Hydrographical measurements in the Fram Strait
This article is part of the series “2024-08: Fram Strait cruise KPH”.
You see the map with Greenland to the left and Svalbard to the right? And all these orange dots?
Each of the dots stands for a cast where an instrument or a set of instruments was lowered into the water. As you see it was quite a lot of casts that the researchers and the crew of the icebreaker Kronprins Haakon conducted in the Fram Strait between 14° W and 10° E within two weeks.
Let’s start with the most prominent hydrographical instrument, the CTD.
CTD stands for Conductivity, Temperature, and Depth. It is a group of instruments to measure these important water properties. The conductivity is directly related to salinity. While the CTD is lowered these three properties are measured continuously which results in so-called “profiles” that show the salinity and the temperature per depth. With these properties oceanographers get the first insight in what kind of sea water it may be. It is Atlantic? Or Arctic? But there are also other sensors mounted to the CTD to measure properties such as fluorescence or current velocities.
Also part of a CTD is the rosette of “Niskin bottles”. These bottles can be opened and closed remotely underwater to fetch sea water in defined depths on the way up. The large CTD has 24 of these bottles. When it is on board again and stands on the euro-pallet in the main hangar on deck 3 then it is time to fetch water samples from the Niskin bottles.
Some of the samples will be processed on land after the cruise but many samples are processed directly. Two examples:
Anne-Marie Wefing (NPI) uses the Winkler titration to measure the concentration of dissolved oxygen in the sea water. My chemical knowledge is too weak to explain this process. For the chemists amongst you I refer to the Wikipedia article for details.
Daniel Koestner (University of Bergen) leads water through different types of filters. The water can pass, particles exceeding a certain size will remain in the filter. While the deeper sea water is quite pure the samples from the upper water column contain more particles. The difference is visible by the naked eye but only land-bound lab work will give quantifiable results. Some of the filters can be transported at room temperature, those for chlorophyll have been frozen at -80 °C.
Daniel Koestner is also part of the team that measures particle concentrations directly in the water using a laser. This is how the instrument of the “optical cast” looks like:
The CTD is a heavy-duty instrument and could be used more or less all the time. Other instruments such as the laser or the MSS are much more fragile and the usage had to be skipped when the waves were to high. Too big the risk, that an expensive instrument would crash. But as far as I know everything has gone well!
Some cruise stats:
CTD casts | 73 |
Optical casts | 44 |
MSS casts | 27 |
Total casts | 144 |
While I have the luxury to work whenever I want the sheer amount of casts has led to a huge amount of lab work and some teams have been working in shifts, because casts have been conducted day and night.
Disclaimer: This is a private blog. I try to stick to the facts as best as I can. However, this article has not been proofread by anyone, so some facts may be slightly inaccurate or even plainly wrong. If you find an error, please let me know and I will correct it. Thank you!