It is now time to say goodbye!! We have been transiting for the last few days and just reached Reykjavík, Iceland. Next, we will take all our science gear off the boat (a process called demobilization), thereby concluding the expedition. Over the past couple of days, we have been frantically packing and cleaning the labs that have been our workplace for the last month.

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!

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.

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!

When I have gone to sea in the past, we have always been far enough offshore that we never see land until arriving back at port. On these trips, you get used to the more subtle array of views the open ocean has to offer—sometimes stormy, sometimes foggy, sometimes calm. When you are lucky you see a whale or dolphin.

On this trip we got close to the coast twice in the mouth of fjords off the coast of southwest Greenland. Both times, we arrived in the early morning, only seeing glimpses of land through dense fog. Slowly the fog lifted, and by early afternoon we got better views of the steep, rocky coast and icebergs around us.
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The Greenland ice sheet (and all glaciers for that matter) slowly grinds up the rock beneath it and deposits fine “glacial flour” at the glacier terminus. One of our scientific goals on this cruise is to understand the impact of these processes on elemental cycles—particularly iron and neodymium—in the ocean.

The geochemistry of glacial fjords provided a great excuse to get close to shore, but seeing Greenland like this will be a once-in-a-lifetime experience for most of us. As we have come close to the coast, I have been reading about the nearby villages. The town of Paamiut, with a population of 1,300, was at the end of the first fjord we went to. About a week later, we went further south to a second fjord near Qaqortoq, population 3,050, capital of the southern Kujalleq municipality, and the 5th largest town in Greenland.
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The weather was clearer near Qaqortoq, and we got views not only of the mountains surrounding the fjord, but of the Greenland ice sheet itself—which at first looks like low clouds hanging over the mountain top. Even when we left the fjord mouth, we had great visibility for the rest of that day and into the next. It was hard to stop looking at the views that surrounded us as we continued to take multicores and seawater samples. Mostly it left me with a sense of awe, but it was hard not to feel the vulnerability of the ice sheet when you actually see it up close. As with all the research cruises I have been on in the past, there will be lots of memorable moments, but I think seeing Greenland will be close to the top of the list for a very long time.

The largest habitat

The deep sea, the largest habitat on the planet, is a mostly unexplored, perpetually dark environment, with crushing pressures and temperatures only a few degrees above freezing. As scientists, we seek to explore this remote, foreboding realm of our planet for many reasons.  One of the primary motivations for our quest for deep sea mud on this trip lies in the fact that deep sea sediments archive valuable clues to the history of changes in ocean conditions and climate. 
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The average depth of the ocean is about 3700 meters (over 12,000 feet), which is about 3670 meters deeper than most scuba divers can go.  Obtaining samples from this remote and extreme environment requires specially designed equipment.  There are several devices that can be used to collect seafloor sediments (including manned submersibles and remotely operated vehicles), but the most commonly used methods send sampling gear to the depths using a winch and a very long wire cable.  The primary device that we chose for sampling the top few inches of the seafloor on this expedition is the “multicorer”.

Just a little off the top

​The Ocean Instruments multicorer MC800 that we are using is designed to collect just the very surface of the seafloor, keeping intact the fluffy, dust-like materials that often lie on top of denser mud.  We are interested in the chemistry, sediment characteristics and microscopic life of these seafloor surface sediments.  An understanding of these surface muds will help us interpret clues about ancient oceans hidden in deeper muds.  Essentially, for this project we get to play with mud!

Ocean views

​On this trip we have added a state-of-the-art camera system to the protected inner area of the teepee.  The Multidisciplinary Instrumentation in Support of Oceanography (MISO) camera system developed at Woods Hole Oceanographic Institution provides live views of the seafloor, and also enables us to watch part of the mud collection process.  Like the multicorer, this camera system is designed to function in the harsh conditions of the deep sea. With this unique camera system, we can view the seafloor several feet away before we decide to take a sample (sometimes there are just too many large rocks and not enough mud).  When we find a good spot, we send the multicorer down the rest of the way to the seafloor.

Eight is enough

Once the “feet” of the metal teepee touches the seafloor, the cable begins to go slack, and the eight tubes are slowly pushed into the sediment by the lead weights.  A “slow-down” cylinder on the multicorer ensures that the tubes go into the seafloor slowly, keeping the sediment-water interface as intact as possible.  Under ideal conditions, the tubes are about half-filled with sediment and half filled with water when the process stops. At this point, the winch starts bringing the multicorer back.

Discovery

​The multicorer typically travels back to the surface at 130 feet/minute, and depending on the water depth, it can take a while to get the multicorer’s sediment cargo up to the ship.  Excitement builds as the multicorer reaches the surface.  We all can’t wait to see what the deep seafloor at this site looks like up close.  There is a strong sense of discovery and wonder when we look through the clear tubes to gaze at small patches of unexplored ocean bottom landscape.