Wednesday, May 18, 2011

All that water in the ocean …

17th May 2011

Being out in the Southern Ocean on a ship has one thing in common with every other seagoing expedition: there is water all around us! It is the wonderful medium we are sailing on, but it is also the ‘air’ the corals breathe. Modern corals build their skeleton out of the ingredients they find in seawater today, and the fossil ones record the seawater chemistry of the past in their skeletons.

In the global ocean each water mass has distinctive physical and chemical properties. If we can measure these properties precisely enough, we can distinguish the different water masses from each other. Here in the Drake Passage we have a set of water masses that play an important role when it comes to understanding the modern ocean circulation that transports heat and carbon around the globe. This pattern of ocean circulation has been different in the past, and it plays an important role in understanding climate change.

But let’s get back to the water issue … In order to make meaningful interpretations of past water chemistry recorded in coral skeletons, we first need to get a good handle on the modern water chemistry. This is one of the reasons why we collect quite a bit of water on this cruise as well. We have two ways of doing so. The standard way of collecting water is to use the ship’s CTD. This is a big carousel with a central unit to measure conductivity, temperature, and depth (CTD), as well as oxygen content, and fluorescence. It has 24 bottles attached to it, which have lids at the top and the bottom that can be opened and closed remotely. All bottles are open when the CTD is lowered into the ocean, and we can ‘fire’ the bottles (close them) at any water depth we like on the way up. The second means we have to collect seawater is from similar bottles, attached to the towed camera system we use to take pictures of the seafloor.

Once the water is onboard we take samples for a large array of chemical measurements ranging from dissolved carbon and other nutrient concentrations, to samples for isotopic measurements of various elements. The only measurement we do right here on the ship is to determine the alkalinity of the water samples. But this is a topic David Case, one of the graduate students on our expedition, will explain in more detail in one of the upcoming blogs.

Today was a great day, as we got our second CTD. Today was also squish cup time! You have no clue what I am talking about? They are great souvenirs scientists like to produce during the process of water sampling, and here is how the story goes… What you need is a white cup made out of Styrofoam, a good selection of colorful sharpie pens, and some creativity. The decorated cup then goes into a meshed bag, which gets attached to the CTD, and descends to the deep ocean. As the pressure increases with water depth, all the air is squeezed out of the cup and it is shrunk to a much smaller size. It’s really a great souvenir to take home and you can see some examples in the pictures.

By: Tina

Weather: temperature 35°F; windchill 5°F; windspeed 25 to 30 knots; cloudy, windy, with some precipitation

Sandy deploying the CTD from the “Baltic Room” of the Palmer (M. Escolar).

MT Stian teaches the night watch how to prepare a CTD for deployment (R. Waller).

Tina sampling the CTD (A. Margolin).

Kate Sampling the CTD (T. van der Flierdt).

Squish cups!! There are some cups unsquished in the background as a comparison (A. Margolin).

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