Behold the mighty conductivity temperature depth (CTD) rosette! This is one of the workhorses of ocean research. Sensors measure conductivity (a proxy for salinity), temperature, light level, and other environmental parameters as they change with depth. These sensors are attached to a circular frame called a rosette that holds an array of sampling bottles that can be triggered to close at specific depths. Other instruments and sensors can be attached to the frame as well, such as the cameras operated by Tobias Strickmann and Anton Theileis.
We deploy two CTDs: one with a stainless steel frame and one with a titanium frame. The titanium CTD is important for trace-metal clean work; many scientists onboard are interested in metals naturally found in low concentrations in seawater, and it is important for their equipment to be free of the metals of interest.
The stainless steel CTD is operated by GEOMAR masters students Paula Damke and Hannah Melzer, and the titanium CTD by Elisabeth Bauer (Caeli Wind GmbH), Thorsten Schott (Briese Research), and Dominik Jasiński (GEOMAR). They oversee the CTD’s journey and ensure the bottles close at the correct depths, which are often determined based on the sensor’s water profile measurements taken while the CTD is descending.
When the titanium CTD returns filled with seawater, the bottles are detached from the frame and transported to a trace-metal-free “clean” container on the deck, where the water is then processed for trace metal measurements. Scientists working in the container must change into special shoes and wear a plastic suit to protect their samples from metal contamination.
Once the stainless steel CTD is secured on deck, scientists gather like parched wildlife around a watering hole. Since trace metals aren’t a concern, the bottles can be sampled while still attached to the rosette. It’s a bit like the office water cooler, except with more options; there is a nozzle at the bottom of each bottle that can be pushed in to start the water flow into the waiting container. But just like on the African savannah—and perhaps the office too—there is a strict order to who gets first water access. The order depends on the sensitivity of the parameters to be measured. At the top of the “hierarchy” are the dissolved gases. Helium can travel through the plastic sampling bottle, so seawater for helium isotope analysis is transferred to a less permeable copper tube as soon as possible. Oxygen, on the other hand, is more likely to enter the seawater solution the longer the water is in contact with air and is therefore measured immediately after collection.
At each station, the titanium CTD is deployed once and the stainless steel CTD is deployed multiple times to ensure there is enough water to go around. Deployments regularly reach 4000-5000 m depth, and the journey lasts for hours, so waiting a while for water is just part of the game—and usually, there is other work to be done in the meantime!
By Charlotte Eckmann, with chemistry consulting by Tabea von Keitz.