A day in the life of a CTD watch - part 1/3

One of the last wandering albatrosses we saw on our cruise, captured here at about 40S in the southern Indian Ocean. This species has a strong preference for the cold, deep waters around Antarctica.

The last series of posts have been about Argo floats. Deploying these floats for SOCCOM was one of my responsibilities onboard the Revelle. However, I have spent the vast majority of my time on this cruise working as a CTD watch stander. I am one of five CTD watch standers on this cruise. We are divided into day and night shifts. Seth and Hannah work on the day shift, from noon to midnight; Natalie, Dave and I work on the night shift, from midnight to noon.

The starry eyed night shift watch. We love to have meetings at the top of the rosette - not really. Photo courtesy of Joseph Gum.

As watch standers, our lives revolve around the rosette. We assist with all stages of the rosette's deployment, recovery and sampling. Additionally, we work closely with the chief scientist and help her in whatever way we can. You can think of us Santa's little helpers. On the busiest days a normal day, we work continuously throughout our shift.

The rosette. Unfortunately, we lost this one, and all of our best instruments, to the ocean; it is currently sitting on top of the Kerguelen Plateau. It was a huge loss at the time, but the crew managed to assemble a second rosette from spare parts in less than 24 hours. It was a remarkable accomplishment.

This is the rosette that took us through most of our cruise. Even though it was a backup, it has served us quite well.

My days begin at around 11pm, an hour before my shift starts. I usually get a cup of tea and a light meal before heading to the lab. At 11:50pm, we switch off with the day shift and pick up wherever they left off. More often than not, we take over in the middle of a sampling session so we have to be ready to go the moment our shift starts.

Natalie inspecting the rosette before deployment.

One of the main tasks of the CTD watch is prepping the rosette for deployment. We begin this process 30 minutes before arriving at a station. This involves cocking each Niskin bottle open, closing all the spigots, sealing all the vents and inspecting the rosette for any potential malfunctions. When that's done, we walk around the rosette to double check then triple check that everything is in order.

Dave doing prep work.

After the rosette is prepped for deployment, we change into our deck gear to help with the launch. This is one of the more fun parts of our job. For the launch, our job is to maintain taut tag-lines as the winch operator raises the rosette over the deck. We take instructions from the on-duty research tech (res-tech for short), who also directs the winch operator.

Rosette deployment on probably the nicest day we had on this cruise. Dave, Natalie and I are working the tag lines. Our job is to stabilize the rosette as the winch lowers it into the water. John (center), the on-duty res-tech, directs the deck operations. Photo courtesy of Cara Nissen.

Rosette launch is one of our favorite parts of the job because it is an excuse for us to be outside and it makes us feel like real seamen :). I have many videos of us in action but I'll have to wait till I get to shore to upload them.

Once the rosette is in the water, we change into our normal clothes and head to the computer lab. In the computer lab, we sit at the CTD console, which is a checkerboard of monitors that displays live readouts from the rosette's instruments. For this part of the job, we monitor the rosette as it moves through the water and log all important events. Additionally, we are in constant communication with the winch operator via a speaker phone and guide him as he lowers the instrument to the seafloor.

The CTD console. This where we sit to monitor the rosette when it is in the water.

For a single cast, we lower the rosette to just above the seafloor then collect water samples as we bring it back to the surface. The trickiest part of this process is the bottom approach. As the name suggests, this is when the rosette approaches the bottom of the ocean. Ideally, we want to stop 10 meters above the seafloor, but this is easier said than done. Even though we have instruments that give us estimates of the ocean depth, none of them have the range and accuracy to be reliable on their own.

With almost $1M USD worth of instruments (literally) on the line, we can't afford to make any mistakes. If we overshoot and hit the bottom, we can damage the instruments onboard the rosette. With so much at stake, we are always supervised when doing bottom approach.

Alison draws near to supervise bottom approach.

We use two main instruments to guide the rosette to the bottom. The first is the shipboard multi-beam sonar. The multi-beam is mounted on the bottom of the ship and faces the seafloor. It works by sending a pulse sound waves at a specific frequency into the ocean then listening for the same pulse after it reflects off the ocean bottom. With knowledge of the speed of sound through seawater, the echo's arrival time, the multi-beam can produce an estimate of ocean depth.

The ship's multi-beam is very useful, but it measures seafloor depth over a relatively large area. If the seafloor terrain is very rugged, the multi-beam's depth estimate may be tens of meters different from depth directly beneath the ship or rosette. To obtain a more localized depth estimate, we use an altimeter that is mounted on the bottom of the rosette.

The altimeter onboard the rosette works in a similar fashion to the shipboard multi-beam. However, it is much smaller than the shipboard multi-beam and not nearly as powerful. The altimeter we have been using for most of the cruise can only detect the ocean bottom from 50 meters away. But once it does, it produces the most accurate estimate of distance to the seafloor.

Hannah working at the CTD console. Courtney provides expert guidance as Charlene looks on.

Under the normal circumstances, the multi-beam's depth estimate is good enough to get the rosette close enough to the bottom for the altimeter to kick in. From there, we rely entirely on the altimeter to safely get us to 10m off the seafloor.

To complicate things even further, both the altimeter and ship board multi-beam are known to "misbehave" and produce incorrect information at any given time. On rare occasions, we have to resort to old bathymetry maps to help us figure out the true ocean depth. This was a bit overwhelming at first, but after a few days on the job we learned how to weigh all pieces of information appropriately and make the right decisions.

Once we safely lower the rosette to 10m off the ocean bottom, we fire the first Niskin bottle to collect a water sample. After logging some notes, we then instruct the winch operator to raise the rosette to a new depth, where we would fire another bottle to collect another water sample. We usually do this for 36 different depths, the last one being at the surface.

Lowering the instrument to the ocean bottom and bringing it back to the surface usually takes about 3 hours. It can be a tedious process, but everyday presents a new challenge (usually in the form of equipment malfunction), which keeps things interesting.

As the rosette approaches the surface, we grab our life vests and helmets to help with the rosette recovery. This where we literally get to snatch the rosette out of the water. It's my favorite part of the job. However,  I'll continue this discussion in my next blog post, where I will also describe how we collect samples once the rosette is back on the ship.

Day shift crew recovering the rosette at the end of a cast.

Until next time,
Earle