CTD & Rosettes

03 October 2013

The CTD/rosette is a device that enables scientists to explore some of the deepest parts of the ocean, without getting wet! The CTD/rosette is an instrument that consists of sampling bottles attached to a metal framework. On the framework there are instruments that measures temperature, salinity, and depth. Other instruments can be attached to the rosette as well.

On a research ship, the CTD/rosette is one of the most important scientific tools. On an average day, activities are centered on deploying the rosette. The research ship steams to a station, a specific location that has been selected to put the rosette in the water. The deployment of the rosette is like a well rehearsed, choreographed dance. The rosette is prepared in the hangar by the deck team and the instruments are checked. In the lab, the scientist on watch is monitoring deck activity and remotely turns on the CTD/rosette power and initializes the software. Now the deck choreography goes into high gear under the guidance of the deck leader, who carefully watches the machines and his crew to see that nothing falters.

Following the deck leader's hand signals, the winch operator takes in a few feet of the CTD/rosette cable to lift the rosette off the platform and extend it over the water. He slowly pays out cable. The deck crew works together to lower the rosette into the water. The deck leader then signals the winch operator to let out cable so that the CTD/rosette slides into the ocean, just below the surface, so that his deck crew can slip off their tag lines, which keep the CTD/rosette from accidentally hitting the side of the ship as it is deployed. Then the deck leader has the winch operator lower the CTD/rosette to 10 meters.

Once the CTD/rosette is 10 meters underwater, the scientist in the lab then takes over, directing the winch to lower the CTD/rosette close to the bottom. This process involves coordination between the lab and winch operator, who are in constant contact via a special two-way hands-free phone. It can take from one to two hours to lower the CTD/rosette. On this cruise it will go as deep as 4700 meters at some stations. The descent can be slow due to the swell of the Southern Ocean, which affects how the ship, the cable, and the CTD/rosette interact. The scientist on watch in the lab monitors the descent, and with the help of the CTD/rosette's on-board altimeter, the scientist can ensure that it doesn't hit the bottom.

During the ascent, the scientist in the lab will have the winch stop at pre-selected depths so that the bottles can be "tripped," or closed, by computer command from the lab. On the particular CTD/rosette being used aboard this ship, there are 36 bottles. The CTD/rosette ascends at a maximum rate of 60 meters per minute, so it may take several hours to reach the surface. Once it reaches the surface, the choreographed deck crew operation runs in reverse, only this time with water samplers standing by to greet the arrival of the CTD/rosette.

Once the CTD/rosette is back on deck and secure back inside the hangar, the students, scientists, and technicians waiting to take water samples good-naturedly jostle each other for their turn to extract samples. The aptly named sample cop is in charge of keeping track of whose turn it is to sample, and ensuring that the samplers follow a specific sampling order. Water samples for dissolved gases can be affected by time and exposure to air, so it is important that the water samplers for these go before team members sampling less time- or exposure-sensitive properties, such as salinity (dissolved salts). The sampling area has a friendly atmosphere, although the scientists take sampling very seriously.

Although it is entertaining to watch the skilled choreography on deck, it is important to remember that it serves a key purpose. Cruises such as this one enable scientists to acquire much-needed data. The coordination of the deck team, the winch operator, and the scientists is an important step in getting this valuable data. The most amazing part is, the deployment of the CTD/rosette is only one part of the astonishing amount of teamwork required to gather data in the remote corners of the ocean.