Problems with the CTD

Hello everyone!

Like I mentioned in my last post, CTDs are one of the most important instruments in studying oceanography. Different sensors on the CTD measure conductivity, temperature, and depth, as it is sent down through the water column in a CTD cast, giving us profiles of the water they move through. This helps give oceanographers get a sense of how water changes down the water column. On the Armstrong, we have a rosette attached to the CTD. This rosette has 24 niskin bottles, which are essentially big tubes with lids on the top and bottom that close on command as the CTD rosette moves back up to the ship from the bottom of the ocean. This allows us to sample water from the bottles, and can test for dissolved oxygen, nitrate, chlorophyll, dissolved inorganic carbon, dissolved gases, and particulate organic carbon, so that we can understand what is happening in the biological carbon pump at different depths. The CTD rosette also has a variety of different sensors which measure things like oxygen, chlorophyll, and salinity. One of our favorite sensors is the dissolved oxygen sensor, because it is one of the main variables we are looking at in our research.

Chris and Amy (wonderful SSSGs) working on fixing our CTD. The instruments are located in the core of the CTD rosette, and the niskin bottles are the the grey cylinders. Photo Credit to Shawnee Traylor. 

However, in a classic example of how things don’t necessarily go to plan at sea, we have been having problems with our CTD rosette. When we sent down the CTD cast for a test run on the second day we were at sea, we received data from the CTD instrument itself and the rest of the sensors attached with it (so we could see depth, temperature, salinity, and chlorophyll), however we couldn’t close the bottles that we needed to practice taking samples with. After a few days the SSSGs (incredible people who help with technical support on the ship) opened the chamber that held all of the hardware for the bottle closing mechanism to find 3 inches of water! The components were would not be able to work because of the salt water, which meant that we will not be able to close the bottles, or take samples from them. This shifted the focus of what we could do for our research. OOI provides incredible infrastructure to oceanographers by providing an array of sensors at different depths in the Irminger Sea. However, our data for studying the biological carbon pump needs extremely precise measurements, so without sampling from the rosette, we would not be able to calibrate the sensors to the extent that we would need to.

Pieces of CTD rosette hardware that have salt crystals on them after getting wet

Closer up picture of salt crystals on CTD rosette hardware 

So far, we have three solutions. The first is to add an oxygen optode (or a sensor that measures dissolved oxygen) like the one we have on our gliders and on the surface mooring to the CTD sensor package so that we can better understand the calibration between multiple instruments. The SSSGs did this the day after we found out we wouldn’t be able to take water samples. However, because one of the main goals of our project was to provide more specific calibration points for our biological carbon pump measurements, we wondered if we could do more.

CTD rosette set up with acoustic releases (yellow cylinders)

Acoustic release mechanism that will close the niskin bottles. This piece is found at the bottom of the acoustic releases

This led us to our second option, which is to try to figure out a way to get a couple of bottles to close with an anchor release. On the night that we found out that the CTD wouldn’t work and people started suggesting all kinds of different solutions, and James, a science crew member, suggested this ingenious solution. On all of the moorings, there is a system that releases the cables and instruments on it from the anchor that held it down for a year. The idea is that we would use the same mechanism that releases the cable from the anchor to close the bottles at specific depths. Last night we put this idea to the test. Using acoustics, Stephanie and James fired the acoustic release, and closed a bottle. We were able to close three bottles, and take samples!

Everyone happily sampling from the niskin bottles last night 

However, the CTD still needs the part that was broken on its test run. Our chief scientist has decided that now that we have finished deploying and retrieving all of the moorings that we need to, we will go up to Greenland to pick up the part that was broken. The current plan is to meet a ship off the coast with someone from WHOI who will be in a smaller boat with the part we need, after having taken an epic journey traveling from Woods Hole to bring it to us. After we pick up the part and fix the CTD, we will then try to sample as many of the sites in the OOI array as we can. Weather could be a lot rougher than it has been so far, so hopefully we will be able to get back out to the array before we start our transit home so that we will be able to collect as many samples as we can.

That’s all for now, wishing us a quick transit to Greenland!