Science to the depth of the ocean (28 May 2023)
Current position (diamond shape). The circles are the positions at previous blog posts, the black line is roughly the route we took (base map from 
ggplot2).
I have already mentioned CTDs and stations a couple of times, and today I want to explain what they are and the overall aim of the cruise.
The cruise is part of the GEOTRACES programme. The aim of this project is to increase the knowledge of trace metals and their isotopes in marine environments. Many trace metals have a positive or negative impact on marine life, and they are also used to better understand biogeochemical cycles and previous climate changes. Therefore, they play an important role in increasing our understanding of the ocean ((here you can find a – not so recent – overview of trace metals in the ocean). ). There are still many gaps in our knowledge, and GEOTRACES aims to fill them.
Pic01: GEOTRACES cruises in the central Pacific. We are currently doing GP11. (Map from https://www.bodc.ac.uk/geotraces/cruises/section_maps/interactive_map/)
We are doing the transect called GP11 along the equator and across the Pacific. As you can see on the map, we have crossovers with other cruises. These are important to compare the data between cruises.
The abbreviation of our cruise is SO298. SO is for our ship, the "Sonne", and the number increases with every cruise the Sonne does. In preparation for the cruise, a logo was designed and stickers were printed.
Stickers with the logo for everyone (Picture: Rieke Schäfer)
As part of GEOTRACES, the focus of this cruise is trace metals. The aim is to better understand their and their isotopes’ distribution, sources and sinks. This region of the Pacific has been studied very little and is therefore especially interesting. Additionally, measurements of ocean chemistry (such as my pH measurements) and biological measurements are taken.
For many of these measurements, water samples from the depth of the ocean are needed. The instrument needed to take these samples from great depths is the CTD.
CTDs
CTD stands for conductivity (used to calculate salinity), temperature, and depth (calculated from pressure). These sensors are part of a much larger frame with many more components. But even so, they are comparatively small, and often everything together is called the CTD. The biggest part of the frame is taken up by the Niskin bottles. These are big bottles with an opening in the top and bottom, and they are used to take the water samples. While lowering the CTD through the water column, they are open, and then, when the CTD comes back up again, they are closed at different depths.
In preparation for the station. The caps of the Niskin bottles still need to be opened.(Picture: Rieke Schäfer)
There are a number of additional sensors in the CTD, for example, an oxygen sensor and a nitrate sensor. At some stations, a camera is fixed to the frame to take pictures of the plankton. (More details in the GEOMAR blog.)
We are using two different CTDs. One “normal” one made from stainless steel. It is used for all samples that do not get contaminated by it. The second CTD is made from titanium and is used for the trace metal samples. As far as possible, this prevents contamination during sampling, and the bottles are then taken to the clean air container for filtering and distribution
Both CTDs are lowered down to the seafloor, even to a depth of more than 4000 m. With a velocity of about 1 m/s, the whole process takes a few hours. While the CTD is in the water, the ship stays at its position or just drifts. The "Sonne" can maintain its position; however, this consumes extra fuel. The positions of the stations were decided before the cruise by Eric, as the lead scientist, and now he is adapting them to the current situation. We get a plan of the next few stations with their positions and (more importantly) the expected times.
Close to the water’s surface, the currents are quite strong. You can see how the CTD is drifting compared to the ship. At larger depths, the current decreases and the cable goes down vertically. (Picture: Rieke Schäfer)
Once the CTD comes up again, it is fastened, cleaned, and the cable connections are dried with air. After that, the sampling starts. As mentioned, for the titanium CTD, the bottles are taken to the clean lab. For the other CTD, carrying the bottles much further is, luckily, not necessary. These samples are taken directly to the hangar. The whole process takes quite some time, and there is a set order of who takes their samples first. Some of the samples get more easily contaminated through contact with air (oxygen for example), while for others, it does not play a major role. To reduce the air contact time for the more easily contaminated samples, they are taken first. In order to have enough water for everyone, who can take how much water is also regulated.
Back from the depth of the ocean (Picture: Rieke Schäfer)
In order to know from which depth the water of each sample came, the Niskin bottles are numbered and get a code for each station. With this code, it is easier to know which results belong together. In preparation for each station, labels with these codes are made ready to reduce confusion and make it easier for everyone to know the correct codes.
If possible, our measurements are taken here on board. Nutrients, for example, can be measured directly. All the samples that can’t be measured here are prepared as much as possible and fixed. They will then be analysed later. In some cases, it might take a year or more before the results are known.
Communication on board
With so many different topics and samples, it is, of course, important to communicate and relay important information. The plan for the next few stations is always shown at a few central points, and additionally, we can see it in the ship’s intranet. There, we can also see the time until the next waypoint, which is usually the next station.
Every Tuesday and Friday we have a short “science meeting”. In this meeting, one of us gives a short presentation on results from previous cruises or experiments. We also get an update from Eric on anything that might be of importance. The talks are always interesting because they give an insight into what people are looking at with the measurements they are doing.
Permits are necessary to take measurements in the exclusive economic zones of other countries. These permits might include some requirements (such as specific cleaning of instruments). I was surprised that these permits are granted at quite short notice. Some of them have only come in while we have been on the cruise, so just a short time before entering the waters. Details about requirements and updates about permits are also given in the science meetings.
Often it is surprisingly difficult to find people. The ship is not that big, but still, I am often looking for someone or other, more or less successfully …
PTB doctoral student 
Rieke Schäfer is blogging here directly from the RV "Sonne" on her way west from South America across the Pacific Ocean.