On the Drake Passage Expedition, we use a variety of equipments, including tow- and drop-cams to get images from the seafloor (see Rhian’s post), and the CTD to get samples from the water column (see Tina’s post). Specimens, alive and fossil, organic and inorganic, are sampled using trawls and dredges. While this relatively simple equipment has been used since the infancy of deep-sea science, it is reliable, efficient, and can be deployed in a variety of situations from different types of research vessels. While the trawl works very well on soft substrates, the dredge is well adapted to sampling on rugged terrain (for example, on the summit and slope of a seamount). The dredge is also safer to use when we don’t know exactly what the area is like, and it is the instrument that we tend to use the most. During a typical operation, the dredge is dropped straight to the bottom, and after giving the wire some slack, the boat slowly advances in a straight line, paying wire at the same time. After about 30 minutes, the boat stops, and we slowly pull the dredge back in. This is when the sampling of the sea floor happens, and when we have to be particularly vigilant. While being pulled, the dredge might be caught on some feature (e.g. boulder). If the dredge does not continue its course, something will eventually break, and we might lose our precious equipment. The entire dredging operation can last from about an hour to an entire day, depending on the depth at which we sample.
Us deep-sea biologists in-training need to know how and when to use the different tools available, yet this is not the kind of knowledge that we get from the classroom. Here, we learn by doing, and we are very lucky to have chief scientists that make sure that everyone on the science team gets a chance to learn some about everything.
A few days ago, Rhian let me “run my first dredge.” In the dry lab, I face a wall of flat screens displaying information (among other things) on the ship course, and information on the winch, the hauling device that connects us to the dredge. Once the dredge is laid on the seafloor, we radio the winch room to start paying out wire at 30 m/min, and the bridge to keep the boat advancing at the same speed, so the wire can be neatly laid on the seafloor behind us. Once our transect is laid out, it’s time to sample some corals. “Winch room, winch room – dry lab. Please start hauling in at one zero.” I watch the tension of the cable: we don’t want to lose the dredge to a big boulder. The tension of the cable is expected to vary between 2,000 and 5,000 pounds; above that: the cable is under significant tension and we might have to stop the winch. A few minutes pass by. Tension is stable under 5,000 pounds. Michelle brings us some cobbler. Woohoo! Peach! Cable tension spikes. Eric’s blood pressure too. It’s alright, says Rhian. We probably hit a rock. Enjoying the cobbler. Tension spikes. 7,500. Ok – no more cobbler... All eyes on monitors. Tension spikes again to 9,200 pounds. We get concerned, because the safety ring that links cable and dredge is designed to snap at 9,000 pounds… We might be dragging the dredge by one side only! Tension is low again… and spikes to 11,400! We must be sampling on very rugged terrain! This is nerve-racking… Have we lost the equipment? At the end of our transect we haul the wire back in, hoping that the dredge is still attached. On the freezing deck, we are burning of anticipation. The water surface bubbles… The dredge comes out in one piece! It is full of rocks and specimens!
Weather: temperature 41 ºF, windchill 12 ºF, windspeed 20-40 knots, cloudy, relatively big seas
|Eric running a dredge. Whoever is driving the dredge has to keep in close radio contact with the winch operator and the bridge (R. Waller).|
|Mark and Michelle pointing out organisms growing on a rock brought up by the first dredge Michelle has run (R. Waller).|
|Kate, Sandy and Skip clean out the dredge, ready for another deployment (A. Margolin).|
|Working inside means lots of prepping, processing and packing samples! This fossil coral is a Caryophyllia antarctica cup coral collected from Interim Seamount, and will be used for paleoclimate reconstruction. (A. Margolin).|