer.  After making our careful decision on the length of core, we then got to assemble them!

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How it’s assembled
 
Step one was to label the long PVC tubes and cut them to size with a specialized tool called, no surprise, a core cutter!  A small round blade, like a pizza cutter, follows the circumference of the tube and is tightened as it cuts through the PVC.  This allows for a clean cut, so no plastic gets shredded into the sample or onto the deck (like it would with a hacksaw)!
 
Step two was to attach the top and bottom gravity core parts to the PVC tube to officially make it a gravity core.  The top part is a metal tube with a plunger in the top to allow for water to escape.  It also creates a seal with suction at the top, like a finger on the top of a straw when you trap liquid.  The bottom addition is actually two parts!  There is a metal cone like piece that is made of flexible metal fingers that is inserted into the tube so that sediment can get into the tube but can’t fall out when we haul it back onboard, like a lobster pot.  The second piece, the actual cutting piece, is added over the core catcher.  This cutter is exactly what it sounds like, it is angled in and bevelled so that when the tube encounters the sediment it can slice into it.  Now the gravity core tube is assembled!
 
Step three was to attach the weights to it because a PVC tube with a few metal bits attached is not going to travel fast enough to the seafloor over 3000 m away, let alone be able to plunge straight into the sediment!  These weights are attached at the top of the gravity core and need to be drilled into place so that everything is sturdy.  The weights are circular lead weights that weigh 25 lbs (11 kg) each and total around 600 lbs. (272 kg)!
 
Step four was to attach the top of the corer to the wire cable (on a spool attached to a winch) and deploy the whole thing overboard and hope that the bottom of the ocean that we surveyed will yield a good core sample!  We will only know when it is back onboard how much we were able to recover, or if we recovered anything at all.  Sometimes the surface of the seafloor is too hard and sometimes the sediment is sandy, and does not stay in the core, even with the core catcher.  Both these scenarios can result in an empty gravity core.

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So how did we do?
 
We were able get several great cores!  We recovered up to 6-8 ft (1.8-2.4 m) of sediment in most of the successful cores.  Once the core was back onboard and we determined that the recovery was successful, we processed the core for storage.  We apply a strict labelling scheme and cut the tube in half carefully, packing any gaps in the end with foam to maintain the core shape of the sediment. 
 
Unfortunately, some of our cores (4 out of the 12 we attempted) came up completely empty!  We think that two misses were because the surface of the seafloor was too hard and the other two were mostly sand.  This still gives us information about the area but in far less detail!

Chief Scientist Sophie Hines celebrating a successful core! Look at the beautiful streaks of mud on the side of the core! Woo! A great sight to see after a few unsuccessful attempts.

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Where do they go after the voyage?
 
To fully finish our gravity core process, we need to trace the label, that we initially did in sharpie, with an engraving tool to permanently etch the essential information into the core tubes.  These cores will be sent to, and kept cool in, the Oregon State University Marine Repository for safe keeping until we are ready to sample them. 
 
The gravity core has been such an amazing tool for us, and we look forward to opening the core tubes up, sampling the muds, and discovering what ancient ocean sediments have to tell us about the history of environmental changes in the regions we visited.

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Author

Kira Sirois