At around 10:00PM on New Years Eve, the ship arrived on station and we finally got out of the computer lab to get our hands dirty. By “on station”, I mean that we positioned ourselves at 90E longitude on an oceanic ridge 3100 meters (2 miles) above the seafloor, instead of the usual ~4200 meters (2.8 miles). Out here in the open ocean it basically looks like we’re in the exact same place, except that now everything is upside down since we’re in the Southern Hemisphere.
Once on station, we immediately started working. We got the CTD device ready and then sent it down to the seafloor and back. The CTD is what we use to collect water samples, and it also tells us about the Conductivity, Temperature, and Depth in the column of water that we’re sampling. Conductivity basically says how salty the water is; temperature and depth are obvious. The gray bottles that are attached to the rosette are how we collect our water samples, and each bottle is hooked up to a trigger so that we can remotely close a bottle, effectively sampling the water from a specific depth. This is called “firing” a bottle, and you have to yell “Fire in the hole” every time you fire one off.
The C,T, and D aspects of the device are electronic and fully automated as the rosette descends and ascends in the water column, but once the the rosette is back on board after a descent, we need to sample the water manually. We transfer a small amount of water from each gray bottle into a glass vial, poison it with Mercuric Chloride (HgCl2), cap the vial and put it away until we can analyze it back in San Diego.
Mercuric Chloride is pretty nasty stuff, which is why we use it. Its LD50 (the amount it takes to kill half of a population of something, usually determined in rats–Lethal Dose of 50%) is 1 mg/kg. That’s about 90mg for a 200lb person (less than half of an advil). So we have to be very careful when using it to poison our water samples. We only have to transfer 50 microliters of the poison into each water sample vial, which is roughly the equivalent volume of 1 raindrop of water and each drop contains 4mg of the mercuric chloride. For perspective, 4mg is enough to kill something that weighs about 20lbs. The poison is necessary because it kills any bacteria or bugs that might be in our water sample; this ensures that photosynthesis/respiration of these little critters won’t alter the as-is properties of the water itself that we’re after.
So after we finished with the poisoning and capping, we prepped for taking a sediment sample of the seafloor. Keep in mind that this process started at 10:30PM; by the time the water sampling was done, it was roughly 1:30AM on New Year’s Day (scientists really know how to ring in the new year properly). As paleoclimatologists (I know that’s a mouthful, but not if you break it down; paleo-climate-ologists::past-climate-investigators), we’re interested in how things change through time, so we want to take a core of sediment that can reach as deep into the seafloor as possible. The mud on top of the core is most recent, and gets older the deeper you go into the sediment. To get our revered mud, we’re using what is called a gravity corer.
It is essentially a glorified PVC pipe. The bottom of the PVC pipe goes into a short cylinder of stainless steel that comes to a point, which allows for easier penetration into the sediment. The top of the PVC pipe is attached to a device that has a spring-loaded plunger, which is what allows us to pull the sediment out of the water.
Wait, what? How does a plunger do that? Think about drinking a beverage with a straw. If you put your thumb over the straw and pull it out of your drink, your thumb creates suction and the liquid stays in the straw. This spring-loaded plunger at the top of the core works in just the same way, but it has to have the spring attached so that as the core penetrates into the seafloor, the plunger can allow water that is already in the pipe to escape.
Then, in order to make sure the corer can actually get deep into the sediment, we add about 600 pounds of steel around it. Remember that this is all done by manpower.
OK, just kidding on that one. Everything gets hooked up to the main winch on the stern of the ship, then it gets lowered down to the seafloor at about 2 miles per hour, and boom! Sediment core. It gets pulled back up onto the ship, we open it up to see how much sediment we collected, then cut it down to size and put it in the refrigerator, to be shipped back to San Diego for analysis. That was all finished around 9AM New Year’s Day, and it was time for some much-needed sleep. Our amazing ResTech Meg stayed up another 5 hours to take one last core in the same area, and we are now underway to our next location for sampling.
This post is already much longer than it should be, so I’ll save the discussion of the science behind coring and water sampling for later!